Xiaolei Pang
email: xiaolei.pang@outlook.com

Franck Bassinot

Sophie Sepulcre

Cleaning method impact on the Mg/Ca of three planktonic foraminifer species: a downcore study along a depth transect

Keywords: Foraminifera, Mg/Ca thermometry, Cleaning protocol, Paleoceanography

We performed a downcore comparison of Mg/Ca ratios obtained using reductive and nonreductive cleaning methods applied to Globigerinoides ruber, Pulleniatina obliquiloculata and Globorotalia menardii picked along two nearby sediment cores retrieved at 2100m and 3875m in the eastern equatorial Indian Ocean. At the shallower site, our results show statistically significant discrepancies in averaged relative Mg/Ca loss induced by the reductive cleaning compared to non-reductive cleaning: G. menardii (8.5%) > G. ruber (6.2%) > P. obliquiloculata (4.4%), suggesting that reductive cleaning impact on relative Mg/Ca loss is species-specific. At the deeper site, differences between the relative Mg/Ca losses for the three species are smaller: G. menardii (3.7%), G. ruber (4.1%) and P. obliquiloculata (6.3%), suggesting the clear impact of in situ preservation state on the cleaning related Mg/Ca loss. The Mg/Ca loss in both relative (%) and absolute (mmol/mol) value of species G. ruber and G. menardii are significantly smaller at the deeper site than the shallower site, implying that these two species which have experienced intense dissolution on the sea-floor are less prone to lose further Mg during the reductive cleaning. P. obliquiloculata, however, show non-significant difference in Mg/Ca loss between the two sites, indicating that the cleaning related Mg/Ca loss in this species is somewhat insensitive to the preservation state at our two sites. Our data reveal that Mg/Ca loss results from partial dissolution induced by the reductive step. We conclude that the non-reductive cleaning approach should be preferred for building inter-laboratory homogeneous datasets of Mg/Caderived temperatures, whereas the reductive step should be only included when severe oxide coating is observed.

Introduction

Foraminiferal Mg/Ca ratios are used for reconstructing past seawater temperatures [START_REF] Anand | Calibration of Mg/Ca thermometry in planktonic foraminifera from a sediment trap time series[END_REF][START_REF] Dai | Influences of Temperature and Secondary Environmental Parameters on Planktonic Foraminiferal Mg/Ca: A New Core-Top Calibration[END_REF][START_REF] Elderfield | Evolution of Ocean Temperature and Ice Volume Through the Mid-Pleistocene Climate Transition[END_REF][START_REF] Fan | Variability of the Indonesian Throughflow in the Makassar Strait over the Last 30 ka[END_REF][START_REF] Xu | Indo-Pacific Warm Pool variability during the Holocene and Last Glacial Maximum[END_REF][START_REF] Yu | Mg/Ca in the benthic foraminifera Cibicidoides wuellerstorfi and Cibicidoides mundulus: Temperature versus carbonate ion saturation[END_REF]. In order to accurately reconstruct Mg/Ca temperatures, it is crucial to clean foraminiferal tests to remove Mg-rich contaminants. Common Mg contaminants are clay minerals, organic matter and ferromanganese oxides [START_REF] Barker | A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry[END_REF][START_REF] Pena | Characterization of contaminant phases in foraminifera carbonates by electron microprobe mapping[END_REF]. The removal of clay minerals and organic matter are the most important steps in Mg/Ca cleaning [START_REF] Barker | A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry[END_REF] and are always included in cleaning methods. In order to remove oxides, a reductive step must be added to the cleaning protocol, with the use of a hydrous hydrazine solution mixed with citrate acid and buffered with ammonia [START_REF] Boyle | Comparison of Atlantic and Pacific paleochemical records for the last 215,000 years: changes in deep ocean circulation and chemical inventories[END_REF][START_REF] Martin | A simple evaluation of cleaning procedures on fossil benthic foraminiferal Mg/Ca[END_REF][START_REF] Yu | Preferential dissolution of benthic foraminiferal calcite during laboratory reductive cleaning[END_REF]. Benefits and pitfalls of removing oxides with such a chemical treatment have been widely debated over these last years, some studies including this reductive step, whereas others preferring not to [START_REF] Martin | A simple evaluation of cleaning procedures on fossil benthic foraminiferal Mg/Ca[END_REF][START_REF] Barker | A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry[END_REF][START_REF] Yu | Preferential dissolution of benthic foraminiferal calcite during laboratory reductive cleaning[END_REF][START_REF] Bian | Investigating the fidelity of Mg/Ca and other elemental data from reductively cleaned planktonic foraminifera[END_REF]. Indeed, despite the ammonia buffering, several studies have provided evidence of dissolution of foraminiferal tests due to this reductive cleaning, resulting in a decrease of Mg/Ca ratios compared to those obtained from a cleaning protocol without a reductive step (e.g. [START_REF] Rosenthal | Interlaboratory comparison study of Mg/Ca and Sr/Ca measurements in planktonic foraminifera for paleoceanographic research[END_REF][START_REF] Johnstone | Effect of preservation state of planktonic foraminifera tests on the decrease in Mg/Ca due to reductive cleaning and on sample loss during cleaning[END_REF]. According to several authors, this decrease in Mg/Ca reflects the inhomogeneous distribution of Mg in the tests and the preferential dissolution of Mg-enriched parts during the reductive cleaning [START_REF] Barker | A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry[END_REF][START_REF] Yu | Preferential dissolution of benthic foraminiferal calcite during laboratory reductive cleaning[END_REF]. However, in some sedimentary environments, not removing contaminant oxides can result in erroneously high Mg/Ca values [START_REF] Pena | Identification and removal of Mn-Mg-rich contaminant phases on foraminiferal tests: Implications for Mg/Ca past temperature reconstructions[END_REF][START_REF] Pena | Characterization of contaminant phases in foraminifera carbonates by electron microprobe mapping[END_REF][START_REF] Weldeab | Comparison of foraminiferal cleaning procedures for Mg/Ca paleothermometry on core material deposited under varying terrigenous-input and bottom water conditions[END_REF].

Paleoceanographers are caught, therefore, between a rock and a hard place on choosing the cleaning protocol (i.e. with or without using a reductive step) and no general consensus has yet emerged. As a result, reductive step is currently employed in some laboratories but omitted in others. This lack of consensus can have significant impacts on paleoceanographic reconstructions because, for instance, building databases of past ocean temperatures requires coherent datasets.

Addressing inter-laboratory biases such as the potential impact of cleaning protocol and how to correct for this are thus key issues to homogenize the published Mg/Ca results.

Several studies have tried to address quantitatively the effects of reductive cleaning on foraminiferal Mg/Ca but have reached contrasted conclusions. As part of an inter-laboratory comparison exercise including a multi-planktonic species study, [START_REF] Rosenthal | Interlaboratory comparison study of Mg/Ca and Sr/Ca measurements in planktonic foraminifera for paleoceanographic research[END_REF] observed an average reduction of ~15% in Mg/Ca ratio obtained from foraminiferal tests cleaned with a reductive step compared to those cleaned without it. [START_REF] Barker | A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry[END_REF] had previously reported a similar 10-15% shift in Mg/Ca between the two cleaning approaches. By studying multi-benthic species, [START_REF] Elderfield | Calibrations for benthic foraminiferal Mg/Ca paleothermometry and the carbonate ion hypothesis[END_REF] reported an average 0.2 mmol/mol Mg/Ca reduction with a reductive step, and concluded that this value was similar to the 10-15% difference found for planktonic foraminifera. When using a multi-planktonic species calibration for Mg/Ca thermometry [START_REF] Anand | Calibration of Mg/Ca thermometry in planktonic foraminifera from a sediment trap time series[END_REF], a 10-15% relative loss is equivalent to 1.1-1.6°C decrease. This amount is rather large considering, for instance, that the glacial-interglacial sea surface temperature change in the tropics could be in the order of 3-4°C(e.g. [START_REF] Linsley | Holocene evolution of the Indonesian throughflow and the western Pacific warm pool[END_REF].

Based on these early studies, a 10% loss is usually used for correcting the cleaning induced Mg/Ca offset. However, following studies on specific planktonic species revealed considerable differences. For instance, [START_REF] Xu | Changes in the thermocline structure of the Indonesian outflow during Terminations I and II[END_REF] reported much smaller cleaning-induced Mg/Ca differences and suggested that Mg-loss could be species dependent, with a ~6.6% Mg/Ca relative decrease for G. ruber, and a smaller ~ 1.8% loss for Pulleniatina obliquiloculata. [START_REF] Fan | Variability of the Indonesian Throughflow in the Makassar Strait over the Last 30 ka[END_REF], on the other hand, recently reported a stronger ~ 6% Mg/Ca decrease for P. obliquiloculata when a reductive step was added to the cleaning protocol. To make things even more puzzling, [START_REF] Johnstone | Effect of preservation state of planktonic foraminifera tests on the decrease in Mg/Ca due to reductive cleaning and on sample loss during cleaning[END_REF] recently reported no Mg/Ca change for P. obliquiloculata after reductive cleaning, and relative Mg/Ca losses of ~4% for G. ruber, ~10% for Neogloboquadrina dutertrei and ~5% for Globigerinoides sacculifer.

