Clara T Bolton
email: bolton@cerege.fr

Emmeline Gray

Wolfgang Kuhnt

Ann E Holbourn

Julia Lübbers

Katharine Grant

Tachikawa Kazuyo

Eelco Gianluca Marino

J Rohling

Anta-Clarisse Sarr

Kazuyo Tachikawa

Eelco J Rohling

Andersen

Secular and orbital-scale variability of equatorial Indian Ocean summer monsoon winds during the late Miocene

come

Introduction

The Asian monsoon is a major hydrological phenomenon driven by the pressure gradients created by asymmetric heating between the equatorial Indian and western Pacific Oceans and the Indo-40

Asian landmass, creating strong seasonally-reversing winds and ocean currents and heavy boreal summer precipitation over the Bay of Bengal and Indian subcontinent (Webster, 1987a, b;[START_REF] Schott | The monsoon circulation of the Indian Ocean[END_REF][START_REF] Gadgil | The Indian monsoon and its variability[END_REF][START_REF] Schott | Indian Ocean circulation and climate variability[END_REF]. Surface winds over the northern Indian Ocean (Arabian Sea and Bay of Bengal) and South China Sea are strong indicators of the strength of the South Asian and East Asian monsoon subsystems, respectively, and precipitation amount and 45 seasonality can also be diagnostic of monsoon strength (e.g., [START_REF] Webster | Monsoon and ENSO: Selectively interactive systems[END_REF][START_REF] Goswami | A broad-scale circulation index for the interannual variability of the Indian summer monsoon[END_REF]. Thus, past monsoon dynamics can be reconstructed using wind, runoff, and precipitation indicators recorded in marine sediments from these core convective regions. The Asian monsoon is known to have varied substantially over short (interannual to suborbital) and long (orbital to geological) timescales in response to forcing factors both external and internal to Earth's climate 50 system (e.g., [START_REF] Wang | Evolution and variability of the Asian monsoon system: state of the art and outstanding issues[END_REF][START_REF] Clemens | A 350,000 year summer-monsoon multi-proxy stack from the Owen Ridge, Northern Arabian Sea[END_REF][START_REF] Farnsworth | Past East Asian monsoon evolution controlled by paleogeography, not CO2[END_REF][START_REF] Kathayat | Indian monsoon variability on millennial-orbital timescales[END_REF].

There is great uncertainty surrounding the timing of Asian monsoon initiation and intensification, and the degree of coupling between regional monsoon subsystems. Discrepancies in part stems 55 from the fact that records come from the South Asian or East Asian monsoon subsystems, which are sensitive to different aspects of regional topography [START_REF] Molnar | Orographic controls on climate and paleoclimate of Asia: thermal and mechanical roles for the Tibetan Plateau[END_REF][START_REF] Clift | Correlation of Himalayan exhumation rates and Asian monsoon intensity[END_REF][START_REF] Clift | A history of the Asian monsoon and its interactions with solid 865 Earth tectonics in Cenozoic South Asia[END_REF][START_REF] Boos | Dominant control of the South Asian monsoon by orographic insulation versus plateau heating[END_REF][START_REF] Acosta | Competing topographic mechanisms for the summer Indo-Asian monsoon[END_REF]Sarr et al., in review). Further, differences in monsoon expression occur even within the core convective region of the South Asian monsoon (e.g., dominance of summer monsoon winds in the southern Bay of Bengal versus 60 monsoonal rainfall/runoff in the northern and eastern parts). Meanwhile, proxies generally record singular aspects of monsoonal climate that are not necessarily coupled on all timescales (e.g., winter versus summer monsoon, wind intensity versus precipitation amount or seasonality). Evidence for strong monsoonal climates (i.e., with strong seasonality of precipitation) exists across Asia during the Paleogene [START_REF] Spicer | Paleogene monsoons across India and South China: Drivers of biotic change[END_REF][START_REF] Licht | Asian monsoons in a 1035 late Eocene greenhouse world[END_REF], yet many terrestrial records from southeast 65

Asia suggest an onset of the monsoon near the Oligocene-Miocene boundary (~24-22 Ma) [START_REF] Guo | Onset of Asian desertification by 22 Myr ago inferred from loess deposits in China[END_REF][START_REF] Sun | How old is the Asian monsoon system?-Palaeobotanical records from China[END_REF][START_REF] Wang | Evolution and variability of the Asian monsoon system: state of the art and outstanding issues[END_REF]. Marine records of drift sedimentation near the Maldives archipelago as well as upwelling and oxygenation indicators from the Arabian Sea, both influenced by wind and surface ocean circulation, suggest an onset of strong seasonally-reversing South Asian (monsoon) winds and Arabian Sea upwelling during the late middle Miocene (~13-10 Ma) [START_REF] Zhuang | Monsoonal upwelling in the western Arabian Sea 1230 since the middle Miocene[END_REF][START_REF] Gupta | Evolution of the South Asian monsoon wind system since the late Middle Miocene[END_REF][START_REF] Betzler | The abrupt onset of the modern South Asian Monsoon winds[END_REF][START_REF] Betzler | CC BY 4.0 License. from the sedimentary archive of the Maldives (Indian Ocean)[END_REF][START_REF] Bialik | Monsoons, upwelling, and the deoxygenation of the northwestern Indian Ocean in response to 810 middle to late Miocene global climatic shifts[END_REF][START_REF] Nigrini | Composition and biostratigraphy of radiolarian assemblages from an area of upwelling (northwestern Arabian Sea, Leg 117)[END_REF].

In contrast, a time interval in which records from different regions and proxies converge somewhat is the late Miocene. Magnetic records from the Chinese Loess Plateau are interpreted to show a 75 long-term intensification of the East Asian summer monsoon (EASM) from ~8.2-2.6 Ma [START_REF] Ao | Late Miocene-Pliocene Asian monsoon intensification linked to Antarctic ice-sheet growth[END_REF]. A late Miocene strengthening of Asian winter monsoons is inferred from South China Sea [START_REF] Holbourn | Late Miocene climate cooling and intensification of southeast Asian winter monsoon[END_REF][START_REF] Jia | Changes in terrestrial ecosystem since 30 Ma in East Asia: Stable isotope evidence from black carbon in the South China Sea[END_REF] and Andaman Sea [START_REF] Lee | Monsoon-influenced variation of clay mineral compositions and detrital Nd-Sr isotopes in the western Andaman 1025 Sea (IODP Site U1447) since the late Miocene[END_REF] records. In the Arabian Sea, an intensification of upwelling and productivity at ~8 Ma is interpreted to reflect a strengthening of the South Asian summer monsoon (SASM) [START_REF] Kroon | G. bulloides was counted in the >150 µm fraction in Huang et al. (white diamonds) and the >125 µm fraction in Kroon et al. (grey circles), g: clay mineralogy (smectite/(illite + chlorite)) at IODP Site U1447 in the Andaman Sea 1360[END_REF]Singh and Gupta, 80 2014;[START_REF] Gupta | Indian Ocean high-productivity event 945 (10-8 Ma): Linked to global cooling or to the initiation of the Indian monsoons?[END_REF], although other studies find evidence contrary to this [START_REF] Tripathi | First evidence of denitrification vis-à-vis monsoon in the Arabian Sea since Late Miocene[END_REF]Huang et al., 2007). Proposed monsoon intensifications during the late Miocene roughly coincide with strong global cooling [START_REF] Herbert | Late Miocene global cooling and the rise of modern ecosystems[END_REF], and a number of studies have implicated cooling and the ramp-up of Antarctic glaciation in monsoon strengthening [START_REF] Ao | Late Miocene-Pliocene Asian monsoon intensification linked to Antarctic ice-sheet growth[END_REF][START_REF] Holbourn | Late Miocene climate cooling and intensification of southeast Asian winter monsoon[END_REF][START_REF] Gupta | Indian Ocean high-productivity event 945 (10-8 Ma): Linked to global cooling or to the initiation of the Indian monsoons?[END_REF]. Until now, a lack of continuous, well-preserved marine sequences from 85 the South Asian monsoon region has stalled our understanding of its complex evolution during the Miocene.

The SASM is thought to have varied strongly on orbital timescales because monsoon strength responds, both directly and via internal feedback mechanisms, to insolation forcing. Model 90 simulations predict a stronger South Asian monsoon during summer insolation maxima at both precession minima and (to a lesser degree) obliquity maxima [START_REF] Bosmans | Response of the Asian summer monsoons to idealized precession and obliquity forcing in a set of GCMs[END_REF]Prell andKutzbach, 1992, 1987;[START_REF] Kutzbach | Monsoon climate of the early Holocene: climate experiment with the earth's orbital parameters for 9000 years ago[END_REF][START_REF] Jalihal | The response of tropical precipitation to Earth's precession: the role of energy fluxes and vertical stability[END_REF][START_REF] Tabor | Interpreting precession-driven δ18O variability in the South Asian monsoon region[END_REF]. Precession is the dominant control on insolation and its seasonal distribution near the equator, and proxy-based Pleistocene SASM records show strong precession-band (19-23 kyr) variability (Kathayat et al., 95 2016;[START_REF] Prell | Monsoon variability over the past 150,000 years[END_REF][START_REF] Clemens | Forcing mechanisms of the Indian Ocean monsoon[END_REF]Clemens et al., 2021;[START_REF] Zhisheng | Glacial-interglacial Indian summer monsoon dynamics[END_REF][START_REF] Bolton | A 500,000 year record of Indian summer monsoon dynamics recorded by eastern equatorial Indian Ocean upper water-820 column structure[END_REF][START_REF] Caley | New Arabian Sea records help decipher orbital timing of Indo-Asian monsoon[END_REF][START_REF] Gebregiorgis | Southern Hemisphere forcing of South Asian monsoon precipitation over the past ~1 million years[END_REF][START_REF] Wang | Evolution and variability of the Asian monsoon system: state of the art and outstanding issues[END_REF][START_REF] Clemens | Late Pleistocene variability of Arabian Sea summer monsoon winds and continental aridity: Eolian records from the lithogenic component of deep-sea 840 sediments[END_REF][START_REF] Rostek | Sea surface temperature and 1130 productivity records for the past 240 kyr in the Arabian Sea, Deep Sea Research Part II[END_REF]. The influence of global boundary conditions related to ice volume and greenhouse gas concentrations on SASM winds and precipitation/runoff is also demonstrated by strong obliquity-and eccentricity-band variance in Plio-Pleistocene records (e.g., Clemens and 100 Prell, 2003;[START_REF] Gebregiorgis | Southern Hemisphere forcing of South Asian monsoon precipitation over the past ~1 million years[END_REF]Clemens et al., 2021;[START_REF] An | Glacial-interglacial Indian summer monsoon dynamics[END_REF] [START_REF] Clemens | A 350,000 year summer-monsoon multi-proxy stack from the Owen Ridge, Northern Arabian Sea[END_REF][START_REF] Caley | New Arabian Sea records help decipher orbital timing of Indo-Asian monsoon[END_REF][START_REF] Singh | Productivity collapses in the Arabian Sea during glacial cold phases[END_REF][START_REF] Shimmield | The inorganic geochemical record of the northwest 1150 Arabian Sea: a history of productivity variation over the last 400 ky from sites 722 and 724[END_REF]Rogalla and 105 Andruleit, 2005;[START_REF] Clemens | Forcing mechanisms of the Indian Ocean monsoon[END_REF][START_REF] Ziegler | Precession phasing offset between Indian summer monsoon and Arabian Sea productivity linked to changes in Atlantic overturning circulation[END_REF]. These records suggest that summer monsoon proxies significantly lag northern hemisphere summer insolation maxima in the precession and obliquity bands due to climate feedbacks. Orbital control on past SASM strength in the pre-Pleistocene, when boundary conditions were different, has so far only been investigated in the Andaman Sea over the latest Miocene-early Pliocene, where seawater oxygen isotope data 110 suggest high-amplitude precession and obliquity forcing of monsoon rainfall with significant phase lags [START_REF] Jöhnck | Variability of the Indian Monsoon in the Andaman Sea across the Miocene-Pliocene transition[END_REF].

