C Creaser

E Bottini

F Erba

H Giraud

Svensen

L M E Percival

L R Tedeschi

R A Creaser

C Bottini

E Erba

F Giraud

H Svensen

J Savian

R Trindade

R Coccioni

F Frontalini

L Jovane

T A Mather

H C Jenkyns

; C Bottini

; E Erba

; F Giraud

; H Svensen

; J Savian

; R Trindade

; R Coccioni

; F Frontalini

; L Jovane

; H C Jenkyns

Determining the style and provenance of magmatic activity during the Early Aptian Oceanic Anoxic Event (OAE 1a)

Determining the style and provenance of magmatic activity during the Early Aptian Oceanic Anoxic Event

(OAE 1a)

1a, and that such volcanism could have caused a fourfold rise in mantle carbon emissions (δ 13 C  -6 ‰; Gales et al., 2020, and references therein). These emissions alone could have plausibly caused a similarly swift 3000 ppm increase in atmospheric pCO 2 levels and the 1.5-2 ‰ negative δ 13 C excursion typically documented in the C3 segment. However, based on pCO 2 trends reconstructed from the Cau section (Betic Cordillera, Spain), Naafs et al. (2016) argued that pCO 2 levels only rose gradually through the first part of OAE 1a and by <300 ppm during the C3 segment. In this case, a much smaller increase in carbon emissions, but with a very isotopically light composition (δ 13 C < -10 ‰), would have been required to cause the C3 negative excursion without greatly increasing atmospheric pCO 2 [START_REF] Adloff | Unravelling the sources of carbon emissions at the onset of Oceanic Anoxic Event (OAE) 1a[END_REF]. Such parameters are inconsistent with a purely mantle-carbon source. Instead, this carbon could have been emitted following metamorphism of organic-rich sediments by intrusive sills during the emplacement of the High Arctic large igneous province (HALIP; Figure 2), or from the destabilisation of methane clathrates (e.g., [START_REF] Jahren | Terrestrial record of methane hydrate dissociation in the Early Cretaceous[END_REF][START_REF] Méhay | A volcanic CO 2 pulse triggered the Cretaceous Oceanic Anoxic Event 1a and a biocalcification crisis[END_REF][START_REF] Polteau | The Early Cretaceous Barents Sea Sill Complex: Distribution, 40 Ar/ 39 Ar geochronology, and implications for carbon gas formation[END_REF][START_REF] Adloff | Unravelling the sources of carbon emissions at the onset of Oceanic Anoxic Event (OAE) 1a[END_REF]. This study investigates the geochemical records of volcanism related to the emplacement of the G-OJP and HALIP during the earliest Aptian and OAE 1a in order to determine the dominant style of magmatic activity operating during those times. This distinction might indicate whether or not a specific igneous province was the foremost source of carbon to the ocean-atmosphere system and likely the key trigger of the event.

Large Igneous Provinces emplaced during the latest Barremian and early Aptian

The G-OJP was an oceanic LIP consisting of several million cubic kilometres of igneous material (largely tholeiitic basalt) emplaced as intrusive and extrusive magma bodies into/onto the ocean crust of the western Pacific (Figure 2; see also [START_REF] Larson | Onset of the Mid-Cretaceous greenhouse in the Barremian-Aptian: Igneous events and the biological, sedimentary, and geochemical responses[END_REF][START_REF] Larson | Onset of the Mid-Cretaceous greenhouse in the Barremian-Aptian: Igneous events and the biological, sedimentary, and geochemical responses[END_REF]. The Ontong-Java Plateau alone has an immense volume of 10s of millions of cubic kilometres (possibly up to 44-57×10 6 km 3 ; [START_REF] Gladczenko | Crustal structure of the Ontong Java Plateau: modeling of new gravity and existing seismic data[END_REF]. When other subsidiary plateaus thought to have initially formed as part of the G-OJP are included (e.g., the Manihiki and Hikurangi; [START_REF] Taylor | The single largest oceanic plateau: Ontong Java-Manihiki-Hikurangi[END_REF] J o u r n a l P r e -p r o o f Journal Pre-proof [START_REF] Hoernle | Age and geochemistry of volcanic rocks from the Hikurangi and Manihiki oceanic Plateaus[END_REF], the total volume of igneous material rises to 59-77×10 6 km 3 (Kerr and Mahoney, 2007). The original province may have been even larger if a part of it was emplaced into/onto the now-subducted Farallon Plate, as has been previously suggested [START_REF] Schlanger | Volcanism and vertical tectonics in the Pacific Basin related to global Cretaceous transgressions[END_REF][START_REF] Larson | Onset of the Mid-Cretaceous greenhouse in the Barremian-Aptian: Igneous events and the biological, sedimentary, and geochemical responses[END_REF]. Thus, the G-OJP could have been an order of magnitude greater in size than the largest-known continental LIPs: the Siberian Traps and Central Atlantic Magmatic Province, which were likely no more than 5×10 6 km 3 each in volume (see review by [START_REF] Bond | Large igneous provinces and mass extinctions: an update[END_REF]. The average eruption rate on the G-OJP remains unknown. However, if Bauer et al.'s (2017) calculation of a six-to ten-fold increase in submarine volcanic activity during OAE 1a compared to pre-OAE background mid-ocean ridge volcanism is correct, and ridge basalt production during Barremian-Aptian times was similar to today (20 km 3 /yr; [START_REF] Cogné | Trends and rhythms in global seafloor generation rate[END_REF], then on average >100 km 3 /yr of igneous material would have been emplaced on the G-OJP during OAE 1a. This average eruption rate is markedly higher than that proposed for any known continental LIP (e.g., [START_REF] Schoene | U-Pb constraints on pulsed eruption of the Deccan Traps across the end-Cretaceous mass extinction[END_REF].

By contrast, Barremian-Aptian HALIP magmatism apparently consisted chiefly of subaerially erupted basalt flows and the intrusion of tholeiitic sills into organic-rich sedimentary rocks [START_REF] Tegner | Magmatism and Eurekan deformation in the High Arctic Large Igneous Province: 40 Ar-39 Ar age of Kap Washington Group volcanics. North Greenland[END_REF][START_REF] Corfu | U-Pb geochronology of Cretaceous magmatism on Svalbard and Franz Josef Land, Barents Sea large igneous province[END_REF][START_REF] Polteau | The Early Cretaceous Barents Sea Sill Complex: Distribution, 40 Ar/ 39 Ar geochronology, and implications for carbon gas formation[END_REF][START_REF] Dockman | Timing and origin of magmatism in the Sverdrup Basin, Northern Canada-Implications for lithospheric evolution in the High Arctic Large Igneous Province (HALIP)[END_REF]. The volume of HALIP igneous material emplaced during Barremian-Aptian times remains poorly constrained, but is thought to have been on the order of 100,000s of cubic kilometres [START_REF] Tegner | Magmatism and Eurekan deformation in the High Arctic Large Igneous Province: 40 Ar-39 Ar age of Kap Washington Group volcanics. North Greenland[END_REF][START_REF] Polteau | The Early Cretaceous Barents Sea Sill Complex: Distribution, 40 Ar/ 39 Ar geochronology, and implications for carbon gas formation[END_REF][START_REF] Dockman | Timing and origin of magmatism in the Sverdrup Basin, Northern Canada-Implications for lithospheric evolution in the High Arctic Large Igneous Province (HALIP)[END_REF], much smaller than the G-OJP but more comparable in volume to many other LIPs. However, the intrusion of organic-rich sediments by HALIP magmatic sills could have acted as an additional source of isotopically light carbon (δ 13 C < -20 ‰) to the ocean-atmosphere system [START_REF] Polteau | The Early Cretaceous Barents Sea Sill Complex: Distribution, 40 Ar/ 39 Ar geochronology, and implications for carbon gas formation[END_REF]c.f., Svensen et al., 2004;[START_REF] Mcelwain | Changes in carbon dioxide during an oceanic anoxic event linked to intrusion into Gondwana coals[END_REF]. By contrast, carbon emissions from the G-OJP would have been almost exclusively magmatic in origin, as that LIP was predominantly emplaced into comparatively volatile-depleted oceanic basalts.

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Crucially, although there is some evidence from phreatomagmatic deposits dated to 120 Ma for subaerial eruptions on the G-OJP during Barremian-Aptian times [START_REF] Chambers | Phreatomagmatic eruptions on the Ontong Java Plateau: an Aptian 40 Ar/ 39 Ar age for volcaniclastic rocks at ODP Site 1184[END_REF][START_REF] Thordarson | Accretionary-lapilli-bearing pyroclastic rocks at ODP Leg 192 Site 1184: a record of subaerial phreatomagmatic eruptions on the Ontong Java Plateau[END_REF], the emplacement of that province into/onto the oceanic crust means that the great majority of volcanic activity associated with it is expected to have been submarine in nature. By contrast, most volatile emissions from HALIP subaerial basalts and thermogenic degassing should have reached the atmosphere. Previous studies have highlighted the fact that geochemical markers of LIP volcanism in stratigraphic archives may be able to distinguish between these styles of magmatic processes, and also yield information on the proximity of the igneous activity [START_REF] Kuroda | Lead isotopic record of Barremian-Aptian marine sediments: Implications for large igneous provinces and the Aptian climatic crisis[END_REF][START_REF] Erba | The Origin, Evolution, and Environmental Impact of J o u r n a l P r e -p r o o f Journal Pre-proof Oceanic Large Igneous Provinces[END_REF]. In particular, [START_REF] Percival | Does Large Igneous Province volcanism always perturb the mercury cycle? Comparing the records of Oceanic Anoxic Event 2 and the end-Cretaceous to other Mesozoic events[END_REF] noted the differing degrees to which styles of volcanism can affect the likelihood of LIP emplacement and associated eruptions being recorded by two key proxies for these phenomena: mercury (Hg) concentrations and osmium-(Os-) isotope ratios (specifically 187 Os/ 188 Os).

Sedimentary 187 Os/ 188 Os ratios as a marker of LIP emplacement during OAE 1a

Large igneous province emplacement can cause a change in the 187 Os/ 188 Os composition of the global ocean by acting as a major source of mantle-derived osmium, through either direct emission of the element during submarine volcanic/hydrothermal activity or weathering/alteration of juvenile LIP basalts (e.g., [START_REF] Cohen | New geochemical evidence for the onset of volcanism in the Central Atlantic magmatic province and environmental change at the Triassic-Jurassic boundary[END_REF][START_REF] Turgeon | Cretaceous oceanic anoxic event 2 triggered by a massive magmatic episode[END_REF]. Importantly, the seawater residence time of osmium (on the order of 10s kyr today: Peucker-Ehrenbrink and [START_REF] Peucker-Ehrenbrink | The marine osmium isotope record[END_REF] means that the global ocean should feature a homogenous 187 Os/ 188 Os composition but still respond to geologically rapid changes in the sources to that inventory, except in hydrographically restricted basins with a very low basin-ocean water-mass exchange where the Os-isotope ratio might be dominated by local inputs (e.g., [START_REF] Paquay | Heterogeneous seawater 187 Os/ 188 Os during the late Pleistocene glaciations[END_REF][START_REF] Dickson | Evidence for weathering and volcanism during the PETM from Arctic and Peri-Tethys osmium isotope records[END_REF].