As can be seen for these few examples, published data are confusing, fueling a neverending debate about cleaning methodologies and how to compare or even combine Mg/Ca results obtained from different laboratories. Difficulties in drawing consistent conclusions from previous works come from the fact that no comprehensive study of cleaning effects has yet combined all the following characteristics: (1) the two cleaning protocols should be applied on the same samples, within the same laboratory, to limit other potential sources of bias, (2) the cleaning protocols should be applied to different planktonic species picked from the same samples in order to test for inter-species differences, (3) the potential impact of the preservation state of foraminiferal tests should be addressed by comparing sedimentary material retrieved at different water depths, (4) the study should include a downcore evaluation of Mg/Ca offsets related to cleaning methods. Such a downcore comparison is mandatory if one wants to decipher whether the cleaning offsets may vary through time or depth, along the same sedimentary record, as a result of changes in the abundance of oxides (reflecting past variations in bottom water oxygenation), past changes in calcite preservation conditions at the seafloor, and/or diagenetic effects. This downcore study is particular important on assessing whether a constant correction factor can be applied along a core over different time periods (e.g. through a glacial-interglacial cycle). In addition, downcore studies provide a large number of samples, making it possible to derive more statistically robust evaluations of cleaning effects.

The present study aims at unraveling the species and preservation dependencies of Mg/Ca-offsets resulting from using non-reductive and reductive cleaning methods. For this purpose, we picked three planktonic foraminifer species (G. ruber, G. menardii and P. obliquiloculata) exhibiting different sensitivities to dissolution for assessing species-dependent effects. Because the preservation state of foraminifer tests is chiefly related to the water depth of deposition [START_REF] Regenberg | Global dissolution effects on planktonic foraminiferal Mg/Ca ratios controlled by the calcitesaturation state of bottom waters[END_REF], we used two cores located in the same area but retrieved at different water depths for addressing the dissolution issues. Although being close from one another, the sites display slightly different hydrographic settings today and past temperature evolution were not identical. Differences in Mg/Ca values between the two sites related to slightly various environmental settings will not be the focus of the present paper, which chiefly deals with cleaning-related Mg/Ca offsets.

Materials and Methods

We selected two adjacent sediment cores retrieved from different water depths in the western Timor Sea (Figure 1): core MD98-2165 (9.65° S, 118.34° E) located at 2100m of water depth, and core MD98-2166 (11.96° S, 120.46° E) located at 3875m of water depth. This choice has been driven by one of our goals in this experiment: to test whether the preservation state of foraminiferal tests exerts an impact on the relative Mg/Ca changes induced by adding a reductive step to the cleaning protocol. The preservation state of planktonic foraminiferal Mg/Ca ratios is controlled by the calcite-saturation state of bottom water [START_REF] Regenberg | Global dissolution effects on planktonic foraminiferal Mg/Ca ratios controlled by the calcitesaturation state of bottom waters[END_REF]. [START_REF] Regenberg | Global dissolution effects on planktonic foraminiferal Mg/Ca ratios controlled by the calcitesaturation state of bottom waters[END_REF] reported a global decrease in planktonic foraminiferal Mg/Ca ratios of ~0.054 mmol/mol per µmol/kg below a threshold for dissolution of ~21.3 µmol/kg. At the two studied sites in this paper, we calculated the calcite-saturation state of bottom waters (∆[CO3 2-]) -the difference between in situ [CO3 2-] and saturation state of [CO3 2-] -using the CO2Sys software [START_REF] Lewis | CO2SYS-Program developed for CO2 system calculations[END_REF] fueled with total alkalinity, temperature, salinity, pH and nutrient values obtained from the nearest GLODAP sites [START_REF] Sabine | GLOBAL OCEAN DATA ANALYSIS PROJECT (GLODAP): RESULTS AND DATA[END_REF]. Estimated ∆[CO3 2-] is about +10 µmol/kg at the water depth of core MD98-2165 and -11 µmol/kg at the water depth of core MD98-2166. As expected, the difference between the two sites and the under-saturation at site MD98-2166 suggest a much stronger dissolution effect on foraminifer tests at the deeper site. This is in agreement with the higher proportion of foraminifer fragments observed in samples from the deeper core MD98-2166. To better constrain the impact of the preservation state on the Mg loss associated to the reductive cleaning step, we will chiefly look at relative cleaning related Mg/Ca loss in %, and we will also provide the absolute cleaning related differences of Mg/Ca values in mmol/mol between the two sites. It is out of the scope of this study to distinguish in what extent the Mg/Ca differences between the two sites reflect in-situ dissolution or/and differences in hydrographic conditions. For core MD98-2165, we focused on the upper part of the core down to 779 cm. Age model of the upper part of MD98-2165 was developed by [START_REF] Waelbroeck | Distant origin of circulation changes in the Indian Ocean during the last deglaciation[END_REF] based on 20 14 C dates. Two anomalous 14 C values (inversion) suggested that the upper 55 cm of the sedimentary are allochthonous and were deposited rapidly [START_REF] Waelbroeck | Distant origin of circulation changes in the Indian Ocean during the last deglaciation[END_REF]. We focused our study, therefore, on the interval 110-779cm, covering the time interval from ~ 21 ka Cal BP (Last Glacial Maximum) to the Late Holocene (~2 ka Cal BP). Samples for the present study were taken every 5 to 10 cm (corresponding to a time resolution average on 300 yr).

For core MD98-2166, we studied the upper sedimentary section down to 540 cm. We sampled the sediment series every 10 cm. The age model for MD98-2166 was developed by [START_REF] Moreno | Influence of orbital forcing and sea level changes on sedimentation patterns in the Timor Sea during the last 260 ka[END_REF] by correlating the G. ruber δ 18 O record of MD98-2166 to the highresolution (i.e., 300 yr mean resolution) G. ruber δ 18 O record of core MD98-2165. This age model indicates that our Mg/Ca records cover the interval from the last glacial period (~35 ka Cal BP) to the middle Holocene (~ 2 ka Cal BP).

We picked three planktonic foraminifera species for this study: G. ruber (white), P. obliquiloculata and G. menardii. G. ruber and P. obliquiloculata are two species commonly used in the Indo-Pacific Warm Pool region for reconstructing surface and upper thermocline water temperatures, respectively (e.g. [START_REF] Xu | Indo-Pacific Warm Pool variability during the Holocene and Last Glacial Maximum[END_REF][START_REF] Bolliet | Mindanao Dome variability over the last 160 kyr: Episodic glacial cooling of the West Pacific Warm Pool[END_REF][START_REF] Fan | Variability of the Indonesian Throughflow in the Makassar Strait over the Last 30 ka[END_REF][START_REF] Wang | Precession-paced thermocline water temperature changes in response to upwelling conditions off southern Sumatra over the past 300,000 years[END_REF]). G. menardii is a deep-dwelling species, which has been used to reconstruct past changes in the thermocline structure [START_REF] Regenberg | Morphological, geochemical, and ecological differences of the extant menardiform planktonic foraminifera Globorotalia menardii and Globorotalia cultrata[END_REF]. These three species have different textures and morphological structures. P. obliquiloculata is characterized by a glassy, outer crust [START_REF] Steinhardt | Profiling planktonic foraminiferal crust formation[END_REF]; G. menardii displays a large and thick surrounding keel and a dense outer crust [START_REF] Regenberg | Morphological, geochemical, and ecological differences of the extant menardiform planktonic foraminifera Globorotalia menardii and Globorotalia cultrata[END_REF]; G. ruber has a thin and porous test. The outer crusts (P. obliquiloculata and G. menardii) and the keel of G. menardii are characterized by lower Mg/Ca ratios compared to inner layer parts [START_REF] Sadekov | Characterization of Mg/Ca distributions in planktonic foraminifera species by electron microprobe mapping[END_REF][START_REF] Sadekov | Surface and subsurface seawater temperature reconstruction using Mg/Ca microanalysis of planktonic foraminifera Globigerinoides ruber, Globigerinoides sacculifer, and Pulleniatina obliquiloculata[END_REF][START_REF] Steinhardt | Profiling planktonic foraminiferal crust formation[END_REF], and are known to be more resistant to dissolution occurring on the sea floor or during the cleaning in laboratory [START_REF] Brown | Variations in Mg/Ca and Sr/Ca ratios of planktonic foraminifera caused by postdepositional dissolution: Evidence of shallow Mg-dependent dissolution[END_REF][START_REF] Johnstone | Effect of preservation state of planktonic foraminifera tests on the decrease in Mg/Ca due to reductive cleaning and on sample loss during cleaning[END_REF]. Because of these characteristics, P. obliquiloculata and G. menardii are dissolution-resistant species, whereas G. ruber tests are more easily removed by dissolution in deep marine sediments.

For each sample, about 60-80 individuals of G. ruber and P. obliquiloculata were handpicked from the 250-355 µm size fraction and about 30 individuals of G. menardii were handpicked from the 355-450 µm size fraction. The samples were gently crushed between two glass plates under a binocular and thoroughly mixed to insure optimum homogeneity of sample aliquots. Samples were then divided as evenly as possible to obtain two sub-samples (Group A and Group B) and transferred to acid-cleaned, 500 µl centrifuge tubes for cleaning.