In this paper, we investigate sediment accumulation and export productivity dynamics at millennial resolution in late Miocene sediments from southern Bay of Bengal (BOB) International Ocean 115Discovery Program (IODP) Site U1443 (5°N, 90°E, Fig. 1). The late Miocene (11.6-5.3 Ma) is an interval of major global climate change, with long-term cooling between ~7.5 and 5.5 Ma [START_REF] Herbert | Late Miocene global cooling and the rise of modern ecosystems[END_REF] culminating in major high-latitude cooling events [START_REF] Holbourn | Late Miocene climate cooling and intensification of southeast Asian winter monsoon[END_REF], and important carbon cycle shifts recorded in the marine and terrestrial realms potentially linked to atmospheric CO2 decline [START_REF] Tauxe | A re-assessment of the chronostratigraphy of late Miocene C3-C4 transitions, Paleoceanography and Paleoclimatology[END_REF][START_REF] Steinthorsdottir | The Miocene: the Future of the Past, 1170 Paleoceanography and Paleoclimatology, e2020PA[END_REF]. The region of 120 Site U1443 is strongly influenced by seasonally reversing monsoon winds today, and primary productivity is tightly coupled to the annual monsoon cycle (Fig. 1a-b, Fig. 2). During the SASM, strong moisture-laden winds blow inland, driving surface circulation changes and increased mixed layer depth (Fig. 1a-b) (Webster, 1987a, b;[START_REF] Schott | The monsoon circulation of the Indian Ocean[END_REF][START_REF] Gadgil | The Indian monsoon and its variability[END_REF][START_REF] Schott | Indian Ocean circulation and climate variability[END_REF]. Strong Ekman pumping mixes nutrients into the surface layer during the South Asian 125 summer monsoon and, to a lesser extent, the winter monsoon, stimulating biological productivity (Fig. 2) [START_REF] Lévy | Basinwide seasonal evolution of the Indian Ocean's phytoplankton blooms[END_REF][START_REF] Mccreary | Biophysical processes in the Indian Ocean[END_REF][START_REF] Koné | Physical and biogeochemical controls of the phytoplankton seasonal cycle in the Indian Ocean: A modeling study[END_REF][START_REF] Behrenfeld | Carbon-based ocean productivity and 800 phytoplankton physiology from space[END_REF][START_REF] Longhurst | Seasonal cycles of pelagic production and consumption[END_REF]. This strong seasonal signal is transferred to the sedimentary record in the form of strong variance at orbital periods because insolation variations drive seasonal contrast. In this paper, we generate a new orbitally tuned age model based on benthic foraminiferal stable isotopes 130 spanning ~9 to 5 Ma. Core-scanning X-Ray Fluorescence (XRF) data are then used to reconstruct bulk, carbonate, and biogenic barium content and mass accumulation rates (MARs), shedding light on secular and orbital-scale export productivity and sedimentation changes over the late Miocene.

Background 135

In the modern southern BOB in waters overlying Site U1443, both primary and export productivity are strongly controlled by seasonally reversing winds associated with the South Asian monsoon. Figure 2 shows the annual cycle of wind stress, mixed layer depth (MLD), net primary productivity (NPP), and biogenic particle export based on recent oceanographic, satellite, and sediment trap data (see methods for details). In boreal summer (JJA) strong southwest winds mix the upper water 140 column, deepening the MLD to ~60m and entraining nutrients into the photic zone, leading to enhanced primary productivity and biogenic particle export (with a lag of ~3-4 weeks) (Fig. 1a-b, Fig. 2). During boreal winter, northeast winds deepen the MLD to a lesser extent, resulting in a second smaller peak in productivity during the winter monsoon (DJF) (Fig. 2). During the intermonsoon seasons, lowest wind stress is recorded leading to a shallow MLD, higher Sea Surface 145 Temperatures (SSTs), and a more stratified upper water column, resulting in increased oligotrophy and reduced biological productivity. The biannual productivity maxima observed in the surface ocean above Site U1443 is characteristic of monsoon-dominated tropical regions [START_REF] Longhurst | Seasonal cycles of pelagic production and consumption[END_REF][START_REF] Lévy | Basinwide seasonal evolution of the Indian Ocean's phytoplankton blooms[END_REF]. Sediment trap data suggest that primary productivity is the dominant control on organic carbon export at a location west of the northern end of the Ninetyeast Ridge (SBBT site; 150 5°N, 87°E, Fig. 1c). However, lithogenic mineral ballasting at this location is not negligible (average ~13% and 15% lithogenic particles in shallow and deep traps respectively) [START_REF] Rixen | The ballast effect of lithogenic matter and its influences on the carbon fluxes in the Indian Ocean[END_REF][START_REF] Unger | CC BY 4.0 License. oceanographic processes[END_REF] and could in part explain the bias towards the late summer peak seen in biogenic fluxes compared to NPP, as maximum concentrations of lithogenic particles at SBBT occur during the summer monsoon. While wind forcing is identified as the dominant factor 155 controlling biogenic particle fluxes at the SBBT site, advection of nutrient-and chlorophyll-rich waters originating from the eastern Arabian Sea via the Southwest Monsoon Current (SMC) may further contribute to the summer productivity peak in this region [START_REF] Unger | CC BY 4.0 License. oceanographic processes[END_REF]. During the summer monsoon, the relatively salty and nutrient-rich SMC flows eastwards south of Sri Lanka then turns northwards into the BOB (Fig. 1c) [START_REF] Schott | Indian Ocean circulation and climate variability[END_REF][START_REF] Jensen | Cross-equatorial pathways of salt and tracers from the northern Indian Ocean: Modelling results[END_REF]. The SMC and 160 associated eddies have been shown to increase chlorophyll concentrations and average phytoplankton size along their paths as far east as 88-90°E, with the current's influence generally restricted to north of 6°N at this longitude [START_REF] Jyothibabu | Phytoplankton size structure in the southern Bay of Bengal modified by the 990 Summer Monsoon Current and associated eddies: Implications on the vertical biogenic flux[END_REF][START_REF] Vinayachandran | Biological response of the sea around Sri Lanka to summer monsoon[END_REF][START_REF] Webber | The dynamics of the Southwest Monsoon current in 2016 from high-resolution in situ observations and models[END_REF]. While river runoff and resultant salinity stratification during the summer monsoon supress primary productivity further north in the BOB (Prasanna [START_REF] Kumar | Why is the Bay of Bengal less productive during summer monsoon compared to the Arabian Sea?[END_REF], 165 seasonal surface salinity variations are very small (<0.2 psu) at 5°N [START_REF] Zweng | c: Mn/Ca ratios used to trace oxygen minimum zone (OMZ) variations from Maldives Site IODP U1471[END_REF].

Accordingly, monsoon impacts on nutrients and productivity in our study area are limited to those driven by surface currents and wind mixing, and biogenic export fluxes during the SASM are similar (Particulate organic carbon, POC) or higher (CaCO3 and biogenic SiO2) than at sites further north in the BOB [START_REF] Unger | CC BY 4.0 License. oceanographic processes[END_REF]. Thus, modern data give us confidence that export 170 productivity at our site is likely a reflection of South Asian (primarily summer) monsoon wind Ma based on initial bio-magneto-stratigraphy. Samples come from lithologic Units Ib and IIa, and sediments mainly consist of light grey to pale yellow nannofossil ooze with clay and foraminifers, and occasional volcanic ash [START_REF] Clemens | Site U1443[END_REF]. To minimise aliasing due to low estimated sedimentation rates (~0.5-2 cm/kyr), 1 cm thick half rounds (~18 cc) were sampled for micropaleontology. Cores were sampled at a depth resolution of 4 cm in the upper part of the study 190 interval (70.06-114.18 m) and 2 cm in the lower part (114.18-122.76 m) where sedimentation rate estimates were lower. U-channels for XRF scanning were sampled from archive halves of sediment cores at Kochi Core Centre (Japan) during the post-cruise sampling party for 39 sections included in the splice between U1443C 9H 2A (69.95 m CCSF) and U1443C 13H 5A (113.58 m CCSF).

195

Modern oceanographic conditions over the seasonal cycle above Site U1443 were assessed using recent datasets 2). Monthly data for wind (Wind Stress, Metop-A ASCAT, 0.25°, Global, Near Real Time, 2009-present) (Fig. 2a), MLD (1969MLD ( -2010) ) [START_REF] Keerthi | Late Cenozoic stable isotope stratigraphy and paleoceanography of DSDP sites from the east equatorial and central North Pacific Ocean[END_REF] (Fig. 2b) and depth-integrated NPP estimated from satellite-derived surface chlorophyll concentrations (Primary Productivity, Aqua MODIS, NPP, Global, 2003-present, EXPERIMENTAL (Monthly 200 Composite) calculated using method of [START_REF] Behrenfeld | Photosynthetic rates derived from satellite-based chlorophyll concentration[END_REF]; [START_REF] Erd | Primary Productivity, Aqua MODIS, NPP, Global, 2003-present[END_REF])) (Fig. 2c) were extracted for a box between ~4.5-5.5°N latitude and 89-91°E longitude (depending on grid Fig. 1c) [START_REF] Unger | CC BY 4.0 License. oceanographic processes[END_REF]. In Fig. 2 we show data from the deep traps (~3000 m, ~21 day sampling intervals) [START_REF] Rixen | The ballast effect of lithogenic matter and its influences on the carbon fluxes in the Indian Ocean[END_REF], but seasonal patterns of biogenic particle flux are very similar in the shallow (~1000 m) SBBT traps [START_REF] Unger | Impact of regional Indian Ocean characteristics on the 1190 biogeochemical variability of settling particles[END_REF][START_REF] Vidya | Influence of physical and biological processes on the seasonal cycle of biogenic flux in the 1200 equatorial Indian Ocean[END_REF].

Data points show fluxes recorded in individual years (1987-1997, plotted against mid-time for the trap deployment), with monthly averages also shown. Monthly wind and NPP data were 210 downloaded from the ERDDAP server (https://coastwatch.pfeg.noaa.gov/erddap/index.html) and Indian Ocean MLD data [START_REF] Keerthi | Late Cenozoic stable isotope stratigraphy and paleoceanography of DSDP sites from the east equatorial and central North Pacific Ocean[END_REF] from http://www.ifremer.fr/cerweb/deboyer/mld/Surface_Mixed_Layer_Depth.php.

Late Miocene benthic foraminiferal stable isotope data 215

Bulk sediment samples were oven-dried at 50°C, weighed, and washed over a 63 µm sieve in tap water at CEREGE. The >63 µm fraction was oven-dried at 50°C on a filter paper and weighed to determine percentage coarse fraction. The <63 µm fraction was centrifuged and dried at 50°C. Depth resolution for the benthic isotope record is 8 cm (70.06 m to 112.87 m CCSF), 4 cm (112.86 m and 114.18 m CCSF) or 2cm , except near splice tie-points where 220 sampling included overlap between cores increasing resolution. Six to twelve specimens of the epibenthic foraminiferal species Cibicidoides wuellerstorfi were picked from the >212µm fraction, with 6-8 well-preserved specimens selected for analysis. Tests were broken into fragments, cleaned in ethanol in an ultrasonic bath, and oven dried at 40°C. Stable carbon and oxygen isotopes were measured on a Thermo Scientific MAT 253 dual-inlet isotope ratio mass spectrometer (DI-IRMS) 225 coupled to Kiel IV carbonate preparation device at the Leibniz Laboratory, University of Kiel.

Based on long-term analysis of international and internal carbonate standards, precision (1σ) is better than ±0.08‰ for δ 18 O and 0.05‰ for δ 13 C. Results were calibrated using the National

Age model

Seven calcareous nannofossil bio-events dated between 5.04 Ma and 9.53 Ma were identified at 235 high-resolution in Site U1443 splice samples (Table S1) to increase the resolution of shipboard biostratigraphy [START_REF] Robinson | Biostratigraphic Summary[END_REF]. To check for orbital periodicities prior to tuning, wavelet analyses were performed on benthic isotope records in the depth domain and on the revised nannofossil-based age model (using a 4 th order polynomial fit, Fig. S1) in R using the biwavelet package [START_REF] Gouhier | Package 'biwavelet[END_REF][START_REF] Grinsted | Application of the cross wavelet transform and 940 wavelet coherence to geophysical time series[END_REF]) (Fig. S2a-d). All time-series were first 240 interpolated to constant depth or age resolution, such that the maximum resolution present was preserved (2 cm and 2 kyr for benthic isotope records in the depth and age domain, respectively).