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Journal Pre-proof Primitive osmium derived from mantle or meteoritic material features 187 Os/ 188 Os 0.13 [START_REF] Allègre | Age of the Deccan traps using 187 Re-187 Os systematics[END_REF], whereas riverine osmium sourced from weathering of the continental crust is typically much more radiogenic (modern-day average riverine 187 Os/ 188 Os 1.4; [START_REF] Peucker-Ehrenbrink | Rhenium-osmium isotope systematics and platinum group element concentrations: Loess and the upper continental crust[END_REF]. Consequently, a significant increase in the flux of mantle-derived Os from LIP activity to the global ocean, relative to riverine runoff of the element from weathering of the continental crust and the comparatively consistent mid-ocean-ridge and cosmogenic inputs, should result in a lower seawater 187 Os/ 188 Os ratio throughout the open ocean. This hydrogenous Os-isotope signature is recorded in seafloor sediments, which can preserve past seawater compositions as long as the sedimentary system remains closed with respect to rhenium and osmium following deposition [START_REF] Cohen | Precise Re-Os ages of organic-rich mudrocks and the Os isotope composition of Jurassic seawater[END_REF].

In all previously studied records of OAE 1a, a pronounced shift is observed in the recorded seawater Os-isotope ratio ( 187 Os/ 188 Os (i) ) towards very unradiogenic compositions (typically <0.2) around the base of the Selli Level or equivalent strata deposited during OAE 1a, together with an increase in sedimentary Os concentrations [START_REF] Tejada | Ontong Java Plateau eruption as a trigger for the early Aptian oceanic anoxic event[END_REF][START_REF] Bottini | Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a[END_REF][START_REF] Adloff | Unravelling the sources of carbon emissions at the onset of Oceanic Anoxic Event (OAE) 1a[END_REF]; see also Figure 1B). These unradiogenic 187 Os/ 188 Os (i) compositions and high Os concentrations generally continue up the stratigraphy through the entire Selli Level (or equivalent), and are thought to document intense LIP volcanism that acted as a source of osmium to the global ocean through one or both of submarine hydrothermal emissions of the element and erosion/alteration of the juvenile basalts formed during the volcanic activity [START_REF] Tejada | Ontong Java Plateau eruption as a trigger for the early Aptian oceanic anoxic event[END_REF][START_REF] Bottini | Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a[END_REF][START_REF] Adloff | Unravelling the sources of carbon emissions at the onset of Oceanic Anoxic Event (OAE) 1a[END_REF]. However, Tethyan records of OAE 1a also document a brief rebound in Os-isotope ratios to higher values in basal Selli Level (or equivalent) strata, potentially highlighting a transient spell of enhanced continental weathering and associated flux of radiogenic Os at the onset of the event that temporarily overprinted the unradiogenic signature [START_REF] Tejada | Ontong Java Plateau eruption as a trigger for the early Aptian oceanic anoxic event[END_REF][START_REF] Bottini | Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a[END_REF]. This transient pulse of enhanced continental weathering early in OAE 1a is supported by strontium-and lithium-isotope excursions (Jones and Jenkyns, 2001;[START_REF] Lechler | Lithium-isotope evidence for enhanced silicate weathering during OAE 1a (Early Aptian Selli event)[END_REF].

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Sedimentary Hg concentrations as a marker of LIP volcanism during OAE 1a

Volcanic eruptions represent one of the largest natural sources of mercury to the Earth's surface in the present day [START_REF] Pyle | The importance of volcanic emissions for the global atmospheric mercury cycle[END_REF][START_REF] Bagnato | Degassing of gaseous (elemental and reactive) and particulate mercury from Mount Etna volcano (Southern Italy)[END_REF]. Mercury is chiefly emitted as a gaseous elemental species that typically has a residence time of 0.5-2 years in the stratosphere, enabling worldwide distribution before the element is ultimately deposited in sediments [START_REF] Schroeder | Atmospheric mercury -An overview[END_REF][START_REF] Ericksen | Accumulation of atmospheric mercury in forest foliage[END_REF][START_REF] Selin | Global Biogeochemical Cycling of Mercury: A Review[END_REF]. The global distribution of volcanic mercury has led several studies of geological events associated with LIPs to utilise sedimentary concentrations of the element as a proxy for the volcanism (see reviews by [START_REF] Grasby | Mercury as a proxy for volcanic emissions in the geologic record[END_REF][START_REF] Percival | Sedimentary mercury enrichments as a tracer of Large Igneous Province volcanism[END_REF]. This approach includes normalising sedimentary mercury concentrations against total organic carbon (TOC) content to account for the association of the element with organic compounds when it is deposited in sediments (Sanei et al., 2012, and references therein). Thus, peaks in sedimentary Hg/TOC ratios are generally interpreted as reflecting an increased input of Hg to the environment from an external source, such as volcanism or wildfires, the latter of which is also known to cause peaks in sedimentary Hg and Hg/TOC values in modern settings (e.g., [START_REF] Daga | Historical records of mercury in southern latitudes over 1600 years: Lake Futalaufquen, Northern Patagonia[END_REF].

However, in euxinic settings where free sulphides precipitate in the water column, Hg may be deposited with that phase rather than organic matter (Shen et al., 2019a;[START_REF] Shen | Sedimentary host phases of mercury (Hg) and implications for use of Hg as a volcanic proxy[END_REF].

Alternatively, in a very well-oxygenated environment, featuring limited burial of both phases, mercury might be adsorbed on to clays (Kongchum et al., 2011;[START_REF] Shen | Sedimentary host phases of mercury (Hg) and implications for use of Hg as a volcanic proxy[END_REF]. Even in settings where Hg contents follow those of TOC, a change in the type of organic matter (e.g., from marine bacterial to terrestrial detrital), or a large influx of organic material from a new source, could alter the Hg/TOC ratio if the affinity of mercury for the various kinds of organic material varies (see [START_REF] Hammer | Are Early Triassic extinction events associated with mercury anomalies? A reassessment of the Smithian/Spathian boundary extinction[END_REF][START_REF] Them | Terrestrial sources as the primary delivery mechanism of mercury to the oceans across the Toarcian Oceanic Anoxic Event (Early Jurassic)[END_REF]. Therefore, robustly demonstrating a global-scale perturbation to the mercury signature caused by volcanism is dependent on documenting enrichment of the element in stratigraphic archives that cover a wide range of geographic areas, and for which the depositional environment and burial history are known.

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Journal Pre-proof To date, three stratigraphic archives of OAE 1a have been investigated for mercury and Hg/TOC trends by [START_REF] Charbonnier | Mercury enrichment indicates volcanic triggering of Valanginian environmental change[END_REF], all of them representing palaeoenvironments from the north-westernmost part of the Tethyan realm: La Bédoule (South Provençal Basin, SE France), Glaise (Vocontian Basin, SE France), and Roter Sattel (Briançonnais Domain, Switzerland). This last record has been shown as being rather thermally mature (Charbonnier et al., 2018a), potentially altering the TOC content and inflating measured Hg/TOC ratios to above those from the time of deposition (as shown for other Cretaceous records by [START_REF] Charbonnier | Effect of intense weathering and postdepositional degradation of organic matter on Hg/TOC proxy in organic-rich sediments and its implications for deep-time investigations[END_REF]. Nonetheless, [START_REF] Charbonnier | Mercury enrichment indicates volcanic triggering of Valanginian environmental change[END_REF] reported increased Hg and Hg/TOC values that were ascribed to G-OJP volcanic activity during OAE 1a (see also Supplementary Figure 1). Interestingly, Barremian-Aptian boundary records from the northwest Tethyan area have also been hypothesized as recording an earlier episode of volcanism associated with that LIP, which took place significantly prior to OAE 1a (Charbonnier et al., 2018b).

Study aims

Whilst mercury-cycle perturbations from Barremian-Aptian times (including OAE 1a) have been reported from the northwest Tethyan area, and linked to volcanism on the G-OJP, it remains unclear whether these disturbances truly represent a global volcanic signal or local influxes of mercury to that specific region. In this context, it is notable that mercury emitted from modern submarine volcanic systems appears to be efficiently scavenged, limiting dispersal of the element from such sources to areas relatively proximal to the point(s) of origin (within 100s km; [START_REF] Bowman | Mercury in the North Atlantic Ocean: The U.S. GEOTRACERS zonal and meridional sections[END_REF]. Indeed, prior investigations of the latest Cenomanian OAE (OAE 2; 94 Ma), which has also been linked with submarine LIP volcanism during oceanic-plateau emplacement (e.g., [START_REF] Turgeon | Cretaceous oceanic anoxic event 2 triggered by a massive magmatic episode[END_REF], have shown that most studied records of that event do not feature Hg enrichments J o u r n a l P r e -p r o o f Journal Pre-proof or elevated Hg/TOC ratios, except perhaps for sites proximal to LIPs [START_REF] Scaife | Sedimentary Mercury Enrichments as a Marker for Submarine Large Igneous Province Volcanism? Evidence from the Mid-Cenomanian Event and Oceanic Anoxic Event 2 (Late Cretaceous)[END_REF]Percival et al., 2018).

Consequently, analyses of stratigraphic archives of OAE 1a from around the world are needed in order to determine whether there was global-scale Hg-cycle perturbation during that event, or localized disturbances unrelated to magmatism. Furthermore, a global perspective of the mercury cycle will help to elucidate whether any volcanic fluxes of mercury were largely derived from G-OJP or HALIP activity. If volatile emissions during OAE 1a were primarily associated with submarine LIP activity of the G-OJP, sedimentary mercury enrichments correlative with the Os-isotope evidence of volcanism would likely be recorded in Pacific sites proximal to that source, but potentially nowhere else due to the limited dispersal range of the element in the marine realm. By contrast, mercury fluxes from subaerial eruptions and/or thermogenic emissions related to the HALIP should certainly be documented in the Arctic, and might also be documented in sites around the world due to those magmatic processes being more likely to emit volatiles directly to the atmosphere.

Here, new mercury data are presented from six records of latest-Barremian-early Aptian age, including OAE 1a (Figure 2): DSDP Site 463 (Mid-Pacific Mountains), the Cismon core (Belluno Basin, Italy), the Poggio le Guaine core (Umbria-Marche Basin, Italy), the Notre-Dame-de-Rosans section (Vocontian Basin, SE France), the DH-1 Longyearbyen core (Boreal Basin, Svalbard), and the Petrobras Well D (Sergipe-Alagoas Basin, NE Brazil). New Os-isotope data are also presented from the Poggio le Guaine core. These sequences include the first OAE 1a records from outside the Tethyan region that have been studied for mercury, offering a more global perspective on any Hgcycle perturbations during that time. Crucially, these study areas include three sites where stratigraphic Hg and Os-isotope trends can be directly correlated (DSDP Site 463 and the Cismon and Poggio le Guaine cores). Additionally, the Mid-Pacific Mountain and Arctic sites would have been relatively proximal to the G-OJP and HALIP, respectively. Comparing these new mercury and osmium-isotope profiles with previously published datasets, and correlating trends in the two proxies amongst records where both have been studied, will give new insights on the dominant style of J o u r n a l P r e -p r o o f Journal Pre-proof volcanic activity that influenced the global Hg cycle during OAE 1a and could potentially identify the LIP that was the primary trigger of that event.