Group A and Group B sub-samples were cleaned following the protocol presented in [START_REF] Barker | A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry[END_REF], and a reductive step was added for samples from Group B. Here is a brief description of cleaning, for further details, please refer to [START_REF] Barker | A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry[END_REF]. Samples were ultrasonically cleaned in Milli-Q water (5 times) then in methanol (2 times) for removing clay minerals. Two additional rinses in Milli-Q water were performed for removing any remaining methanol. Samples were then checked under a binocular and any coarse silicates and darkcolored particles were removed before continuing with the oxidative cleaning step. For Group B samples, a reductive step was added prior to the oxidative cleaning. For this reductive step, we followed the protocol initially developed by [START_REF] Boyle | Comparison of Atlantic and Pacific paleochemical records for the last 215,000 years: changes in deep ocean circulation and chemical inventories[END_REF] and later used or slightly modified in other studies (eg. [START_REF] Martin | A simple evaluation of cleaning procedures on fossil benthic foraminiferal Mg/Ca[END_REF][START_REF] Yu | Preferential dissolution of benthic foraminiferal calcite during laboratory reductive cleaning[END_REF]Brian & Martin, 2010).

Samples were put in 0.12M citrate buffered with ammonium hydroxide and hydrazine solution in a boiling water bath for 30 minutes. Samples were ultra-sonicated every 5 minutes for promoting the reaction. Samples were then rinsed twice with Milli-Q water. For the rest of cleaning procedure, samples in Group A and Group B were treated the same way. They were soaked into alkali-buffered 1% H2O2 solution in a boiling water bath for 15 minutes (1 time). Every 3 minutes, samples were put into an ultrasonic bath for a few seconds for releasing CO2 bubbles building up during the oxidative reaction. Samples were then rinsed twice with Milli-Q water for removing remaining oxidative reagent. The cleaning protocol ends up with a rapid leaching in a diluted nitric acid solution (HNO3 0.001M) for removing any adsorbed contaminants, followed by thorough rinsing with Milli-Q water. All the water was removed through pipetting and samples left in the centrifuge tubes.

Prior to analysis, each sample was dissolved in 0.075M HNO3 and then centrifuged for removing any undissolved particulate impurities. The supernatant solution was transferred to a new acid-cleaned, 500 µl centrifuge tube. Mg/Ca ratios were measured by an inductively coupled plasma mass spectrometry (ICP-MS, Analytik Jena PlasmaQuant MS Elite). Aliquots of samples were first analyzed for Ca concentrations and the remaining solutions were diluted to reach a Ca concentration of ~100 ppm in order to minimize possible matrix effects. A standard solution was analyzed every 3 samples to monitor the instrument stability and correct for drifts [START_REF] Yu | Determination of multiple element/calcium ratios in foraminiferal calcite by quadrupole ICP-MS: FORAMINIFERAL CALCITE[END_REF]. The long-term reproducibility of Mg/Ca measurements estimated from replicate analyses of a standard solution (Mg/Ca=3.69 mmol/mol) is ±1.4% (n=144 1σ relative standard deviation).

In each run, the Group A and Group B subsamples of the same initial sample were analyzed sequentially (i.e. A1B1, A2B2, …, AnBn) for insuring a better inter-comparison of paired-results.

By doing this, we aimed at minimizing potential differences in Mg/Ca ratios between pairedsubsamples that could result from instrumental instabilities. Together with Mg and Ca, we measured Al and Fe, to check for potential contamination by clay minerals (e.g., [START_REF] Barker | A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry[END_REF], and we measured Mn, to detect the presence of oxide coating.

The impact of adding a reductive cleaning step on Element/Ca (e.g. Mn/Ca, Mg/Ca) has been estimated by computing the relative Elements/Ca loss in % compared to non-reductively cleaned paired samples. It is calculated as follow: Element/Ca-loss (%) = (Element/Ca without reductive step -Element/Ca with reductive step) / Element/Ca without reductive step) *100. The species-specific Mg/Ca temperature sensitivities of the three species studied in this paper are similar, around 9% increase in Mg/Ca per 1 ºC increase [START_REF] Anand | Calibration of Mg/Ca thermometry in planktonic foraminifera from a sediment trap time series[END_REF][START_REF] Regenberg | Calibrating Mg/Ca ratios of multiple planktonic foraminiferal species with delta O-18-calcification temperatures: Paleothermometry for the upper water column[END_REF]. Thus, the cleaning-related Mg/Ca-losses (%) are direct indicators of the impact of adding a reductive step on the temperature reconstructions. In other words, the larger the Mg/Ca-loss (%) is, the larger the temperature difference resulting from the two cleaning methods will be. Together with relative Mg/Ca-losses (%), the absolute cleaning related Mg/Ca offsets in mmol/mol will be also shown in this study. Comparing both relative (%) and absolute (mmol/mol) Mg/Ca-losses on the same species from the two different sites makes it possible to better understand the importance of in situ preservation on cleaning-related Mg/Ca differences.

Results

Contaminants

The low levels of Al/Ca (mean value ~ 0.01 mmol.mol -1 , maximum values < 0.2 mmol.mol -1 ) indicate that the cleaning procedure successfully removed any clay phase in the foraminifer samples. No relationships exist between higher concentrations of Al/Ca and high Mg/Ca values.

Except for several reductively cleaned samples of G. ruber, most of Fe/Ca ratios rang between 0.6 and 1.2 mmol.mol -1 for the three species in core MD98-2165 (Figure 234). In comparison, Fe/Ca ratios for the three species in core MD98-2166 are no higher than 0.6 mmol.mol -1 , with showing rather similar variations around a mean value at ~0.4 mmol/mol in all three species (Figure 234). Fe/Ca values show little impact from reductive step in core MD98-2165, adding reductive step result in averaged Fe/Ca decrease 7.2% for G. ruber, 2.9% for P. obliquiloculata and 2.5% for G. menardii. In comparison, Fe/Ca values of core MD98-2166 show negligible impact from reductive step in all three species. Overall, Mn/Ca ratios remain low, ranging from un-detectable up to a maximum value of 0.65 mmol/mol (non-reductively cleaned G. ruber in core MD98-2166) (Figure 234). Mn/Ca values clearly show the impact of reductive cleaning on oxide removal. Adding a reductive step results in a systematic 56 to 66% drop (average values) in Mn/Ca values in B sub-samples compared to the A group aliquots cleaned with the non-reductive protocol (Figure 234). For instance, average Mn/Ca in G. ruber from core MD98-2166 drops from 0.52 mmom.mol -1 in the non-reductively-cleaned, A-batch, to 0.22 mmol/mol in the reductively cleaned, B-batch (Figure 2d).

Mg/Ca results

G. ruber

For G. ruber, average Mg/Ca values range between ~ 3.2 and 4.7 mmol/mol (Figure 2e) displaying a clear glacial/interglacial structure in both core MD98-2165 and MD98-2166, with low Mg/Ca values during the glacial period increasing from around 20 ka Cal BP to reach a maximum at the onset of Holocene (~11 ka Cal BP). Core MD98-2165, which has a more complete Holocene section, shows decreasing Mg/Ca values towards a minimum centered around ~ 5 ka Cal BP and then increasing upward (~ 4.5 mmol/mol at the core top). Overall, the G. ruber Mg/Ca records from both cores are rather similar in terms of absolute Mg/Ca values and relative glacial/early Holocene evolution. This suggests that sea surface temperatures might have been slightly higher at the site MD98-2166, considering the fact Mg/Ca ratios at this deeper site should have been more reduced due to the stronger in situ dissolution [START_REF] Johnstone | Improving temperature estimates derived from Mg/Ca of planktonic foraminifera using X-ray computed tomography-based dissolution index, XDX[END_REF][START_REF] Regenberg | Global dissolution effects on planktonic foraminiferal Mg/Ca ratios controlled by the calcitesaturation state of bottom waters[END_REF].

As can be readily seen from Figure 2 (e-f, and i-j), except for a few odd samples, Mg/Ca values obtained with the reductive cleaning protocol (blue curves) are systematically lower in both cores than values obtained without the reductive step (red curves). Mg/Ca values decrease in average by ~0.26 mmol/mol in core MD98-2165 when adding a reductive step, whereas the Mg/Ca drop is only ~0.16 mmol/mol in core MD98-2166 (Figure e-f). This corresponds to a relative Mg/Ca-loss averaging 6.2 ± 3.5% (1σ, n=67) in core MD98-2165, while it is 4.1 ± 4.6 % (1σ, n=43) for the deeper core MD98-2166 (Figure 2 i-j). These results suggest that the reductive step has a stronger impact on Mg/Ca ratios of G. ruber from the shallower core. In core MD98-2166, there are a few, limited samples for which the offset is opposite (i.e. higher Mg/Ca values for reductively cleaned samples), but these samples remain scarce and with limited impact on the average Mg/Ca difference. Those odd values could result from the presence of small impurities and/or result from analytical variability. The Mg/Ca differences do not display a clear pattern with age (or depth) in cores MD98-2165 and MD98-2166. In (a)-(f), the red circles shows Element/Ca ratios obtained without the reductive step, the blue circles corresponds to Mg/Ca ratios obtained with the reductive step. In (e) and (f), the numbers on the graphs represent the average Mg/Ca decreases in mmol/mol due to the reductive cleaning.

(g)-(j), Mn/Ca and Mg/Ca loss in %; in (g) and (h), the values and the horizontal dashed line indicate the average Mn/Ca-loss in %. In (i) and (j), averaged Mg/Ca-loss (%) ± 1 sigma uncertainty are displayed, with n the total number of samples; horizontal dashed line indicates the average value, horizontal grey line represents the 0% loss.

