Expression of Early Cretaceous global anoxic events in Northeastern Brazilian basins

Abstract

In different basins around the world, organic-rich layers from the Cretaceous are being associated with perturbations in the global carbon cycle that reflect abrupt climatic and oceanic changes responsible for increased anoxia. In the northeastern part of Brazil, organic-rich rocks from the Parnaíba and São Luís basins—deposited during the late Aptian-early Albian and associated with the Codó Formation—offer an interesting stratigraphic record to evaluate the influence of oceanic anoxic events on restricted marine paleoenvironments. Therefore, an integrated study using different geochemical proxies (total organic carbon (TOC), Rock-Eval pyrolysis, δ13Corg, δ15Ntotal, and biomarkers) was conducted on sedimentary sections of the São Luís and Parnaíba basins (Codó Formation), which are assigned to the upper Aptian-lower Albian. The δ13Corg data obtained were compared with the isotopic δ13Corg profile from the section of the Santa Rosa Canyon (Mexico) where the record of the anoxic Aptian-Albian event (oceanic anoxic event 1b) was characterized. The stratigraphic record of the Codó Formation in both the basins presented the C11–C12 Cretaceous isotopic segments with clear 13Corg depletion associated with a spike in TOC that reached 10% at Parnaíba Basin and 15% at São Luís Basin. At Parnaíba Basin, biomarker results showed a high abundance of the terpane gammacerane, suggesting conditions of water-mass stratification and hypersalinity, setting that took place in the southern hemisphere during OAE 1b and enhanced preservation of organic matter in different basins. Also, marine sources were evidenced by the predominance of the C27 ααα(R) sterane and by the identification of the C30 ααα(R) sterane. In addition, the presence of the biomarkers C27 trisnorhopane 17β, C29αβ and C29 ααα (R) sterane reflect the contribution of terrigenous material to organic matter, which is commonly observed during anoxic events due to the acceleration of the hydrological cycle. Nitrogen isotopes revealed a relatively positive value (greater than 3%) which suggests anaerobic reduction of NO3- to N2 under suboxic to anoxic conditions (denitrification). Otherwise, a negative δ15N trend was observed in samples from the negative δ13Corg trend which represented a high organic carbon content. This δ15N signature suggests a decrease in the available NO3- (beginning of exhaustion) and an enhanced level of nitrogen fixation by microbial organisms during the anoxic event, which is also supported by a higher abundance of prokaryotic biomarkers (hopanes) in the organic matter.