STUDY AREAS

DSDP Site 463 (Mid-Pacific Mountains, W Pacific Ocean)

DSDP Site 463, drilled in the Mid-Pacific Mountains in 1978, records a large part of the uppermost Barremian to lower Aptian stratigraphic interval [START_REF] Thiede | Site 463: Western Mid-Pacific Mountains[END_REF]. The lithology generally consists of pelagic limestones with some chert-rich intervals [START_REF] Mélières | Organic-matter-rich and hypersiliceous Aptian sediments from western Mid-Pacific Mountains[END_REF]. There is a switch to more clay-rich marlstones between 625-615 mbsf that marks the OAE 1a stratigraphic interval [START_REF] Thiede | Site 463: Western Mid-Pacific Mountains[END_REF]Sliter, 1989). However, black shales akin to those observed in most Tethyan archives are largely absent from this site and only appear in upper Selli Level Equivalent strata, with a modest increase in TOC content (to 1.5 wt%, this study; although previous works have reported values locally up to 7-8 wt%; [START_REF] Thiede | Site 463: Western Mid-Pacific Mountains[END_REF][START_REF] Van Breugel | Synchronous negative carbon isotope shifts in marine and terrestrial biomarkers at the onset of the early Aptian oceanic anoxic event 1a: Evidence for the release of 13 C-depleted carbon into the atmosphere[END_REF][START_REF] Bottini | Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a[END_REF]. Age constraints (including the position of the Barremian-Aptian boundary) are based largely on magneto-and biostratigraphy [START_REF] Tarduno | M-sequence reversals recorded in DSDP sediment cores from the western Mid-Pacific Mountains and Magellan J o u r n a l P r e -p r o o f Journal Pre-proof Rise[END_REF]. Several studies have investigated the Barremian-Aptian carbon-isotope trends at DSDP Site 463 (e.g., [START_REF] Price | New constraints upon isotope variation during the early Cretaceous (Barremian-Cenomanian) from the Pacific Ocean[END_REF][START_REF] Ando | Timing and magnitude of early Aptian extreme warming: Unraveling primary δ 18 O variation in indurated pelagic carbonates at Deep Sea Drilling Project Site 463, central Pacific Ocean[END_REF][START_REF] Bottini | Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a[END_REF], identifying the C2-C7 segments that can be used to define the Selli Level Equivalent strata deposited during OAE 1a. Osmium-isotope trends and trace-metal enrichments have been interpreted as evidence for intense volcanic activity during Barremian-Aptian times (particularly before and during OAE 1a), with the volcanism generally attributed to the nearby G-OJP [START_REF] Bottini | Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a[END_REF][START_REF] Erba | The Origin, Evolution, and Environmental Impact of J o u r n a l P r e -p r o o f Journal Pre-proof Oceanic Large Igneous Provinces[END_REF]. Volcanic eruptions proximal to DSDP Site 463 are supported by sporadic preservation of thin tuffaceous layers locally preserved within the Barremian-Aptian sediments, particularly within the Selli Level Equivalent [START_REF] Hein | Origin and alteration of volcanic ash and pelagic brown clay, Deep Sea Drilling Project Leg 62, north-central Pacific[END_REF][START_REF] Vallier | Volcanogenic Sediments from Hess Rise and the Mid-Pacific Mountains, Deep Sea Drilling Project Leg[END_REF][START_REF] Thiede | Oxygen-deficient depositional paleoenvironments in the mid-Cretaceous tropical and sub-tropical central Pacific Ocean[END_REF]. The provenance of these tuffs remains unknown, but given the proximity of the

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Journal Pre-proof Mid-Pacific Mountains to the G-OJP, eruptions on that volcanically active LIP would be a plausible source.

Cismon core (Belluno Basin, Italy)

The Cismon core was drilled in the Southern Alps north-west of Treviso (Italy) in 1995, and is one of the best-studied latest Barremian-Aptian Tethyan records [START_REF] Erba | The Cismon APTICORE (Southern Alps, Italy): a" reference section" for the Lower Cretaceous at low latitudes[END_REF]. The core largely consists of pale-coloured pelagic carbonates deposited on the slope of the Belluno Basin.

However, the Selli Level Equivalent is marked by a clear lithological change to organic-rich marlstones and shales interbedded with sporadic radiolarian-rich beds [START_REF] Erba | The Cismon APTICORE (Southern Alps, Italy): a" reference section" for the Lower Cretaceous at low latitudes[END_REF]. The increase in TOC, together with elevated sulphur contents and preservation of the biomarker isorenieratane in some Selli Level Equivalent beds, strongly supports the development of at least periodically euxinic conditions in the Belluno Basin during OAE 1a [START_REF] Van Breugel | Synchronous negative carbon isotope shifts in marine and terrestrial biomarkers at the onset of the early Aptian oceanic anoxic event 1a: Evidence for the release of 13 C-depleted carbon into the atmosphere[END_REF][START_REF] Bottini | Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a[END_REF]. Magneto-, bio-, and carbon-isotope stratigraphy provide excellent temporal constraints, with all the δ 13 C excursions and C1-C8 segments well preserved [START_REF] Menegatti | High-resolution δ 13 C stratigraphy through the early Aptian "Livello Selli" of the Alpine Tethys[END_REF][START_REF] Erba | Integrated stratigraphy of the Cismon Apticore (southern Alps, Italy); a "reference section" for the Barremian-Aptian interval at low latitudes[END_REF][START_REF] Channell | Early Cretaceous magnetic stratigraphy in the APTICORE drill core and adjacent outcrop at Cismon (Southern Alps, Italy), and correlation to the proposed Barremian-Aptian boundary stratotype[END_REF]. A pronounced decline in the abundance of nannoconids has been documented just below the Barremian-Aptian boundary, with a further reduction (to almost nothing) of these nannofossils near the base of the Selli Level Equivalent [START_REF] Erba | Nannofossils and superplumes: the early Aptian "nannoconid crisis[END_REF]. These changes in the fossil record highlight the biotic impact of the environmental perturbations that took place prior to and during OAE 1a (e.g., [START_REF] Bralower | Timing and paleoceanography of oceanic dysoxia/anoxia in the Late Barremian to Early Aptian (Early Cretaceous)[END_REF][START_REF] Erba | Nannofossils and superplumes: the early Aptian "nannoconid crisis[END_REF][START_REF] Erba | Nannofossil carbonate fluxes during the Early Cretaceous: Phytoplankton response to nutrification episodes, atmospheric CO 2 , and anoxia[END_REF][START_REF] Erba | Calcareous nannoplankton response to surface-water acidification around Oceanic Anoxic Event 1a[END_REF].

Age modelling of lower Aptian strata from the Cismon core have indicated dates for the Barremian-Aptian transition and OAE 1a that overlap with G-OJP basalt ages (see Malinverno et al., 2012;[START_REF] Erba | The Origin, Evolution, and Environmental Impact of J o u r n a l P r e -p r o o f Journal Pre-proof Oceanic Large Igneous Provinces[END_REF]. Furthermore, both osmium-isotope trends (see above) and enrichments in several trace metals of probable mafic derivation in Barremian-Aptian strata provide direct evidence of LIP activity at that time [START_REF] Bottini | Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a[END_REF][START_REF] Erba | The Origin, Evolution, and Environmental Impact of J o u r n a l P r e -p r o o f Journal Pre-proof Oceanic Large Igneous Provinces[END_REF].

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Poggio le Guaine core (Umbria-Marche Basin, Italy)

The Poggio le Guaine (PLG) core was drilled in 2010 in the Northern Apennines at a site 6 km to the west of the city of Cagli [START_REF] Coccioni | Umbria-Marche Basin, Central Italy: A reference section for the Aptian-Albian interval at low latitudes[END_REF], close to a previously studied lower Aptian outcrop that includes sediments of OAE 1a age deposited as the Selli Level sensu stricto (e.g., [START_REF] Lowrie | Lower Cretaceous magnetic stratigraphy in Umbrian pelagic carbonate rocks[END_REF]Coccioni et al., 1987[START_REF] Coccioni | Stratigrafia, micropaleontologia e mineralogia delle Marne a Fucoidi delle sezioni di Poggio le Guaine e del Fiume Bosso (Appennino umbro-marchigiano). Atti 2° Convegno Internazionale "Fossili, Evoluzione, Ambiente[END_REF][START_REF] Baudin | Organic matter characterisation of the Selli Level (Umbria-Marche Basin, central Italy)[END_REF]. Like the outcrop section, the core represents a well-preserved and apparently continuous Aptian-Albian succession that records a pelagic environment in the Umbria-Marche Basin of the north-western Tethyan area. The lithology is dominated by white and grey nannofossil-foraminiferal pelagic limestones, with rare cherts, radiolarian-rich beds, and green limestones [START_REF] Coccioni | Umbria-Marche Basin, Central Italy: A reference section for the Aptian-Albian interval at low latitudes[END_REF][START_REF] Savian | The Barremian-Aptian boundary in the Poggio le Guaine core (central Italy): Evidence for magnetic polarity Chron M0r and oceanic anoxic event 1a[END_REF]. Stratigraphic age constraints on the position of the Barremian-Aptian boundary are based on magnetostratigraphy and calcareous planktonic foraminiferal biostratigraphy [START_REF] Coccioni | Umbria-Marche Basin, Central Italy: A reference section for the Aptian-Albian interval at low latitudes[END_REF][START_REF] Savian | The Barremian-Aptian boundary in the Poggio le Guaine core (central Italy): Evidence for magnetic polarity Chron M0r and oceanic anoxic event 1a[END_REF].

Intercalated organic-rich shales and radiolarian-rich beds, an overall relative elevation in TOC contents, and a negative (C3) and recovery/broad positive excursion (C4-C6) in carbonate δ 13 C ratios all define the Selli Level (89.24-91.29 m; [START_REF] Coccioni | Umbria-Marche Basin, Central Italy: A reference section for the Aptian-Albian interval at low latitudes[END_REF][START_REF] Savian | The Barremian-Aptian boundary in the Poggio le Guaine core (central Italy): Evidence for magnetic polarity Chron M0r and oceanic anoxic event 1a[END_REF] this study).

However, within the OAE 1a strata there is a black-shale unit containing insufficient carbonate for carbon-isotope analysis, leaving a gap in the δ 13 C trends that hinders precise placement of the C4-C5 and C5-C6 boundaries (89-91 m).