P. obliquiloculata

The P. obliquiloculata Mg/Ca record from core MD98-2165 displays a clear deglaciation increase, with glacial values around 2 mmol/mol, increasing to ~ 2.7 mmol/mol at the beginning of the Holocene (Figure 3e). Then, Mg/Ca values decrease upward to reach ~ 2.1 mmol/mol (similar to glacial values) at the core top, with the most rapid drop in Mg/Ca occurring around 5 ka Cal BP. While the structure and values of Mg/Ca records were rather similar in both cores for G. ruber, the Mg/Ca records from cores MD98-2165 and MD98-2166 cores are significantly different for P. obliquiloculata. The deeper core MD98-2166 shows lower values than the shallower core MD98-2165 (Figure 3e-f) and does not reveal a strong glacial/interglacial structure, maximum Holocene Mg/Ca values (around 2 mmol/mol) remaining very similar to glacial values, around 1.8 mmol/mol. Lower Mg/Ca in Core MD98-2166 may partly result from the poorer preservation at this deeper site [START_REF] Johnstone | Improving temperature estimates derived from Mg/Ca of planktonic foraminifera using X-ray computed tomography-based dissolution index, XDX[END_REF][START_REF] Regenberg | Global dissolution effects on planktonic foraminiferal Mg/Ca ratios controlled by the calcitesaturation state of bottom waters[END_REF]. We interpret the different time-evolution of the two P. obliquiloculata Mg/Ca records as resulting from the regional complexity of the thermocline structure and its specific evolution at the two sites. This contrasts with the coherent evolution of sea-surface conditions at the two sites revealed by the similarity of G. ruber Mg/Ca records.

As was already observed for G. ruber, Mg/Ca ratios obtained on aliquots treated with the reductive step are lower in average than Mg/Ca ratios obtained on aliquots cleaned without the reductive step, with showing an averaged loss of ~0.11mmol/mol decrease in core MD98-2165 and ~0.13mmol/mol decrease in core MD98-2166. The corresponding relative Mg/Ca losses between the two cleaning methods average 4.4 ± 6.8% (1σ, n=71) for core MD98-2165, and 6.3 ± 8.8% (1σ, n=46) for the deeper core MD98-2166 (Figure 3i-j). Those differences show, however, a higher degree of scattering for P.obliquiloculata than for G. ruber. As can be observed in Figure 3, part of that variability rests on a few "odd" samples for which reductively cleaned Mg/Ca are higher than the non-reductively cleaned aliquots. The variation of cleaning related Mg/Ca differences do not display a clear pattern with age (or depth) in cores MD98-2165 and MD98-2166 (Figure 3i-j). ,d,f). In (a)-(f), the red circles shows Element/Ca ratios obtained without the reductive step, the blue circles corresponds to Mg/Ca ratios obtained with the reductive step. In (e) and (f), the numbers on the graphs represent the average Mg/Ca decreases in mmol/mol due to the reductive cleaning. (g)-(j), Mn/Ca and Mg/Ca loss in %; in (g) and (h), the values and the horizontal dashed line indicate the average Mn/Ca-loss in %. In (i) and (j), averaged Mg/Ca-loss (%) ± 1 sigma uncertainty are displayed, with n the total number of samples; horizontal dashed line indicates the average value, horizontal grey line represents the 0% loss.

G. menardii

For G. menardii, averaged Mg/Ca values range between ~ 1.8 mmol/mol and ~2.7 mmol/mol in core MD98-2165, and between ~ 1.3 and 2.2 mmol/mol in core MD98-2166 (Figure 4e-f). No clear Mg/Ca evolution corresponding to the deglaciation can be seen in the records, potential glacial/interglacial structure being overshadowed by short-scale, high amplitude oscillations and rapid excursions. As was already observed for P. obliquiloculata, G. menardii Mg/Ca values are significantly lower in core MD98-2166 than in the shallower core MD98-2165 and their temporal evolutions are different too. As was concluded for P. obliquiloculata, the lower Mg/Ca values in G. menardii from Core MD98-2166 may partly result from the poorer preservation at this deeper site [START_REF] Johnstone | Improving temperature estimates derived from Mg/Ca of planktonic foraminifera using X-ray computed tomography-based dissolution index, XDX[END_REF][START_REF] Regenberg | Global dissolution effects on planktonic foraminiferal Mg/Ca ratios controlled by the calcitesaturation state of bottom waters[END_REF].

However, we interpret the different time-evolution of the two G. menardii Mg/Ca records as resulting from the regional complexity of the thermocline structure and its specific evolution at the two sites, which result in different temperature conditions recorded at our two sites.

Similarly, to what was observed for the two previous planktonic species, Mg/Ca ratios obtained with the reductive cleaning protocol (blue curves) are usually lower than Mg/Ca ratios obtained without the reductive cleaning protocol (red curves), showing an averaged ~0.21 mmol/mol decrease in core MD98-2165 and a smaller ~0.08 mmol/mol decrease in core MD98-2166. However, the variance is higher than for the two other species. In the shallow core MD98-2165, the Mg/Ca cleaning differences average 8.5 ± 6.3% (1σ, n=48), whereas they average 3.7% with a particularly high variance of ±11.1% (1σ, n=53) in the deeper core MD98-2166 (Figure 4i-j). Such particularly strong variance in the deeper core is largely controlled by a few odd samples, which display much higher reductively-cleaned Mg/Ca values than their nonreductively cleaned aliquots (up to -30% difference; Figure 4 Overall, when considering the three planktonic species together, Mg/Ca ratios from Group B (with reductive step) are significantly reduced compared to those from Group A (Figures 2, 3 and4), and Mg/Ca ratios from Group B aliquots being lowered by 6 ± 7% (1 , n=328) compared to aliquots from Group A. (g)-(j), Mn/Ca and Mg/Ca loss in %; in (g) and (h), the values and the horizontal dashed line indicate the average Mn/Ca-loss in %. In (i) and (j), averaged Mg/Ca-loss (%) ± 1 sigma uncertainty are displayed, with n the total number of samples; horizontal dashed line indicates the average value, horizontal grey line represents the 0% loss.

Discussion

Statistical robustness of our estimates of cleaning-related Mg loss

Considering the rather high scattering of datasets, it is important to decipher first whether we can confidently compare and rank the averaged Mg/Ca loss in both % and mmol/mol between the three species within a specific core and between the two sites for a given species.

We applied Shapiro-Wilk tests to our data. Results indicate that Mg/Ca differences between paired A-B aliquots are random, showing normal distributions. Thus, we are allowed to run t-test to check for the statistical robustness of similarity/dissimilarity between average Mg/Ca losses.

Prior to performing those tests, we run F-tests to decipher whether variances of the paired datasets we want to compare are similar or dissimilar, which made it possible for us to select the most appropriate t-test (i.e. Student test for equal variance cases, Welch's t-test for unequal variance cases).

When comparing two species at a time, those t-tests show that the averaged Mg/Ca-loss in % obtained for G. ruber and P. obliquiloculata, and for P. obliquiloculata and G. menardii at site MD98-2165 are significantly different from one another (at a 95% confidence level), as are the averaged Mg/Ca-loss in mmol/mol. However, the Mg/Ca loss of G. ruber (0.26 mmol/mol) and G. menardii (0.21 mmol/mol) cannot be significantly discriminated from one another. For site MD98-2166, the t-tests show that the averaged Mg/Ca-loss in % obtained for the three species are not significantly different, except for G. ruber (0.16 mmol/mol) and G. menardii (0.08 mmol/mol), which can be significantly discriminated.

As for the comparison between sites of the three species, Mg/Ca loss in both % and absolute mmol/mol of G. ruber and G. menardii are significantly different and higher at the shallower site MD98-2165 than at the deeper site MD98-2166. The t-test indicate, however, that the difference in Mg/Ca loss between the two sites is not significant for P. obliquiloculata.

Analytical scattering, instrumental stability, inhomogeneity of foraminiferal tests

Despite careful sample preparation and analytical precautions (i.e. sequential ICP-MS analyses of paired Group A and Group B aliquots, drift correction), Mg/Ca differences show scattering and variability, which are of the same order of magnitude and sometimes higher than the averaged offsets, and which are much larger than long-term instrumental uncertainty (i.e. ±1.4%, 1σ). The lowest variability is observed for G. ruber, which shows a standard deviation of ±3.5% (1σ) around the 6.2% mean difference at site MD98-2165 (Figure 2i) and shows a standard deviation of ± 4.6% (1σ) around the 4.1% mean at site MD98-2166 (Figure 2j). On the other side of the spectrum, the highest variance is observed for G. menardii, with a standard deviation of ±6.3% (1σ) around a mean value 8.5% at site MD98-2165 (Figure 4i), and a staggering standard deviation of ±11.1% (1σ) around a mean value of 3.7% at the deeper site MD98-2166 (Figure 4j).

As can be readily seen in Figures 2, 3 and4, standard deviations, in each data set, integrate a few "anomalous" data points for which the batch A-batch B aliquot differences are negative, meaning that the reductively cleaned sub-samples show higher Mg/Ca ratios than their corresponding non-reductively cleaned aliquots. These anomalous values are particularly high in the G. menardii dataset from core MD98-2166, with one sample showing a non-reductively cleaned A-aliquot with 29% less Mg than the corresponding B-aliquot (Figure 4j).

