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

Abstract

Records of the lithium isotopic composition of seawater (δ7Lisw), preserved in the lithium isotopic composition of shallow-marine carbonate sediments (δ7Licarb), 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 δ7Lisw 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 (δ44/40Ca, 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, δ44/40Ca values, and/or element abundances. These trends differ from coeval planktonic foraminifera records of δ7Lisw 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.