Notre-Dame-de-Rosans (Vocontian Basin, SE France)

The Vocontian Basin was one of a number of large epicontinental depocentres located in the north-western area of the Cretaceous Tethys Ocean, formed by tectonic processes related to the opening of the Bay of Biscay (e.g., [START_REF] Hibsch | Evénements tectoniques crétacés dans la partie méridionale du bassin subalpin (massif Ventoux-Lure et partie orientale de l'arc de Castellane, SE France). Implications géodynamiques[END_REF], with basinal clay-rich calcareous hemipelagic sediments preserved today in south-eastern France [START_REF] Bréhéret | L'Aptien et l'Albien de la fosse vocontienne (des bordures au bassin): Evolution de la sédimentation et enseignements sur les événements anoxiques[END_REF]. The Notre-Dame-de-Rosans J o u r n a l P r e -p r o o f Journal Pre-proof section provides an excellent record of OAE 1a in the Vocontian Basin, comprising interbedded clayrich calcareous marls and fine-grained turbidites [START_REF] Giraud | The palaeoceanographic crisis of the Early Aptian (OAE 1a) in the Vocontian Basin (SE France)[END_REF]. The stratigraphy can be correlated with other archives from that region, and elsewhere, based on biostratigraphic constraints, its carbon-isotope record (clearly documenting the C2-C7 segments), and the preservation of less calcareous, darker coloured, TOC-enriched grey marls in the upper part of the Selli Level Equivalent [START_REF] Giraud | The palaeoceanographic crisis of the Early Aptian (OAE 1a) in the Vocontian Basin (SE France)[END_REF], locally named the Niveau Goguel [START_REF] Bréhéret | Episodes de sédimentation riche en matière organique dans les marnes bleues d'âge aptien et albien de la partie pélagique du bassin vocontien[END_REF]. This organic matter is immature and appears to be largely composed of marine algal/bacterial material, but potentially with some contribution from degraded/terrestrially derived debris [START_REF] Giraud | The palaeoceanographic crisis of the Early Aptian (OAE 1a) in the Vocontian Basin (SE France)[END_REF]. Based on the identification of <6 μm framboidal pyrite and cyanobacterial biomarkers in the Niveau Goguel strata of the nearby Les Sauzeries section (<50 km away), it has been inferred that anoxic-euxinic settings developed during the latter part of OAE 1a [START_REF] Ando | Paleoproductivity of dinoflagellate and cyanobacteria during the mid-Cretaceous oceanic anoxic events in the Vocontian Basin, SE France. 26[END_REF][START_REF] Giraud | The palaeoceanographic crisis of the Early Aptian (OAE 1a) in the Vocontian Basin (SE France)[END_REF]. However, such conditions were likely intermittent, with the turbidite flows reventilating the marine environment [START_REF] Caillaud | Turbidite-induced re-oxygenation episodes of the sediment-water interface in a diverticulum of the Tethys Ocean during the Oceanic Anoxic Event 1a: The French Vocontian Basin[END_REF].

DH-1 core, Longyearbyen (Boreal Basin, Svalbard, Norway)

The western part of Svalbard records a very near-shore shallow-marine palaeoenvironment in an epicontinental basin, with a lithology consisting of mudstone-siltstone beds, fluvial-marginal marine sandstone lenses, and coals [START_REF] Midtkandal | The Aptian (Early Cretaceous) oceanic anoxic event (OAE1a) in Svalbard, Barents Sea, and the absolute age of the Barremian-Aptian boundary[END_REF][START_REF] Vickers | Stratigraphic and geochemical expression of Barremian-Aptian global climate change in Arctic Svalbard[END_REF]. Macroscopic higher-plant debris in DH-1 core samples supports a dominantly terrestrial source of organic matter, further evidenced by low measured hydrogen index (HI) values, although the low HI could partly/completely result from the high thermal maturity of the sediments [START_REF] Midtkandal | The Aptian (Early Cretaceous) oceanic anoxic event (OAE1a) in Svalbard, Barents Sea, and the absolute age of the Barremian-Aptian boundary[END_REF]. A limited quantity of marine palynomorphs allows for dinoflagellate biostratigraphy, from which a broadly Barremian-Aptian age for the interval of the DH-1 core studied here has been determined [START_REF] Midtkandal | The Aptian (Early Cretaceous) oceanic anoxic event (OAE1a) in Svalbard, Barents Sea, and the absolute age of the Barremian-Aptian boundary[END_REF]. A negative δ 13 C excursion and positive shift immediately stratigraphically above it (150.47-133.37 m) are thought to be equivalent to the C3 and C4-C6 segments, respectively, and have been interpreted as marking the Selli Level Equivalent strata in the core (Midtkandal et al., J o u r n a l P r e -p r o o f Journal Pre-proof 2016). Further subdivision of the C4-C6 segments is not possible due to the low resolution of the δ 13 C dataset. Interestingly, TOC contents decrease to an average of 1.8 wt% within the Selli Level Equivalent, compared to mean quantities of 3.8 wt% and 3.3. wt% in sediments stratigraphically above and below, respectively [START_REF] Midtkandal | The Aptian (Early Cretaceous) oceanic anoxic event (OAE1a) in Svalbard, Barents Sea, and the absolute age of the Barremian-Aptian boundary[END_REF]. For this study, mercury analyses were limited to mudstone layers, avoiding sandstone/coal beds, in order to maintain relative consistency in terms of the lithology and organic-matter content of the studied samples, as previous works have shown that major lithological variations can strongly influence Hg concentrations and Hg/TOC variations independently of any potential external source (e.g., [START_REF] Percival | Does Large Igneous Province volcanism always perturb the mercury cycle? Comparing the records of Oceanic Anoxic Event 2 and the end-Cretaceous to other Mesozoic events[END_REF].

Petrobras Well D (Sergipe-Alagoas Basin, NE Brazil)

The Sergipe-Alagoas Basin was one of a number of rift-basins along the eastern and southern part of Brazil that formed during the Early Cretaceous as a result of the opening of the South Atlantic Ocean (Chaboureau et al., 2013, and references therein). The Petrobras Well D was drilled through the Muribeca and Riachuelo Formations of the basin, consisting of sandy and conglomeratic siliciclastics interbedded with siltstones and mudstones, the last of which are locally calcareous.

Taken together, the stratigraphic sequence is interpreted as having been deposited in a continentalcoastal setting that was initially dominated by fluvio-deltaic systems, but gradually transitioned towards lacustrine/lagoonal settings due to continuing basin subsidence as rifting of the South Atlantic proceeded [START_REF] Tedeschi | Aptian carbon-isotope record from the Sergipe-Alagoas Basin: New insights into oceanic anoxic event 1a and the timing of seawater entry into the South Atlantic[END_REF]. Elsewhere in the Sergipe-Alagoas Basin, the Riachuelo Formation has been dated as late Aptian in age on the basis of planktonic foraminiferal biostratigraphy (e.g., [START_REF] Koutsoukos | Late Aptian to Maastrichtian foraminiferal biogeography and palaeoceanography of the Sergipe Basin, Brazil[END_REF], with the underlying Muribeca Formation generally accepted as also having been deposited (earlier) in the Aptian [START_REF] Tedeschi | Aptian carbon-isotope record from the Sergipe-Alagoas Basin: New insights into oceanic anoxic event 1a and the timing of seawater entry into the South Atlantic[END_REF]. Consequently, a series of pronounced carbon-isotope excursions near the bottom of the Muribeca Formation in the Petrobras Well D core has been interpreted as marking the Selli Level Equivalent, with a negative excursion overlain by two positive shifts interpreted as the C3 and C4-C6 segments, respectively [START_REF] Tedeschi | Aptian carbon-isotope record from the Sergipe-Alagoas Basin: New insights into oceanic anoxic event 1a and the timing of seawater entry into the South Atlantic[END_REF]. TOC contents are variable, ranging from <0.1 wt% up to 5.2 wt%, with mean J o u r n a l P r e -p r o o f Journal Pre-proof values highest in the postulated C5 strata, and primarily composed of detrital terrestrial organic matter that has locally been oxidized, based on the determined hydrogen and oxygen indices and the nature of the recorded palaeoenvironment [START_REF] Tedeschi | Aptian carbon-isotope record from the Sergipe-Alagoas Basin: New insights into oceanic anoxic event 1a and the timing of seawater entry into the South Atlantic[END_REF]. New TOC data were generated by Rock-Eval 6 analysis for samples from DSDP Site 463, the Cismon core, and Notre-Dame-de-Rosans, at the University of Oxford, after the methodology of [START_REF] Behar | Rock-Eval 6 technology: performances and developments[END_REF]. Repeated measurements of an internal mudrock standard SAB134 (calibrated to the International reference material IFP 160000) were used to assess analytical accuracy and repeatability, and yielded an average value of 2.81±0.07 wt%, consistent with long-term measurements for the laboratory (2.87±0.11 wt%;Storm et al., 2020), and indicating analytical uncertainty better than 0.1 wt% (1σ). New TOC data for PLG core samples were determined on a Strohlein Coulomat 702 at the University of Oxford, using the procedure in [START_REF] Jenkyns | The early Toarcian (Jurassic) anoxic event: Stratigraphic, sedimentary, and geochemical evidence[END_REF]. TOC contents have been previously determined using Rock Eval and Leco SC-632 instruments for all samples from the DH-1 core and Petrobras Well D analyzed for mercury in this study, at the Institute J o u r n a l P r e -p r o o f Journal Pre-proof for Energy Technology, Kjeller (Norway) for the DH-1 core [START_REF] Midtkandal | The Aptian (Early Cretaceous) oceanic anoxic event (OAE1a) in Svalbard, Barents Sea, and the absolute age of the Barremian-Aptian boundary[END_REF], and at the Universities of Oxford and Universidade do Estado do Rio de Janeiro for Petrobras Well D [START_REF] Tedeschi | Aptian carbon-isotope record from the Sergipe-Alagoas Basin: New insights into oceanic anoxic event 1a and the timing of seawater entry into the South Atlantic[END_REF]. The new data from the Cismon core were combined with published values that were generated at the Open University (UK) using a Leco CNS-2000 elemental analyser [START_REF] Bottini | Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a[END_REF].