Our results also show that there is no apparent co-variance between relative Mg/Ca loss in the three planktonic species along each two cores (see panels (i) and (j) in Figures 2, 3 and4).

It has been estimated that the ∆[CO3 2-] change in the deep Indian Ocean water over a glacialinterglacial cycle was about 7 µmol/kg [START_REF] Kerr | The effect of ocean alkalinity and carbon transfer on deep-sea carbonate ion concentration during the past five glacial cycles[END_REF]. This is only about a third of the ~21 µmol/kg ∆[CO3 2-] offset that exists today between the two studied sites as a result of their difference in water depth. The largest, average difference in Mg/Ca-loss that we attribute to dissolution difference between cores MD98-2165 and MD98-2166 is ~ 4.8% for G. menardii.

Taken at face value, this suggests that the glacial-interglacial change in bottom water ∆[CO3 2-] may have only accounted for change of ~ 1.6% (one third of 4.8%) along the cores. Due to the rather large analytical uncertainties, such a glacial-interglacial variance in the Mg/Ca-cleaning offset cannot be resolved from our data as is clearly indicated by our observations, which failed to highlight a coherent downcore evolution of the Mg/Ca-cleaning offset.

The above observations suggest that most of the variance that we observe in cleaning A-B Mg/Ca differences along a given core is random, reflecting inhomogeneities between aliquots and analytical uncertainties, and does not reflect any strong, stratigraphic pre-conditioning of the samples (e.g. through evolution of dissolution conditions at the seafloor during the deposition of sedimentary material). Such random scattering around the mean values is also confirmed by Shapiro-Wilk tests (i.e. null hypothesis of normal distribution with p>0.05), which indicate that Mg/Ca differences between paired A-B aliquots can be considered as normal distributions. So, our results suggest that past changes in the carbonate preservation state at either site were not strong enough to strongly imprint downcore changes in the cleaning-related Mg loss. We have to stress out, however, that such a result may not necessarily apply to sediment material from other areas of the deep ocean.

Although we cannot reject the possibility that scattering and these anomalous negative values (i.e. Mg/Ca in non-reductively cleaned samples < Mg/Ca in reductively cleaned samples) result to some extent from analytical noise, this should be only marginal as there is no evidence of severe and rapid instable ICP-MS behavior during runs. Anomalous Mg/Ca paired-values are not associated, for instance, with anomalous values of other elements measured together with Mg and Ca. Thus, we believe that variances of several percent around the averaged Mg/Ca differences chiefly result from inhomogeneity between aliquots from the same samples, despite our efforts to mix thoroughly crushed foraminifer tests before splitting them into paired-aliquots.

In order to avoid excessive loss of calcite material during the ultra-sonification of the cleaning protocols, foraminifer tests were only slightly crushed to open shell chambers but not finely ground [START_REF] Barker | A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry[END_REF]. It is known that Mg is heterogeneously distributed in the shells of the three species used in this study: G. ruber [START_REF] Eggins | Mg/Ca variation in planktonic foraminifera tests: implications for reconstructing palaeo-seawater temperature and habitat migration[END_REF][START_REF] Sadekov | Surface and subsurface seawater temperature reconstruction using Mg/Ca microanalysis of planktonic foraminifera Globigerinoides ruber, Globigerinoides sacculifer, and Pulleniatina obliquiloculata[END_REF], P. obliquiloculata [START_REF] Sadekov | Surface and subsurface seawater temperature reconstruction using Mg/Ca microanalysis of planktonic foraminifera Globigerinoides ruber, Globigerinoides sacculifer, and Pulleniatina obliquiloculata[END_REF][START_REF] Steinhardt | Profiling planktonic foraminiferal crust formation[END_REF] and G. menardii [START_REF] Sadekov | Characterization of Mg/Ca distributions in planktonic foraminifera species by electron microprobe mapping[END_REF]. Large fragments of shell chambers may not allow for a thorough enough homogenization and could result in slightly different Mg/Ca composition between A and B aliquots. It is noteworthy that the two species of our data sets (namely P. obliquiloculata and G. menardii)

showing the highest variance in terms of Mg-cleaning loss and extreme anomalous negative values, are two species with complex structures characterized by a very heterogeneous distribution of Magnesium in the tests. P. obliquiloculata has a thick and dense outside cortex with lower Mg/Ca than the inside of the test [START_REF] Steinhardt | Profiling planktonic foraminiferal crust formation[END_REF]. G.menardii shows a heavy outer crust as well as thick keel [START_REF] Regenberg | Morphological, geochemical, and ecological differences of the extant menardiform planktonic foraminifera Globorotalia menardii and Globorotalia cultrata[END_REF], which are characterized by low Mg/Ca ratios (~ 2 mmol.mol -1 ) that contrast with the higher Mg/Ca (ranging from 4 to 6 mmol.mol -1 ) of the inner chamber calcite [START_REF] Sadekov | Characterization of Mg/Ca distributions in planktonic foraminifera species by electron microprobe mapping[END_REF]. G. ruber, on the other hand, is a bi-lamellar species: each new layer of calcite is constituted by an inner, Mg-rich primary layer and an outer, Mg-poorer layer. Thus, contrary to G. mernardii and P. obliquiloculata -whose heterogeneity occurs at the test-scale -Mg heterogeneity in G. ruber occurs at the wall-scale, and breaking open the tests should not result in significant Mg/Ca differences between the large fragments.

This may explain the lower standard deviation shown by Mg-difference between aliquots of G. ruber samples compared to the two other species.

We conclude from the above observations that Mg/Ca differences between paired A-B aliquots of a given species chiefly reflect the excess-dissolution of Mg-rich layers induced by the reductive cleaning procedure [START_REF] Barker | A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry[END_REF][START_REF] Rosenthal | Interlaboratory comparison study of Mg/Ca and Sr/Ca measurements in planktonic foraminifera for paleoceanographic research[END_REF][START_REF] Yu | Preferential dissolution of benthic foraminiferal calcite during laboratory reductive cleaning[END_REF], with no clear impact from the removal of oxides (i.e. no relationship between Mg and Mn relative losses). Our data show that a rather large scattering can be superimposed on the relative Mg/Ca offset between the cleaning methods. We attribute the particularly high scattering to inhomogeneities in Mg distribution within the foraminifer test (i.e. and, thus, between the fragments), which randomly affect the Mg difference between A and B aliquots, leading (i) to a higher scattering in G. menardii and P.obliquiloculata datasets than in G.ruber dataset and (ii) resulting in odd, negative offsets for some samples (i.e. Mg/Ca in A aliquots< Mg/Ca in B aliquots).

What controls the relative loss of Mg during reductive cleaning?

As indicated above, the reductive cleaning protocol results in a systematic and important relative loss of manganese in batch-B aliquots compared to batch-A aliquots. The relative losses of Mn/Ca range from 56% for G. menardii in core MD98-2166 (mean Mn/Ca dropping from 0.23 mmol/mol to 0.11 mmol/mol when adding the reductive step) up to 66% for P. obliquiloculata in core MD98-2166 (the mean Mn/Ca dropping from 0.22 mmol/mol to 0.08 mmol/mol). The Mn losses average 61% when computing together data from the three species at both sites. Such an important Mn loss traduces the efficient removal of Mn-rich oxides when using the reductive cleaning protocol. The fact that reductively cleaned samples still contain a small but measurable amount of Mn suggests that this element is also present in the calcite lattice. Whether this Mn is located in the foraminifer test or in a diagenetic Mn-rich carbonate phases [START_REF] Boyle | Manganese Carbonate Overgrowths on Foraminifera Tests[END_REF][START_REF] Pena | Identification and removal of Mn-Mg-rich contaminant phases on foraminiferal tests: Implications for Mg/Ca past temperature reconstructions[END_REF] is beyond the scope of this paper. Importantly, there is no relation between Mn and Mg contents and there is no relationship between the relative loss of Mn and that of Mg (Figure 5), suggesting that in our two cores the cleaning-related loss of Mg does not chiefly result from the removal of Mn-rich oxides that may have contained Mg among other metals. Thus, our data confirm that the decrease of Mg/Ca ratios associated with the reductive cleaning likely results from cleaning-induced, calcite dissolution, which preferentially removes the more labile and Mg-richer parts of foraminifer tests [START_REF] Barker | A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry[END_REF][START_REF] Rosenthal | Interlaboratory comparison study of Mg/Ca and Sr/Ca measurements in planktonic foraminifera for paleoceanographic research[END_REF][START_REF] Yu | Preferential dissolution of benthic foraminiferal calcite during laboratory reductive cleaning[END_REF]. Recently published scanning electron microscopy (SEM) images provide direct evidence of such cleaning-induced dissolution effects on foraminiferal microstructures (e.g., [START_REF] Johnstone | Effect of preservation state of planktonic foraminifera tests on the decrease in Mg/Ca due to reductive cleaning and on sample loss during cleaning[END_REF][START_REF] Peral | Updated calibration of the clumped isotope thermometer in planktonic and benthic foraminifera[END_REF].

