INVESTIGATING THE MIDDLE ORDOVICIAN (DARRIWILIAN) REDOX STRUCTURE OF THE ARGENTINE PRECORDILLERA WITH SULFUR ISOTOPE AND IRON SPECIATION PROXIES

Greenhouse conditions of the early Paleozoic contributed to increased ocean stratification and the persistence of deep-water anoxia through the Middle Ordovician (Darriwilian). An expanding geochemical dataset from marine sedimentary rocks from the Argentine Precordillera suggest changing nutrient availability under fluctuating climatic conditions. A dramatic shift in the S-isotope composition of carbonate associated sulfate (CAS) in the Darriwilian is interpreted as rapid ventilation of anoxic deep-water driven by global cooling and invigorated ocean circulation. The S-isotope compositions of CAS and pyrite are explained by a dual reservoir model with oxidized surface and deep-water (anoxic) reservoirs that permit isolation of redox reactions between reservoirs. This model is based on the shallow-water carbonate record and requires further appraisal with data from deep-water facies to assess whether pyrite is formed in an anoxic portion of the water column or in sedimentary pore-waters under an oxic water column.

New Fe-speciation and trace metal data from the Gualcamayo black shale provide further constraints on Darriwilian environmental conditions. Highly reactive iron phases (Fe_HR) including pyrite, iron carbonate, iron oxides, and magnetite were determined via sequential extraction techniques. Fe-speciation data indicates the shales of the Gualcamayo Formation were deposited under dominantly anoxic (ferruginous) conditions through the lower Eoplacognathus suecicus conodont Zone. However, anomalously low concentrations from the pyrite fraction (<0.05 wt %) and high concentrations of Fe-oxides (up to 3 wt. %) suggest the potential for pyrite oxidation in these outcrop samples. To evaluate the potential for alteration we determined Mo concentrations, which are generally resistant to depletion during oxidative weathering. Mo concentrations fall within the range of intermittently euxinic (14-100 ppm) in the modern ocean, but below the accepted threshold for ancient euxinic environments (25 ppm). Combined, these data suggest that the Gualcamayo was deposited in non-euxinic waters with sulfide restricted to sedimentary pore-waters, that Mo drawdown occurred elsewhere, or that the sulfate/sulfide reservoirs were small enough to limit Mo drawdown under anoxic conditions.