Caracterização geoquímica molecular de sedimentos marinhos abissais ao oeste do Oceano Atlântico Sul

Abstract

Abyssal marine sediments, at depths greater than 3000 m, may contain organic matter linked to the genesis of organic input from different origins, through relative contributions from marine and continental sources that have accumulated over thousands of years, providing a wide spectrum of organic substance composition. In this context, this research aims to apply organic geochemistry techniques to evalue and understand, at the molecular level, of organic matter in abyssal sediments from a short sample collected west of the South Atlantic Ocean, in a region adjacent to the Rio Grande Rise (RGR). This sample was collected at a depth of 3287m and dates from the late Pleistocene to the Holocene period, between 16,200 and 4,700 years ago. To obtain this sample, the abyssal sediment cores were sectioned every 1 cm, and these were initially analyzed by Rock-Eval® pyrolysis to obtain data on carbonate concentration (mgg-1), percentage of mineral carbon, and total organic carbon (TOC). A molecular-level investigation of chemical compositional variations was then conducted using gas chromatography coupled with mass spectrometry (GC/MS) to identify organic substances in the total extract of each sediment section, to understand the variations in depositional environment and deposition of organic matter in the sediments over time. Rock-Eval® Pyrolysis results, with TOC values < 0.12% and carbonates ~ 30%, in addition to the low soluble extract content (<0.12%), indicate that the marine sediment is related to a region of low productivity. The lower values of carbonate and mineral carbon content observed in the older samples, from the Pleistocene, indicate greater stagnation of bottom waters and high acidity, with greater dissolution of carbonates. Analysis of molecular biomarkers revealed compounds belonging to different organic classes, such as n-alkanes, hopanoids, steradynes, carboxylic acids, amides, and nitriles, allowing for the geochemical reconstruction of the depositional environment and the characterization of organic matter. The presence of long-chain n-alkanes, including C27, C29, and C31, with a prevalence of odd-numbered homologs over evennumbered ones, indicates the contribution of higher plants to the organic matter. Steradiene profiles, such as cholesta-3,5-diene (C27) and stigmasta-3,5-diene (C29), indicated a decrease in terrigenous contribution from the base to the top of the core. The presence of pentacyclic terpane compounds, such as α-amyrin and pentacyclic triterpenoid methyl ester isomers, are metabolized by plants, both found in leaf waxes. Hopanoids, such as neohopenes (C27 and C29), associated with anoxic conditions, and hopanone (C27), show evidence of initial diagenesis processes, with diagenetic precursors, from diploptene and bacteriohopanotetrol, respectively, of bacterial origin. The presence of short-chain carboxylic acids (<C18) indicates a major contribution from bacterial products, facilitated by secondary production and reworking of organic matter. The presence of n-alkyl amides may be associated with the origin of higher plants, although these may also be a product of biomass burning. Thus, the geochemical data of hopanoids indicate a paleoenvironment that favors anoxic conditions for the oldest sediments, suggesting an immature and preserved sediment of biogenic origin, including products of early diagenesis. Amides and n-alkanes stood out as significant biomarkers in this study, presenting geochemical profiles with variations that coincide with the transition period between the Pleistocene and the Holocene. These changes in concentration levels and the contribution of substances indicate a direct influence of climatic changes that occurred during this geological interval. The composition of organic matter reveals a strong indication of plant input.