Mn/Ca ratios were often used to monitor cleaning efficacy. Low Mn/Ca ratios (< 0.1mmol/mol) was suggested as a threshold for indicating negligible problem from oxide contaminants on foraminiferal Element/Ca analysis [START_REF] Boyle | Manganese Carbonate Overgrowths on Foraminifera Tests[END_REF]. Our down core Mn/Ca ratios are usually higher than 0.1mmol/mol for non-reductively cleaned samples. Even for the samples cleaned with the reductive step, there is still a considerable number of samples with Mn/Ca ratios in excess of 0.1mmol/mol. However, there is no evidence showing that Mg/Ca ratios for the two studied cores are affected by oxide contaminants. This suggests that Mn/Ca values might not always be a good indicator for monitoring cleaning efficacy for Mg/Ca analysis. plotted versus non-reductively cleaned values (X). Since there is no evidence of a coherent evolution of A-B Mg/Ca differences along the cores (panels (i) and (j) in Figures 2, 3 and4), such a synthetic plot provides the best graphic solution to summarize the distribution and relative contribution of each species and core to the overall estimate of Mg/Ca loss (Figure 6). The slope obtained through a simple linear regression approach has been used in many previous studies to provide an estimate of the average Mg differences between A and B aliquots [START_REF] Barker | A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry[END_REF][START_REF] Rosenthal | Interlaboratory comparison study of Mg/Ca and Sr/Ca measurements in planktonic foraminifera for paleoceanographic research[END_REF][START_REF] Yu | Preferential dissolution of benthic foraminiferal calcite during laboratory reductive cleaning[END_REF]. When taking into account the three species together, at the two sites, the overall Mg-loss is ~ 5.7% (i.e. slope 0.943 of the green regression line, Figure 6). This loss is much smaller than the 15% loss found by [START_REF] Rosenthal | Interlaboratory comparison study of Mg/Ca and Sr/Ca measurements in planktonic foraminifera for paleoceanographic research[END_REF] from their multi-planktonic foraminifera study. It is noteworthy that their estimate of a 15% decrease due to reductive cleaning was obtained from a linear regression approach without forcing the intercept to zero, whereas we chose to force the intercept to zero in Figure 6. Forcing the regression to zero assumes that the linear tendency we observe over the 1.5-4.5 mmol/mol Mg/Ca range of our dataset can be strictly extrapolated down to a zero Mg loss in a pure calcite (i.e. no Ca substitution). By forcing the intercept to zero, the average loss we obtained through this approach is similar to that obtained by computing a simple arithmetic mean of the whole dataset. We used published data from Rosenthal et al. (2004, Figure 5 therein), and run a linear regression with an intercept forced to zero. The re-evaluated slope indicates a reductive Mg loss of 6% for the multi-species, foraminifer Mg/Ca database of [START_REF] Rosenthal | Interlaboratory comparison study of Mg/Ca and Sr/Ca measurements in planktonic foraminifera for paleoceanographic research[END_REF]; a result fully consistent with our own. This suggests that differences between published Mg-loss may not only result from different protocols or different species or assemblages being analyzed, but may also depend upon the approach used to estimate the average Mg/Ca decrease.

Species-dependence of Mg loss during reductive cleaning

Merging all the data together provides an estimate of cleaning Mg/Ca loss (~ 5.7%) that is coherent with the revisited Mg/Ca loss (6%) using the database of [START_REF] Rosenthal | Interlaboratory comparison study of Mg/Ca and Sr/Ca measurements in planktonic foraminifera for paleoceanographic research[END_REF].

Such multi-species estimates of cleaning loss mask, however, species-dependent differences, which appear statistically significant when applying Student or Welch's t-tests.

In order to highlight the potential species-dependent differences in Mg/Ca resulting from the cleaning process, we focus on the data from the shallower of our two cores, MD98-2165, which provides relatively well-preserved planktonic tests. Among the three species we studied, P. obliquiloculata shows the smallest average loss of Mg (~ 4.4%, Figure 3i). This average Mgloss is significantly higher than the 1.8% Mg-loss found by [START_REF] Xu | Changes in the thermocline structure of the Indonesian outflow during Terminations I and II[END_REF] and the 0% loss estimated by [START_REF] Johnstone | Effect of preservation state of planktonic foraminifera tests on the decrease in Mg/Ca due to reductive cleaning and on sample loss during cleaning[END_REF] for this species. But the average loss we found is slightly smaller than the ~ 6% loss published by [START_REF] Fan | Variability of the Indonesian Throughflow in the Makassar Strait over the Last 30 ka[END_REF] for P. obliquiloculata tests picked from two Makassar Strait cores (Indonesian Archipelago) and retrieved at shallower water depths (1185m and 1984m) than MD98-2165 (2100m).

Our data indicate that G. menardii shells lose almost twice as much Mg (8.5%) than P. obliquiloculata (4.4%) in core MD98-2165 when a reductive step is added to the cleaning protocol (Figure 4i). The third species studied, G. ruber, shows an intermediate, cleaning-related Mg loss, which averages 6.2 %. This result is consistent with the 6.6% decrease observed by [START_REF] Xu | Changes in the thermocline structure of the Indonesian outflow during Terminations I and II[END_REF] using material from a nearby sediment core retrieved at a similar water depth (1783m) in the Indonesian Archipelago. Interestingly, [START_REF] Xu | Changes in the thermocline structure of the Indonesian outflow during Terminations I and II[END_REF] and the present study obtain the same Mg-loss for G. ruber (~ 6%), whereas the two studies differ significantly with regard to the P. obliquiloculata Mg-loss (see above).

The glossy P. obliquiloculata test is one of the most dissolution-resistant among planktonic foraminifers [START_REF] Berger | Foraminiferal Ooze: Solution at Depths[END_REF][START_REF] Johnstone | Effect of preservation state of planktonic foraminifera tests on the decrease in Mg/Ca due to reductive cleaning and on sample loss during cleaning[END_REF]. Increasing the substitution of Ca by Mg is known to degrade calcite crystallinity and increase its sensitivity to dissolution [START_REF] Bassinot | Crystallinity of foraminifera shells: A proxy to reconstruct past bottom water CO3= changes? Geochemistry[END_REF][START_REF] Brown | Variations in Mg/Ca and Sr/Ca ratios of planktonic foraminifera caused by postdepositional dissolution: Evidence of shallow Mg-dependent dissolution[END_REF][START_REF] Nouet | Dissolution effects on the crystallography and Mg/Ca content of planktonic foraminifera Globorotalia tumida (Rotaliina) revealed by X-ray diffractometry[END_REF]. We believe that the smaller decrease of Mg during the reductive cleaning (i.e. higher resistance to dissolution) shown by P. obliquiloculata compared to the two other species, results from the lower Mg content of its calcite test (with Mg/Ca ranging from ~ 2.0 mmol.mol -1 to 2.7 mmol.mol -1 in core MD98-2165).

Interestingly, G. menardii not only losses twice as much Mg (8.5%) than P. obliquiloculata (4.4%) when cleaned with the reductive protocol, but it also losses more Mg than G. ruber (6.2%). This higher relative loss of Mg from G. menardii seems to be at odd with the fact that G. ruber is considered a dissolution-sensitive planktonic species, whereas G. menardii is considered a dissolution-resistant one [START_REF] Berger | Foraminiferal Ooze: Solution at Depths[END_REF]. However, it is important to recall that the capacity of G. menardii to resist dissolution is chiefly due to the presence of a thick and low-Mg keel [START_REF] Sadekov | Characterization of Mg/Ca distributions in planktonic foraminifera species by electron microprobe mapping[END_REF]. This keel can be preserved in sediments as easily identifiable, crescent-shaped fragments, even when the chamber walls have long been dissolved and G. menardii tests reduced to fragments [START_REF] Mekik | Tracing deep-sea calcite dissolution: Agreement between the Globorotalia menardii fragmentation index and elemental ratios (Mg/Ca and Mg/Sr) in planktonic foraminifers[END_REF][START_REF] Mekik | A novel approach to dissolution correction of Mg/Ca-based paleothermometry in the tropical Pacific[END_REF]. If not for this resistant keel, G. menardii should not be considered a dissolution-resistant species since its walls, which are two to three times richer in Mg than the keel [START_REF] Sadekov | Characterization of Mg/Ca distributions in planktonic foraminifera species by electron microprobe mapping[END_REF], can show very early signs of dissolution (i.e. holes starting to develop early in the external walls of the last foraminifer chambers and rapidly increasing in size with proceeding dissolution). The early impact of dissolution on its chamber walls, associated with the resistance of its keel make G. menardii an interesting candidate for developing semi-quantitative proxies of dissolution intensity at the seafloor [START_REF] Mekik | Tracing deep-sea calcite dissolution: Agreement between the Globorotalia menardii fragmentation index and elemental ratios (Mg/Ca and Mg/Sr) in planktonic foraminifers[END_REF]. We believe that it is this capacity of chamber walls to dissolve rapidly that explains why G. menardii tests from core MD98-2165 losses more Mg than P. obliquiloculata and even more than G. ruber when subject to the reductive cleaning.

The effect of in situ dissolution in modulating Mg/Ca cleaning reduction

When considering separately Mg/Ca data from the two cores (Figure 6), planktonic foraminifers picked from the shallow core MD98-2195 show a slightly higher Mg-loss during reductive cleaning (slope of 0.937, i.e. loss of ~ 6.3‰,) than foraminifers from the deeper MD98-2166 core (slope of 0.952, i.e. ~ 4.8% loss). The difference is small but consistent with the hypothesis that foraminifers having undergone in situ dissolution are likely to lose less Mg during cleaning process than better-preserved tests [START_REF] Barker | A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry[END_REF]. However, the impact of in situ dissolution is actually a bit more complex than just a reduced cleaning loss of Mg with increasing water depth of deposition. Our data suggest, here again, that there are some speciesrelated differences in the way in situ dissolution impacts Mg loss during cleaning.

