Evaluation of oxygen isotopes and trace elements in planktonic foraminifera from the Mediterranean Sea as recorders of seawater oxygen isotopes and salinity

The Mediterranean Sea is characterized by a relatively strong west to east salinity gradient, which makes it an area suitable for testing the effect of salinity on foraminiferal shell geochemistry. We collected living specimens of the planktonic foraminifer Globigerinoides ruber albus to analyse the relation between element /Ca ratios, stable oxygen isotopes of their shells, and surface seawater salinity, isotopic composition and temperature. The oxygen isotopes of sea surface water also correlate with salinity in the Mediterranean during winter, which is when sampling for this study took place. Seawater oxygen and hydrogen isotopes are positively correlated in both the eastern and western Mediterranean Sea, although the relationship differs from previously reported values, especially in the eastern region. The slope between salinity and seawater oxygen isotopes is lower than previously published results. Still, despite the rather modest slope, seawater and foraminiferal carbonate oxygen isotopes are correlated in our dataset, albeit with large residuals and high residual variability. This scatter could be due to either biological variability in vital effects or environmental variability. Numerical models backtracking particles show that ocean-current-driven mixing of particles of different origins might dampen sensitivity and could result in an offset caused by horizontal transport. Results show that Na /Ca is positively correlated with salinity and independent of temperature. As expected, foraminiferal Mg /Ca increases with temperature, which is in line with earlier calibrations, and in the high salinity environment. By using living foraminifera during winter, the previously established Mg /Ca–temperature calibration is extended to temperatures below 18 C, which is a fundamental prerequisite of using single foraminifera for reconstructing past seasonality.

3 different environmental conditions as they have been transported to the sampling location. This may add to the uncertainty in the variable to cross-correlate against or even introduce a bias in the resulting calibration. Recently this has been suggested for dinoflagellate cysts (Nooteboom et al., 2019) and planktonic foraminifera, collected from the water column (Ganssen and Kroon, 1991), from sediment (van Sebille et al., 2015) and also from sediment traps (Steinhardt et al., 2014), but can also be applied to specimens collected living from the sea surface. 70 Here we used a plankton pump and sea water samples collected from the Mediterranean Sea in January and February of 2016 to test viability of deconvolving salinity from combined temperature and sea water oxygen isotope reconstructions. We also investigate the potential of the newly developed salinity proxy Na/Ca in the Mediterranean Sea. Using samples collected in winter we also extent the calibration of Mg/Ca to sea water temperature for G. ruber albus towards its lower temperature tolerance limits (14°C; Bijma et al., 1990), which is essential for the application of this species for past seasonality 75 reconstructions.

