Global variability in seawater Mg:Ca and Sr:Ca ratios in the modern ocean

The substantial spatial horizontal/vertical deviations from homogeneity, and the natural variability in modern seawater Mg:Ca and Sr:Ca ratios presented in this study, are in line with several local and regional studies that document deviations from the accepted average oceanic values (Mg:Ca ∼ 5.10 mol:mol, Sr:Ca ∼ 8.50 mmol:mol). Higher deviations from the means are expected in areas with active processes along the dissolution and calcification lines (Fig. 4), as well as near hydrothermal vents, regions of sea-ice formation, anoxic basins, coastal areas, and river mouths (12). Nevertheless, paleo-proxy model calibrations and experimental laboratory work tend to assume conservative behavior, homogeneity, and steady state of modern ocean seawater Mg:Ca and Sr:Ca ratios following the MORT concept. Our results suggest that modern ocean variability (Figs. 3 and 4) should be considered, and that caution should be taken when reconstructing past seawater conditions, paying attention to the ecosystem from which the sample is being recovered. The inclusion of confidence intervals based on modern ocean elemental ratios’ variability (Fig. 5 and SI Appendix, Fig. S2) could offer a valid tool to communicate past seawater Mg:Ca and Sr:Ca ratios’ uncertainty. Yet, it is true that most reconstructions (7) reflect open-ocean bottom water masses appropriately, with a possible extrapolation to surface waters if corrections for biological activity are considered. Our results also support published discussions (4) suggesting that there is a need in the scientific community to have a detailed and better understanding of the variability in seawater Mg:Ca and Sr:Ca ratios from the past, and thus more detailed research needs to be conducted at the ecosystem and habitat level (Figs. 3 and 4). The assumption of homogeneity in elemental ratios as currently used in marine sciences is mostly applicable to open-ocean conditions (but not in all cases), additionally to the deep ocean, and, in particular, to bottom water masses or at MOR flanks. Yet, many samples used in, for example, paleo-reconstruction studies have been obtained in shelves and marginal seas aiming to reconstruct open ocean or extrapolating results to the global ocean (SI Appendix, Fig. S2). These studies are mostly focused on temporal variability around a fixed location, while our dataset sheds light on the spatial ratios’ variability at a fixed point in time, to then consider the likely variability that occurred in the past. The open and deep ocean represent the majority of the oceanic water by volume, but many key biogeochemical processes impacting the ratios directly or indirectly take place in continental margins, including slope, shelf, and neritic areas, where, for example, large TA fluxes and consumption occur, coupled to anoxia events, nutrient loads, freshwater mixing, and ice melting. Previously reported results on seawater Mg:Ca and Sr:Ca ratio reconstruction should strictly correspond to the water masses and the biogeographical region where the samples formed or the processes took place (7, 22, 23). The further we go back in time, the less reliable measurements are, and fewer data are available, and thus it seems optimistic to extrapolate these data to the various past ocean regions/areas and ecosystems as a whole. This becomes obvious when considering that the variability in modern oceanic seawater Mg:Ca and Sr:Ca ratios ranges more than 2 units, which is large compared to the low variability needed for paleo reconstructions, depending on the reconstructed ecosystem and habitat of interest. It is reasonable to assume that a similar variability occurred throughout the past 600 My depending on the prevailing conditions and processes (Fig. 5), which could be one of the reasons why different ratio reconstructions, based on different taxa, techniques, and environments, do not agree on the end result/secular trend (discussed below).