As just seen above, G. menardii tests from the shallow core MD98-2165 show a significantly higher cleaning-related Mg/Ca decrease (8.5%) than the other two species (6.2% for G. ruber, 4.4% for P. obliquiloculata). In the deeper core MD98-2166, however, G. menardii exhibits a similar or lower cleaning-related relative Mg/Ca decrease (3.7%) than the other two species (4.1% for G. ruber, 6.3% for P. obliquiloculata). This implies that different preservation states of foraminiferal tests between the two sites exert an impact on Mg loss during the reductive cleaning step. Both the relative (%) and absolute (mmol/mol) Mg/Ca losses for G. menardii are significantly higher for the shallower core MD98-2165 (8.5%, 0.21 mmol/mol) than the deeper core MD98-2166 (3.7%, 0.08 mmol/mol) (Figure 4 e-f and i-j). We interpret the strong difference in G. menardii cleaning Mg-loss between the shallow core MD98-2165 and the deep core MD98-2166 as an indication that in situ dissolution, which has presumably removed some part of Mg-rich chamber walls (and, thus, has increased the relative contribution of the Mg-poor, dissolution resistant keel), significantly reduces the capacity of G. menardii to lose further Mg during the reductive cleaning.

Similar to what we observe for G. menardii, G. ruber data reveal a potential "preconditioning" effect of in-situ dissolution on the cleaning-related loss of Mg. Our data show that cleaning-related offsets in Mg/Ca ratios are smaller for G. ruber (4.1%, 0.16 mmol/mol) in the deeper core MD98-2166 than from the shallower core MD98-2165 (6.2%, 0.26 mmol/mol). This difference is seen as statistically robust when running a t-test. This suggests that, similarly to what we observed for G. menardii, the tests of G. ruber from the deeper core, which have experienced more intense dissolution on the seafloor than the tests from the shallow core, are less prone to lose further Mg during reductive cleaning.

Contrary to what is observed for the two other species, P. obliquiloculata exhibits a stronger, cleaning-induced Mg/Ca decrease in the deeper core MD98-2166 (6.3%, 0.13 mmol/mol), than in the shallower core MD98-2165 (4.4%, 0.11 mmol/mol) (Figure 3). But this difference is not statistically robust when running a t-test. This suggests that the decrease in Mg/Ca due to reductive cleaning step is insensitive to the preservation state for P. obliquiloculata.

Including or not the reductive step in the cleaning method?

There is no doubt that using a reductive step during cleaning is necessary for those specific elements that are abundant in oxide coatings (e.g. Cd, U and Ba; [START_REF] Boyle | Comparison of Atlantic and Pacific paleochemical records for the last 215,000 years: changes in deep ocean circulation and chemical inventories[END_REF][START_REF] Yu | Preferential dissolution of benthic foraminiferal calcite during laboratory reductive cleaning[END_REF], as well as, apparently, for Mg/Ca analyses performed on samples particularly affected by oxide contamination, such as those from in the Panama basin [START_REF] Pena | Identification and removal of Mn-Mg-rich contaminant phases on foraminiferal tests: Implications for Mg/Ca past temperature reconstructions[END_REF][START_REF] Pena | Characterization of contaminant phases in foraminifera carbonates by electron microprobe mapping[END_REF]. For most of non-problematic regions, like our study area in the Indonesian Archipelago, adding a reductive step does not improve Mg/Ca measurements. We observed no relationship between cleaning-related loss of Mg and Mn, and there are no outliers showing anomalously high Mg/Ca ratios associated to high Mn/Ca values in the non-reductively cleaned (A) samples. This suggests that the amount of Mg associated to oxides is negligible compared to the amount of Mg that is contained in the foraminiferal calcite, an observation already made by [START_REF] Barker | A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry[END_REF]. Reductive cleaning does not apparently improve Mg/Ca results with respect to the removal of oxides, but it significantly reduces the Mg/Ca ratios through the preferential dissolution of Mg-rich parts of the foraminiferal tests. In addition, as can be readily seen from Figures 2, 3 and 4, anomalous Mg/Ca values are more often found in the reductively cleaned datasets, an observation which could reflect the fact that the reductive cleaning adds more manipulations (i.e. increased likelihood of contamination) and is more difficult to conduct properly (i.e. it requires a precise timing of the reductive step to avoid variability in dissolutioninduced changes). Thus, we believe that the reductive cleaning should be used only in sediments with severe oxide coating of the particles and/or when the paucity of planktonic foraminifers makes it impossible to conduct separate analyses for Mg/Ca and other elements that are significantly contaminated by oxides (e.g. Cd, Ba). If one is interested chiefly in Mg/Ca, we believe that it is best to use the non-reductive approach for cleaning the foraminiferal tests. In cases when the reductive step must be included, attention should be paid to Mg/Ca results from species having strongly non-homogeneous distribution of Mg. Our results show, for instance, that in order to obtain reliable Mg/Ca-based temperatures from G. menardii, it is mandatory to work on well-preserved samples, and the use of reductive step during cleaning should be better avoided.

Correcting for the Mg/Ca offsets induced by the cleaning methods

When dealing with spatial temperature distribution and trying to map temperature anomalies at a given period of the past (e.g. Last Glacial Maximum), it is crucial to insure the compilation of the most coherent temperature estimates possible. In such cases, efforts should be made to use as much as possible Mg/Ca data obtained with the same cleaning protocol. However, to date, no agreement has been reached regarding the cleaning protocol for Mg/Ca analyses. Some laboratories add a reductive step, while others do not include it. As a consequence, when comparing data from different laboratories or dealing with the construction of Mg/Ca-Tº databases, one has to cope with the potential problem of mixing inhomogeneous Mg/Ca datasets (e.g. [START_REF] Xu | Indo-Pacific Warm Pool variability during the Holocene and Last Glacial Maximum[END_REF][START_REF] Fan | Variability of the Indonesian Throughflow in the Makassar Strait over the Last 30 ka[END_REF][START_REF] Wang | Precession-paced thermocline water temperature changes in response to upwelling conditions off southern Sumatra over the past 300,000 years[END_REF]. This problem does not only concern direct Mg/Ca-Tº reconstruction. Mg/Ca-derived temperatures are also used, for instance, to reconstruct other paleoceanography proxies, such as the δ 18 O of seawater [START_REF] Legrande | Global gridded data set of the oxygen isotopic composition in seawater[END_REF][START_REF] Thirumalai | Constraining past seawater δ 18 O and temperature records developed from foraminiferal geochemistry: PSU Solver Algorithm[END_REF], or to study the thermocline structure based on the Mg/Ca-Tº gradient between shallow and deep-dwelling species such as G. ruber and P. obliquiloculata (e.g., [START_REF] Holbourn | Indonesian Throughflow variability during the last 140 ka: the Timor Sea outflow[END_REF][START_REF] Fan | Variability of the Indonesian Throughflow in the Makassar Strait over the Last 30 ka[END_REF][START_REF] Hollstein | Variations in Western Pacific Warm Pool surface and thermocline conditions over the past 110,000 years: Forcing mechanisms and implications for the glacial Walker circulation[END_REF][START_REF] Wang | Precession-paced thermocline water temperature changes in response to upwelling conditions off southern Sumatra over the past 300,000 years[END_REF]. In all these cases, cleaning-induced differences in Mg/Ca can bias the interpretations. It is therefore crucial to decipher how we can minimize cleaning-induced Mg/Ca differences between datasets.

Our results and those from previous studies show that simple linear corrections can be applied. In Figure 7 (a-b), we compare temperature records obtained in core MD98-2165 using the initial Mg/Ca results for G. ruber (panel a) and P. obliquiloculata (panel b) converted to temperatures using the empirical equation Mg/Ca = 0.38 exp (0.09*Tº) for G. ruber and Mg/Ca = 0.328 exp (0.09*Tº) for P. obliquiloculata [START_REF] Anand | Calibration of Mg/Ca thermometry in planktonic foraminifera from a sediment trap time series[END_REF]. Without any correction for cleaning effects (upper curves), the average temperature difference between reductively cleaned and non-reductively cleaned samples is ~0.7 ºC for G. ruber and ~ 0.5°C for P. obliquiloculata.

As expected these differences cancel out (lower curves) when correcting reductively cleaned Mg/Ca values with species-specific offsets (i.e. +6% for G. ruber and +4% for P. obliquiloculata) prior to temperature conversion. Using the same multi-species, average offset factor (5.7%, Figure 6) to correct Mg/Ca data from P.obliquiloculata would have resulted in slightly over-corrected P. obliquiloculata Mg/Ca-thermocline T°, thus reducing artificially the vertical temperature gradients that can be deduced from comparing the temperatures obtained with the G. ruber Mg/Ca-SST. Yet, using a common, multi-species correction factor does not modify the downcore evolution of the G. ruber and P.obliquiloculata temperature records, and does not affect, therefore, the interpretation of the relative evolution of vertical temperature gradient at site MD98-2165.

