Environmental and vital effects on lithium isotopes and trace element ratios in biogenic carbonates: Insights from an estuarine salinity gradient

Isotope and element ratios in the carbonate skeletons of calcifying marine organisms can be used to reconstruct past environments and understand physiological processes occurring before calcification. The Li isotope composition (δ⁷Li) of biogenic carbonates is thought to reflect the δ⁷Li values of the water in which the calcifying organisms grew, allowing for the use of carbonate fossils to estimate δ⁷Li of ancient seawater, which is known to record past silicate rock weathering congruency. To determine whether δ⁷Li in biogenic carbonates is truly a faithful archive of past seawater, it is crucial to elucidate any environmental or biological factors that could influence carbonate δ⁷Li and cause deviations from seawater δ⁷Li. Here we show that δ⁷Li in the low-Mg calcite shell of the eastern oyster, Crassostrea virginica, tracks aqueous δ⁷Li with an offset of +1.9 (Formula presented) 1.5‰ and is not influenced by salinity in an estuarine environment. This contrasts with inorganic calcite, in which δ⁷Li is known to be lower than seawater values, suggesting a prevalent vital effect in oyster calcite. We ascribe this vital effect to a combination of possible physiological isotope fractionation mechanisms, including preferential passive removal of ⁶Li from the extrapallial fluid (EPF) and preferential transport of ⁶Li into the EPF via a Na⁺/H⁺ pump, both of which have been discussed by previous studies. Overall, we conclude that understanding the mechanism(s) driving the vital effect in biogenic calcite is crucial to establishing δ⁷Li as a faithful paleo-archive and a tool for reconstructing past weathering and climate.

Duke Scholars

Author Michael Kipp Earth and Climate Sciences

Author Thomas F. Schultz Marine Science and Conservation

Author Joshua Samuel Osterberg Marine Science and Conservation