Records were then filtered to remove signals with periods longer than one third of the length of the dataset using the "bandpass" function in the R package Astrochron [START_REF] Meyers | Astrochron: An R Package for Astrochronology[END_REF]. Wavelets indicate that obliquity-driven cycles are present throughout (~0.53 m,41 kyr and 53 kyr,245 Fig. S2 c-d), confirming that the U1443 record is suitable for orbital tuning. Using revised nannofossil datums (Table S1, Fig. S1) and shipboard magnetostratigraphy [START_REF] Clemens | Site U1443[END_REF] as preliminary age-depth tie-points (Fig. 3), an astronomical age model was constructed by tuning our monospecific benthic δ 18 O record to an eccentricity plus tilt (ET) target curve [START_REF] Laskar | A long-term numerical solution for the insolation quantities of the Earth[END_REF]) (Fig. 4). We did not include precession in our tuning target so as not to introduce assumptions 250 related to which hemisphere was controlling climate at our site, and because the temporal resolution of our benthic record does not permit accurate resolution of precession cycles in some intervals. We used a minimal tuning approach, tying ET maxima to benthic δ 18 O minima, with at most one tiepoint per ~100 kyr and often one every 200-300 kyr (Fig. 4, Fig. S1), so as not to artificially introduce frequency modulations [START_REF] Zeeden | Testing astronomically tuned age models[END_REF]. 255

XRF Scanning and Calibration

U-channels were scanned at 1 cm intervals at The Australian National University (ANU) on a third generation Avaatech XRF core scanner. All cores were scanned sequentially and standards measured daily were consistent across all runs. Core sections were covered with 4 micron-thick 260 Ultralene film and measured at 10 kv with a 500 µA current and no filter, then at 30 kv with a 200 µA current and Pd thin filter, and finally at 50 kv with a 50 µA current and Cu filter. A 30 sec count time was used for all runs. Late Miocene XRF data generated at ANU (72.75-113.56m CCSF) were spliced with data from Lübbers et al. (2019) (112.80m to 122.76m CCSF), also scanned on an Avaatech XRF core scanner but with different machine settings. To splice the records, we rescaled 265 the raw [START_REF] Lübbers | The middle to late Miocene "Carbonate Crash" in the equatorial 1040 Indian Ocean[END_REF] Agilent 7500 ce). Twenty samples from the scanned late Miocene interval, selected to cover the range of values in the raw XRF count data for elements of interest, were analysed and concentrations of Al, K, Ca, Ti, Mn, Fe, Rb, Sr, and Ba were determined. Prior to analysis, samples were dried and homogenised in a pestle and mortar. About 30 mg of sediment was completely dissolved by acid digestion using a 2:1 mixture of ultrapure acids (15 M HNO3 and 22 M HF with 275 HClO4) on a hot plate. Blank contribution was estimated to be negligible. The accuracy of measurements was evaluated using analysis of geostandards MAG-1(marine mud) and BE-N (basalt). The typical analytical uncertainty was better than 5%. XRF-derived element counts were converted into element concentrations by direct linear calibration. This allowed us to reduce uncertainties related to the variable matrix effect and physical properties such as moisture content 280 that typically change downcore. Additionally, we used calibrated XRF data to calculate biogenic barium concentrations and to estimate % CaCO3 and "carbonate-free basis" (cfb) elemental concentrations, permitting evaluation of the extent to which dilution by the dominant sediment constituent (here CaCO3) is driving trends and variability of more minor constituents in our records.

To represent the relative contributions of CaCO3 versus terrigenous sediment components, we use 285 the log count ratio of Ca/(∑( Al, K, Ti, Fe, Rb)), termed log(Ca/Terr).

Ba-based export productivity proxies

The accumulation of biogenic barium in sediments is a reliable proxy for export production in certain environments [START_REF] Paytan | Marine barite: Recorder of variations in ocean export productivity[END_REF]. Micron-sized barite (BaSO4) crystals are the main 290 carriers of particulate barium in the ocean, with a maximum in concentration occurring just below the euphotic zone [START_REF] Bishop | The barite-opal-organic carbon association in oceanic particulate matter[END_REF][START_REF] Dehairs | Discrete suspended particles of barite and the barium cycle in the open ocean[END_REF]. Although the exact mechanisms governing the precipitation of barite in the water column are only now coming to light [START_REF] Martínez-Ruiz | Barite formation in the ocean: Origin of amorphous and crystalline precipitates[END_REF], its formation is thought to be associated with decaying organic matter. Depth profiles of dissolved Ba suggest that passive adsorption of barite onto mainly biogenic particles as they sink 295 through the water column, combined with vertical mixing of dissolved Ba from the deep ocean and riverine input, can best explain Ba's nutrient-like water column distribution [START_REF] Dehairs | Discrete suspended particles of barite and the barium cycle in the open ocean[END_REF][START_REF] Cao | Constraining the oceanic barium cycle with stable barium isotopes[END_REF]. [START_REF] Goldberg | Chemistry of Pacific pelagic sediments[END_REF] first hypothesised that an increase in Ba accumulation rate in sediments underlying the equatorial Pacific divergence zone was directly linked to overlying high productivity, followed by similar observations in equatorial Indian Ocean 300 sediments [START_REF] Schmitz | Barium, equatorial high productivity, and the northward wandering of the Indian continent[END_REF]. Subsequently, evidence for strong correlations between fluxes of Ba and organic carbon in Atlantic and Pacific sediment traps led to algorithms relating new productivity to particulate Ba flux [START_REF] Dymond | Barium in deep-sea sediment: A geochemical proxy for 900 paleoproductivity[END_REF][START_REF] Francois | Biogenic barium fluxes to the deep sea: Implications for paleoproductivity reconstruction[END_REF]. A further study focusing on the accumulation of barite (Babarite) extracted from core-top and late Pleistocene sediments refined its use as a proxy for export productivity (Paytan et al., 1996). Although 305 significant Ba regeneration occurs in the uppermost few millimetres of sediment (Paytan and Kastner, 1996), barite dissolution is thought to cease after burial due to supersaturation in interstitial waters [START_REF] Gingele | Discrete barite particles and barium as tracers of paleoproductivity 925 in South Atlantic sediments[END_REF][START_REF] Dymond | Barium in deep-sea sediment: A geochemical proxy for 900 paleoproductivity[END_REF] and barite is not subject to burial diagenesis in oxic sediments [START_REF] Paytan | Marine barite as a monitor 1090 of seawater strontium isotope composition[END_REF]. Ocean sedimentary Ba has both a biogenic (Babio) and a terrigenous (Badetrital) component, so estimates of past export productivity using barium 310 must distinguish between these sources. This can either be done by chemical leaching of bulk sediment (assuming that all barite is Babio) e.g. (Paytan et al., 1996), or by determination of total barium (Batotal) and subtraction of Badetrital using Al content and the terrigenous Ba/Al ratio, resulting in a record of Baxs (Batotal -Badetrital), see equation 1 [START_REF] Dymond | Barium in deep-sea sediment: A geochemical proxy for 900 paleoproductivity[END_REF].

315 [Baxs]ppm = [Batotal]ppm -(Ba/Alterrigenous)*[Altotal]ppm (1)
Direct comparisons of measurements of Babarite and Baxs suggest that non-barite phases of barium may be included in the calculation of Baxs; nevertheless Baxs is most representative of Babarite and therefore export productivity in oxic carbonate-rich sediments with low terrigenous, biogenic silica, 320 and organic carbon contents [START_REF] Eagle | A comparison between excess barium and barite as indicators of carbon export[END_REF][START_REF] Averyt | A comparison of multiple proxies for export production in the equatorial Pacific[END_REF][START_REF] Gonneea | Phase associations of barium in marine sediments[END_REF]. The use of bulk Ba/Ti, Ba/Al, and Ba/Fe ratios is another approach to evaluate relative changes in export productivity (i.e., normalisation to an element presumed to be predominantly of terrigenous origin), but without precisely predefining the Ba/terrigenous ratio that could vary over time, and also removing the effect of dilution by a dominant sedimentary component such as 325

CaCO3 [START_REF] Murray | Export production and 1070 carbonate dissolution in the central equatorial Pacific Ocean over the past 1 Myr[END_REF].

Here, we reconstruct changes in export productivity at Site U1443 over the late Miocene using XRF-derived Ba data and compare elemental count ratios of log(Ba/Fe), log(Ba/Ti), and log(Ba/Al), with [Ba]cfb and [Ba]xs calculated following equation (1), using a Ba/Alterrigenous value of 330 0.0075 g/g following [START_REF] Dymond | Barium in deep-sea sediment: A geochemical proxy for 900 paleoproductivity[END_REF]. To verify consistency of trends, we also calculate

Mass Accumulation rates 335

MARs of bulk sediment, CaCO3, [Ba]xs, and summed terrigenous elements (Al, K, Ti, Fe, and Rb) were calculated by multiplying concentrations by linear sedimentation rates (in cm/kyr) derived from our new age model and dry bulk densities (in g/cm 3 ). Dry bulk density values were estimated from high-resolution shipboard Gamma Ray Attenuation bulk density scanning data, and the linear relationship between all shipboard U1443 wet bulk density and dry bulk density measurements 340 (n=164, r 2 >0.99) [START_REF] Clemens | Site U1443[END_REF]. Units are g/cm 2 /kyr for bulk and CaCO3 MAR, µg/cm 2 /kyr for [Ba]xs MAR and mg/cm 2 /kyr for terrigenous MAR.

Time series analysis

Spectral analyses of benthic isotope and XRF data against tuned age were performed on filtered 345 records with a constant time step as described in Section 2.3 (0.5 kyr for XRF records and 2 kyr for isotope records). Multi-taper method (MTM) spectral analyses using a robust red-noise model were performed using Acycle [START_REF] Li | Acycle: Time-series analysis software for paleoclimate research and education[END_REF]. Blackman-Tukey cross-spectral analyses were performed in Arand to assess phase and coherence [START_REF] Howell | ARAND Time-Series Analysis Software[END_REF]. Wavelet analyses were performed in R using the biwavelet package [START_REF] Gouhier | Package 'biwavelet[END_REF][START_REF] Grinsted | Application of the cross wavelet transform and 940 wavelet coherence to geophysical time series[END_REF]. To illustrate precession-350 band variance and amplitude modulation, certain records (with identified significant precession variance) were filtered using a Tanner-Hilbert filter centred on 43 cycles/myr with bandwidth ±3 (designed to include both the ~24 kyr and ~22 kyr precession periods) in Acycle [START_REF] Li | Acycle: Time-series analysis software for paleoclimate research and education[END_REF]. S1) and magnetochron boundaries. Our tuned benthic δ 18 O and δ 13 C records, shown in Figure 4, span the interval 4.96 Ma to 8.99 Ma and our age model shows excellent agreement with revised 360 biostratigraphic and shipboard magnetostratigraphic datums (Fig. S1). Sedimentation rates generally vary between 1 and 1.7 cm/kyr, with a minimum of ~0.5-0.7 cm/kyr in the oldest part of the record (8.6 to 9 Ma) and a maximum of ~1.9 cm/kyr at 7.8-8 Ma (Fig. 4d). We note that between 112.86 m and 121 m CCSF (8.7-8.1 Ma), our age model differs by up to 60 kyr from that of [START_REF] Lübbers | The middle to late Miocene "Carbonate Crash" in the equatorial 1040 Indian Ocean[END_REF], which is based on correlation of Site U1443 benthic δ 18 O and δ 13 C data to 365 the orbitally tuned ODP Site 1146 δ 13 C record [START_REF] Holbourn | Late Miocene climate cooling and intensification of southeast Asian winter monsoon[END_REF] using a limited number of tie points (Site locations in Fig. 1c). Wavelet analyses (Fig. 4 f,g) as well as spectral analyses (Fig. S2e,f) of the tuned benthic records reveal significant orbital periodicities (>99% significance level) of ~405 kyr and 41 kyr for δ 18 O and ~405 kyr, 125 kyr, 95 kyr, 53 kyr and 41 kyr for δ 13 C, and filtered isotope records show good correspondence with filtered ET (Fig. 4h). Cross-spectral 370 analysis between δ 18 O and δ 13 C reveals >95% coherency in the 41 kyr and 405 kyr bands (Fig. S3a). Our age model is supported by close agreement between Site U1443 benthic δ 18 O and δ 13 C data and independent orbitally-tuned benthic isotope records from the South China Sea (ODP 1146) [START_REF] Holbourn | Late Miocene climate cooling and intensification of southeast Asian winter monsoon[END_REF]Holbourn et al., In Press), equatorial Pacific (IODP Sites U1338 and 1337) [START_REF] Drury | Evaluating climatic response to external 885 radiative forcing during the late Miocene to early Pliocene: New perspectives from eastern equatorial Pacific (IODP U1338) and North Atlantic (ODP 982) locations[END_REF][START_REF] Drury | Deciphering the state of the late Miocene to early Pliocene equatorial Pacific, 890[END_REF][START_REF] Drury | Late Miocene climate and time scale reconciliation: Accurate orbital calibration from a deep-sea perspective[END_REF], and equatorial Atlantic (ODP Sites 926 375 and 999) [START_REF] Bickert | Late Neogene benthic stable isotope record of ODP Site 999: Implications for Caribbean paleoceanography, organic carbon burial and the Messinian Salinity Crisis[END_REF][START_REF] Shackleton | The Late Miocene stable isotope record, Site 926[END_REF][START_REF] Drury | Late Miocene climate and time scale reconciliation: Accurate orbital calibration from a deep-sea perspective[END_REF][START_REF] Zeeden | Revised Miocene splice, astronomical tuning and calcareous plankton biochronology of ODP Site 926 between 1225 5 and 14.4 Ma[END_REF] (Figs S4, S5).