The necessity for as precise as possible quantitative estimates of temperatures is linked to our interest in comparing those temperatures between different sites of the same region. In order to illustrate potential misinterpretations that may result from cleaning issues and the interest of correction for these effects, we compared Mg/Ca-SST that we obtained from G. ruber in the shallow core MD98-2165, with G. ruber Mg/Ca-SST obtained from nearby core SO139-74KL [START_REF] Wang | Precession-paced thermocline water temperature changes in response to upwelling conditions off southern Sumatra over the past 300,000 years[END_REF] located to the South off Sumatra (Figure 1). Mg/Ca ratios in SO139-74KL

were measured from G. ruber in the 250-350 µm size fraction and the cleaning method included a reductive step [START_REF] Wang | Precession-paced thermocline water temperature changes in response to upwelling conditions off southern Sumatra over the past 300,000 years[END_REF]. Mg/Ca ratios were converted to SST by using, for both records, the same empirical equation for G. ruber: Mg/Ca = 0.38 exp (0.09*Tº) [START_REF] Anand | Calibration of Mg/Ca thermometry in planktonic foraminifera from a sediment trap time series[END_REF]. As can be seen from Figure 7c, Mg/Ca-SST obtained using the same cleaning method (i.e. with reductive step) indicate roughly comparable SST between the two cores during the early onset of deglaciation (20 -18 ka Cal BP) and the early Holocene (starting around 10ka), with a strong temperature gradient developing during the deglaciation itself. However, when we compare Mg/Ca-SST obtained using different cleaning methods (i.e. using MD98-2165 data obtained without the reductive step), the data suggest that a lateral temperature gradient always existed between the two sites since the LGM and only collapsed sometimes during the mid-Holocene (Figure 7d). Thus, paleo-interpretation is clearly affected by the cleaning effects on Mg/Ca records. We applied the G. ruber +6% correction to reductively-cleaned Mg/Ca data from core SO139-74KL (Figure 7e). After correction, the evolution of both Mg/Ca-SST records is fully coherent with the interpretation resulting the comparison of Mg/Ca-SST obtained with the same cleaning technique (Figure 7c). This example demonstrates the necessity to adjust the cleaning-offset when precise, absolute Mg/Ca-based water temperatures are needed (i.e., vertical or lateral gradients) for paleo-interpretation.

Although a multi-species correction factor (i.e., 5.7%, see Figure 6) is practical to use to minimize cleaning-offsets, we suggest using species-dependent correction factors when possible (i.e. 6 % for well-preserved samples of G. ruber and 4 % for well-preserved samples of P. obliquiloculata). For the poorly-preserved samples of G. ruber (such as the case in MD98-2166), a smaller correction factor of 4% should be more appropriate. For samples of G. menardii and the poorly-preserved samples of P. obliquiloculata, it would be inappropriate to correct the cleaning-offset by a universal factor, as their large variance and weak correlation between Mg/Ca ratios cleaned with and without the reductive step; in fact, it is as well as pointless to correct them because they have lost the ability to reflect the water temperature due to the altered Mg/Ca ratios caused by the in situ dissolution. For those other species not mentioned in this study, the multi-species correction factor of 5.7% could be employed.

Conclusions

In this study, we compared the Mg/Ca ratios obtained using two cleaning methods on three planktonic species, G. ruber, P. obliquiloculata and G. menardii, picked from two sediment cores retrieved at different water depths. The Mg/Ca records cover the last ~21ka Cal BP for core MD98-2165 (2100m) and ~35ka Cal BP for core MD98-2166 (3875m).

Our results show that adding a reductive step decreases the Mg/Ca ratio for all three species. The overall Mg/Ca loss averages 6% (0.16 mmol/mol). In the shallow, well-preserved core MD98-2165, t-tests show that differences between averaged Mg/Ca % losses in the three species are statistically robust, indicating that the cleaning loss is species-specific to some extent.

The cleaning-related differences in Mg/Ca (in both relative % and mmol/mol) of G. ruber and G. menardii are higher in the shallower core MD98-2165 than in the deeper core MD98-2166, suggesting that foraminifera tests which have experienced intense dissolution on the sea-floor are less prone to lose further Mg during the reductive cleaning. P. obliquiloculata, however, show non-significant difference in Mg/Ca loss between the two sites, indicating that cleaning related Mg/Ca loss in this species is insensitive to the preservation state.

Well-preserved G. menardii shells retrieved in core MD98-2165 shows the strongest cleaning-induced Mg/Ca decrease of our study (8.5%), which is likely due to the fragile nature of the wall calcite that tends to dissolve easily. G. menardii specimens from Core MD98-2166, which have experienced some dissolution at the seafloor, show a marked reduction in cleaningrelated Mg/Ca loss (3.7%) compared to specimens from the shallow core. We interpret this result as being indicative of the strong resistance to dissolution of the remaining outer crust and keel, characterized by low Mg/Ca composition [START_REF] Sadekov | Characterization of Mg/Ca distributions in planktonic foraminifera species by electron microprobe mapping[END_REF]. This suggests that wellpreserved G. menardii specimens (retrieved from shallow cores, always above the lysocline) are mandatory for reconstructing unbiased Mg/Ca-based thermocline temperature, and non-reductive cleaning procedure is clearly preferable for this species to prevent partial dissolution.

For all species studied, reductive cleaning results in a clear Mg/Ca decrease compared to non-reductively cleaned values. However, there is no sign of significant changes in the relative Mg/Ca variations along a given core and for a given species when comparing the curves obtained by the two cleaning methods. This suggests that past changes in the carbonate preservation state over a glacial-interglacial cycle at either site were not strong enough to result in noticeable downcore changes in the cleaning-related Mg loss. In other words, relative seawater temperature changes reconstructed along a given sediment records (e.g. the temperature change between

LGM and Holocene, for instance) are not affected by the choice of the cleaning method.

The averaged 5.7% correction factor that we obtained from merging together Mg/Ca-loss from the three studied species at the two sites can be used for correcting cleaning effects and may prove valid for other foraminifer species, although this assumption will need to be tested in future studies. However, for minimizing the cleaning related Mg/Ca-offsets, we rather recommend using species-dependent correction factors when possible, such as 6 % for wellpreserved samples of G. ruber, 4 % for well-preserved samples of P. obliquiloculata and 8.5% for well-preserved samples of G. menardii.

It is the authors' contention that non-reductive cleaning approach should be preferred for building inter-laboratory homogeneous datasets of Mg/Ca-derived temperatures, whereas the reductive step should be only included when severe oxide coating is observed.

Figure 1 .

 1 Figure 1. Locations of sediment cores discussed in this study; MD98-2165 (red dot, this study), water depth 2100m; MD98-2166 (red dot, this study), water depth 3875m; SO139-74KL (yellow dot)(Wang et al., 2018), water depth 1690m.

Figure 2 .

 2 Figure 2. Element/Ca of G. ruber from core MD98-2165 (a, c, e) and core MD98-2166 (b, d, f).

Figure 3 .

 3 Figure 3. Element/Ca of P. obliquiloculata from core MD98-2165 (a, c, e) and core MD98-2166 (b, d, f). In (a)-(f), the red circles shows Element/Ca ratios obtained without the reductive step,

  j). Similarly, to what we observed for P. obliquiloculata in the deeper core MD98-2166, the combination of high variance in the Mg/Ca difference, and the low amplitude evolution of Mg/Ca along the core result in a very poor linear relationship between the reductively cleaned and non-reductively cleaned Mg/Ca values in MD98-2166 (R 2 =0.24).

Figure 4 .

 4 Figure 4. Element/Ca of G. menardii from core MD98-2165 (a, c, e) and core MD98-2166 (b, d, f). In (a)-(f), the red circles shows Element/Ca ratios obtained without the reductive step, the blue

Figure 5 .

 5 Figure 5. Comparison of Mg/Ca-loss (%) versus Mn/Ca-loss (%) of all three species (i.e. G. ruber, P. obliquiloculata and G. menardii) from core MD98-2165 (a) and core MD98-2166 (b). The dashed lines indicate the best linear fits. Note the absence of significant relationships between Mg/Ca-loss and Mn/Ca-loss at both sites. 4.4 Cleaning effect on Mg/Ca: a multi-species perspective Figure 6 compares all our data in a X-Y plot, with reductively cleaned Mg/Ca values (Y)

Figure 6 .

 6 Figure 6. Comparison of Mg/Ca ratios obtained with cleaning methods including and excluding the reductive step. Solid line, y=x; dashed line, linear best fits forced through the origin for samples from MD98-2165 (red), from MD98-2166 (blue) and all the data from the two sediment cores (green).

Figure 7 .

 7 Figure 7. (a) Comparison of G. ruber Mg/Ca-SST with using different cleaning methods in core MD98-2165, and cleaning-offset correction; SST is the short of sea surface temperature; TWT is the short of thermocline water temperature; NRC represents non-reductively cleaned samples and RC represent reductively cleaned samples. (b) Comparison of reductively and nonreductively cleaned P. obliquiloculata Mg/Ca-TWT in core MD98-2165, and cleaning-offset correction. (c)(d)(e) Comparison of G. ruber Mg/Ca-SST between core MD98-2165 and SO139-

Acknowledgments

This research is founded by INSU/LEFE MAGICS project. X. Pang acknowledges financial support from the China Scholarship Council. The data archiving is underway, the complete dataset of this study will be available on www.pangaea.de.