Origin and Alteration of Organic Matter in Hydrate-Bearing Sediments of the Rio Grande Cone, Brazil: Evidence from Biological, Physical, and Chemical Factors

The Rio Grande Cone is a major fanlike depositional feature in the continental slope of the Pelotas Basin, Southern Brazil. Two representative sediment cores collected in the Cone area were retrieved using a piston core device. In this work, the organic matter (OM) in the sediments was characterized for a continental vs. marine origin using chemical proxies to help constrain the origin of gas in hydrates. The main contribution of OM was from marine organic carbon based on the stable carbon isotope (δ 13 C-org) and total organic carbon/total nitrogen ratio (TOC:TN) analyses. In addition, the 14 C data showed important information about the origin of the OM and we suggest some factors that could modify the original organic matter and therefore mask the “real” 14 C ages: (1) biological activity that could modify the carbon isotopic composition of bulk terrestrial organic matter values, (2) the existence of younger sediments from mass wasting deposits unconformably overlying older sediments, and (3) the deep-sediment-sourced methane contribution due to the input of “old” (>50 ka) organic compounds from migrating fluids.