It is of critical importance to understand at a global level what is causing the variability in the ratios, what the implications are, and what can be learned from a deeper understanding of the underlying mechanisms to improve their use in marine sciences. It has been previously reported that anomalies in seawater Ca concentrations are due to removal from the upper ocean via calcification, enrichment from carbonate dissolution during sinking, dissolution in surface layers in regions of low pH, associated with terrestrial inputs, and groundwater Ca fluxes (also observed in this study; Figs. 3 and 4). How these essential processes drive the elemental concentrations and ratios can be observed in the cloud dispersion around the IAPSO reference point along the TA gradient, considering all data points together (Fig. 3). The TA is of crucial importance in this study because it represents the charge balance in seawater and the buffer capacity to resist changes in pH. Thus, it has a strong relationship with salinity, being indirectly related to the seawater elemental composition. Dissolution of carbonates forms Ca²⁺ and CO₃²⁻; Ca⁺² does not influence TA, but CO₃²⁻ increases TA by 2 units, and thereby shows an indirect relationship between seawater Ca²⁺ concentrations and TA (Fig. 3A). For example, in tropical regions of the Indian Ocean, Ca anomalies arise due to direct consumption by coral reefs during biogenic calcification (25), but it is unknown if the anomalies prevail over time or if they remain transient in nature. This process was observed in our data from the coral reefs in the Mozambique Channel, with anomalies driving a higher seawater Mg:Ca ratio despite the reefs being located in semi open ocean areas with continous water exchange (Fig. 4 A and B, light blue dots). Additionally, the cycling of Ca due to dissolution and precipitation of calcium carbonates modifies the TA in the surrounding seawater within coral reef lagoons (26, 27) but also in the open ocean and coastal environments via the carbonate/TA relation (Fig. 3A). The TA and DIC relation, in particular with seawater Mg:Ca, was magnified in continental margin upwelling areas, which in this dataset correspond to the east Pacific in the California Current system (NOAA cruises) (Figs. 3 and 4). In this particular case, seawater has a higher acidity (higher DIC and CO₂, lower pH), which in turn dissolves carbonates faster due to a shallower saturation horizon for calcite and aragonite. This results in very high seawater Mg:Ca (> 6 mol:mol) and Sr:Ca (> 9 mmol:mol) ratios with respect to the reference IAPSO standard, with a few exceptions (Fig. 4 A and B). As a general rule, coastal seas water masses in the Indian, Pacific, and Atlantic Oceans exhibited higher seawater Mg:Ca and Sr:Ca ratios than open ocean regions, driven by a combination of biological and chemical processes (Fig. 4C).

Regions of riverine and glacial meltwater input influence TA concentrations as well as seawater elemental cycling, acting as competing sources and sinks. Freshwater discharges act to dilute seawater carbonate and bicarbonate (the principal components of seawater TA), lowering salinity and thus reducing TA. However, these water sources supply dissolved minerals that increase TA and indirectly shift seawater Mg:Ca and Sr:Ca ratios, as seen in this dataset, for example in the river discharge and glacial melting samples around the Atlantic Ocean continental margins (Fig. 3). In polar regions, carbon cycling in sea-ice brines leads to the abiotic precipitation of carbonates in the ice, which are released to the upper ocean during ice melt, shifting also seawater Mg:Ca and Sr:Ca ratios. This, at the local/regional level, is an additional TA source when the precipitated carbonate is also dissolved. TA is semiconservative and can be used as a tracer for ocean circulation and mixing as different water masses have different TA signatures as well as seawater Mg:Ca and Sr:Ca ratios (assuming semiconservative behavior in the open ocean). Deep and bottom waters are typically TA-rich from remineralization and dissolution of carbonate/organic sinking particles and sediment resuspension. The Ca concentration in seawater is also modified via the production and dissolution of biogenic carbonates from several calcareous organisms, for example foraminifera tests, pteropod shells, and coccoliths (28) that mainly inhabit open-ocean water masses and upwelling regions. This induces further variability in seawater Mg:Ca and Sr:Ca ratios in spatiotemporal terms (Figs. 3 and 4). The fundamental reasons for Mg and Sr anomalies at the elemental level are more complex. Their variability is linked to profiles of phosphate (PO₄³⁻) and nitrate (NO₃⁻) that reflect biological consumption patterns (3), mostly in the open ocean and upwelling regions (e.g., in meridional transects in the Atlantic and Pacific Ocean, as well as the California Current), accounting for up to 5% of the variation in the seawater Mg:Ca and Sr:Ca ratios (29). In continental margins, the ratios are mainly governed by regional upwelling and mixing of different water masses with riverine and groundwater/lacustrine inputs, which usually exhibit different elemental concentrations than the surface/surrounding waters (30) (SI Appendix, Table S1 and Figs. 1 and 3C). Our results indicate greater variability (above 5%) for seawater Sr:Ca compared to previous work (8) (2 to 3% change), which is attributed to the inclusion of more data encompassing a wider spatial scale. Open-ocean surface seawater Sr:Ca ratios are overall more variable than Mg:Ca ratios. This may be explained by the abundance of acantharians and Sr use, in contrast to the relatively small amount of Mg that is used by coccolithophores and foraminifera during biogenic calcification (31). Frequently, the maximum density of acantharians in the oceans is found between 50- and 200-m depth (32). This accounts for changes in seawater Sr composition in the upper ocean since acantharians utilize a substantial fraction of Sr to produce their tests. Sinking tests of upper ocean organisms provide a continuous vertical flux of Mg and Sr particulates out of the surface ocean, which dissolve while sinking, enriching the pool of dissolved Mg and Sr in deeper waters. If these enriched deep waters are upwelled, anomalies in the ratios occur (e.g., California Current; Fig. 3). Similarly, aeolian inputs induce variations in dissolved mineral ion fluxes (33). Mg can be enriched in dust of African, Asian, or North American origin, being deposited in all oceans by between 0.5 and 2.3 g/100 g (34). Also abundant in dust are other metals, specifically iron (Fe-compounds), which can fertilize phytoplankton blooms and enhance further biological uptake of Ca, Mg, and Sr, shifting the seawater ratios. At large, the trade-off between input and consumption processes affects the final seawater Mg:Ca and Sr:Ca ratios. Overall, Ca, Mg, and Sr are taken up during biological processes in the surface ocean and then dissolve as they sink through the water column. Below 1,000 m, ratio changes are a function of water mass circulation and deep carbonate dissolution, which is driven by the saturation horizons.

