A NEW MODEL-DATA COMPARISON: PROMISES AND PITFALLS OF PALEOREDOX PROXIES IN CARBONATE ROCKS (Invited Presentation)

Carbonate rocks and sediments are a popular archive for geochemical paleoredox proxies because they have been deposited nearly continuously since 3.8 billion years ago. Because their precipitation can be directly linked to seawater chemistry, their geochemistry can record time intervals of elevated organic carbon accumulation and/or heterogeneous redox conditions on both global and spatial scales. Moreover, the fidelity of these paleoredox records—which include carbonate-associated sulfur isotopes (δ³⁴S_CAS), ‘stable’ uranium isotopes (δ²³⁸U), chromium isotopes (δ⁵³Cr), iodine content (I/Ca), and the cerium anomaly (Ce/Ce*)—in tracking the degree of oxygenation depends on assessing the post-depositional, or diagenetic, alteration of the studied carbonate sediments and rocks. Here, we present (1) insights from simple numerical models of carbonate geochemical alteration for paleoredox proxies in carbonates, and (2) a model-data comparison to a compilation of Late Neogene proxy data measured in the same carbonate samples. Carbonates in fluid-buffered environments where seawater is the diagenetic fluid exhibit muted alteration trajectories, whereas meteoric or anoxic seawater fluids can exhibit distinctive trends away from the carbonates’ original composition. Differences in the residence time, fluid geochemical compositions, sensitivities to redox conditions, and biogeochemical behaviors among these tracers define the varying utilities of these proxies and highlight the potential of multi-proxy studies to reveal the relative roles of seawater and diagenetic fluids. Our results present an updated framework for evaluating carbonate geochemical tracers for tracking ancient anoxia and the carbon cycle.