METHODS

Mercury

New osmium-isotope data were determined for ten samples from the PLG core following the methodology outlined in Kendall et al. (2015), with eight samples taken from the Selli Level, and two others from beneath the base of it. Sample preparation utilized Carius-tube digestion with Cr VI O 3 -H 2 SO 4 , with subsequent Os purification using established solvent extraction (by chloroform) and microdistillation techniques [START_REF] Selby | Re-Os geochronology of organic rich sediments: an evaluation of organic matter analysis methods[END_REF]. Rhenium purification was carried out with solvent extraction using sodium hydroxide and acetone, and subsequent anion exchange chromatography [START_REF] Cumming | Anoxia in the terrestrial environment during the late Mesoproterozoic[END_REF]. Isotopic compositions and concentrations of rhenium and osmium were determined by isotope dilution and negative thermal ionisation mass spectrometry (N-TIMS) on a Thermo Triton instrument at the Department of Earth and Atmospheric Sciences, University of Alberta (Canada). Total procedural blanks for osmium and rhenium were 0.3 and 15 pg, respectively, whilst the 187 Os/ 188 Os composition of the blanks was 0.20. In-house standard solutions for osmium (AB2; see e.g., [START_REF] Selby | Direct Rhenium-Osmium age of the Oxfordian-Kimmeridgian boundary, Staffin bay, Isle of Skye, UK, and the Late Jurassic time scale[END_REF][START_REF] Finlay | Tracking the Hirnantian glaciation using Os isotopes[END_REF] and rhenium (ICP-MS standard rhenium solution of normal isotopic composition) yielded values in agreement with previous studies (van [START_REF] Van Acken | Constraining the depositional history of the Neoproterozoic Shaler Supergroup, Amundsen Basin, NW Canada: Rhenium-osmium dating of black shales from the Wynniatt and Boot Inlet Formations[END_REF]Kendall et al., 2015): 187 Os/ 188 Os of 0.10684±0.00015 (1σ) for osmium, and 185 Re/ 187 Re of 0.59778±0.00077 (1σ) for rhenium.

The past seawater composition at the time of deposition ( 187 Os/ 188 Os (i) ) is determined from the modern-day 187 Os/ 188 Os ratio of a sedimentary rock sample using its age and its Re and Os concentrations to account for the post-depositional decay of rhenium ( 187 Re) to 187 Os [START_REF] Cohen | Precise Re-Os ages of organic-rich mudrocks and the Os isotope composition of Jurassic seawater[END_REF]. The osmium concentration of a sedimentary rock at the time of its deposition is determined from the modern-day osmium concentration after accounting for the post-depositional decay of 187 Re to 187 Os, by using the difference between the measured modern-day 187 Os/ 188 Os ratio and the J o u r n a l P r e -p r o o f Journal Pre-proof calculated 187 Os/ 188 Os (i) value, together with the 192 Os content of the rock and the known natural 192 Os/ 188 Os isotope ratio (see Supplementary Text).

RESULTS

Os-isotope and osmium concentration data from the PLG core

Following correction of the measured modern-day Os-isotope ratio for decay of rhenium since deposition at 120 Ma, the recorded 187 Os/ 188 Os (i) values show a clear shift from 0.6 to 0.2 in the basal Selli Level strata (Figure 3). There is also a notable increase in the calculated [Os (i) ] concentrations across that horizon, from 149 ppt in the stratigraphically lowest analyzed sample to a maximum of 3719 ppt in the middle of the Selli Level. These trends are broadly consistent with osmium trends from Gorgo a Cerbara, Cismon (both Italy), DSDP Site 463 (Pacific), and Cau (Spain) reported by previous studies [START_REF] Tejada | Ontong Java Plateau eruption as a trigger for the early Aptian oceanic anoxic event[END_REF][START_REF] Bottini | Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a[END_REF][START_REF] Adloff | Unravelling the sources of carbon emissions at the onset of Oceanic Anoxic Event (OAE) 1a[END_REF].

Interestingly, the transient shift to more radiogenic 187 Os/ 188 Os (i) values documented from other Tethyan records (Cismon and Gorgo a Cerbara; [START_REF] Tejada | Ontong Java Plateau eruption as a trigger for the early Aptian oceanic anoxic event[END_REF][START_REF] Bottini | Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a[END_REF] see Figure 1) is not observed in the PLG data. This lack of 187 Os/ 188 Os (i) spike in the PLG core may result from the absence of data from below the Selli Level at that site (Figure 3). Alternatively, the 187 Os/ 188 Os (i) spike may not be captured by the PLG dataset due to its relatively low-resolution, or the presence of a small hiatus/condensed layer at the base of the Selli Level, although there is no sedimentological evidence for such a stratigraphic gap.

Hg concentration and Hg/TOC ratio data

J o u r n a l P r e -p r o o f

Journal Pre-proof At DSDP Site 463, there is a clear increase in Hg content from the average of 16.4 ppb below the Selli Level Equivalent to a mean of 101 ppb within it, with a first increase up to 168.5 ppb in C3 (625.28-623.96 mbsf) and a second peak up to 471.5 ppb in the C4-lowest C5 segments (622.22-620.10 mbsf), before returning to much lower (mean 18.4 ppb) values in sediments above the Selli Level Equivalent (Figure 4A). By contrast, the TOC contents of DSDP Site 463 samples that were also measured for mercury are relatively consistent, and low: typically <0.5 wt%, apart from for three samples in the middle of the Selli Level Equivalent that have values close to or above 1 wt%.

Interestingly, no sample measured in this study had TOC contents of 7-8 wt% as reported for some stratigraphic layers at DSDP Site 463 by previous works (see section 2.1). Consequently, the two increases in Hg concentrations are largely reproduced by Hg/TOC ratios, with peaks up to 802 ppb/wt% and 689 ppb/wt% in C3 and lowest C5 strata, respectively (Figure 4A). Crucially, both peaks greatly exceed the range of error of background samples, confirming that they do not result from analytical uncertainty. A small number of samples have a TOC content that is too low to confidently interpret Hg/TOC ratios (Hg/TOC ratios based on TOC contents < 0.2 wt% are generally deemed unreliable due to the high percentage error in the TOC measurement: see [START_REF] Grasby | Mercury anomalies associated with three extinction events (Capitanian crisis, latest Permian extinction and the Smithian/Spathian extinction) in NW Pangea[END_REF].

However, the inclusion or exclusion of these data does not greatly affect the recorded stratigraphic trends.

Hg concentrations in the Cismon core are extremely variable throughout the studied interval, ranging between 3.7-334 ppb with a number of peaks: either side of the Barremian-Aptian boundary (at 34.21 m and 27.85 m), within the C3 to C4 strata (23 m), and again in the C6 segment between 19.87-18.83 m (Figure 4B). TOC contents of samples below the Selli Level Equivalent are typically very low (only three samples exceed 0.2 wt%), resulting in highly variable Hg/TOC values between 113-1637 ppb/wt% that also show peaks either side of the Barremian-Aptian boundary, although interpreting these data is greatly hindered due to the normalisation by extremely low TOC contents. By contrast, the typically higher TOC contents within the Selli Level Equivalent result in generally lower, though still variable, ratios between 7.9-468 ppb/wt% (Figure 4B). However, the C3-C4 peak

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Journal Pre-proof in Hg concentrations is only reproduced by a two data-point peak in Hg/TOC, with no increase in Hg/TOC ratios in the C6 segment.

Similarly to the Cismon core, the PLG record features rather variable sedimentary Hg concentrations between 1.4-201 ppb, with a number of peaks below the Selli Level and another peak within the lower part of the C4-C6 strata within it (Figure 4C). Concentrations across the Barremian-Aptian boundary and above the Selli Level are particularly low (averaging 9.9 ppb). TOC contents increase markedly from typically <0.5 wt% below the Selli Level to values in excess of 2 wt% within it; one sample reaching 10.7 wt% (Figure 4C). The various peaks in Hg are maintained to some degree following normalisation against TOC, with three peaks of between 200-600 ppb/wt%: one at 93.8-93.6 m, a second at 92.2 m (both below the OAE 1a horizon), and finally at 90.4 m (lowest C4-C6 strata), albeit consisting of just one or two data points (Figure 4C). However, it should be noted that several PLG Hg data points are normalized against low (but >0.2 wt%) TOC contents, creating a high level of uncertainty in the Hg/TOC stratigraphic trend. Many of the PLG Hg/TOC peaks do not significantly exceed this range of error; thus, they cannot be unambiguously interpreted as marking an external influx of mercury.

Hg concentrations are consistently low through most of the Notre-Dame-de-Rosans strata (averaging 28.6 ppb), apart from a peak up to 156.5 ppb (mean 60.0 ppb) spanning the uppermost C3, all of C4, and lower C5 segments (18.8-24.9 m; Figure 4D). Newly measured TOC contents in the samples analyzed for mercury show comparable trends to those previously published by [START_REF] Giraud | The palaeoceanographic crisis of the Early Aptian (OAE 1a) in the Vocontian Basin (SE France)[END_REF], averaging approximately 0.5 wt% throughout most of the studied record, except in the C6 upper part of the Selli Level Equivalent (the Niveau Goguel), where values rise to between 1-2 wt% (Figure 4D). Consequently, the peak in Hg concentrations largely remains following normalisation by TOC, with an increase in Hg/TOC ratios up to 333 ppb/wt% in C4 strata, compared to consistently lower values averaging 64 ppb/wt% throughout the rest of the studied interval.

J o u r n a l P r e -p r o o f

Journal Pre-proof Hg concentrations are variable throughout the studied interval of the DH-1 core, but show a systematic increase across the Selli Level Equivalent. The basal (C3) strata are marked by a peak up to 202 ppb at 150.47 m, which is succeeded by a broader increase averaging 73.8 ppb across the upper C4-C6 segments (144.3-136.02 m), compared to much lower contents that are typically <10 ppb throughout the rest of the record (Figure 4E). When normalized against the TOC contents of these samples (previously published by [START_REF] Midtkandal | The Aptian (Early Cretaceous) oceanic anoxic event (OAE1a) in Svalbard, Barents Sea, and the absolute age of the Barremian-Aptian boundary[END_REF], the C3 peak in Hg largely disappears, but the elevated Hg contents across C4-C6 strata are reflected by a rise in average Hg/TOC values to 40 ppb/wt% in these strata, compared to 12.1 ppb/wt% for the rest of the record (Figure 4E).

Hg concentrations are consistently very low (averaging 13.9 ppb) throughout the studied interval of the Petrobras Well D, and show no increase across the postulated Selli Level Equivalent (Figure 4F). As for the DH-1 core, TOC contents of these samples have been previously published [START_REF] Tedeschi | Aptian carbon-isotope record from the Sergipe-Alagoas Basin: New insights into oceanic anoxic event 1a and the timing of seawater entry into the South Atlantic[END_REF], with a marked increase from <0.1 wt% below the OAE 1a level to generally between 1-3 wt% (up to a maximum of 5.22 wt%) within it. Consequently, Hg/TOC ratios are generally also very low (averaging <20 ppb/wt%) with a few higher values near the base of the studied interval caused by low TOC contents (Figure 4F). There is one sample (from 201.43 m) with anomalously high Hg concentrations and Hg/TOC ratios of 373 ppb and 158 ppb/wt%, respectively, but this is significantly above the top of the Selli Level Equivalent.