Processes related to the removal and addition of seawater Ca, Mg, and Sr, as well as the relation to TA, work at different spatiotemporal scales, including the ecosystem and habitat levels (Figs. 3 and 4). At large, riverine discharge in continental shelves, hydrothermalism, and MORs in the deep ocean dominate elemental (Ca, Mg, and Sr) fluxes. In certain ecosystems such as coral reefs or calcifying planktonic communities, organisms may significantly influence these fluxes and the ratios via calcification, dissolution, presence of calcite/aragonite/Mg-calcite, and the relation with TA (via hydrothermalism or neritic interactions, for example mangroves and seagrass meadows fluxes). The relation of TA to the elements is particularly important because TA fluxes are indirectly balanced via Ca increase owing to the CO₃²⁻ and also Mg from rock weathering (35). An increased sink flux of Ca during the last 30 My rather than changes in Mg and Sr is viewed as the force driving correlated increases of seawater Mg:Ca and Sr:Ca ratios (Fig. 3). Yet, this does not prevent local and regional processes from impacting the ratios on shorter time scales at the ecosystem and habitat levels.

While the importance of organisms’ carbonate Mg:Ca and Sr:Ca composition has been previously noted, less considered are the effects of in situ modern seawater Mg:Ca and Sr:Ca ratios on the calcification process and carbonate composition (35, 36). In coccoliths, a change of seawater Mg:Ca values from 5.10 to 5.90 mol:mol changes calcite Mg:Ca from 0.05 to 0.25 mmol:mol, which can override paleothermometry reconstructions based on them, or ancient seawater proxies on the assumption of constant seawater ratios (17). Similarly, changes in seawater Sr:Ca from 4.00 to 10.00 mmol:mol increases the calcite Sr:Ca from 1.3 to 3.5 mmol:mol, depending on the seawater ratio (36–38), which challenges the application as paleo-productivity proxy (39, 40), because the modern variability exceeds the reconstructed one. This demonstrates the necessity for routinely incorporating a normalization of carbonate elemental ratios to seawater elemental ratios, which corresponds to the partitioning coefficient (comparison of the solubilities of two solutes in a solution) for each element (D_Mg and D_Sr) (3, 5). This is especially relevant for the comparison of carbonate samples from different ocean regions, ecosystems, habitats, and taxa, which is often needed in global studies.

