Evaluation of early diagenetic signatures of lithium and stable strontium isotopes in shallow marine carbonate sediments

Abstract

Lithium (δ7Li) and stable strontium isotope (δ88/86Sr) ratios of shallow marine carbonate sediments serve as valuable proxies for reconstructing both seawater δ7Li and δ88/86Sr behavior and diverse geological processes. However, interpretations of these isotope values can be complicated by diagenesis. We conducted mineralogical, petrographic, and geochemical analyses of different sediment size fractions from two cores collected from distinct shallow marine depositional settings in the Bahamas, with the aim of evaluating the impact of early diagenetic processes on shallow water carbonate δ7Li and δ88/86Sr values. We found that the finest grain size fraction (i.e., < 63 μm or mud) is particularly vulnerable to recrystallization, leading to notable alteration of both δ7Li and δ88/86Sr values of mud-sized sediments relative to bulk sediment composition and expected fractionations from seawater. We interpret these data to reflect greater extents of recrystallization and secondary carbonate formation in the mud-sized fraction, linked to higher surface area to volume ratios. Although the alteration of the mud fraction appears to have minimally impacted bulk-sediment δ88/86Sr or δ7Li values, these findings expand the framework for interpreting the δ7Li and δ88/86Sr values of carbonate sediments.