A 13 million-year record of Li isotope compositions in island carbonates: Constraints on bulk inorganic carbonate as a global seawater Li isotope archive

Lithium isotope (d7Li) variations in marine carbonate sediments and rocks have been widely used to reconstruct global seawater Li isotope compositions and then trace the paleo-weathering processes. However, there are still debates on whether ancient carbonates can faithfully document original seawater d7Li signals, as d7Li values of shallow-water carbonates are tightly related to carbonate mineralogy and diagenetic alteration. In this study, we present high-resolution d7Li, trace element and carbonate mineralogy data of shallow-water carbonate deposits from two shallow drillcores (Jiuzhang A and B) and a deep drillcore (XK-1) in the South China Sea. We compare these new d7Li data to the ca. 13 Myr history of seawater d7Li evolution from the middle Miocene to Pleistocene, in order to better constrain the effects of carbonate mineralogy and early diagenesis on d7Li values of bulk inorganic carbonates. We observe that the d7Li values of primary carbonate deposits in Jiuzhang A, B and the uppermost XK-1 drillcores (24.6% +/- 1.6%, n = 37, 1r) are significantly lower than modern seawater d7Li values (D7Liprimary-seawater = --6.0%). In contrast, marine diagenetic carbonates in the XK-1 drillcore exhibit d7Li values of 28.7% +/- 0.7% (n = 54, 1r), approaching coeval seawater values (D7Limarine diagenetic-seawater = -2.0%) and much higher than those of primary carbonate deposits. Meteoric diagenetic carbonates in the XK-1 drillcore show d7Li values of 22.4% +/- 1.6% (n = 46, 1r), close to that of primary carbonate deposits in Jiuzhang A, B and the uppermost XK-1 drillcores, but significantly lower than coeval seawater d7Li values (D7Limeteoric diagenetic-seawater = -5.0% to -9 parts per thousand for modern and Miocene seawater). Such d7Li variations in shallow-water carbonates are attributed to variations in carbonate mineralogy (i.e., aragonite, high-Mg calcite and low-Mg calcite) and diagenetic regimes (fluid- or sediment-buffered conditions of marine and meteoric diagenesis). By comparing d7Li data from the South China Sea to those from the Bahamas, we suggest strongly fluid-buffered conditions for Li isotopes during marine diagenesis, resulting in d7Li values of marine limestones and dolostones approaching ambient seawater d7Li signals. In contrast, meteoric diagenetic carbonates, likely marked by sediment-buffered conditions, may inherit the original d7Li signals of primary carbonate deposits. Hence, full considerations of the carbonate mineralogy and diagenesis facilitate a better use of bulk inorganic carbonate-archived d7Li to reconstruct paleo-weathering evolution in deep time lacking the skeletal fossil records.(c) 2023 Elsevier Ltd. All rights reserved.