DISCUSSION

Tethyan Hg-cycle disturbances prior to OAE 1a

Volcanic activity related to G-OJP emplacement has been previously proposed as having occurred during latest Barremian times, based on enrichments in mercury and other trace metals across the Barremian-Aptian boundary, especially in Tethyan stratigraphic archives (Erba et al.,

J o u r n a l P r e -p r o o f

Journal Pre-proof 2015; Charbonnier et al., 2018b). Some increase in submarine volcanism and/or basalt-seawater interaction might also be registered by a small shift towards mantle compositions in Pb and Os isotopes across the Barremian-Aptian boundaries of the Shatsky Rise and Cismon records, respectively [START_REF] Kuroda | Lead isotopic record of Barremian-Aptian marine sediments: Implications for large igneous provinces and the Aptian climatic crisis[END_REF][START_REF] Bottini | Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a[END_REF]. It has been further suggested that this G-OJP volcanism contributed to the decline in nannoconid fauna during the Barremian-Aptian transition [START_REF] Erba | The Origin, Evolution, and Environmental Impact of J o u r n a l P r e -p r o o f Journal Pre-proof Oceanic Large Igneous Provinces[END_REF]. Whilst Hg concentrations and Hg/TOC ratios are highly variable in pre-OAE 1a strata from the Cismon and PLG cores, peaks around the Barremian-Aptian boundary (and particularly strata of the magnetic chron CM0) might support a mercury-cycle disturbance in the Tethyan region at that time (Figure 4B-C). However, interpretation of the Hg/TOC ratios for both the Cismon and PLG cores is greatly hindered by the very low TOC contents of most samples (and attendant large range of uncertainty) from strata below the Selli Level or equivalent. Indeed, given the relatively low organic-matter and (in the case of Cismon) sulphur contents in Barremian-Aptian boundary sediments (Figure 4B-C; Supplementary Figure 2B), a lithological control on mercury deposition, whereby the element is associated with clay minerals, is likely for these strata. This conclusion is reinforced by the absence of Hg peaks in strata above the PLG Selli Level, when argillaceous layers (which make up much of the Selli Level and are interbedded with limestones beneath it) discontinue, leaving only calcareous lithologies.

Even if there were one or more Hg-cycle perturbations (volcanically stimulated or otherwise) in the Tethyan realm during the Barremian-Aptian transition prior to OAE 1a, there is no evidence that such phenomena were occurring outside of this region. Hg concentrations and Hg/TOC ratios remain relatively low throughout the (admittedly incomplete) strata below the Selli Level Equivalent at DSDP Site 463 (Figure 4A). Given how proximal this Mid-Pacific Mountains site was to the G-OJP, if volcanism on that LIP were capable of dispersing sufficient Hg to reach the northwest Tethys, the expectation would be for it to be recorded in more proximal sedimentary records as well. Nor can the absence of Barremian-Aptian Hg enrichment at DSDP Site 463 be attributed to overprinting by excess TOC, as the Hg concentrations themselves largely remain low. Thus, whilst activity on the G-OJP during the latest Barremian is not discounted as the cause of the nannoconid decline, if the LIP

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Journal Pre-proof was volcanically active at that time then it apparently did not perturb either the regional or global mercury cycle significantly. Volcanism on the north-eastern Tethyan margin has been suggested as a potential cause of Hg peaks in Valanginian sedimentary records from the northwest Tethys [START_REF] Charbonnier | Mercury enrichment indicates volcanic triggering of Valanginian environmental change[END_REF], but there is no clear indication that such eruptions took place during latest Barremian to early Aptian times. Thus, without evidence of north-western Tethyan eruptions or G-OJP mercury emissions during the Barremian-Aptian transition, the Hg/TOC variations documented stratigraphically below the Selli Level (or equivalent strata) in the Cismon and PLG cores cannot be conclusively linked to volcanism.

Evidence for submarine G-OJP volcanism at the onset of OAE 1a

The shift in 187 Os/ 188 Os (i) values towards unradiogenic compositions in basal Selli Level (C3) strata of the PLG core matches published Os-isotope stratigraphic trends from other sites in the Tethys and Pacific (Figure 5). This similarity supports the commencement of intense LIP volcanism, or at least some form of basalt-seawater interaction, during the onset of OAE 1a. There is a clear correlation between the 187 Os/ 188 Os (i) shift and the increased Hg and Hg/TOC values at DSDP Site 463; however, no such relationship is documented in the Cismon or PLG cores (Figure 5). Whilst no 187 Os/ 188 Os (i) data exist for the other six sites studied for mercury here and by [START_REF] Charbonnier | Mercury enrichment indicates volcanic triggering of Valanginian environmental change[END_REF], it can be seen that although most of those locales record some enrichment(s) in sedimentary mercury, none of them feature a clear peak in Hg and Hg/TOC in the C3 segment comparable to that in coeval strata of DSDP Site 463 (Figure 4 and Supplementary Figure 1).

Given that by far the clearest sedimentary mercury enrichment during the onset of OAE 1a is at DSDP Site 463, it is possible that this single-site peak results from a lithological change in that archive, such as increased clay content or burial with pyrite (c.f., [START_REF] Shen | Sedimentary host phases of mercury (Hg) and implications for use of Hg as a volcanic proxy[END_REF]. Alternatively, it may reflect a local input of Hg from increased wildfire activity/runoff of terrestrial organic matter (c.f. [START_REF] Daga | Historical records of mercury in southern latitudes over 1600 years: Lake Futalaufquen, Northern Patagonia[END_REF][START_REF] Grasby | Isotopic signatures of mercury contamination in latest Permian oceans[END_REF][START_REF] Them | Terrestrial sources as the primary delivery mechanism of mercury to the oceans across the Toarcian Oceanic Anoxic Event (Early Jurassic)[END_REF]. However, the C3 Hg enrichment at J o u r n a l P r e -p r o o f Journal Pre-proof DSDP Site 463 does not correlate with an increase in total clay or pyrite content (Supplementary Figure 2A), ruling out an association between mercury and such phases, and thereby discounting lithological/redox changes as the cause of the Hg/TOC increase. Organic-matter oxidation/degradation has been reported from the DSDP Site 463 Aptian record, but [START_REF] Van Breugel | Synchronous negative carbon isotope shifts in marine and terrestrial biomarkers at the onset of the early Aptian oceanic anoxic event 1a: Evidence for the release of 13 C-depleted carbon into the atmosphere[END_REF] noted that the sediments from the basal part of the Selli Level Equivalent are less thermally mature than the rock layers stratigraphically below and above, making it unlikely that the C3 Hg/TOC peak merely results from post-depositional TOC depletion (c.f., [START_REF] Charbonnier | Effect of intense weathering and postdepositional degradation of organic matter on Hg/TOC proxy in organic-rich sediments and its implications for deep-time investigations[END_REF]. Both marine and terrestrially derived organic matter are known to be present in Selli Level Equivalent sediments of DSDP Site 463, but an increased abundance in terrestrial material specifically in the C3 Segment has not been robustly established [START_REF] Mélières | Organic-matter-rich and hypersiliceous Aptian sediments from western Mid-Pacific Mountains[END_REF][START_REF] Dean | Accumulation of organic matter in Cretaceous oxygendeficient depositional environments in the central Pacific Ocean[END_REF][START_REF] Van Breugel | Synchronous negative carbon isotope shifts in marine and terrestrial biomarkers at the onset of the early Aptian oceanic anoxic event 1a: Evidence for the release of 13 C-depleted carbon into the atmosphere[END_REF].

Notably, however, the Hg and Hg/TOC increase at the base of the Selli Level Equivalent stratigraphically correlates with the appearance of abundant tuffaceous layers (Figure 6). Thus, it is likely that the mercury peaks at DSDP Site 463 were caused by nearby volcanic activity proximal to the G-OJP. The correlation between Hg enrichments, tuffaceous layers, and recorded shift in 187 Os/ 188 Os (i) ratios to unradiogenic compositions further suggests that all three could have been caused by the same volcanic process. Given that the volcanic activity that caused the shift in seawater Os-isotope ratio represented a six-to tenfold increase on pre-event background activity [START_REF] Bauer | Quantifying the volcanic emissions which triggered Oceanic Anoxic Event 1a and their effect on ocean acidification[END_REF], it is highly unlikely that the volcanism responsible for the recorded Hg enrichment and 187 Os/ 188 Os (i) shift at DSDP Site 463, which appears to have occurred at least proximally to the G-OJP, can have been anything other than intense activity on that LIP. Moreover, given the intensity of this LIP volcanism, the fact that the mercury enrichment only appears in a site proximal to the G-OJP, and not correlative with 187 Os/ 188 Os (i) shifts in the Tethyan realm, supports predominantly submarine eruptions, as also concluded by previous studies based on trends in other trace-element concentrations and lead-isotope signatures [START_REF] Kuroda | Lead isotopic record of Barremian-Aptian marine sediments: Implications for large igneous provinces and the Aptian climatic crisis[END_REF][START_REF] Erba | The Origin, Evolution, and Environmental Impact of J o u r n a l P r e -p r o o f Journal Pre-proof Oceanic Large Igneous Provinces[END_REF]. The differing seawater residence times of osmium (10s kyr) and mercury (100s yr) would have enabled unradiogenic Os sourced from J o u r n a l P r e -p r o o f Journal Pre-proof this volcanism to mix throughout the global ocean, whilst rapid Hg scavenging limited any evidence of a mercury flux to near the G-OJP, as documented in this study.

The slight possibility that the correlative Hg enrichment and 187 Os/ 188 Os (i) shift at DSDP Site 463 were caused by HALIP volcanism is ruled out by the absence of a similar Hg peak in the C3 sediments of the Tethyan and (especially) DH-1 archives. It is possible that the one data-point Hg spike at the base of the C3 level in the DH-1 core represents a volcanic Hg flux from the HALIP that was then overprinted by excess TOC burial (as previously proposed for other sites by [START_REF] Percival | Globally enhanced mercury deposition during the end-Pliensbachian extinction and Toarcian OAE: A link to the Karoo-Ferrar Large Igneous Province[END_REF]Charbonnier andFöllmi, 2017), but this interpretation is speculative, and there is also no evidence for such an overprinting of increased Hg concentrations at any other site (Figure 4 and Supplementary Figure 1). If the HALIP had emitted sufficient mercury to the atmosphere to reach the Pacific, it is highly unlikely that there would be little clear record of such Hg output in any of the comparatively proximal Arctic or European/Tethyan sedimentary records (see Figure 2). Thus, the G-OJP is more likely to have been the primary source of mercury to the Mid-Pacific Mountains (and unradiogenic osmium to the global ocean as a whole).