Materials and Methods
During two cruises (NESSC Cruises 64PE406 and 64PE407, RV Pelagia) between January 12th and February 25th in 2016, a total of 98 plankton samples were collected from the surface waters of the Mediterranean Sea along an east-west transect using a plankton pump system (Ottens, 1992). Surface water was continuously pumped on board from 5m water depth and lead 80 through a plankton net with 100 μm mesh size. Replacing the cod-end every 6h (filtering 57m3 of sea water on average, constantly monitored using a water gauge), accumulated samples were washed out of the net into a 90 μm sieve, rinsed thoroughly with deionized water to remove smaller particles as well as salts, and subsequently stored onboard at -80°C. At NIOZ all plankton samples were then freeze-dried, and dry oxidation by low temperature ashing (100°C) was used to combust the organic material while minimizing potential impacts on carbonate trace metal concentrations and δ 18 O (Fallet et al., 2009). 85 After ashing, samples were rinsed again thoroughly with de-ionized water and ethanol to remove potential ash residues. A variety of samples containing specimens of G. ruber albus (Morard et al., 2019) was selected to cover a large range in salinities and temperatures. Specimens used for analyses were selected from the size fraction 150 -250 μm, even though it has been reported that at this size fraction G. ruber albus and Globigerinoides elongatus cannot always be confidently distinguished due to similar morphology (Aurahs et al., 2011). Surface seawater samples for stable oxygen isotopes were collected every 60 90 minutes from the same pump, resulting in a set of 309 samples. A volume of 2 ml was stored without headspace at 4°C during the cruise to be analyzed at the home laboratory.
The elemental ratio of the final foraminiferal chamber (named the F-chamber) of individual shells were measured by laser ablation quadrupole inductively coupled plasma mass spectrometry (LA-Q-ICP-MS) using a circular spot with a diameter of 60-80 µm, depending on the size of the last chamber. The laser system (NWR193UC, New Wave Research) at Royal NIOZ 95 was used in combination with a two-volume sample cell (TV2), which allows detecting variability in elemental ratios within the foraminiferal chamber wall due to a short wash-out time of 1.8s (van Dijk et al., 2017). Ablating only F-chambers minimizes sampling of older carbonate that might have formed under different environmental conditions due to lateral and vertical transport. All specimens were ablated with an energy density of 1±0.1 J/cm2 and a repetition rate of 6Hz in a helium environment. A 0.7L/m helium flow transported the resulting aerosol to an in-house-built smoothing device before entering 100 the quadrupole ICP-MS (iCAP-Q, Thermo Fisher Scientific). Masses 7Li, 11B,23Na,24Mg,25Mg,27Al,43Ca,44Ca,57Fe, 88Sr, 137Ba and 238U were monitored, 44Ca served as an internal standard for quantification of the associated elements. The synthetic carbonate standard MACS-3 was used for calibration, in addition carbonate standards JCp-1, JCt-1, NFHS1 (NIOZ foraminifera house standard; Mezger et al., 2016) as well as glass standards SRM NIST610 and NIST612 were used for monitoring data quality. Accuracy of the analyses was 97%, while precision was 3.0% for Mg and 2.4% for Na measurements. 105 Stable oxygen and carbon isotopes of foraminiferal calcite were measured on groups of whole specimens different from those used for LA-Q-ICP-MS, using an automated carbonate device (Thermo Kiel IV) which was connected to Thermo Finnigan MAT 253 Dual Inlet Isotope Ratio Mass Spectrometer (IRMS). The NBS 19 limestone was used as a calibration standard, the NFHS1 standard was used for drift detection and correction. The standard deviation and offset of the NBS19 and the NFHS-1 were always within 0.1‰ for δ 18 O. 110 Due to the large amount of material required (20 to 40 μg) and the small amount of specimens present in the samples, specimens from different samples sometimes needed to be combined. This resulted, for example, in combining 12 and 8 µg of foraminifera from two adjacent transects and hence, the average temperature, salinity and δ 18 Oseawater for these transects was calculated based on the relative contribution of the foraminiferal weight of the individual transects (i.e. 60 and 40 % respectively). Sea water oxygen and hydrogen stable isotopes were analysed with the Liquid Water Isotope Analyser (LWIA; Los Gatos Research 115 Model 912-0008). This system measures the water samples using Off-Axis Integrated-Cavity Output Spectroscopy (OA-ICOS). The LWIA was connected with a GC PAL from CTC Analytics to inject 1µl per measurement. To achieve this, the GCPAL was equipped with a 1.2 µl Hamilton syringe. In-House standards (S35, S45, NSW, LGR5 and double distilled water) were calibrated against VSMOW2-, VSLAP2-and GISP2-standard water obtained from IAEA in Vienna, using the same setup. The use of standard water VSMOW2, which has δ 18 O values identical to the older SMOW standard, allows for simple 120 comparison with older data that was calibrated using SMOW, without additional corrections. Every sample and standard was measured 14 times sequentially, the first four runs were only used to flush the system while the last 10 measurements were used for the analysis. Additionally, between every sample or standard, the sample introduction line was rinsed with double distilled water. Data were processed with LGR LWIA Post Processor Software v. 3.0.0.88. Average standard deviation per sample was 0.14‰ for oxygen isotope measurements and 0.71‰ for hydrogen isotope measurements. 125 The likely provenance of the foraminifera sampled was computed by backtracking virtual particles in a high-resolution ocean model. For this, we used the Copernicus Marine Environmental Monitoring Service (CMEMS) Global Reanalysis model. The ocean surface currents, temperature and salinity are available at daily resolution and 1/12 degree horizontal resolution. In these fields, we backtracked particles using the OceanParcels v2.1.1 software (Lange and Sebille, 2017; Delandmeter and van Sebille, 2019). We released 10,000 particles equally spaced between the start and end locations of 25 of the transects (i.e. all 130 for which there were sufficient foraminiferal specimens for isotope analysis), on the day these transects were sampled, and tracked the particles back for 30 days with a 4th order Runge-Kutta algorithm with a 1 hour time step, storing local temperature, salinity and location for each particle every day. To avoid beaching of particles, we used an unbeaching Kernel similar to that in Delandmeter and van Sebille (2019). The full code of the simulations is available at https://github.com/OceanParcels/MedForams_Daemmer/. 135

