Reconstructing the lithium isotopic composition (d7Li) of seawater from shallow marine carbonate sediments

Records of the lithium isotopic composition of seawater (delta(7)Lisw), preserved in the lithium isotopic composition of shallow-marine carbonate sediments (delta Li-7(carb)), provide information about the links between silicate weathering and clay formation, the global carbon cycle, and Earth's climate on geologic time-scales. However, the record of delta Li-7(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 delta Li-7 values paired with a suite of stable isotope systems (delta Ca-44/40, delta Mg-26) 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 set-tings and exhibit large stratigraphic trends in delta Li-7 values that correlate with mineralogy, delta Ca-44/40 values, and/or element abundances. These trends differ from coeval planktonic foraminifera records of delta Li-7(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 delta Li-7 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 delta Li-7 value of seawater in the geologic past from shallow-water marine carbonate sediments. (c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

The records of lithium isotopic composition in seawater preserved in shallow-marine carbonate sediments are valuable for studying the links between silicate weathering, clay formation, global carbon cycle, and Earth's climate over geological time. This study presents measurements of lithium isotopic composition in Neogene shallow-marine carbonates and demonstrates the effects of mineralogy and diagenetic alteration on the variability of bulk sediment δ Li-7 values. The findings suggest that robust 'snapshots' of seawater δ Li-7 values in the past can be obtained from shallow-water marine carbonate sediments.