SULFUR-CYCLING IN THE DARRIWILIAN (MIDDLE ORDOVICIAN) OF ARGENTINA: IMPLICATIONS FOR OCEAN CIRCULATION AND THE REDOX EVOLUTION OF THE ORDOVICIAN OCEAN

Peak diversity during the Great Ordovician Biodiversification Event (GOBE) is reached in the Darriwilian. Prior to the GOBE, sluggish ocean circulation may have contributed to the formation of deep-water anoxia and the development of stratified water bodies. A growing body of evidence suggests low marine sulfate (<2-10 mM) and the persistence of euxinic conditions during this time, yet availability of biospheric oxygen is vital for the development and diversification of advanced metazoans. Therefore, understanding the redox conditions of the Middle Ordovician is critical to unravelling the relationship between the redox state of the oceans and animal evolution.

Dramatic changes to the marine sulfur cycle are suggested by the marine S-isotope record from Argentina, Newfoundland, and China, derived from carbonate-associated sulfate (CAS) and pyrite (Kah et al., 2016). Changes within the sulfur isotope system are best explained by a dual reservoir model for S-cycling, that records the reduction of ocean stratification and oxidation of deep-water in the Darriwilian, potentially associated with increased ocean circulation and delivery of cool, oxygenated water to deeper portions of the water column.

Previous studies in Argentina have focused on the Las Chacritas and Las Aguaditas Formations (Thompson and Kah, 2012), which preserve carbonate and interbedded shale on a gradually deepening slope. Here we expand the S-isotopic record of the Middle Ordovician of Argentina with addition of high-resolution δ³⁴S_CAS and δ³⁴S_PY from the Gualcamayo Formation. Black shale of the Gualcamayo Formation reflect deposition within an anoxic water body down slope from the Las Chacritas and Las Aguaditas formations, and thus represent an opportunity to investigate coeval shallow to deep depositional environments. The additional S-isotope data is used to further constrain the utility of a dual-reservoir model for S-cycling in the Early Paleozoic and ocean ventilation in the Darriwilian.