Results

Mean temporal resolution of the Site U1443 benthic isotope record is 4.2 kyr. Between 9 Ma and 7.6 Ma, mean benthic δ 18 O values vary between 2.5 and 2.8 ‰, with an overall decreasing trend 380 culminating in minimum values averaging ~2.4 ‰ between 7.6 and 7 Ma (Fig. 4c). Between 7 and 6.5 Ma, mean δ 18 O values increase by ~0.25 ‰, and between 6.5 and 5 Ma, mean values vary between 2.55 and 2.8 ‰. Between 6 and 5 Ma, a number of prominent benthic δ 18 O maxima are identified in the U1443 δ 18 O record, namely TG2, TG12, TG14, TG20 and TG22 (following nomenclature of Shackleton et al. (1995)) (Fig. 4c). Between 7.7 and 6.9 Ma, strong obliquity 385 modulation of the U1443 δ 18 O record is seen (Fig. 4f), as also noted at Sites U1337 [START_REF] Drury | Late Miocene climate and time scale reconciliation: Accurate orbital calibration from a deep-sea perspective[END_REF] and 1146 [START_REF] Holbourn | Late Miocene climate cooling and intensification of southeast Asian winter monsoon[END_REF] (Fig. S4). Long-term trends are similar to those recorded at Pacific sites, with benthic δ 18 O values at Indian Ocean Site U1443 ~0.1‰ heavier than at Pacific Sites U1337/U1338 and ~0.2-0.3‰ heavier than at South China Sea Site 1146 (Fig. 5a).

390

Mean benthic δ 13 C values at Site U1443 vary between 0.7 and 1.1‰ from 9 to 7.6 Ma, then decrease from ~1 to -0.2‰ between 7.6 and 6.7 Ma, reflecting the globally recognised Late Miocene Carbon Isotope Shift (LMCIS) [START_REF] Keerthi | Late Cenozoic stable isotope stratigraphy and paleoceanography of DSDP sites from the east equatorial and central North Pacific Ocean[END_REF][START_REF] Keigwin | Uppermost Miocene carbon isotope stratigraphy of a piston core in the equatorial Pacific[END_REF]) (Fig. 4e). From 6.7 to 5 Ma, mean values vary between -0.2 and 0.4 ‰. The timing of the LMCIS at Site U1443 (~7.6 Ma to ~6.7 Ma) is synchronous with the event in independent orbitally-tuned high-395 resolution records [START_REF] Drury | Deciphering the state of the late Miocene to early Pliocene equatorial Pacific, 890[END_REF][START_REF] Drury | Late Miocene climate and time scale reconciliation: Accurate orbital calibration from a deep-sea perspective[END_REF][START_REF] Holbourn | Late Miocene climate cooling and intensification of southeast Asian winter monsoon[END_REF][START_REF] Drury | Evaluating climatic response to external 885 radiative forcing during the late Miocene to early Pliocene: New perspectives from eastern equatorial Pacific (IODP U1338) and North Atlantic (ODP 982) locations[END_REF] (Fig. 5b, Fig. S5), and its magnitude (~1‰ decrease in δ 13

XRF data and calibration

Scanning XRF results are shown in Figure 6. Linear calibration between element counts and concentrations in discrete samples over the late Miocene interval (~8.15 to 5 Ma) showed significant coefficients of determination, with R 2 values ranging from 0.68 (Al) to 0.87 (Fe) (Fig. 405 S6), excluding Sr and Ca (see explanation below). Raw and calibrated elemental data show consistent trends and amplitude variability (Fig. 6). For Ti, Ba, Al, and Mn, the re-scaled counts/sec values in the 113.37-122.76 m CCSF interval [START_REF] Lübbers | The middle to late Miocene "Carbonate Crash" in the equatorial 1040 Indian Ocean[END_REF] fell outside of our calibration range, thus data below 113.6m (~8.15 Ma) were not converted to concentrations (Fig. 6). In brief, Al, Si, Ti, Fe, Rb, and K show similar trends, with a long-term small increase in concentrations 410 from 8.15 Ma to 5 Ma and spikes (particularly pronounced in Rb and K) corresponding in some cases to described ash layers [START_REF] Clemens | Site U1443[END_REF]. Ca and Ba show a minor long-term decrease over the study interval, while Sr and Mn increase from ~8.15 Ma to 6 Ma, then stabilise or decrease slightly. All elements show high-frequency variability throughout. For Ca and Sr, R 2 values were lower (0.39 and 0.42 respectively, Fig S6) due to the consistently high Ca and Sr contents and small 415 variability in the selected calibration samples. To estimate percent CaCO3, we therefore used a Ca/Fe ratio calibration rather than a direct linear calibration. We first used the linear relationship between Ca/Fe counts and Ca/Fe as determined by ICP-MS (Fig. S6, R 2 =0.93). Then %CaCO3 was calculated assuming that all Ca was contained in CaCO3 -a reasonable assumption at Site U1443 given the relatively low clay content in lithological subunits Ib and IIa [START_REF] Clemens | Site U1443[END_REF]. 420

Confidence in our method is provided by very good agreement with independent %CaCO3 estimates for the middle and early late Miocene interval of Site U1443 based on calibration of XRFderived counts of (Ca/∑(Ca, Al, Si, K, Ti, Mn, Fe, S)) to discrete CaCO3 measurements [START_REF] Lübbers | The middle to late Miocene "Carbonate Crash" in the equatorial 1040 Indian Ocean[END_REF], including an overlapping interval based on an alternate splice from 112.8 and 113.6 m CCSF (Fig. 7b). 425

CaCO3 content, sediment accumulation patterns and Ba proxies

Late Miocene estimated % CaCO3 varies between ~60 and 90% with a small long-term decrease over the 9-5 Ma interval (Fig. 7b). This long-term trend is also visible in the log(Ca/Terr) record (Fig. 7c), implying a small increase in the contribution of terrigenous material relative to CaCO3 in 430 Site U1443 sediments over time. Three % CaCO3 and log(Ca/Terr) minima between 5 and 6 Ma occur in identified ash layers. Log(Ba/Al), log(Ba/Fe) and log(Ba/Ti) show identical long-term and orbital-scale trends (Fig. S7), therefore we only discuss log(Ba/Fe) in the main text. Log(Ba/Fe) shows a long-term decrease between 9 and 5.3 Ma, and a smaller increase from 5.3 to 5 Ma (Fig. 7d). [Ba]xs shows identical variability whether calculated using Al or Ti (Fig. 7e), and generally 435

shows similar patterns to log(Ba/Fe) where records overlap (8.15 to 5 Ma). Values of [Ba]xs generally vary between 400-800 ppm, however carbonate-free [Ba]xs concentrations are typically 1000 to 4000 ppm (Fig. S7). A peak in log(Ba/Fe) between 7.6 and 7.3 Ma is less pronounced in the [Ba]xs record, but is prominent in the carbonate-free [Ba]xs record (Fig. S7, grey shading), suggesting that this peak is supressed in the [Ba]xs record as a result of dilution by carbonate. The 440 trough between 7.6 and 7.9 Ma seen in log(Ba/Fe), [Ba]xs and to a lesser extent in % CaCO3, log(Ca/Terr), and carbonate-free [Ba]xs appears not to be an artefact of dilution.

Bulk sediment MARs vary between 0.5 and 2.1 g/cm 2 /kyr with a step increase from ~0.5 to 1.5 g/cm 2 /kyr occurring at 8.66 Ma (Fig. 7g), concurrent with a major sedimentation rate increase (Fig. 445 4d). CaCO3 MARs range from 0.4 to 2 g/cm 2 /kyr and co-vary with bulk sediment MARs, with the increasing difference between the two records reflecting a small long-term increase in non-CaCO3 components (Fig. 7g). This small increase is reflected in terrigenous element MARs, which vary between ~20-80 mg/cm 2 /kyr (excluding volcanic ash layers) (Fig. 7i). We note that absolute values of terrigenous MAR should be interpreted with caution, because this calculation does not include Si 450 as this element was not quantified in discrete samples. Nevertheless, a significant correlation between Al and Si counts (R 2 = 0.8, Fig. 6) suggests that Si is primarily of terrigenous origin, therefore trends in log(Ca/Terr) and terrigenous MAR in Fig. 7 are likely robust despite the exclusion of Si. From 8.3-5 Ma, [Ba]xs MAR shows similar patterns to CaCO3 MAR, with no clear long-term trend and maximum values driven by higher sedimentation rates in the intervals 5-5.2 455 Ma, 6.1-6.3 Ma, 7.5-7.7 Ma, and 7.8-8 Ma (Fig. 7f).

Spectral analyses reveal significant orbital periods in all late Miocene XRF records (Fig. 8). The kyr (>95%). Percent CaCO3 contains significant (>95%) variability at 22.5 kyr, with additional peaks at 68 kyr and 37 kyr. In summary, all records show highly significant variability in the precession band (22-24 kyr), with variability at the 22.5 kyr period and the ~125 kyr period most strongly associated with the biogenic component of Ba and with CaCO3. Wavelet analyses of log(Ba/Fe) and [Ba]xs confirm significant precession-scale variability in these records throughout 470 the 9-5 Ma interval (Fig. 9).

