PALEOREDOX CONDITIONS DURING THE CENOMANIAN-TURONIAN IN CENTRAL MEXICO AND THEIR RELATION TO OCEANIC ANOXIC EVENT 2

The Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE 2) is an exceptional episode of accelerated global. In central Mexico (Huayacocotla Basin), the Upper Tamaulipas and Agua Nueva formations contains the Cenomanian-Turonian transition and are mainly constituted by black to dark-gray laminated limestone with interbedded shale and bentonite. These units also contain organic matter-rich sediments and pyritic layers. In this study, an integrated approach combining sedimentological, petrographic and geochemical analyses provides information about the paleoenvironmental conditions and their relation to the OAE 2. The studied section (Tlacolula section, ~80 m thick) was constrained by U‒Pb zircon geochronology and biostratigraphic data to the latest Cenomanian-early Turonian and consists of pelagic mudstone/wackestone containing radiolarians and planktonic foraminifera, with chert nodules, intercalated with thin calcareous black shale and greenish bentonite horizons. Pyrite is present in the form of framboids, occurring as nodules, disseminations, thin horizons or filling burrows. The lower and middle portion of the section is dominated by laminated and poorly bioturbated (bedding-parallel burrows) facies with a high TOC content (up to 3.96 wt%) whereas the upper part is characterized by highly bioturbated beds poor in organic matter. Although the size of framboids varies slightly from sample to sample, they have a size mostly around 2 μm. Moreover, pyrite has highly negative δ³⁴S (V-CDT) values ranging from −9.5 to −36‰. Altogether, these data suggest that bottom redox conditions were poorly oxygenated, changing from reduced to partially oxygenated. This is consistent with the calculated U/Th ratios mostly >0.75, pointing to dysoxic and anoxic regimes, and the enrichment in several redox sensitive trace elements. On the other hand, the ³⁴S-depleted values indicate pyrite formation mediated by microbial sulfate reduction in an open system with available sulfate, favored by a high organic matter burial and the oxygen-depleted conditions. The more permanent reducing environments in the lower and middle part of the section are likely reflecting oxygen deficient conditions associated with OAE 2, superimposed to the anoxic/dysoxic bottom-water regime inherent to the Huayacocotla Basin.