Despite an apparent lack of impact on the Hg cycle, it cannot be excluded that the HALIP (as well as other carbon sources such as methane clathrates; e.g., [START_REF] Jahren | Terrestrial record of methane hydrate dissociation in the Early Cretaceous[END_REF] still contributed to causing the global C-cycle perturbation associated with OAE 1a. Such a scenario would likely have relied upon HALIP magmatic sills intruding organic-rich sediments with a low Hg content, causing massive emissions of isotopically light carbon, but little mercury output. However, it is not currently known whether such lithologies exist around the HALIP sills. Moreover, this scenario is at odds with the documentation of Hg-cycle perturbations in stratigraphic records of several events linked with LIPs thought to have featured thermogenic volatile emissions (e.g., [START_REF] Percival | Globally enhanced mercury deposition during the end-Pliensbachian extinction and Toarcian OAE: A link to the Karoo-Ferrar Large Igneous Province[END_REF][START_REF] Percival | Mercury evidence for pulsed volcanism during the end-Triassic mass extinction[END_REF][START_REF] Hammer | Are Early Triassic extinction events associated with mercury anomalies? A reassessment of the Smithian/Spathian boundary extinction[END_REF]Shen et al., 2019b), and with preliminary investigations of carbon and mercury contents in shales intruded by LIP magmas in South Africa, which show clear evidence for remobilisation of both volatiles [START_REF] Svensen | Release of mercury from black shale during contact metamorphism and the implications for mercury as a volcanic proxy[END_REF]. Therefore, it is more likely that the role played by the HALIP in initiating OAE 1a (if any) was minor compared to that of G-OJP activity.

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Volcanic vs terrestrial sources of Hg to marine environments after the onset of OAE 1a

More plausibly, the widespread sedimentary Hg enrichments documented across C4 (or even the very uppermost C3) to C5 strata of seven out of nine OAE 1a sites studied here and by [START_REF] Charbonnier | Mercury enrichment indicates volcanic triggering of Valanginian environmental change[END_REF] might have been triggered by a perturbation of the atmospheric mercury inventory following emissions of the element to the atmosphere from one or both of HALIP volcanism and sill intrusion of organic-rich shales. Given that subaerial LIP volcanism is thought to have emitted large volumes of Hg to the atmosphere at other times in Earth's history (e.g., [START_REF] Sanei | Latest Permian mercury anomalies[END_REF][START_REF] Percival | Mercury evidence for pulsed volcanism during the end-Triassic mass extinction[END_REF][START_REF] Percival | Does Large Igneous Province volcanism always perturb the mercury cycle? Comparing the records of Oceanic Anoxic Event 2 and the end-Cretaceous to other Mesozoic events[END_REF][START_REF] Grasby | Mercury as a proxy for volcanic emissions in the geologic record[END_REF], similar eruptions on areas of the G-OJP that emerged above the sea surface could also have produced this effect. However, because the C4-C5 strata postdate the onset of OAE 1a by on the order of 10s kyr [START_REF] Li | Toward an orbital chronology for the early Aptian Oceanic Anoxic Event (OAE1a, ~120 Ma)[END_REF][START_REF] Malinverno | Orbital tuning as an inverse problem: Chronology of the early Aptian oceanic anoxic event 1a (Selli Level) in the Cismon APTICORE[END_REF], the sedimentary Hg peaks might also result from lithological/redox changes or an increased input of terrigenous material following environmental responses to the climate disruption associated with the event.

As outlined in section 1.3, mercury can be deposited in sediments bound to sulphides, particularly in euxinic water columns [START_REF] Shen | Sedimentary host phases of mercury (Hg) and implications for use of Hg as a volcanic proxy[END_REF]. In this context, the fact that the Cismon core records the development of euxinic conditions during OAE 1a (e.g., [START_REF] Van Breugel | Synchronous negative carbon isotope shifts in marine and terrestrial biomarkers at the onset of the early Aptian oceanic anoxic event 1a: Evidence for the release of 13 C-depleted carbon into the atmosphere[END_REF], and that C4 (and C6) strata from that archive marked by increased Hg concentrations also feature elevated sulphur contents (Supplementary Figure 2B), may indicate a switch to mercury burial with sulphides in the Belluno Basin during OAE 1a. If this association between Hg and S was the case, it could suggest that redox changes and pyrite-deposition controlled sedimentary Hg contents at that location during OAE 1a, rather than any external influx. And if so, it is likely that a similar situation existed for the PLG core, given the similar palaeoenvironments and Hg/TOC records of the Cismon and PLG records (only one or two high data points that do not exceed pre-event levels, or at least their range of uncertainty). Whilst this local redox/sulphide control on mercury for these two sites remains J o u r n a l P r e -p r o o f Journal Pre-proof unproven, the relative paucity of high Hg/TOC values nonetheless means that robust evidence of a consistent external flux of mercury to the Belluno and Umbria-Marche basins during OAE 1a is currently lacking. However, C4-C5 strata, specifically, of Notre-Dame-de-Rosans, La Bédoule, Glaise, and DSDP Site 463 record no evidence of euxinic conditions under which deposition of mercury with sulphides would have been most likely [START_REF] Mélières | Organic-matter-rich and hypersiliceous Aptian sediments from western Mid-Pacific Mountains[END_REF][START_REF] Van Breugel | Synchronous negative carbon isotope shifts in marine and terrestrial biomarkers at the onset of the early Aptian oceanic anoxic event 1a: Evidence for the release of 13 C-depleted carbon into the atmosphere[END_REF][START_REF] Westermann | Rapid changes in the redox conditions of the western Tethys Ocean during the early Aptian oceanic anoxic event[END_REF][START_REF] Giraud | The palaeoceanographic crisis of the Early Aptian (OAE 1a) in the Vocontian Basin (SE France)[END_REF], although DSDP Site 463 does feature an increased pyrite abundance in the C4 segment [START_REF] Mélières | Organic-matter-rich and hypersiliceous Aptian sediments from western Mid-Pacific Mountains[END_REF]. Euxinic conditions are extremely unlikely to have developed in the nearshore shallow-marine environment recorded by the DH-1 core, and a reasonably oxic (or at least not severely anoxic) setting is supported for that record by low sedimentary molybdenum and uranium concentrations, which show little or no enrichment above average-shale values except for two samples above the Selli Level Equivalent (this study; see Supplementary Figure 2C). There is also no correlation between the C4-C5 Hg enrichments and abundance of clays/Al 2 O 3 at any of DSDP Site 463, the DH-1 core, La Bédoule, or Glaise (see Supplementary Figure 2, and Charbonnier and Föllmi, 2017). These trends support a lack of association between mercury and clay minerals in the C4-C5 strata at those sites, which would be expected due to the abundant TOC content in these same sediments, given the likely greater affiliation for mercury with organic matter over clays when both phases are present [START_REF] Shen | Sedimentary host phases of mercury (Hg) and implications for use of Hg as a volcanic proxy[END_REF]. Thus, for at least four or five locations spanning the Arctic, Pacific, and north-western Tethyan realms presented here and in [START_REF] Charbonnier | Mercury enrichment indicates volcanic triggering of Valanginian environmental change[END_REF], there is no evidence that mercury was deposited with a phase other than organic matter, making it likely that the C4-C5 peaks in Hg and Hg/TOC values were caused by an increased influx of mercury rather than lithological/redox changes.

Charcoal particles potentially indicative of wildfire activity have been reported from strata of Aptian age [START_REF] Brown | Cretaceous wildfires and their impact on the Earth system[END_REF][START_REF] Wang | Characteristics of Early Cretaceous wildfires in peat-forming environment, NE China[END_REF], although not from the Selli Level Equivalent specifically, or in any of the nine sites studied here and by [START_REF] Charbonnier | Mercury enrichment indicates volcanic triggering of Valanginian environmental change[END_REF]. Thus, there is no positive evidence that the widespread Hg influxes were the result of wildfires, although J o u r n a l P r e -p r o o f Journal Pre-proof such a source cannot be completely ruled out. Nonetheless, terrestrially derived organic matter is abundant in the DH-1 core, and is thought to be present in some mixture with marine material at all of DSDP Site 463, Notre-Dame- de-Rosans, and Glaise (van Breugel et al., 2007;[START_REF] Westermann | Rapid changes in the redox conditions of the western Tethys Ocean during the early Aptian oceanic anoxic event[END_REF][START_REF] Midtkandal | The Aptian (Early Cretaceous) oceanic anoxic event (OAE1a) in Svalbard, Barents Sea, and the absolute age of the Barremian-Aptian boundary[END_REF][START_REF] Giraud | The palaeoceanographic crisis of the Early Aptian (OAE 1a) in the Vocontian Basin (SE France)[END_REF]. Organic-matter contents are too low at La Bédoule to reliably determine their composition. Thus, an increased input of terrigenous organic matter during a time of increased riverine runoff is a theoretically plausible source of the mercury to these sites during the early-middle part of OAE 1a, as previously proposed for the Early Jurassic Toarcian OAE [START_REF] Them | Terrestrial sources as the primary delivery mechanism of mercury to the oceans across the Toarcian Oceanic Anoxic Event (Early Jurassic)[END_REF]. However, peak weathering rates during OAE 1a are thought to have been during C3 rather than C4 [START_REF] Tejada | Ontong Java Plateau eruption as a trigger for the early Aptian oceanic anoxic event[END_REF][START_REF] Bottini | Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a[END_REF][START_REF] Lechler | Lithium-isotope evidence for enhanced silicate weathering during OAE 1a (Early Aptian Selli event)[END_REF]. Thus, if the mercury were derived from runoff of terrestrial material, an enrichment would be expected in the C3 segment rather than C4-C5 strata. Moreover, there is no evidence for an enhanced flux of mercury to the depositional setting of the Petrobras Well D core, despite an increase in the content of terrestrially derived organic matter in the upper part of the OAE 1a level [START_REF] Tedeschi | Aptian carbon-isotope record from the Sergipe-Alagoas Basin: New insights into oceanic anoxic event 1a and the timing of seawater entry into the South Atlantic[END_REF].

Consequently, whilst local terrestrial runoff cannot be discounted as the cause of the widespread C4-C5 Hg-cycle perturbations, LIP-related emissions of mercury to the atmosphere provide an equally plausible explanation. In the latter scenario, overprinting of Hg by excess TOC might have muted any record of Hg/TOC peaks at sites such as Cismon or PLG (see [START_REF] Charbonnier | Mercury enrichment indicates volcanic triggering of Valanginian environmental change[END_REF]. As noted above, the lack of clear evidence of volcanic activity in the northwest Tethys during latest Barremian to early Aptian times means that local eruptions cannot be unambiguously stated as the cause of the observed C4-C5 Hg enrichments in Tethyan stratigraphic archives. Future studies incorporating mercury-isotope analyses may aid determination of the pathways taken by the element to reach depositional environments, and potentially indicate whether LIP activity or local processes such as terrestrial runoff and/or wildfires were the main cause of the widespread Hg enrichments recorded in the C4-C5 segments (c.f., [START_REF] Grasby | Isotopic signatures of mercury contamination in latest Permian oceans[END_REF][START_REF] Them | Terrestrial sources as the primary delivery mechanism of mercury to the oceans across the Toarcian Oceanic Anoxic Event (Early Jurassic)[END_REF].

