Exploring biomarker signatures of methane hydrates in the Amazon Cone

The study of organic matter in sediments is crucial for advancing energy resource exploration and understanding geological and biogeochemical processes. This study focuses on the Amazon Cone (Brazil), a region of significant interest following the last decade's discovery of gas hydrates in Brazilian waters. Methane, the main gas released by dissociating hydrates, is a potent greenhouse gas with biogenic or thermogenic origins. Therefore, understanding its pathways in sedimentary environments is fundamental for energy exploration and climate sciences. Building on data from the 2023 Amaryllis-Amagas Oceanographic Mission aboard the Marion Dufresne research vessel, this work investigates gas hydrate systems on Brazil’s equatorial margin through biomarker analysis. A total of 89 samples from seven piston cores were analyzed by Rock-Eval Pyrolysis, and 20 samples were selected based on Total Organic Carbon (TOC) values and core positions (top, middle, and bottom). Soxhlet extraction with dichloromethane/methanol (8:2) was followed by liquid chromatography to separate saturated hydrocarbons, aromatic hydrocarbons, and polar fractions, and GC/MS (Gas Chromatography-Mass Spectroscopy) was used for compound identification. N-alkanes ranging from n-C18 to n-C35 were identified with a predominance of long-chain n-alkanes (n-C25 to n-C35) with a marked odd-over-even carbon number preference (e.g., the greater abundance of nC27, nC29, nC31, and nC33), which indicate an input of terrestrial plant-derived organic matter. Furthermore, pristane and phytane are present in very low abundance. Terpanes distribution points to anoxic depositional conditions, and the domination of ββ-C30, ββ-C31, and ββ-C32 compounds corresponds to a low level of thermal maturity. The steranes analysis also observed low maturity, showing a predominance of biological isomers, while the diasteranes DIA27S > DIA27R ratio emphasizes clay-catalyzed processes in a clastic, clay-rich sedimentary environment, characteristic of the Amazon Cone. The terrestrial input coupled with evidence of bacterial activity highlights the role of microbial processes in shaping the organic matter composition. Also, the low thermal maturity of the organic matter aligns with favorable conditions for biogenic methane production. Likewise, the clay-rich environment of the Amazon Cone facilitates the trapping and preservation of gas hydrates by providing structural stability to the sediments. Combined with the anoxic conditions inferred from the biomarkers, these findings are consistent with the microbial pathways critical for methane production and gas hydrate stability. Therefore, the Amazon Cone appears to be a region where microbial and geological processes converge to create and maintain methane hydrate deposits. This underscores the potential of the area not only as a site of scientific interest but also as a candidate for future energy exploration, with the added significance of understanding methane’s role in global carbon cycling and greenhouse gas emissions.