Applications of the Osmium Isotopic System and Rhenium-Platinum Group Element Concentrations in Organic-Rich Mud Rocks from the Eagle Ford Formation, Texas: A Geochemistry and Redox Study

The concentration of Re and platinum group elements (Re-PGE) and 187Os/188Os ratios have been measured from ocean anoxic event 2 (OAE 2; ~94.87 - ~93.92 Ma) sediments from a West Texas core. The paleo-location of the core is the Comanche shelf near the southern mouth of the Cretaceous Western Interior Seaway (KWIS). Osmium geochemical data indicates a massive input of unradiogenic material to the oceans immediately before the onset of OAE 2. During OAE 2, two additional pulses of unradiogenic Os entered the oceans. One of these pulses correlates in time with the precipitation of mafic trace metals (e.g. Co, Cr, Sc) and the PGEs Pt, Ir, Ru, and Pd from ~94.60 Ma – 94.48 Ma, peaking at ~94.55 Ma. Osmium concentration has its third peak immediately before the peak of these elements at ~94.56 Ma, likely showing a high-resolution indication of when this PGE and trace metal-rich source entered the KWIS. These enrichments have been documented in other locations globally but the timings are not synchronous. In the Comanche Shelf, the time interval of the PGE and trace metal enrichment correlates with a shift to more oxic waters and cooling of global temperature. Some evidence includes bioturbation, a positive δ18O shift, and the reappearance of northern fauna Cyclonephelium compactum-C. membraniphorum into the KWIS via southward migration. The redox behavior of the PGEs in oceanic depositional settings is not well understood, but Cr is well studied. Chromium enrichment in oxic conditions is unusual because it creates mostly soluble species in such conditions. However, Cr6+ can precipitate in oxic conditions if replacing Mg in detrital sediments which is abundant in this interval. Iridium is known to complex with ferromanganese and organic material in oxic conditions, thus seeing a concentration increase in Ir makes sense. The source of these enrichments is most likely volcanic but an extraterrestrial source is also possible. Based on 187Os/188Os data, the end of massive LIP input to the oceans occurs at ~94.49 Ma and it takes ~570 Kyr for δ13C to shift back to pre-excursion values at ~93.92 Ma.