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

Abstract

Seawater Mg:Ca and Sr:Ca ratios are biogeochemical parameters reflecting the Earth–ocean–atmosphere dynamic exchange of elements. The ratios’ dependence on the environment and organisms' biology facilitates their application in marine sciences. Here, we present a measured single-laboratory dataset, combined with previous data, to test the assumption of limited seawater Mg:Ca and Sr:Ca variability across marine environments globally. High variability was found in open-ocean upwelling and polar regions, shelves/neritic and river-influenced areas, where seawater Mg:Ca and Sr:Ca ratios range from ∼4.40 to 6.40 mmol:mol and ∼6.95 to 9.80 mmol:mol, respectively. Open-ocean seawater Mg:Ca is semiconservative (∼4.90 to 5.30 mol:mol), while Sr:Ca is more variable and nonconservative (∼7.70 to 8.80 mmol:mol); both ratios are nonconservative in coastal seas. Further, the Ca, Mg, and Sr elemental fluxes are connected to large total alkalinity deviations from International Association for the Physical Sciences of the Oceans (IAPSO) standard values. Because there is significant modern seawater Mg:Ca and Sr:Ca ratios variability across marine environments we cannot absolutely assume that fossil archives using taxa-specific proxies reflect true global seawater chemistry but rather taxa- and process-specific ecosystem variations, reflecting regional conditions. This variability could reconcile secular seawater Mg:Ca and Sr:Ca ratio reconstructions using different taxa and techniques by assuming an error of 1 to 1.50 mol:mol, and 1 to 1.90 mmol:mol, respectively. The modern ratios’ variability is similar to the reconstructed rise over 20 Ma (Neogene Period), nurturing the question of seminonconservative behavior of Ca, Mg, and Sr over modern Earth geological history with an overlooked environmental effect.

An ICP‐AES method for routine high‐precision measurement of seawater Sr/Ca ratios to validate coral paleothermometry calibrations

“…This is effective but more laborious and results in a substantially increased error bar (Fig. 3), consistent with the findings of Lebrato et al (2020), who used a dilution factor of 50 (cf. Fig.…”

“…A partial solution that cleverly combines our approach to the blank problem with that of Watanabe et al (2020) was reported recently by Lebrato et al (2020). These authors correct their calibration standards for Ca using a matrix‐matched blank similar to ours, rightly assuming that the Ca contribution from the Sr standard is negligible since its concentration is so much lower.…”

“…A convenient method for measuring seawater Sr/Ca ratios with a precision matching that of common coral aragonite analyses (< 0.2%) is ostensibly needed to identify and possibly avoid locations where this issue may arise. A recently developed ICP‐AES method (Lebrato et al 2020) does not yet meet this goal, as it reports a short‐term precision for seawater Sr/Ca of 0.4–0.8%.…”

“…In their ICP‐AES method, Lebrato et al (2020) chose to normalize their measured seawater Sr/Ca and Mg/Ca ratios to IAPSO Standard Seawater. This seawater standard is certified for conductivity and forms the basis of the Practical Salinity Scale (Millero et al 2008).…”

Carbon isotope evidence for the global physiology of Proterozoic cyanobacteria

“…The Our middle Proterozoic ε P distribution encompasses a variety of marine environments and atmospheric conditions over the course of 800 million years, and therefore, p CO 2 and dissolved CO 2 could have exhibited wide variation in time and space over this interval. Estimates of p CO 2 over the past ≈70 million years, for example, span a relative range of ≈150-fold (60 to 8900 ppm by volume; https://www.paleo-co2.org ), while dissolved CO 2 in the modern ocean varies over a relative range of ≈370-fold [8 to 2900 μmol kg −1 ( 43 )]. If atmospheric or marine CO 2 in the middle Proterozoic varied similarly then planktic cyanobacteria with β-carboxysomes could produce the full range of middle Proterozoic ε P values because of the strong dependence of their net carbon isotope fractionation on CO 2 concentrations ( Fig.…”