The lithium isotope composition (δ⁷Li) of seawater records important information on the evolution of the carbon and silicon cycles over geologic time. Here we report δ⁷Li values in a suite of Neogene dolostones (23–3.4 Ma, n = 39) from the South China Sea. Over the Neogene, dolostone δ⁷Li values have increased from 24.9‰ to 30.1‰, overlapping previously published foraminifera records and documenting seawater δ⁷Li without applying any isotopic fractionation factor correction. Our results suggest that fabric-retentive dolostones are good proxies for providing seawater δ⁷Li records even after early diagenesis. Importantly, these results provide a high-resolution dolostone record rather than a foraminifera record to confirm the observed ~5‰ increase in Neogene seawater δ⁷Li. We predict that early marine diagenetic dolostones with fabric textures abundant in the Proterozoic may record changes in seawater δ⁷Li and continental weathering, providing great insights into the Precambrian global carbon cycle.

Marine carbonates, precipitating from seawater through inorganic geochemical and biogeochemical processes, are considered to have recorded the seawater geochemical signatures reflecting the marine environmental conditions during their formation. However, they are susceptible to post-depositional diagenetic alteration. The redox conditions and chemical composition of the diagenetic fluid may be different from those of the overlying seawater. Therefore, assessing whether carbonate rocks that have experienced variable diagenesis could still preserve primary seawater geochemistry is a prerequisite before inferring ancient marine environments using geochemical tracers such as the cerium anomaly (Ce/Ce*). Here, we investigate rare earth elements plus yttrium (REY) geochemical features of reefal carbonates from the XK-1 core in the Xisha Islands of the South China Sea. We aim to evaluate whether island carbonates have the potential to preserve reliable primary seawater REY geochemical characteristics after experiencing meteoric diagenesis, marine burial diagenesis, or dolomitization. The results show that even after variable diagenetic alteration, all carbonate samples exhibit seawater-like REY patterns, which are characterized by negative Ce anomalies (Ce/Ce* < 1), distinctively high Y/Ho ratios (>44), and uniform depletion of light rare earth elements (LREE) relative to heavy rare earth elements (HREE) ((Pr/Yb)_N < 1). This suggests that the original seawater REY signatures are retained, regardless of varying degrees of changes in the mineralogical composition, diagenetic fluid composition, and redox state. The unmodifiable REY characteristics in carbonates during diagenesis can be attributed to three aspects: (1) during meteoric diagenesis, the low REY content of meteoric fluids and the short-term reactions between fluids and carbonates make it difficult to significantly alter the REY composition of carbonates; (2) during marine burial diagenesis, the ubiquitous cementation creates a relatively closed environment that facilitates the inheritance of REY signatures from primary carbonates; (3) during dolomitization, the dolomitizing fluids derived from penecontemporaneous seawater would not destroy but rather promote the preservation of the original seawater REY signatures in dolostones. The Ce/Ce* variations indicate that the Xisha carbonates have been deposited under constantly oxic conditions since the Neogene, consistent with paleontological and redox-sensitive element geochemical evidence.

Marine carbonates, including shallow-water carbonates and pelagic carbonates, precipitating from seawater and representing the largest sink of calcium (Ca) and carbon (C) at Earth’s surface, could record seawater chemistry. Therefore, marine carbonates help regulate atmospheric carbon dioxide (CO ₂ ) concentration and eventually Earth’s long-term climate change. Calcium stable isotope geochemistry is a powerful tool to reconstruct paleo-seawater Ca isotopic compositions (δ ^44/40 Ca _sw ) and to constrain the global Ca and C cycles over geological history. Here, we present a Neogene record of δ ^44/40 Ca from pure dolomites of the core XK-1 in the South China Sea. We propose that the dolomites formed in seawater-like fluid-buffered diagenetic environments near the seawater-sediment interface. We demonstrate that XK-1 dolomites display no Ca isotope fractionation from seawater, and hence may serve as good archives of contemporaneous seawater δ ^44/40 Ca. Further, we quantify respective contributions of shallow-water and pelagic carbonates in sequestering carbon over the Neogene, using a Ca isotope mass balance box model. We find that more CO ₂ may have been sequestered by pelagic carbonate burial during global cooling. The enhanced continental weathering, global cooling, sea-level fall, seawater chemical changes, and pelagic carbonate burial are tightly linked.