Reconstructing the lithium isotopic composition (δ⁷Li) of seawater from shallow marine carbonate sediments

Records of the lithium isotopic composition of seawater (δ⁷Li_sw), preserved in the lithium isotopic composition of shallow-marine carbonate sediments (δ⁷Li_carb), provide information about the links between silicate weathering and clay formation, the global carbon cycle, and Earth's climate on geologic timescales. However, the record of δ⁷Li_sw values in shallow marine carbonates is complicated by the effects of mineralogy (e.g., calcite vs aragonite) and diagenesis. Here we present measurements of bulk carbonate δ⁷Li values paired with a suite of stable isotope systems (δ^44/40Ca, δ²⁶Mg) and element abundance ratios (Li/(Ca + Mg), Sr/(Ca + Mg), Mg/(Ca + Mg)) in Neogene shallow-marine carbonates from sites in the Bahamas and southwest Australia. The studied sites span a range of depositional and diagenetic settings and exhibit large stratigraphic trends in δ⁷Li values that correlate with mineralogy, δ^44/40Ca values, and/or element abundances. These trends differ from coeval planktonic foraminifera records of δ⁷Li_sw and instead predominantly reflect local processes. We show, using a suite of geochemical analyses and a numerical model of early marine diagenesis, that the observed variability in bulk sediment δ⁷Li values can be quantitatively explained by the effects of mineralogy and diagenetic alteration under both fluid-buffered and sediment-buffered conditions. Using this framework, we show that it is possible to produce robust and accurate ‘snapshots’ of the δ⁷Li value of seawater in the geologic past from shallow-water marine carbonate sediments.