Mediterranean Sea geochemistry
The sampled East-West transect spans a salinity gradient from 39.2 to 36.2 and an accompanying temperature gradient from 19°C (east) to 14°C. The 6 hour-intervals represented on average a distance of 57 kilometres (min 0 km, max 117 km). On average, this resulted in an internal variability of 0.14 salinity units and 0.33°C for each of the 98 transects.

Na/Ca vs Salinity
Na/Ca values measured on individual F-chambers of G. ruber albus from the Mediterranean Sea range from 6.8 to 12.7 mmol/mol and are positively correlated with sea surface salinity (Fig. 4 a). The variability between individuals (1-2 mmol/mol) observed within transects is orders of magnitude higher than the analytical uncertainty (RSD of 5%) and is also higher than 155 the uncertainty in the slope of the Na/Ca-salinity calibration (Fig. 4 a).

Mg/Ca vs Temperature
Mg/Ca-values measured on individual F-chambers of G. ruber albusfrom the Mediterranean Sea range from 1.34 to 7.63 mmol/mol and are positively correlated with in-situ measured sea surface temperatures, although the temperature range sampled during winter time was rather narrow (Fig. 4 b).

Particle backtracking
Particle backtracking shows that foraminifera collected at each transect might actually have travelled long distances within the 30days prior to sampling at the sample locations. The length of the modelled trajectories varies greatly from location to location, ranging between 200-500km. This resulted in a variabilities (SD) within one transect ranging from 0.11 to 1.0°C and 0.03 to 0.4 salinity units. 165

Salinity, δ 18 O and δD of the sea water
A single uniform and stable trend in sea water stable isotopes with salinity is a prerequisite for reconstructing past salinities. This is important not only when using the stable oxygen isotopes measured on foraminiferal shell carbonates, but also for the interpretation of the hydrogen isotopic composition of alkenones, which are also used as proxies for paleo-salinity (Schouten 170 et al., 2006;Vasiliev et al., 2013;Weiss et al., 2017).
The data presented here substantially increases the amount of data on the relation between salinity and water isotopes of the Mediterranean (Fig. 2). Although the new data clearly overlap with existing data, we also observe slight but statistically significant differences in the average salinity to δ 18 O relationship for the different data sets. The overall lower δ 18 O values of sea water measured here compared to the combined set of surface sea water isotopes from Stahl and Rinow (1973), Pierre et 175 al. (1986), Gat et al. (1996), Pierre (1999) and Cox (2010) of approximately 0.3‰ (Fig. 2) may be explained by inter-decadal, seasonal and geographical variability between sample sets, or a combination of these factors. Importantly such offsets also give a first order indication of the limit to the accuracy and precision of reconstructions of past salinity using a combined temperature-stable isotope approach from the primary relationship used.
Although Gat et al. (1996) reported a markedly different δD/δ 18 O relationship for the Eastern Mediterranean Sea compared to 180 that of the Western Mediterranean Sea, our results show no sign of such a longitudinal discontinuity for the same area ( Fig.   2). This implies that the water isotopic composition of the entire Mediterranean Sea can primarily be described by a single mixing line between two end-members, with high versus lower salinity, respectively. The remarkable trend between δD/δ 18 O observed previously by Gat et al. (1996) was explained as a deuterium excess effect due to a combination of the composition of the lowermost air vapor and mixing with the enriched surface waters, most notable in winter months. The discrepancy in 185 δD/δ 18 O relationship observed between our data and those of Gat et al. (1996) may be due to inter-decadal variability in the hydrological cycle or by differences in seasonal coverage. Potentially the observations of Gat et al. (1996) were hence either related to unusual conditions, spatially restricted features not covered by our sampling locations or the hydrological cycle in the eastern Mediterranean has recently changed considerably. Either way the observed offset between the western and the eastern basin is apparently not stable and should therefore probably not be considered when using Mediterranean stable isotope 190 signatures for reconstructing paleo-salinities.