Discussion

Late Miocene sedimentation patterns in the southern Bay of Bengal

We first examine the drivers of changes in sediment MAR identified in our record, and their 475 possible link to regional and global productivity trends. The three-fold increase in CaCO3 MAR at 8.66 Ma at Site U1443, originally described in [START_REF] Lübbers | The middle to late Miocene "Carbonate Crash" in the equatorial 1040 Indian Ocean[END_REF], could result from improved preservation and/or increased carbonate export by pelagic calcifiers (coccolithophores and/or foraminifera). Based on CaCO3 percentages, MARs, and benthic to planktic foraminiferal ratios, [START_REF] Lübbers | The middle to late Miocene "Carbonate Crash" in the equatorial 1040 Indian Ocean[END_REF] identified the mid to late Miocene "carbonate crash" in Site U1443 sediments 480 between ~12.2 Ma and 10 Ma, with a slow recovery from ~10 to 8.7 Ma, favouring an interpretation that the increase in CaCO3 MAR at 8.66 Ma reflects improved preservation. A record of planktic foraminiferal fragmentation between 9 and 8 Ma generated in the present study, interpreted to reflect a decrease in carbonate dissolution [START_REF] Le | Carbonate dissolution fluctuations in the western equatorial Pacific during the late Quaternary[END_REF], supports this interpretation (Fig. 7h). We see no change in log(Ba/Fe) concurrent with the CaCO3 MAR increase 485 at 8.66 Ma, which suggests that total export productivity at Site U1443 remained stable over this transition. However, our data suggest that an increase in coccolithophore production may have occurred. The contribution of foraminifera to total CaCO3 over our study interval is low (see >63 µm MAR in Fig. 7g), leading us to infer that higher CaCO3 MARs between 8.66 and 5 Ma are primarily driven by coccoliths. A 3-fold increase in sediment accumulation rate (SAR) at ~8.6 Ma 490 with no change in CaCO3 content (%), implying a large increase in CaCO3 MARs, is also seen at shallower (2247 m) Deep Sea Drilling Project (DSDP) Site 216 on the NER near the equator (Fig. 1c) [START_REF] Bukry | Coccolith and silicoflagellate stratigraphy, eastern Indian Ocean, Deep Sea Drilling 830 Project Leg 22 in: Initial Reports of the Deep Sea Drilling Project[END_REF][START_REF] Mcneill | Understanding Himalayan erosion and the significance of the Nicobar Fan[END_REF]Pimm, 1974). This suggests that production played a role in coccolithophore productivity and calcification driven by weathering alkalinity inputs and regional nutrient changes [START_REF] Si | Reduced continental weathering and marine calcification linked to late 1155 Neogene decline in atmospheric CO 2[END_REF].

500

Interestingly, the increase in bulk MARs at 8.66 Ma is driven by both CaCO3 and to a lesser extent non-CaCO3 components (clays), implicating another mechanism as well as improved carbonate preservation and increased coccolith export productivity affecting sedimentation at Site U1443. We suggest that an increase in coccolith CaCO3 flux to the seafloor could have led to increased scavenging by sinking biogenic aggregates of fine clays. Fine clays are present in the southern BOB 505 water column as a direct result of riverine flux [START_REF] Rixen | The ballast effect of lithogenic matter and its influences on the carbon fluxes in the Indian Ocean[END_REF][START_REF] Ramaswamy | Lithogenic Fluxes to the Northern Indian Ocean[END_REF], and in nepheloid layers above the NER where high clay concentrations occur due to proximity to the sedimentary fan systems to the east (Nicobar Fan) and west (Bengal Fan) [START_REF] Stow | Sediment facies and processes on the distal Bengal Fan[END_REF].

Recent studies of sedimentation patterns on the Bengal and Nicobar Fans, separated by the NER, interpret a large increase in SAR both on the NER and the Nicobar Fan at ~10-8 Ma to reflect 510 increased lithogenic sediment flux to the eastern Indian Ocean [START_REF] Mcneill | Understanding Himalayan erosion and the significance of the Nicobar Fan[END_REF]Pickering et al., 2020b). Our data from the NER show that >75% of the 3-fold increase in SAR at 8.66 Ma is driven by biogenic CaCO3, thus we caution against using SAR at Site U1443/758 as representative of changes in sediment flux to the Bengal-Nicobar Fan system. Data from Site U1443, as well as from nearby DSDP Site 216 [START_REF] Bukry | Coccolith and silicoflagellate stratigraphy, eastern Indian Ocean, Deep Sea Drilling 830 Project Leg 22 in: Initial Reports of the Deep Sea Drilling Project[END_REF][START_REF] Mcneill | Understanding Himalayan erosion and the significance of the Nicobar Fan[END_REF]Pimm, 1974) (Fig. 1c), suggest that 515 increases in biogenic carbonate accumulation on the NER are decoupled, both temporally and mechanistically, from the increase in sediment delivery to the Nicobar Fan system. The gradual increase in terrigenous element and non-CaCO3 MARs over the 9-5 Ma interval seen at Site U1443 (Fig. 7g,i) is part of a longer-term trend of increasing mineral flux in this region of the NER from the Miocene to the Pleistocene, beginning at ~12 Ma at ODP Site 758, that is thought to reflect 520 increased Himalayan erosion [START_REF] Ali | Persistent provenance of South Asian Monsoon induced silicate weathering over the past 27 million years[END_REF][START_REF] Hovan | The Cenozoic record of continental mineral deposition on Broken and Ninetyeast Ridges, Indian Ocean: southern African aridity and sediment delivery from the Himalayas[END_REF].

Increases in the MAR of biogenic components (CaCO3, opal, organic carbon, phosphorus) between ~9 and 4 Ma have been measured in sediments from the Pacific, Indian and Atlantic Oceans [START_REF] Farrell | The middle Miocene climatic transition: East Antarctic ice sheet development, deep ocean circulation and global carbon cycling, Palaeogeography, 915 palaeoclimatology[END_REF][START_REF] Lyle | Simulation of long eccentricity (400-kyr) cycle in ocean carbon reservoir during Miocene Climate Optimum: Weathering and nutrient response to orbital change[END_REF][START_REF] Van Andel | Cenozoic History and Paleoceanography of the Central Equatorial Pacific Ocean: A Regional Synthesis of Deep Sea Drilling Project Data[END_REF][START_REF] Grant | Coupled productivity and carbon isotope records in the southwest Pacific Ocean during the late Miocene-early Pliocene biogenic bloom[END_REF]Delaney and 525 Filippelli, 1994;[START_REF] Hermoyian | Late Miocene-early Pliocene biogenic bloom: Evidence from low-productivity regions of the Indian and Atlantic Oceans[END_REF][START_REF] Dickens | The latest Miocene-early Pliocene biogenic bloom: a revised Indian Ocean perspective[END_REF][START_REF] Drury | Climate, cryosphere and carbon cycle controls on Southeast Atlantic orbital-scale carbonate deposition since the Oligocene (30-0 Ma)[END_REF]. This period of increased biogenic sedimentation, supported by independent paleoproductivity proxies (e.g., Diester [START_REF] Diester Haass | In search of the late Miocene-early Pliocene 880 "biogenic bloom[END_REF], is thought to reflect higher biological productivity and was dubbed the "biogenic bloom" by [START_REF] Farrell | The middle Miocene climatic transition: East Antarctic ice sheet development, deep ocean circulation and global carbon cycling, Palaeogeography, 915 palaeoclimatology[END_REF]. A low-resolution CaCO3 MAR record from Site 758 shows higher values between 8 and 4 Ma, which in the absence of evidence for an increase in 530 carbonate dissolution at 4 Ma, could suggest an end to the biogenic bloom at this site in the early Pliocene [START_REF] Dickens | The latest Miocene-early Pliocene biogenic bloom: a revised Indian Ocean perspective[END_REF][START_REF] Pierce | Proceedings of the Ocean Drilling Program[END_REF][START_REF] Si | Reduced continental weathering and marine calcification linked to late 1155 Neogene decline in atmospheric CO 2[END_REF], although improved age control for the Pliocene interval as well as independent paleoproductivity reconstructions are needed to verify this. Hypotheses to explain the biogenic bloom invoke a change in global nutrient cycling; i.e., a global increase in nutrient input, and/or redistribution of nutrients between basins 535 [START_REF] Grant | Coupled productivity and carbon isotope records in the southwest Pacific Ocean during the late Miocene-early Pliocene biogenic bloom[END_REF], although the asynchronous timing of the biogenic bloom between regions, its variable expression, and its differentiation from the carbonate crash recovery complicate its interpretation. Diester [START_REF] Diester Haass | Late Miocene carbon isotope records and marine biological productivity: Was there a (dusty) link?[END_REF] hypothesised that changes in reconstructed productivity were correlated to the LMCIS at four Indo-Pacific sites, and tentatively proposed a link to a strengthened wind regime at this time. At Site U1443, we find no clear link between export 540 productivity or carbonate sedimentation and the LMCIS (Fig. 7). In the northern Indian Ocean, the influence of possible concurrent changes in monsoon strength on paleoproductivity and biogenic MARs must also be considered, and these are discussed in Section 4.3. The 24 kyr period stands out as highly significant in all records (>99%, except for %CaCO3 and [Ba]cfb where >95%). In contrast, the 125-kyr and 22.5 kyr peaks that dominates the [Ba]xs and log(Ba/Fe) spectra (>99% significant), are also highly significant only in the [Ba],

Orbital forcing of late Miocene South Asian summer monsoon winds

[Ba]cfb, %CaCO3, and log(Ca/Terr) records. This suggests that strong variability at the 125-kyr 555 (eccentricity) and 22.5 kyr (precession) periods is related to biological productivity (i.e. Babio and not Badetrital, as well as biogenic CaCO3). The 30-kyr peak in [Ba]xs is also seen in [Ba] but not in [Fe], [Al], or [Ti], so we similarly interpret this period as being related to biological productivity.

The 23.6-kyr and 22.3-kyr periods (highlighted together in Fig. 8 as one grey band spanning 22-24 kyr) are primary periods of Earth's precession, whereas the 53-kyr and 41-kyr periods are related to 560

Earth's obliquity [START_REF] Laskar | A long-term numerical solution for the insolation quantities of the Earth[END_REF]. seawater δ 18 O in the nearby Andaman Sea [START_REF] Jöhnck | Variability of the Indian Monsoon in the Andaman Sea across the Miocene-Pliocene transition[END_REF]. These authors suggest that, prior to 565 a distinct switch to obliquity-driven variability around 5.55 Ma, their records reflect strong precessional (insolation) control on South Asian monsoon rainfall from 6.2-5.55 Ma, with significant phase lags between proxies and precession. Wavelet analyses of log(Ba/Fe) and [Ba]xs show that precession-band (22-24 kyr) variability dominated throughout our 9-5 Ma study interval at Site U1443 (Fig. 9). Although phase relationships with insolation should be interpreted with 570 caution because of errors inherent to our late Miocene age model, export productivity appears to be coherent and in phase (within error) with the summer inter-tropical insolation gradient (SITIG, the insolation gradient between 23°N and 23°S on June 21 st ) (Fig. S3d). The SITIG has been proposed as a primary control on the strength of SASM winds, because a stronger SITIG increases the pressure gradient between the two limbs of the winter hemisphere Hadley cell, which drives 575 monsoon winds into the summer hemisphere [START_REF] Bosmans | Obliquity forcing of low-latitude climate[END_REF]. Our new Ba-based export productivity records corroborate the hypothesis that insolation played a dominant role in driving late Miocene South Asian summer monsoon wind variability in the equatorial sector of the SAM region, as predicted by general circulation models [START_REF] Bosmans | Response of the Asian summer monsoons to idealized precession and obliquity forcing in a set of GCMs[END_REF], with internal climate processes such as ice volume playing a more minor role than in the Late Pleistocene when large 580 glacial-interglacial cycles and related feedbacks drove variability in the Asian monsoon on 100-kyr timescales [START_REF] Clemens | Precession-band variance missing from East Asian monsoon runoff[END_REF]Clemens et al., 2021) and SASM wind proxies from the Arabian Sea and southern BOB record up to ~9 kyr phase lags relative to precession [START_REF] Bolton | A 500,000 year record of Indian summer monsoon dynamics recorded by eastern equatorial Indian Ocean upper water-820 column structure[END_REF][START_REF] Caley | New Arabian Sea records help decipher orbital timing of Indo-Asian monsoon[END_REF][START_REF] Clemens | A 350,000 year summer-monsoon multi-proxy stack from the Owen Ridge, Northern Arabian Sea[END_REF][START_REF] Clemens | Forcing mechanisms of the Indian Ocean monsoon[END_REF].