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Implications for the impact of magmatic CO 2 emissions from LIPs on the global carbon cycle

Even if the widespread Hg-cycle disturbances during the early-middle (C4-C5) part of OAE 1a were related to subaerial volcanism and/or HALIP-related thermogenic volatile emissions, this activity apparently did not commence until after the onset of the event. Thus, it is unlikely that these processes played a major role in triggering the crisis. Instead, the combined Hg enrichments at DSDP Site 463 and lack of clear C3 peaks from elsewhere strongly support submarine volcanism on the G-OJP as the dominant form of LIP activity during the onset of OAE 1a, suggesting magmatic CO 2 output from that province as the main driver of the carbon-cycle perturbations that initiated climate change. Such a scenario supports the model of [START_REF] Bauer | Quantifying the volcanic emissions which triggered Oceanic Anoxic Event 1a and their effect on ocean acidification[END_REF] that G-OJP mantle-carbon emissions were sufficient to cause the pronounced negative CIE documented in C3 strata that record the onset of OAE 1a. However, it remains unclear whether G-OJP volcanism caused a global carbon-cycle perturbation simply due to the huge volume (59-77×10 6 km 3 ; Kerr and Mahoney, 2007), and high basaltic emplacement rate (possibly >100 km 3 /yr; see introduction) of that LIP, particularly if it was combined with activity on the previously postulated twin province emplaced onto the Farallon Plate [START_REF] Schlanger | Volcanism and vertical tectonics in the Pacific Basin related to global Cretaceous transgressions[END_REF][START_REF] Larson | Onset of the Mid-Cretaceous greenhouse in the Barremian-Aptian: Igneous events and the biological, sedimentary, and geochemical responses[END_REF]. If carbon emissions associated with the uniquely voluminous/rapid G-OJP magmatism were solely responsible for the negative CIE at the onset of OAE 1a, it would not detract from previous hypotheses that most (smaller) LIPs do not emit sufficient magmatic CO 2 to drive such disturbances unless combined with thermogenic volatiles and/or methane hydrate release (e.g., [START_REF] Hesselbo | Massive dissociation of gas hydrate during a Jurassic oceanic anoxic event[END_REF][START_REF] Self | Volatile fluxes during flood basalt eruptions and potential effects on the global environment: A Deccan perspective[END_REF]. However, these results might also highlight the possibility that all LIPs can drive such disturbances through magmatic carbon emissions alone (as previously hypothesized by e.g., [START_REF] Saunders | Two LIPs and two Earth-system crises: the impact of the North Atlantic Igneous Province and the Siberian Traps on the Earth-surface carbon cycle[END_REF][START_REF] Gutjahr | Very large release of mostly volcanic carbon during the Palaeocene-Eocene Thermal Maximum[END_REF]. Further studies of (intrusive and extrusive) LIP basalts and sediments intruded by magmatic sills, both for the G-OJP, HALIP and other provinces, are needed in order to constrain the potential outputs of different volatile J o u r n a l P r e -p r o o f Journal Pre-proof species from magmatic and thermogenic sources, and to determine the extent to which these different sources could have perturbed the cycling of different elements at the Earth's surface during different geological periods.

CONCLUSIONS

This study has investigated the influence of different magmatic processes related to the development of large igneous provinces (LIPs) on the global environment, using OAE 1a as a case study. By combining new mercury data for six records of that event, including the first sites studied outside of the Tethyan realm, and new osmium-isotope evidence from one of these six (Poggio le Guaine) with previously published datasets, the types of LIP activity likely to have been most prevalent during the onset and main body of the event have been determined.

Although osmium-isotope trends are broadly consistent across all new and previously studied sites in documenting intense LIP activity throughout OAE 1a, records of mercury-cycle disturbance show considerable geographical variability. The only unambiguous Hg perturbation during the onset of OAE 1a (matching Os-isotope evidence of LIP activity) is recorded in the Mid-Pacific Mountains very proximal to the Greater Ontong-Java Plateau (G-OJP). No such clear evidence for mercury-cycle perturbations at that time is recorded at Tethyan or South Atlantic sites, or from the Arctic region proximal to the High Arctic Large Igneous Province (HALIP). This pattern supports G-OJP volcanism as the dominant form of LIP activity at the onset of OAE 1a, causing the globally documented Os-isotope shift but only localized Hg enrichment. These findings also re-emphasise previous hypotheses that oceanic LIPs only influence mercury cycling in immediately proximal regions, due to the limited dispersal range of Hg emitted by submarine volcanism and other subaqueous basalt-seawater interactions, and likely do not cause a global-scale perturbation.

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Journal Pre-proof Crucially, whilst submarine LIP activity on the G-OJP during the onset of OAE 1a is clearly documented by osmium-isotope and mercury evidence, there is no clear indication for HALIP magmatism at that time. More widespread, if generally less pronounced, mercury-cycle perturbations appear to have taken place during the early-middle (C4-C5) part of OAE 1a. These disturbances could have been linked to a transient pulse in subaerial eruptions or thermogenic emissions related to the emplacement of the HALIP, or an intensification of/switch to subaerial eruptions on the G-OJP.

However, a non-volcanic cause of these widespread Hg-cycle disturbances after the onset of OAE 1a, resulting from local environmental degradation during the event, cannot be ruled out. Even if these widespread mercury enrichments do signify sub-aerial LIP activity, the observation that they are not recorded globally in sediments that mark the onset of OAE 1a strongly suggests that such processes could not have caused the event. Instead, the results appear to confirm that submarine volcanism active on the Greater Ontong-Java Plateau, rather than sill intrusions of organic-rich sediments by the High Arctic LIP, was likely the primary trigger for OAE 1a. [START_REF] Erba | Integrated stratigraphy of the Cismon Apticore (southern Alps, Italy); a "reference section" for the Barremian-Aptian interval at low latitudes[END_REF][START_REF] Bottini | Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a[END_REF]. The grey shaded area indicates the stratigraphic extent of Selli Level Equivalent sediments deposited during OAE 1a; the dashed grey line the Barremian-Aptian boundary at the base of magnetozone M0 [START_REF] Erba | Integrated stratigraphy of the Cismon Apticore (southern Alps, Italy); a "reference section" for the Barremian-Aptian interval at low latitudes[END_REF][START_REF] Channell | Early Cretaceous magnetic stratigraphy in the APTICORE drill core and adjacent outcrop at Cismon (Southern Alps, Italy), and correlation to the proposed Barremian-Aptian boundary stratotype[END_REF][START_REF] Bottini | Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a[END_REF]. The stratigraphic levels of both the decline and crisis of nannoconids [START_REF] Erba | Integrated stratigraphy of the Cismon Apticore (southern Alps, Italy); a "reference section" for the Barremian-Aptian interval at low latitudes[END_REF] are indicated by thin dashed black lines, and the δ 13 C segments (from [START_REF] Menegatti | High-resolution δ 13 C stratigraphy through the early Aptian "Livello Selli" of the Alpine Tethys[END_REF] by solid thin black lines. See methods section for details on the calculation of initial seawater Os-isotope ratios ( 187 Os/ 188 Os (i) ) and sedimentary osmium concentrations ([Os (i) ]). Lithology, biostratigraphy, magnetostratigraphy, and carbon-isotope segments are sourced as for Figure 4.

  data for all records were generated using a RA-915 Portable Mercury Analyzer with PYRO-915 Pyrolyzer, Lumex, at the University of Oxford (UK). Analyses were carried out following the methodology in Percival et al. (2017), with at least two analyses conducted for each sample. Two reference materials were utilized as standards for machine calibration and drift check throughout a set of analyses: NIMT/UOE/FM/001 -Inorganic Elements in Peat (169 ppb Hg) and NIST-SRM2587 -Trace Elements in Soil Containing Lead from Paint (290 ppb Hg). Analytical uncertainty based on repeated measurements of the reference materials was ±15 ppb. Concentrations of other metals in the DH-1 core samples were measured by ICP-AES at Imperial College London (UK) following preparation by lithium metaborate fusion and hydrofluoric/perchloric acid digestion, after the methods in Neumann et al. (2013).

FIGURE CAPTIONS Figure 1 :

 CAPTIONS1 FIGURE CAPTIONS

Figure 2 :

 2 Figure 2: Palaeogeographic map of the Barremian-Aptian world (120 Ma). The positions of the HALIP and G-OJP are indicated by dark red areas. Locations A-F mark sites of sedimentary records investigated for mercury in this study (black squares = mercury and osmium-isotope data presented; black circles = mercury data presented). Locations G-I (black triangles) indicate sites previously studied for mercury

ACKNOWLEDGEMENTS

We greatly appreciate feedback from two anonymous reviewers that has improved this manuscript. We gratefully acknowledge John Farmer and the University of Edinburgh for provision of geochemical standard material, Stéphane Reboulet for fieldwork assistance at the Notre-Dame-de-Rosans section, and Steve Wyatt for aiding laboratory analyses. The Poggio le Guaine (PLG) core drilling was financially supported by the Fundação de Apoio à Universidade de São Paulo and Petrobras grant 2405. Peter Szatmari is thanked for conducting the logistical arrangements required to perform rhenium-osmium analyses on PLG samples at the University of Alberta. The UNIS CO2 Lab is thanked for providing access to the DH-1 core samples, and we greatly appreciate assistance from Sverre Planke, Ivar Midtkandal, and Stéphane Polteau in sampling the core. We thank the UK Natural Environment Research Council (NERC) grant NE/G01700X/1 (to Tamsin Mather), the European Research Council consolidator grant ERC-2018-COG-B18717-V-ECHO (to Tamsin Mather), NERC PhD studentship NE/L501530/1 (to Lawrence Percival), the Flanders Research Foundation (FWO) grant no. 12P4519N (to Lawrence Percival), the MIURPRIN (Ministero dell'Istruzione, dell'Università e della Ricerca-J o u r n a l P r e -p r o o f Journal Pre-proof Progetti di Ricerca di Interesse Nazionale: grant no. PRIN 2017RX9XXXY to Elisabetta Erba), the Research Council of Norway Centres of Excellence (Project 223272 to Henrik Svensen), Petrobras (for financial support of the PLG and Petrobras Well D analyses and doctoral student funding for Leonardo R. Tedeschi), the Vrije Universiteit Brussel, and the Leverhulme Trust for funding.

by [START_REF] Charbonnier | Mercury enrichment indicates volcanic triggering of Valanginian environmental change[END_REF]. Global map is adapted from [START_REF] Van Breugel | Synchronous negative carbon isotope shifts in marine and terrestrial biomarkers at the onset of the early Aptian oceanic anoxic event 1a: Evidence for the release of 13 C-depleted carbon into the atmosphere[END_REF], with the western Tethys inset adapted from [START_REF] Giraud | The palaeoceanographic crisis of the Early Aptian (OAE 1a) in the Vocontian Basin (SE France)[END_REF]. ratios based on TOC contents <0.2 wt% (below the limit of reliability recommended by [START_REF] Grasby | Mercury anomalies associated with three extinction events (Capitanian crisis, latest Permian extinction and the Smithian/Spathian extinction) in NW Pangea[END_REF]. Yellow lines mark the stratigraphic positions of tuffaceous layers [START_REF] Thiede | Site 463: Western Mid-Pacific Mountains[END_REF]

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: