Tracking Anoxia in Ancient Oceans: Potential and Limitations of Paleoredox Proxies in Carbonate Rocks

Abstract

Oxygenation and deoxygenation of the oceans strongly influences organic carbon burial, habitability for marine biota, and biogeochemical cycling of nutrients and critical redox-sensitive elements. The spatial and temporal patterns of redox conditions in the oceans are therefore relevant to past hydrocarbon source rock formation and future environmental conditions. Methods for reconstructing anoxia through Earth history include various paleontological, sedimentological, biomarker, and inorganic geochemical indicators. Of these, inorganic geochemical proxies—a broad category that includes concentrations of redox-sensitive elements and their isotopic ratios—offer the potential to track redox conditions across a range of temporal and spatial scales. Distinct from the other indicators mentioned above, inorganic geochemical records are integrated over longer time scales and can represent global redox conditions. 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 globally.

In this talk, we will present an overview of proxies in carbonates that can capture local and global paleoredox conditions, including carbonate-associated sulfur isotopes (δ³⁴S_CAS), ‘stable’ uranium isotopes (δ²³⁸U), chromium isotopes (δ⁵³Cr), iodine content (I/Ca), and the cerium anomaly (Ce*). This specifically includes literature compilation and comparison of proxy trends in the Phanerozoic carbonate rock record, as well as discrete case studies of individual events. Differences in the residence time, environmental specificities, sensitivities to redox conditions, and biogeochemical behaviors among these approaches define the varying utilities of the tracers and highlight the potential of multi-proxy studies. Moreover, the fidelity of these paleoredox records in tracking the degree of oxygenation depends on diagenetic alteration of carbonate sediments and rocks. To evaluate the role of diagenesis, we present (1) a simple numerical model for carbonate geochemical alteration that compares the susceptibility of each proxy to different diagenetic conditions, and (2) a review of diagenetic impacts from modern examples where the proxies are measured in the same carbonate rocks. In combination, this work will highlight the opportunities and challenges in using carbonate geochemistry in tracking anoxia and the carbon cycle.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019