Na/Ca vs Salinity
The Na/Ca ratios measured on the carbonate shells of G. ruber albus from the Mediterranean Sea are significantly and linearly correlated to salinity (Fig. 4 a). This relationship is similar to the one reported previously for plankton pump-collected G. ruber 195 albus from the Red Sea (Mezger et al., 2016). Mezger et al. (2016) suggested that there might a combined effect of different environmental factors such as carbonate chemistry, salinity and temperature on the Na/Ca values in the field-collected specimens. In the Red Sea it is not possible to decouple these factors as they are strongly related. Since in contrast to the Red Sea where there is a strong negative correlation between salinity and temperature, the Mediterranean sea surface salinity and temperature are positively correlated to each other, comparing our data to that of Mezger et al. (2016) allows to decouple 200 temperature from salinity (Fig. 5). This shows that the correlation between foraminiferal Na/Ca values and temperature observed in the Red Sea was not causal and more likely caused by salinity (Mezger et al. 2016). If temperature would have a significant effect on the Na/Ca values in G. ruber albus, we would expect different slopes and/or offsets for the Na/Ca to salinity calibrations for the Mediterranean Sea and Red Sea. This implies that temperature has no or only a minor impact on Na/Ca ratios in G. ruber albus shells, which is in line with similar findings showing a lack of temperature effect on the Na/Ca 205 of T. sacculifer (Bertlich et al., 2018).The average standard deviation in Na/Ca values for a given salinity corresponds approximately to 2 salinity units, using the calibration given here (Fig. 4 a). This large variability is similar to the inter-chamber and inter-specimen variability in other El/Ca ratios, such as for example in in Mg/Ca reported by Sadekov et al. (2008) and appears to be inherent to single-chamber El/Ca (de Nooijer et al., 2014b). It has been suggested that such variability between individuals and also between different chambers of the same individual, may be caused by differences in living depth (and 210 hence environmental conditions (Mezger et al., 2018b)), lateral transport (van Sebille et al., 2015) or variability in element incorporation during biomineralization due to vital effects (Erez, 2003;de Nooijer et al., 2014a;Spero et al., 2015) or individual timing of chamber formation (Dämmer et al., 2019). Since specimens used here were collected from surface waters and add new chambers very frequently, vertical or literal migration into waters with significantly different conditions as suggested by Mezger et al. (2018) and Van Sebille et al. (2015) appears to be an unlikely cause for heterogeneity between specimens in this 215 case. The relatively large scatter in Na/Ca values observed for single chambers (Fig. 4 a) implies that accurate and precise reconstructions of salinity can only be based on combining a substantial number of specimens (Wit et al., 2013).
If salinity is reconstructed from the Na/Ca measurements using the calibration published by Mezger et al. (2016) and compared versus salinity measured in situ in the Mediterranean Sea, the reconstructed salinity follows the in situ measurements closely almost 1:1. The largest deviation from this 1:1 relationship occurs in the lower salinity range, at a salinity of 36.52 the 220 reconstructed salinity estimates underestimate salinity by 0.71 salinity units. The average difference between in situ salinity measurements and salinity reconstructed based on one single-chamber measurement is an underestimation of salinity by 0.46 salinity units. This is still higher than the theoretical uncertainty associated when combining foraminiferal δ 18 O and temperatures derived from Mg/Ca measured at exactly the same specimens (Rohling, 2007). An uncertainty (1SD) of 1 °C in the Mg/Ca-temperature calibration (which may be particularly optimistic at high seawater temperatures), would result in an 225 uncertainty of ~0.37 units for the reconstructed difference between two salinities. This approach will lead to an improved salinity reconstruction when the (change in) past temperatures are determined more precisely, for example by reducing the error through increased sample size. The same applies for salinity reconstructions based on Na/Ca, for which not many calibrations are available and hence, leaves room for improvement.
While these reconstructions as well as the lack of a strong temperature effect are very encouraging results for the use of Na/Ca 230 as a salinity proxy, the incorporation of Na into foraminiferal calcite does not appear to be homogenous across the entire shell.
It has been shown that the majority of Na in G. ruber albus is located in the spines (Mezger et al., 2018a(Mezger et al., , 2018b, which are not well preserved in the fossil record.