Significant variability at obliquity and precession periods has been identified in high-resolution late

585

In addition to the periods discussed above, a number of non-primary orbital periods termed heterodynes, which result from non-linear interactions between variables operating at Earth's primary orbital periods [START_REF] Rial | Understanding nonlinear responses of the climate system to orbital forcing[END_REF][START_REF] Thomas | Heterodynes dominate precipitation isotopes in the East Asian monsoon region, reflecting interaction of multiple climate factors[END_REF][START_REF] Clemens | Orbital-scale timing and mechanisms driving Late 845 Pleistocene Indo-Asian summer monsoons: Reinterpreting cave speleothem δ18O[END_REF], stand out in our late Miocene records (24,26,30,37,49,and 68 kyr periods;Fig. 8). For example, the 1/24 kyr heterodyne, prominent in all our records, could result from the interference of eccentricity 590 with precession, and the 1/30 kyr heterodyne seen in Ba records from an interaction between obliquity and precession. Several of these heterodynes have been previously identified in spectra of seawater δ 18 O that reflect Asian monsoon precipitation and runoff, both in the Andaman Sea (30 and 130 kyr during the Pleistocene, 27 and 30 kyr in the latest Miocene) [START_REF] Gebregiorgis | Southern Hemisphere forcing of South Asian monsoon precipitation over the past ~1 million years[END_REF][START_REF] Jöhnck | Variability of the Indian Monsoon in the Andaman Sea across the Miocene-Pliocene transition[END_REF] and in the East China Sea (29 and 69 kyr during the Pleistocene) (Clemens et 595 al., 2018), interpreted to suggest a strongly non-linear response of the monsoon to orbital forcing.

We favour the interpretation that the strong 24-kyr variability in our records reflects a primary period of precession, because precession filters of [Ba] Amplitude modulation of precession-scale variability in our productivity records broadly follows that of the SITIG (Fig. 10), suggesting a direct response of SASM winds to cross-equatorial insolation gradients during the late Miocene.

Cross-spectral analysis of our [Ba]xs productivity record with the Site U1448 seawater δ 18 O record 605 over the interval 4.95-6.19 Ma (where records overlap) shows >80% coherency and an in-phase relationship at the 30-kyr period, suggesting that during this time monsoon winds and precipitation/runoff in the BOB were to some degree coupled on orbital timescales (Fig. S3c), as is the case in the late Pleistocene (Clemens et al., 2021). Nevertheless, our [Ba]xs record over this time window contains stronger primary precession (22-24 kyr) and obliquity (41 kyr) signals than the 610 Site U1448 seawater δ 18 O record that cannot be explained by differences in resolution, highlighting that different climatic processes and feedbacks operating on orbital timescales must contribute to the two records (interpreted to reflect runoff/precipitation and wind, respectively) to different extents. In the late Pleistocene, strong obliquity-band and precession-band variance is found in Andaman Sea records of monsoon precipitation/runoff [START_REF] Gebregiorgis | Southern Hemisphere forcing of South Asian monsoon precipitation over the past ~1 million years[END_REF], whereas records 615 of upper ocean stratification controlled by South Asian monsoon wind mixing at Site 758 (~100 m from Site U1443) show only precession-band variance [START_REF] Bolton | A 500,000 year record of Indian summer monsoon dynamics recorded by eastern equatorial Indian Ocean upper water-820 column structure[END_REF]. The significant 41-kyr variability seen in late Miocene terrigenous elements at Site U1443 (Fig. 8a-c) could also suggest obliquity control on monsoon runoff into the BOB at this time, inferring a partial decoupling between monsoon winds (controlling open-ocean productivity) and runoff (controlling terrigenous 620 sedimentation and salinity/seawater δ 18 O) on orbital timescales, although additional records from regions closer to river sediment and freshwater sources are needed to corroborate this idea. It is important to note that whilst SAM expression above Site U1443 in the southern BOB is dominated by summer monsoon winds driving surface ocean currents and deeper mixing, oceanographic conditions in the northern and eastern BOB (e.g., Sites U1447 and U1448) are instead primarily 625 controlled by summer monsoon freshwater inputs [START_REF] Jöhnck | Variability of the Indian Monsoon in the Andaman Sea across the Miocene-Pliocene transition[END_REF][START_REF] Kuhnt | Miocene to Pleistocene Palaeoceanography of the Andaman Region: Evolution of the Indian Monsoon on a Warmer-Than-Present Earth[END_REF]. Runoff and direct precipitation during the SASM lead to strong salinity stratification in the northern parts of the BOB in the late summer and autumn that prevents upper ocean mixing (e.g., [START_REF] Sengupta | Near-surface salinity and stratification in the north Bay of Bengal from moored observations[END_REF]). These regional differences in the manifestation of the monsoon must be considered when interpreting records from the heterogeneous BOB, and records from multiple locations and proxies 630 are needed to achieve a representative picture of the SAM subsystem. The 405 kyr eccentricity modulation of precession-scale export productivity variability broadly coincides with 405-kyr cycles in benthic δ 13 C at Site U1443, with higher export productivity occurring during benthic δ 13 C minima and eccentricity maxima on these timescales (Fig. 10). 635

Cross-spectral analysis indicates that log(Ba/Fe) and benthic δ 13 C are > 95% coherent at the ~405 kyr and 22-24 kyr periods, with an in-phase relationship in the precession band (-13°±25) and a near antiphase relationship on 405-kyr timescales (151°±27) (Fig. S3b). 405-kyr modulation of the ocean carbon cycle, primarily recorded in carbonate content and benthic δ 13 C records [START_REF] Herbert | A long marine history of carbon cycle modulation by orbital-climatic changes[END_REF][START_REF] Drury | Climate, cryosphere and carbon cycle controls on Southeast Atlantic orbital-scale carbonate deposition since the Oligocene (30-0 Ma)[END_REF][START_REF] De | High-latitude biomes and rock weathering mediate climate-carbon cycle feedbacks on eccentricity timescales[END_REF][START_REF] Westerhold | An astronomically dated record of Earth's climate and its predictability over the last 66 million years[END_REF][START_REF] Pälike | A Cenozoic record of the equatorial Pacific carbonate compensation depth[END_REF]640 Paillard, 2017;[START_REF] Holbourn | Orbitally-paced climate evolution during the middle Miocene "Monterey" carbon-isotope excursion[END_REF] but also in productivity and monsoon-related dust records [START_REF] Rickaby | Coccolith chemistry reveals secular variations in the global ocean carbon cycle?[END_REF][START_REF] Wang | Obscuring of long eccentricity cyclicity in Pleistocene oceanic carbon isotope records[END_REF], has been observed throughout the Cenozoic and Mesozoic sedimentary record. In middle Miocene records, poor carbonate preservation noted during eccentricity maxima is interpreted as indicating transient shoaling of the carbonate compensation depth [START_REF] Holbourn | Orbitally-paced climate evolution during the middle Miocene "Monterey" carbon-isotope excursion[END_REF]Flower and Kennett, 1994). Here, we see a broad positive correlation 645 between log(Ba/Fe) and log(Ca/Terr) records on 405-kyr timescales (Figs. 9a,b), suggesting that carbonate content fluctuations at Site U1443 in the late Miocene were more strongly related to biogenic production than to dissolution on long eccentricity timescales. The coincidence of late Miocene eccentricity maxima with productivity maxima and benthic δ 13 C minima at Site U1443 is compatible with the hypothesis that during eccentricity maxima, a strengthened monsoon induced 650 405-kyr cycles in the marine carbon cycle via increased weathering and nutrient inputs, leading to enhanced marine biological productivity and deep-ocean organic carbon burial (Ma et al., 2011).

Late Miocene monsoon evolution

On long timescales, our Site U1443 biogenic Ba records show relatively stable (9 to 6.5 Ma) or 655 slightly decreasing (6.5-5.3 Ma) export productivity between 9 and 5 Ma (Fig. 7d-f, Fig. S7). Based on sediment colour properties and XRF-derived Ba/Ti ratios in the preceding interval (~13.5-8.3 Ma), [START_REF] Lübbers | The middle to late Miocene "Carbonate Crash" in the equatorial 1040 Indian Ocean[END_REF] suggested that a shift towards a higher productivity regime at Site U1443 occurred at ~11.2 Ma, significantly earlier than the onset of the biogenic bloom at other sites and 2.5 Ma before the rise in CaCO3 MAR, and that this shift was potentially linked to an intensification 660 of the South Asian monsoon. An increase in export productivity at ~11 Ma is coherent with longterm changes in benthic foraminiferal assemblages at Site 758 [START_REF] Gupta | Indian Ocean high-productivity event 945 (10-8 Ma): Linked to global cooling or to the initiation of the Indian monsoons?[END_REF][START_REF] Nomura | Paleogene to Neogene deep-sea paleoceanography in the eastern Indian Ocean: benthic foraminifera from ODP Sites 747, 757 and 758[END_REF] and at sites in the western tropical Indian Ocean [START_REF] Smart | Middle-late Miocene benthic foraminifera in a western equatorial Indian Ocean depth transect: paleoceanographic implications[END_REF]. Reconstructions of Arabian Sea upwelling, export productivity, and deoxygenation [START_REF] Bialik | Monsoons, upwelling, and the deoxygenation of the northwestern Indian Ocean in response to 810 middle to late Miocene global climatic shifts[END_REF][START_REF] Gupta | Evolution of the South Asian monsoon wind system since the late Middle Miocene[END_REF]Huang et al., 2007;[START_REF] Zhuang | Monsoonal upwelling in the western Arabian Sea 1230 since the middle Miocene[END_REF], as well as the abrupt appearance of drift sediments in the 665 Maldives Archipelago at ~13 Ma [START_REF] Betzler | The abrupt onset of the modern South Asian Monsoon winds[END_REF], point towards an intensification of seasonally-reversing South Asian monsoon winds between 13-11 Ma, consistent with Site U1443 export productivity records. Our new data suggest that similar levels of export productivity to those seen from 11.2 Ma to 9 Ma persisted until at least 5 Ma at Site U1443.