The response of carbonate Mg:Ca and Sr:Ca ratios of benthic organisms to changes in seawater Mg:Ca and Sr:Ca ratios was recently assessed (3) (Fig. 5) to study taxa-specific calcification processes during Earth geological times. In experimental work, a source of error can be introduced when an organism with a fast generation/reproduction time is isolated from an estuary, a neritic/subtidal setting, shelf, or oceanic environment, and is then placed into experimental incubation media that use the locally available seawater or a source that is not the organism’s original habitat. Thus, the newly precipitated carbonate structures may considerably differ from the previously formed carbonate that was precipitated under the seawater source of differing elemental composition. This effect may significantly alter the bulk carbonate Mg:Ca and Sr:Ca ratios. Organisms inhabit regions with seawater Mg:Ca and Sr:Ca ratios that sometimes are different from those assumed in empirical work and are used for extrapolation in paleo reconstructions or, for example, experiments to test the effects of ocean acidification. Hence, when other variables are modified (e.g., temperature or CO₂) results may also incorporate a response to an a priori believed conservative property like seawater Mg:Ca and Sr:Ca ratios, confounding the meaning of the response. For many organisms, changes within the observed range of modern seawater ratios impact physiology, mineralogy, reproduction, skeleton stiffness, growth rate, and productivity (3), which, in turn can affect the skeletal carbonate ratios. Our dataset suggests that the application of constant seawater elemental ratios needs to be carefully considered and could be a confounding factor in experimental work, if the regional and ecosystem dimensions are not considered.

Another challenge is that modern interpretations of past seawater conditions rely on the fossil carbonate record’s being related to a homogeneous seawater composition (23, 24, 41), but they do not account for the environmental conditions and variability experienced by those organisms. Factors such as a life history in varying seawater ratios, or organisms living in ancient coastal regions or estuaries with seawater elemental ratios different from the assumed open-ocean conditions, are two examples where fossil signals could be potentially misinterpreted. For instance, the projected variability of calcite Mg:Ca and Sr:Ca ratios in many organisms over the range of modern ocean seawater ratios is between 1 and 2 units, which is equivalent to a temperature range greater than 5 °C for foraminifera (3) (Fig. 5). There are numerous examples of the use of calcite Mg:Ca to reconstruct paleo-environmental parameters. Foraminifera calcite Mg:Ca and Sr:Ca ratios have been calibrated vs. temperature using linear equations (42) and isotopic signatures (43), as well as molluscs’ aragonite Mg:Ca ratios vs. temperature, also using linear equations (44). Echinoderms have been used to reconstruct seawater Mg:Ca ratios based on fossilized ossicles, using an exponential equation with respect to the skeleton mole percent MgCO₃ and the calcite Mg:Ca ratio (23). In a recent review (3), numerous benthic and planktonic organisms’ calcite Mg:Ca ratios were compared to seawater Mg:Ca to discuss the use of their skeletons to reconstruct past seawater conditions, focusing on the fractionation coefficient and temperature. Other techniques use calcium carbonate vein (CCV) fluids plotted against temperature to reconstruct seawater Mg:Ca and Sr:Ca ratios. The seawater ratios from which the fluids originated are determined from the fluid evolution trend. Finally, the CCV age is determined from the isotopic curves (7). In these cases, the reconstructed seawater does reflect regional and local conditions, and extrapolations were always transferred to the global ocean. Elemental ratios from foraminifera represent open-ocean water-column and seabed conditions, those from mollusks reflect continental shelf and neritic areas, those from echinoderms (tropical) reflect continental shelf and neritic areas, except if there is evidence that they were deep-sea mollusks/echinoderms, CCVs represent open ocean, MORs, and deep seabed water masses. In the case of the echinoderm’s reconstruction, ossicles were treated individually, and then a pooled number of tropical species were used to represent the modern ocean, giving a seawater Mg:Ca range. Our global seawater Mg:Ca range down to 4,000 m varies from 4.80 to 5.60 mol:mol, which surprisingly agrees with the range in seawater Mg:Ca ratios determined using tropical echinoderms ossicles, where it was concluded that modern seawater Mg:Ca ratios range from 4.00 to 6.40 mol:mol (23). This is in total agreement with our measurements (Figs. 4 and 5) and in line with our suggestion that even if the averaged reconstructed seawater ratios are reliable they must include an error similar to that estimated from the modern ocean ratios and can only act as an approximation. In a recent study, model equations on foraminiferal calcite were adjusted to improve seawater Mg:Ca reconstructions, and it was proposed that a seawater Mg:Ca error of 0.5 mol:mol could be applied for the last 60 My. This correction would mostly agree with other paleo reconstructions based on different organisms and techniques (24). Based on our dataset, we support this error correction and further propose that a seawater Mg:Ca ratio confidence interval of between 1 and 1.5 mol:mol may be more appropriate to include most environmental and ecosystem-based variability. For the seawater Sr:Ca ratio, this goes up to 1.9 mmol:mol (Fig. 5) to account for possible environmental variability.