G. ruber albus Mg/Ca values
The increase in Mg/Ca in G. ruber albus with temperature ( Fig. 4 b) fits recent calibration efforts for Mg-incorporation and temperature (e.g. Gray et al., 2018). Since salinity and inorganic carbon chemistry also both affect Mg incorporation in this species (Kisakürek et al., 2008;Gray et al., 2018), and the Mediterranean exhibits large gradients in these parameters, it is necessary to correct measured Mg/Ca values for these parameters. After normalizing Mg/Ca values to a sea water salinity of 240 35, using the calibration of Gray et al. (2018), the dependency of the Mg/Ca on temperature is similar to previously reported calibrations (e.g. Gray et al., 2018), although the Mg/Ca values at the lower most temperatures appear to be higher than expected (Fig. 6). This could potentially be caused by a combination of an underestimation of the salinity effect in these highly saline waters, since salinities observed here are well outside the calibration range used by Gray et al. (2018), and low temperatures, impacting the foraminiferal Mg/Ca comparatively little. 245 Adding our results to published Mg/Ca-temperature-calibrations for G. ruber albus (Anand et al., 2003;Babila et al., 2014;Fallet et al., 2010;Friedrich et al., 2012;Gray et al., 2018;Haarmann et al., 2011;Huang et al., 2008;Kisakürek et al., 2008;Mathien-Blard and Bassinot, 2009;McConnell and Thunell, 2005;Mohtadi et al., 2009) now extends the combined calibration to lower temperatures (i.e. < 18°C), maintaining a comparatively low temperature sensitivity in the colder part of the calibration (Fig. 6). This not only increases confidence in the application of Mg/Ca in this species as a paleotemperature reconstruction 250 tool for colder temperatures, but also support application of individual foraminiferal Mg/Ca values for reconstructing seasonality (Wit et al., 2010). Although low densities were reported previously for G. ruber albus in the Mediterranean Sea during winter time, including being absent in large areas (Pujol and Grazzini, 1995;Bárcena et al., 2004) our finding implies that lowest values in Mg/Ca can be related to winter temperatures. G. ruber albus is not only present throughout the year as also shown by Rigual-Hernández et al. (2012) and Avnaim-Katav et al. (2020), but it also registers the in-situ temperature, 9 also during seasons which are close to its lower temperature limit. Admittedly the large scatter also observed at one single sampling time (i.e. season) makes the deconvolution of seasonality from analyzing single specimen Mg/Ca values challenging.