670

Compiled Asian monsoon proxy records spanning the 9 to 5 Ma interval show relatively stable long-term SASM strength (Fig. 11, see Fig. 1c for site map). In line with records from the Maldives and Arabian Sea [START_REF] Tripathi | First evidence of denitrification vis-à-vis monsoon in the Arabian Sea since Late Miocene[END_REF]Huang et al., 2007;[START_REF] Betzler | The abrupt onset of the modern South Asian Monsoon winds[END_REF][START_REF] Zhuang | Monsoonal upwelling in the western Arabian Sea 1230 since the middle Miocene[END_REF], Site U1443 records do not corroborate the hypothesis that SASM winds intensified at ~7-8 Ma as suggested by some studies [START_REF] Kroon | G. bulloides was counted in the >150 µm fraction in Huang et al. (white diamonds) and the >125 µm fraction in Kroon et al. (grey circles), g: clay mineralogy (smectite/(illite + chlorite)) at IODP Site U1447 in the Andaman Sea 1360[END_REF][START_REF] Singh | Miocene history of Indian monsoon: A review of marine records, Indian Miocene: A Geodynamic and chronologic framework for palaeobiota, sedimentary 1160 environments and palaeoclimates[END_REF][START_REF] Gupta | Evolution of the South Asian monsoon wind system since the late Middle Miocene[END_REF]An et al., 675 2001). A recent modelling study suggests that the emergence of the Arabian Peninsula played a key role in the establishment of modern Somali Jet structure above the western Indian Ocean, initiating strong upwelling along the Oman margin during the Miocene (Sarr et al., in review). This provides a potential explanation for the temporal decoupling between western Arabian Sea records and SASM records from other regions, and suggests that Arabian Sea upwelling records should not be 680 interpreted as exclusive recorders of SASM strength on geological timescales. The long-term trend in our record shows broad agreement with a low-resolution clay mineralogy record from Site U1447 in the Andaman Sea showing gradual long-term decrease in smectite/(illite and chlorite) over the late Miocene (Fig. 11g), indicating strengthened physical weathering and/or weakened chemical weathering, attributed to the South Asian winter and summer monsoons respectively (Lee 685 et al., 2020). Also at Site U1447, records of potassium content (%K, Fig. 11x) are interpreted to show a shift in sediment provenance and/or an increase in physical weathering and erosion in the sediment source region between ~7 and ~6 Ma (Fig. 11h), potentially linked to an increase in monsoon rainfall intensity and global cooling [START_REF] Kuhnt | Miocene to Pleistocene Palaeoceanography of the Andaman Region: Evolution of the Indian Monsoon on a Warmer-Than-Present Earth[END_REF]. Between 6.2 and 5 Ma, our equatorial Indian Ocean wind records show good long-term agreement with a seawater δ 18 O record 690 from the Andaman Sea [START_REF] Jöhnck | Variability of the Indian Monsoon in the Andaman Sea across the Miocene-Pliocene transition[END_REF], with a minimum in export productivity at ~5.3 Ma at Site U1443 coinciding with a maximum in seawater δ 18 O at Site U1448 (Fig. 11i,j). One interpretation of this could be a coupled reduction in both SASM wind intensity and runoff/precipitation over this interval, although [START_REF] Jöhnck | Variability of the Indian Monsoon in the Andaman Sea across the Miocene-Pliocene transition[END_REF] In contrast to the relatively stable South Asian monsoon between 9 and 5 Ma, evidence for a step 700 strengthening of the East Asian winter monsoon during the late Miocene (~7 Ma) comes from the South China Sea [START_REF] Holbourn | Late Miocene climate cooling and intensification of southeast Asian winter monsoon[END_REF] (Fig. 11a), whereas records from a site on the Chinese Loess Plateau suggest a more gradual intensification of the East Asian summer monsoon from 8.2 to 2.6 Ma (Fig. 11b). The step change in South China Sea surface water geochemistry is interpreted to reflect drying and cooling of the Asian interior and a related southward shift of the ITCZ leading 705 to an intensified dry winter monsoon over southeast Asia [START_REF] Holbourn | Late Miocene climate cooling and intensification of southeast Asian winter monsoon[END_REF]. The long-term increase in East Asian summer monsoon strength inferred from magnetic records in Chinese loess sequences is attributed to progressive Antarctic glaciation that drives an increased pressure gradient between the Australian High and Asian Low pressure cells, a mechanism supported by numerical simulations [START_REF] Ao | Late Miocene-Pliocene Asian monsoon intensification linked to Antarctic ice-sheet growth[END_REF]. However, we note that these single site records may not be 710 representative of the East Asian monsoon subsystem as a whole. Conversely, the apparent insensitivity of equatorial SASM wind intensity to global late Miocene sea surface cooling, which began at ~7.5 Ma and culminated in an SST minimum at 6 to 5.5 Ma (Fig. 11k) [START_REF] Herbert | Late Miocene global cooling and the rise of modern ecosystems[END_REF], is consistent with climate modelling studies that show limited impact of different pCO2 scenarios on SASM wind patterns and strength [START_REF] Kitoh | Simulated changes in the Asian summer monsoon at times of increased atmospheric CO2[END_REF]Sarr et al., in review). The South 715

Asian monsoon is widely considered to be a thermally direct circulation, driven by the thermodynamic contrast between the Indian subcontinent and the equatorial Indian Ocean that develops in summer, with changes in this gradient impacting the strength of onshore SASM monsoon flow [START_REF] Lutsko | Modulation of monsoon circulations by crossequatorial ocean heat transport[END_REF][START_REF] Acosta | Competing topographic mechanisms for the summer Indo-Asian monsoon[END_REF]. The lack of a long-term trend in wind and surface circulation proxies over the 9 to 5 Ma interval (Fig. 11) suggests a relatively constant 720 land-sea temperature gradient despite global cooling. Thus, our data add to the body of evidence suggesting decoupling between the East Asian and South Asian monsoons on long timescales.

Summary & Conclusions

We present new equatorial Indian Ocean benthic δ 13 C and δ 18 O records and an age model spanning 725 the interval between 9 and 5 Ma (late Miocene-earliest Pliocene), and analyse sedimentation and productivity trends and cyclicity using XRF-derived records and MARs. Biogenic sediment MARs reveal a modest imprint of the late Miocene biogenic bloom at Site U1443 lasting until at least 5 Ma, primarily driven by fine-fraction (coccolith) CaCO3 accumulation, as noted at other sites [START_REF] Si | Reduced continental weathering and marine calcification linked to late 1155 Neogene decline in atmospheric CO 2[END_REF]. Nevertheless, the carbonate MAR record of Site U1443 is clearly influenced 730 by carbonate preservation as well as production over the late Miocene, so independent productivity proxies must be considered when defining the duration of the biogenic bloom. Nicobar-Bengal Fan system around the same time [START_REF] Mcneill | Understanding Himalayan erosion and the significance of the Nicobar Fan[END_REF]Pickering et al., 2020a).

Our data show no long-term increase in export productivity between 9 and 5 Ma (and by inference no intensification of SASM winds), therefore these data support existing evidence for an early late Miocene (~13-10 Ma) establishment of strong seasonally reversing South Asian monsoon winds and Arabian Sea upwelling, with relatively stable or slightly weakening SASM winds over the 740 remainder of the late Miocene and earliest Pliocene between 9 and 5 Ma. Spectral and crossspectral analyses of XRF-based biogenic barium records reveal that export productivity in waters overlying Site U1443 was consistently paced by precession, with amplitude modulation of the precession signal on ~405 kyr timescales and no significant variability at glacial-interglacial (obliquity) timescales. Coeval late Miocene productivity maxima and benthic δ 13 C minima during 745 eccentricity maxima at Site U1443 provides support for the hypothesis that the monsoon may have paced changes in the carbon cycle on ~405 kyr timescales (Ma et al., 2011). Significant coherence and an in-phase relationship at the precession band between biogenic barium and the SITIG suggests direct forcing of South Asian monsoon winds by insolation gradients over the late Miocene, relatively unaffected by glacial boundary conditions and long-term global cooling trends 750 [START_REF] Herbert | Late Miocene global cooling and the rise of modern ecosystems[END_REF]. In contrast, East Asian summer and winter monsoons appear to have intensified during the late Miocene in response to global cooling and Antarctic ice sheet growth and related feedbacks [START_REF] Ao | Late Miocene-Pliocene Asian monsoon intensification linked to Antarctic ice-sheet growth[END_REF][START_REF] Holbourn | Late Miocene climate cooling and intensification of southeast Asian winter monsoon[END_REF], although more continuous records over the late Miocene are needed to understand regional trends due to the heterogeneous nature of Asian monsoon expression. 755 S1), and shipboard magnetostratigraphy is also shown for the interval over which it could be reliably determined. Between ~90 and 128m CCSF, sediments in cores from all holes showed scattered directional signals during pass-through magnetic remanence 1270 measurement, which hindered any determination of polarity patterns in this interval across the whole site [START_REF] Clemens | Site U1443[END_REF]. Black and white zones = normal and reversed polarity, respectively; grey zones = magnetic polarity not clearly determined. All numbers are age assignations for boundaries/nannofossil events in Ma [START_REF] Gradstein | The geologic time scale[END_REF] Astronomical time series from [START_REF] Laskar | A long-term numerical solution for the insolation quantities of the Earth[END_REF]. c: tuned benthic δ 18 O vs age, d: sedimentation 1280 rates, e: tuned benthic δ 13 C vs age. Tie-points between a and b are shown in orange (see Table 1). [START_REF] Holbourn | Late Miocene climate cooling and intensification of southeast Asian winter monsoon[END_REF] with age model revised in (Holbourn et al., In Press), equatorial Pacific IODP Sites U1338 [START_REF] Drury | Deciphering the state of the late Miocene to early Pliocene equatorial Pacific, 890[END_REF]Drury et al., 1295 2016) and U1337 [START_REF] Drury | Late Miocene climate and time scale reconciliation: Accurate orbital calibration from a deep-sea perspective[END_REF], and ODP Site 926 [START_REF] Shackleton | The Late Miocene stable isotope record, Site 926[END_REF][START_REF] Drury | Late Miocene climate and time scale reconciliation: Accurate orbital calibration from a deep-sea perspective[END_REF][START_REF] Zeeden | Revised Miocene splice, astronomical tuning and calcareous plankton biochronology of ODP Site 926 between 1225 5 and 14.4 Ma[END_REF]. All records are shown on their latest independent orbitally-tuned chronologies. We have excluded Caribbean ODP Site 999 from this figure because it is bathed in intermediate water masses due to basin geometry and sill depths [START_REF] Bickert | Late Neogene benthic stable isotope record of ODP Site 999: Implications for Caribbean paleoceanography, organic carbon burial and the Messinian Salinity Crisis[END_REF]. Deep South Atlantic Site 704 [START_REF] Müller | Late Miocene to earliest Pliocene (9.8-4.5 Ma) paleoceanography of the subantarctic southeast atlantic: stable isotopic, sedimentologic, and microfossil evidence[END_REF] data are not plotted due to clear age model discrepancies 1300 when compared to orbitally-tuned records. All δ 18 O records are based on Cibicidoides wuellerstorfi or C. mundulus therefore no corrections are applied, following [START_REF] Jöhnch | Oxygen isotope offsets in deep-water benthic foraminifera[END_REF]. The Site 926 record includes δ 13 C corrections for some samples due to the multispecific nature of the record [START_REF] Drury | Late Miocene climate and time scale reconciliation: Accurate orbital calibration from a deep-sea perspective[END_REF]. No correction was applied to C. wuellerstorfi or C. mundulus δ 13 C values. 1305 [START_REF] Laskar | A long-term numerical solution for the insolation quantities of the Earth[END_REF], b: benthic δ 13 C (bandpassed as in Fig. 9) and its 405-kyr filter (above) (note reversed y-axes). c: Lowess-detrended log(Ba/Fe) (window = 0.1 Ma) and 22-25 kyr filter (above), d: Lowess-detrended Baxs (window = 1340 0.1 Ma) and its 22-25 kyr filter (above), e: The summer inter-tropical insolation gradient (SITIG, calculated as the insolation difference between 23°N and 23°S on 21 st June using orbital solution of [START_REF] Laskar | A long-term numerical solution for the insolation quantities of the Earth[END_REF]) and its 22-25 kyr filter and amplitude modulation (above); For panels c-e, raw datasets were filtered using a Tanner-Hilbert filter centred on 43 cycles/myr with bandwidth ±3 (designed to include both the ~24 kyr and ~22 kyr precession periods). For panel b, a Tanner-1345

Hilbert filter centred on 2.47 cycles/myr with bandwidth ±0.8 was applied.

  . Our current knowledge of orbital-resolution past productivity fluctuations in the South Asian monsoon region and their relationship with local (insolation) or remote (global ice volume, greenhouse gases) forcing mechanisms comes almost entirely from Pleistocene Arabian Sea sedimentary records (Clemens https://doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License.

  https://doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License.

  https://doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License. strength, via its control on MLD and nutrient entrainment into the mixed layer as well as on surface ocean currents. Samples used in this study are from Site U1443 (Latitude 5°23 N, Longitude 90°21 E, water depth 2935 m), drilled during IODP Expedition 353 on the crest of Ninetyeast Ridge (NER) (Clemens et al., 2016) (Fig. 1c). Site U1443 is located ~100 m southeast of Ocean Drilling Program (ODP) Site 758 and is a re-drill of this legacy site (Shipboard Scientific Party, 1989). At Site U1443, the use of 180 Advanced Piston Coring (APC) and half-length APC drilling techniques down to >200 m CSF (core depth below sea floor) in three holes allowed recovery of a compete, spliced Neogene sedimentary section spanning 0-195 m CCSF (core composite depth below sea floor). Late Miocene records cover the interval 70.06 m CCSF (U1443B 7H 5W 75-76 cm) to 122.76 m CCSF (U1443C 15H 4W 148-149 cm), following the revised shipboard splice (CCSF-D), spanning the interval ~5 Ma to 9.5 185

  resolution) and binned by month. Scatter thus reflects a combination of spatial variability within our small box and interannual variability; monthly means over each time series are also shown. POC, https://doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License. biogenic SiO2, and CaCO3 fluxes (Fig. 2d, e) are from SBBT sediment trap samples (~5°N, 87°E, 205

  Institute of Standard and Technology (NIST) carbonate isotope standard NBS (National Bureau of Standard) 19, and are reported on the Vienna PeeDee Belemnite (VPDB) scale. C. wuellerstorfi 230 isotope data below 112.86 m CCSF were originally published in Lübbers et al. (2019), and a lowresolution version of the long-term δ 13 C record is included in Bretschneider et al. (submitted).

  https://doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License.

  elemental count data so that absolute values and variance matched our data, based on an overlapping interval between 112.80 and 113.56m CCSF. Quantitative chemical compositions of a subset of discrete bulk sediment samples were determined at CEREGE after total digestion by Inductively Coupled Plasma Mass Spectrometer (ICP-MS 270 https://doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License.

  https://doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License.

