Periods of continentalization as control for carbon storage during the Aptian-Albian oceanic anoxic event (OAE 1b) in the Parnaíba Basin (NE Brazil)

In some basins, the anoxic events have been identified based on the presence of black shales enriched in organic matter (OM). The Aptian/Albian boundary interval may be associated with one of these Cretaceous oceanic perturbations, the Oceanic Anoxic Event (OAE) 1b. It is characterized by a long duration (~3.8 Myr) and periods of oxygen-depleted bottom water conditions, which caused a global carbon-cycle perturbation. The organic-rich shales from the Codó Fm. (Parnaíba Basin, Brazil) are a sedimentary archive for this time slice, but with mixed signals that complicate a clear assignment of the depositional environment. The OM is predominantly marine but also contains lacustrine and terrigenous records. In this context, this study aims to unravel the continental input during the deposition of shales from the Codó Fm. and how this influenced carbon storage during OAE 1b. Fourteen samples were collected from the base to the top of an outcrop located in a gypsum mine in Grajaú, Northeast of Brazil. Bulk (total organic carbon, TOC%; sulfur content, S%; and Rock-Eval) and gas chromatographic analyses (GC-FID and GC-MS) were carried out. The TOC (0.50 to 1.44 wt.%) and S content (0.27 to 3.71 wt.%), and the kerogen type II (marine) indicate a favorable preservation of OM. The exceptionally low values of Tmax (372 to 397 ºC) co-occur with high concentrations of pristane (Pr) and phytane (Ph) but low abundances of n-alkanes. These data, as well as hopenes and steranes with R configuration, are indicators of exceptionally preserved low maturity since the Lower Cretaceous. Pr/Ph ratios < 1 and the presence of benzohopanes further prove the anoxic environment that might have reached the photic zone euxinia (PZE), as evidenced by the detection of arylisoprenoids in all samples. The detection of methyltrimethyltridecylchromans (MTTCs) also suggests PZE, as they originate from photosynthetic organisms that thrive in this environment. Increasing gammacerane/C30 hopane ratios suggest progressively developing hypersaline conditions, whereas decreasing Pr/Ph ratios may point to the stabilization of anoxia during the deposition of the Codó shales. The data obtained here resemble and support the development of progressively hypersaline conditions, whereas decreasing Pr/Ph ratios may indicate saline periods. Moreover, intensified chemical weathering on land increased the continental influence and enhanced the supply of nutrients and terrigenous OM. Evidence for such admixing is bicyclic sesquiterpenes and the predominance of C29 among the steranes. Additionally, higher plant-derived aromatic biomarkers, including cadalene, retene, and α-ionene, are present and indicate forestation as a source, a typical feature in Aptian-Albian sediments. A decreasing trend was observed for the arylisoprenoids ratio (AIR; C13-17/C18-22) and the higher plant parameter [HPP; retene/(retene + cadalene)], from the base to the top, suggesting a transition from episodic to a persistent PZE and an introduction to more arid conditions. The low thermal maturity of the Codó samples enables a precise reconstruction of how OM mixing is controlled by climate change. The low maturity allows for deciphering a variety of nearly unaltered biological markers. The combination of signals from anoxia under a dry climate, stored in low-mature rocks, makes the shales of the Codo Fm. an excellent showcase for reconstructing complex source-to-sink relationships.