Role of lateral transport on δ 18 Oforaminifer 260
Horizontal transport of planktonic foraminifera may increase exposure to variable environmental conditions, including different temperatures, salinities and seawater stable isotope compositions (van Sebille et al., 2015). Comparing the sampled transects with the calculated back tracking trajectories shows that especially close to the straits (Alboran Sea and Strait of Sicily) the area where the foraminifera might be derived from, potentially extends over considerable distances and therefore, variability in environmental parameters. With the surface variability in temperature and salinity during the sampling period, 265 the calculated variability in these parameters varied between 0.11 and 1.03°C per transect and 0.04 and 0.39 salinity units per transect (Fig. 7 b, c). This means that the majority of foraminifera experienced a variability of approximately 0.5°C and 0.15 salinity units.
When considering calibrations, this is not affecting the measured proxy variables as the as the difference may be unbiased, but adds to the uncertainty of the environmental parameter to be reconstructed. Since foraminifera grow by periodically adding 270 chambers and since the size of the added chambers increases exponentially in many species, the carbonate added closer to the sampling location makes up a larger proportion of the total shell mass than carbonate added at earlier life stages. Therefore, chambers formed early during a foraminifer's life, have less impact on average shell composition and hence the calibration and the back tracking trajectories (Fig. 7 a-c) thus indicate the largest possible range of conditions experienced by a single foraminifer. This is relevant when considering whole-shell chemistry (i.e. oxygen isotopes; Fig. 4 a) and to a lesser extent also 275 when considering the elemental composition of the final chamber (Fig. 4 b). The last chamber is affected by a much smaller range in environmental conditions, i.e. only the timespan during which the final chamber was built, not more than a few days prior to sampling.
Since δ 18 O of the calcite could not be measured on F-chambers only, like for element/Ca ratios, and several specimens were needed for a single analysis, results reflect average composition of foraminiferal populations at the sampling areas. The 280 averaging effectively cancels out differences due to inter-and intra-individual variability, but not offsets due to lateral transport. When transport directions are largely uniform, this result in biases and should not add to the scatter in the calcite's isotope composition. Hence this transport affects the calibration, but does not affect precision.

Implications for proxies 285
Combining existing calibrations for foraminiferal Mg/Ca and temperature (Gray et al., 2018) and calibrations relating δ 18 Oforaminifera with temperature (Mulitza et al., 2003), the δ 18 Oseawater can be calculated. With our dataset we here assess the quality of such reconstructions by comparison to measured δ 18 Oseawater (Fig. 8). The Mg/Ca values used here were not corrected for salinity effects, since salinity is the target parameter that has to be reconstructed and is thus treated as unknown. Even though there is a carbonate ion effect on the Mg/Ca in G. ruber albus (Evans et al., 2016;Gray et al., 2018;Kisakürek et al., 290 2008), the measured values were not corrected for this, since this factor is also unknown in paleo-reconstructions. Calculated and measured δ 18 Oseawater do not follow a 1:1 correspondence which could be caused by uncertainties in the different proxy calibrations, analytical uncertainties, heterogeneous element and isotope composition within and between specimens, variability in the location and timing of their calcification and the effect of salinity and pH on Mg/Ca. The lack of a strong correlation between calculated and measured δ 18 Oseawater in our dataset implies that calculating salinity from reconstructed 295 δ 18 Oseawater values will not yield meaningful salinity reconstructions, since reconstructed values for δ 18 Oseawater are not well correlated to in situ measured δ 18 Oseawater. Calculating salinities from δ 18 Oseawater clearly adds much uncertainty due to spatial and temporal variability in the correlation of these two parameters (Conroy et al., 2017;LeGrande and Schmidt, 2006;McConnell et al., 2009).
It is important to note that the scatter in the foraminiferal chemistry can only to a small degree be explained by lateral transport 300 (Fig. 7). This effect may be larger in areas where the environmental conditions vary more strongly over the distance travelled by the foraminifer, and/or in basins where there is simply more lateral transport over the foraminifer's lifetime. In our exercise, the calculated trajectories add only a minor component to the uncertainty in T (often within 0.75 °C; Fig. 7) and salinity (often within 0.25 salinity units).
In our dataset, the uncertainty in salinity estimates based on δ 18 Oseawater is much smaller when using in situ measured 305 temperatures (Fig. 8). The sum of squares of the residuals (difference between reconstructed and measured values) is 9.04 when using temperatures derived from Mg/Ca and δ 18 Oforaminifera, but only 3.56 when using temperatures measured in situ, indicating a better reconstruction.
This shows that the uncertainty or offset in temperatures derived from Mg/Ca, even though the Mg/Ca-temperature relationship is studied relatively extensively for G. ruber albus, is most likely the most limiting step. Even though in our dataset 310 temperatures reconstructed from Mg/Ca deviated less than 2°C from the measured temperature, these small offsets have a large effect on the reconstructed δ 18 Oseawater. It is therefore crucial to choose temperature proxies carefully, use a large enough number of specimens for analysis, be aware about potential effects of lateral particle transport as well as other environmental parameters, and to be conscious about how errors propagate in paleoclimate reconstructions.
Combining all foraminiferal shell chemistry results show that salinities based on δ 18 O and Mg/Ca may under some specific 315 conditions allow calculating past salinity, but the uncertainties in δ 18 Oseawater are large even in a setting with a large salinity gradient such as the Mediterranean Sea. This is in line with predictions of uncertainty based on theoretical considerations (Rohling, 2007). The most limiting step in these calculations is the reconstruction of past temperatures, which should be better than 2 degrees. The development, validation and improvement of other, more direct salinity proxies such as foraminiferal Na/Ca therefore remains crucial for more reliable paleo-salinity reconstructions. 320