[

  Ba]xs using[Ti] to represent Badetrital, applying a Ba/Titerrigenous ratio of 0.183 g/g[START_REF] Mclennan | Relationships between the trace element composition of sedimentary rocks and upper continental crust[END_REF], and carbonate-free [Ba]xs. https://doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License.

  model and benthic foraminiferal isotope data U1443 benthic δ 18 O and δ 13 C data between 70.06 m and 122.76 m CCSF are shown in the depth domain in Figure 3 alongside calcareous nannofossil datums (revised herein, Table

  https://doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License.

  C in smoothed record) is similar to that recorded in Pacific Ocean sediments from Sites U1338, U1337 and 1146. The Site U1443 δ 13 C https://doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License. record shows a consistent positive offset of 0.15-0.25 ‰ relative to South China Sea Site 1146 over the 9-5 Ma interval (Fig. 5b). 400

  https://doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License.

  ~405 kyr period is >99% significant in[Al],[Ba],[Ba]cfb,[Ba]xs, and log(Ba/Fe), whereas the ~125 kyr period is significant (>95% or >99%) in [Ba], [Ba]cfb, [Ba]xs, log(Ba/Fe), and % CaCO3 records. 460 At higher frequencies, the spectral signatures of [Fe], [Al], [Ti],[Ba], are dominated by significant peaks at 24 kyr (>99%) and 41 kyr (>90%), with[Ba] additionally showing peaks at 22.5 kyr (>99%) and at 26 and 30 kyr (>95%). Log(Ba/Fe), [Ba]cfb, and [Ba]xs show dominant (>99% significant) 22.5 kyr variability, with additional >95% significant peaks at 24 kyr and 30 kyr (for [Ba]xs only). Log(Ca/Terr) shows significant peaks at 24 and 22.5 kyr (both >99%), and also at 68 465 https://doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License.

  driving regional carbonate MAR increases as well as improved preservation at deeper sites. A recent study decoupling coccolith and foraminiferal MARs in relatively shallow, globally-495 distributed sites (minimally affected by dissolution) records a late Miocene pulse in coccolith MARs beginning at ~8-9 Ma and persisting until ~3-4 Ma, interpreted to reflect high https://doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License.

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545

  On orbital timescales, time series analyses reveal dominant precession-band (22-24 kyr) variance in late Miocene export productivity records (Fig. 8, 9). Spectral analyses of individual calibrated timeseries of [Ba], [Al], [Fe], [Ti], [Ba]cfb and %CaCO3 allow us to tease apart the effects of sediment dilution and the competing influence of Baterr and Babio on our Ba proxies, [Ba]xs and log(Ba/Fe). The 41 kyr obliquity period is most significant (>90%) in the terrigenous element 550 records (Al, Fe, Ti, Ba), and absent or less significant in [Ba]xs, log(Ba/Fe), log(Ca/Terr), and %CaCO3 (Fig 8).

  Miocene-early Pliocene records of precipitation/runoff based on planktic foraminiferal δ 18 O and https://doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License.

  xs and log(Ba/Fe) spanning 22-25 kyr show https://doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License. strong amplitude modulation of the precession signal at a period of ~405 kyr, which results from the interaction of the 23.6-kyr and 22.3-kyr periods (1/[(1/22.3) -(1/23.6)] = 404.8 kyr) (Fig. 10). 600

  https://doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License.

  https://doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License.

  invoke an increase in local evaporation and/or a change in precipitation source to explain the decreasing seawater δ 18 O values 695 between 5.6 and 5.2 Ma. High-resolution records of precipitation and runoff from the SASM region further back in time are needed to verify to what extent monsoon winds and precipitation/runoff are coupled on long timescales. https://doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License.

  Our results indicate https://doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License. that step increases in SAR recorded on the Ninetyeast Ridge at ~8-9 Ma primarily reflect an increase in CaCO3 accumulation, and that this is likely independent from the increase in lithogenic sediment flux recorded in nearby Nicobar Fan sites, itself related to sediment re-routing within the 735

Figure 1 :

 1 Figure 1: Seasonal contrast (February vs August) in wind stress (a) and mixed layer depth (b). Yellow dots indicate the modern location of IODP Site U1443. Maps were created on the ERDDAP website using the datasets Wind Stress, Metop-A ASCAT, 0.25°, Global, Near Real Time, 2009-1245 present (Monthly) and Ocean Climatology Ocean Mixed Layer Depth MLD T02 kriging (see methods for details). c: Regional bathymetric map showing modern locations of marine and terrestrial sites discussed in this study (white squares) and the location of Site U1443, a redrill of Site 758, in the modern ocean (yellow square) and its paleolatitude at 10 Ma (yellow star, ~2°N, data from paleolatitude.org). The red circle shows the location of the SBBT sediment trap. Green 1250 arrows show surface ocean circulation during the summer monsoon (July/August) and the eastward flow of waters from the Arabian Sea into the BOB via the Southwest Monsoon Current (SMC), after Schott et al. (2009).

Figure 2 :Figure 3 :

 23 Figure 2: Modern, seasonal oceanographic variability above Site U1443 in the southern BOB. a: wind stress, b: mixed layer depth (MLD), c: net primary productivity (NPP), d: particulate organic carbon (POC) flux, e: biogenic silica (bSi) and calcium carbonate (CaCO3) flux. See Section 2.1 for details of individual datasets and sources. Points represent individual months, diamonds and 1260triangles with lines represent monthly mean values over entire time series. Months (x-axis) run from 1 (1 st January) to 12 (1 st December). JJA = June, July August, DJF = December, January, February.

  doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License.

Figure 4 :

 4 Figure 4: Astronomical (minimal) tuning of Site U1443 benthic δ 18 O record to ET target. a: benthic δ 18 O record on depth scale with nannofossil-based age constraints, b: ET tuning target (1:1 weighting, normalised). ETP (1:0.5:-0.4 weighting, normalised) is also shown in grey.Astronomical time series from[START_REF] Laskar | A long-term numerical solution for the insolation quantities of the Earth[END_REF]. c: tuned benthic δ 18 O vs age, d: sedimentation 1280 rates, e: tuned benthic δ 13 C vs age. Tie-points between a and b are shown in orange (see Table1). An age-depth plot showing ET tie-points and good agreement with biostratigraphic age control is shown in Fig S1. f and g: wavelet analyses of tuned isotope records; white shaded area shows cone of influence and contours show 95% significance level. Main orbital periods are shown on the right in kyr. h: Filtered tuned benthic isotope records compared to filtered ET (as in b). Top: 100-kyr 1285 filtered benthic δ 13 C (blue; Gaussian filter centred on 100 kyr with bandwidth ±25 kyr to include 95 and 125 kyr peaks) compared with filtered ET (grey, identical filter design). Bottom: 41-kyr filtered

Figure 5 :

 5 Figure 4: Astronomical (minimal) tuning of Site U1443 benthic δ 18 O record to ET target. a: benthic δ 18 O record on depth scale with nannofossil-based age constraints, b: ET tuning target (1:1 weighting, normalised). ETP (1:0.5:-0.4 weighting, normalised) is also shown in grey.Astronomical time series from[START_REF] Laskar | A long-term numerical solution for the insolation quantities of the Earth[END_REF]. c: tuned benthic δ 18 O vs age, d: sedimentation 1280 rates, e: tuned benthic δ 13 C vs age. Tie-points between a and b are shown in orange (see Table1). An age-depth plot showing ET tie-points and good agreement with biostratigraphic age control is shown in Fig S1. f and g: wavelet analyses of tuned isotope records; white shaded area shows cone of influence and contours show 95% significance level. Main orbital periods are shown on the right in kyr. h: Filtered tuned benthic isotope records compared to filtered ET (as in b). Top: 100-kyr 1285 filtered benthic δ 13 C (blue; Gaussian filter centred on 100 kyr with bandwidth ±25 kyr to include 95 and 125 kyr peaks) compared with filtered ET (grey, identical filter design). Bottom: 41-kyr filtered

  doi.org/10.5194/cp-2021-77 Preprint. Discussion started: 29 June 2021 c Author(s) 2021. CC BY 4.0 License.

Figure 6 :Figure 7 :Figure 8 :

 678 Figure 6: Raw counts/sec (grey lines) and calibrated concentration (blue lines) scanning XRF elemental data over the late Miocene interval. Pink circles show samples used for calibration (See Fig. S6).

Figure 9 :

 9 Figure 9: Orbital-scale variability of productivity and CaCO3 proxies at Site U1443. All records shown here are bandpassed as described in the methods. (a) benthic δ 18 O, (b) benthic δ 13 C, (c) 1330 log(Ca/Terr), (d) log(Ba/Fe), (e) [Ba]xs, (f) ETP. For c-e, wavelet analyses are shown, illustrating dominant precession-scale (22-24 kyr) variability in Ba proxies and both precession and obliquity (41-kyr) variability in log(Ca/Terr).

Figure 10 :

 10 Figure 10: Detrended and filtered late Miocene U1443 records to illustrate precession-band variance and amplitude modulation. a: eccentricity[START_REF] Laskar | A long-term numerical solution for the insolation quantities of the Earth[END_REF], b: benthic δ 13 C (bandpassed as in Fig.9) and its 405-kyr filter (above) (note reversed y-axes). c: Lowess-detrended log(Ba/Fe) (window = 0.1 Ma) and 22-25 kyr filter (above), d: Lowess-detrended Baxs (window = 1340 0.1 Ma) and its 22-25 kyr filter (above), e: The summer inter-tropical insolation gradient (SITIG, calculated as the insolation difference between 23°N and 23°S on 21 st June using orbital solution of[START_REF] Laskar | A long-term numerical solution for the insolation quantities of the Earth[END_REF]) and its 22-25 kyr filter and amplitude modulation (above); For panels c-e, raw datasets were filtered using a Tanner-Hilbert filter centred on 43 cycles/myr with bandwidth ±3 (designed to include both the ~24 kyr and ~22 kyr precession periods). For panel b, a Tanner-1345Hilbert filter centred on 2.47 cycles/myr with bandwidth ±0.8 was applied.

Figure 11 :

 11 Figure 11: Compilation of late Miocene (9-5 Ma) Asian monsoon reconstructions, showing representative records from different regions and their published interpretations (bars on left). a: 1350 Seawater δ 18 O showing an increase in East Asian winter monsoon strength at ~7.4 Ma (Holbourn et al., 2018; Holbourn et al., In Press), b: Stacked magnetic records of the East Asian summer

Acknowledgements

This research used samples and data provided by the International Ocean Discovery Program (IODP). We thank the science party, technical staff and crew of IODP Expedition 353. Funding for this research was provided by French ANR project iMonsoon ANR-16-CE01-0004-01 (CTB), 775 IODP France (CTB), and Deutsche Forschungsgemeinschaft grant Ku649/36-1(WK). KMG is supported by Australian Research Council grant DE190100042. CTB thanks Anna Joy Drury and Tim Herbert for age model feedback, and Luc Beaufort, Baptiste Suchéras-Marx, and Ian Bailey for discussions that helped improve the manuscript. Marta Garcia Molina, Jean-Charles Mazur, and Christine Pailles are thanked for technical laboratory support at CEREGE.

Data Availability

All data will be available on the www.pangaea.de database (submitted 18.06.2021, awaiting validation)

Supplement

Competing Interests

The authors declare that they have no conflict of interest.