The notion that different procedures to reconstruct seawater ratios resulted in secular variability for different ecosystems seems logical when comparing the various methods and marine taxa, which consequently reflect the inhabited environment (Fig. 5). Published studies and equations use benthic/planktonic foraminifera, bivalves and belemnites, turritellid snails, CCVs, halite fluid inclusions, and echinoderm ossicles to reconstruct seawater ratios. There are also reliable linear equations for echinoid spines and plates, worm tubes, crabs, Halimeda algae, coralline algae, coccolithophores, and scleractinian corals (3). Considering the life history, ecological, and geochemical differences associated with the taxa and the reconstruction techniques, we suggest that there is nothing substantially erroneous with the way equations and proxies have been used, except in the assumption of homogeneous seawater ratios in all ecosystems and habitats that may lead to oversimplified estimations. If the reconstruction of secular changes requires assumptions of homogenous global elemental distribution, it is necessary to identify the potential source of error related to the environment/ecosystem/habitat being reconstructed. A separation of the ratios gives a range that does not sustain the notion of constant ratios, except in the open ocean, and even there not in all areas. In the open-ocean environment, paleothermometry and paleoproductivity proxies are robust because seawater Mg:Ca and Sr:Ca ratios are well constrained with depth, ranging from 4.80 to 5.30 mol:mol and 7.70 to 8.70 mmol:mol (Fig. 4), respectively, with some exceptions. However, if a research study requires low variability/constant ratios, which does not happen in the modern ocean and likely did not happen in the ocean over Earth geological times (Fig. 5), the results need to clearly indicate error ranges and potential environmental variability associated with the organisms and method used for the reconstruction. The narrowest variability we detected, even in the open and deep ocean, is seawater Mg:Ca from 4.95 to 5.25 mol:mol and Sr:Ca 7.90 to 8.50 mmol:mol. This means that even the open ocean experiences major deviations from homogeneity. Changes in the ratios in the modern ocean are similar to the reconstructed Neogene rise (20 Ma), and well above Quaternary changes (2.5 Ma), because they could be reflecting environmental variability. This suggests that Neogene records and samples could also reflect to a certain extent local/regional conditions, being an artifact of depositional environments, with variable seawater ratios, similar to the modern ocean. The problem is not related to the marine archives themselves but to the proxy being used and how it is interpreted in relation to the reconstructed environment. For example, some of the most reliable seawater ratio reconstruction records are derived from halite fluid inclusions, which are formed under abnormal marine conditions. This begs the question whether they reflect global oceanic conditions or only those of the open ocean; we suggest they reflect the nearby environment from where they were isolated. The question remains as to how spatial variability in seawater ratios compares to temporal variability of the ratios within individual ecosystems and habitats. This research subject needs urgent attention, in particular in continental margins, where unknowns on how different taxa, ecosystems, and environmental variability may play a role in proxy estimations.

The general assumption of conservative and constant seawater ratios in the fossil record and sediment samples (45) comes into question, especially when considering that the variability of seawater Mg:Ca and Sr:Ca ratios in the modern ocean is in the same range as the reconstructed rise of these ratios over 20 Ma in the Neogene Period. Consequently, it should be acknowledged that in Earth geological time periods the oceanic seawater Mg:Ca and Sr:Ca ratios also exhibited larger variability than commonly presumed. We suggest that different seawater Mg:Ca and Sr:Ca reconstructions based on different taxa, techniques, and abiotic processes, which have given rise to different results, do not necessarily contradict each other but show potential environmental paleo variability, with potential ranges similar to those of the observations made in the modern ocean.