Conclusion
Using plankton pump samples from the Mediterranean Sea, we showed that 1) the relationship of Mg/Ca in G. ruber albus and sea water temperature at lower temperatures follows an exponential relationship, therefore the proxy can now also be applied to lower temperature ranges (<18°C) than before, covering almost the entire temperature tolerance range of that 325 species, though sensitivity of the calibration is comparatively low at low temperatures, 2) the combination of foraminiferal δ 18 O and Mg/Ca together with assumptions about δ 18 Oseawater values and δ 18 Oseawater -salinity relationships does not lead to useful reconstructions of seawater salinity 3) foraminiferal Na/Ca correlates well with sea surface salinity and is independent from temperature, making it a potentially valuable tool for salinity reconstructions.

330
Data availability: Upon publication, the data on which this manuscript is based will be available at the 4TU.Centre for Research .
Author contribution: LKD, LdN and GJR designed the study and performed the sample collection. LKD and JGH prepared and processed the samples and the corresponding data. EvS performed the particle backtracking. All authors were involved in 335 data interpretation. LKD drafted the manuscript with contributions from all authors.  the western Mediterranean can be described as δDwater =4.72*δ 18 Owater+1.67 (dark green). The eastern Mediterranean is very similar to the western basin, the relationship between sea water δ 18 O and δD is δDwater =5.19* δ 18 Owater+1.68 (light green) here. Statistically they cannot be told apart. This was determined using a bootstrapping approach that generated 100 slopes and intercepts for both the eastern and the western dataset and subsequent t-testing using the mean and standard deviation of both groups of slopes and intercepts, which resulted in p-values > 0.05. In both areas the relationship is different from the observations made by Gat et al.

495
The relationships can be described using the following equations: δ 18 Oforaminifera=0.28*S-10.59 (adjusted R 2 = 0.42) and δ 18 Oforaminifera =0.95* δ 18 Owater-0.89 (adjusted R 2 = 0.24). The relationship between δ 18 Owater and salinity in this subset of samples is linear and comparable to that of the entire dataset (δ 18 Owater = 0.13*S -3.91; p-value < 0.05, R 2 = 0.37).  510 showed a negative relationship between temperature and foraminiferal Na/Ca in specimens collected from the Red Sea, the addition of new data from the Mediterranean Sea shows clearly that the previously hypothesized negative impact of temperature on Na/Ca is likely an artefact of the negative relationship of temperature and salinity in the Red Sea and that temperature has no significant impact on Na/Ca.    . ruber albus). The relationship shown with dashed lines and cross shaped markers represents values calculated using foraminiferal δ 18 O as well as Mg/Ca as an additional temperature proxy to decouple the effect of temperature and salinity on δ18O. The relationship shown with the continuous lines and circular markers shows the same samples, but instead of using temperature values derived from foraminiferal Mg/Ca ratios, in situ measurements for temperature were used, the relationship can be described as δ 18 Owater_reconstructed=2.62(±0.69)*δ 18 Owater_measured-63.99(±26.11) with an adjusted R 2 of 0.37. The temperature gradient 535 was 2.2°C.