Constraints on the continental weathering intensity through the Permian using lithium isotopes of well-preserved brachiopod shells

Abstract

Lithium isotopic compositions (δ⁷Li) in marine carbonates provide a powerful means to track continental weathering intensity through geological history and promote our understanding of how and why Earth remained habitable. However, δ⁷Li values in ancient carbonates can be affected by factors such as primary carbonate mineralogy (e.g., aragonite, high-Mg calcite, low-Mg calcite) and post-depositional diagenesis, which may alter their primary signals. Modern articulate brachiopod shells have shown a constant δ⁷Li offset compared to contemporary seawater, suggesting that fossil brachiopod shells could serve as robust archives for this geochemical proxy due to their diagenetically resistant low-Mg calcite composition. Despite this potential, few studies have explored this topic. In this study, we analyze and compare the geochemical compositions of fossil brachiopod shells and their enclosing carbonate rocks from Permian strata in South China to investigate whether brachiopod shells can reliably preserve the δ⁷Li values of ancient seawater and, if they do, to obtain new constraints on changes in the continental weathering regimes through the Permian. We used several diagenetic screening methods, including scanning electron microscopy, cathodoluminescence microscopy, and elemental concentrations to detect potential diagenetic alteration. Our results indicate that 32 out of 49 shells preserve primary δ⁷Li values. Notably, we observed that δ⁷Li offsets between brachiopod shells and their enclosing carbonate rocks vary across the studied sections, likely due to diagenetic alteration of the bulk carbonates. Using the δ⁷Li values from well-preserved brachiopod shells, we reconstructed δ⁷Li variations of Permian seawater. We identified three distinct intervals: (1) extremely low δ⁷Li_sw values (as low as 6.7 ± 2.9‰, n = 12, 1sd) during the Asselian and Sakmarian stages, (2) high δ⁷Li_sw values (32.6 ± 4.5‰, n = 3, 1sd) similar to modern seawater during the Kungurian stage, and (3) relatively low δ⁷Li_sw values (13.6 ± 3.9‰, n = 14, 1sd) from the Capitanian to the Changhsingian stages. These significant changes in δ⁷Li values of Permian seawater indicate dramatic changes in continental weathering regimes during the Permian, possibly linked to major climate changes, such as the termination of the Late Paleozoic Ice Age and warming associated with the emplacement of the large igneous provinces.