Organic matter sulfurization on protracted diagenetic timescales: The possible role of anaerobic oxidation of methane

- The chemical composition of kerogens from Cariaco Basin spanning 130 ky is analyzed.
- Sulfurization is significant in all the studied kerogens.
- Sulfurization is modulated by redox conditions and conditioned by TOC content.
- Maximum sulfurization rate is reached at depth of anaerobic oxidation of methane.
- Maximum sulfurization occurs in sediments at the base of long anoxic sequences.

Kerogens from the anoxic sediments of the Cariaco Basin spanning the last 130 ky were analyzed in order to determine the organic matter (OM) preservation pathways, and the evolution of these pathways with time. The kerogens were isolated using a protocol avoiding the formation of artificial melanoidins. Bulk elemental content, spectroscopic and pyrolytic features of the kerogens show that sulfurization is an important process in the insolubilisation of the OM, even in the most recent sample studied (~ 400 a). The degree of OM sulfurization on the studied interval depends on the redox conditions of the sediment during deposition and is largely correlated to the total organic carbon content of the sediment. A trend of downward increasing sulfurization of the organic matter is observed in the sediments of the studied two interglacials. From the available porewater analyses of Cariaco sediments and comparison with Black Sea sediments, the data suggest that downward progressive sulfurization of the OM is mainly due to the production of H2S by sulfate-dependent anaerobic oxidation of methane occurring at the sulfate/methane transition zone located a few meters below sea floor.