Effects of turbidity flows on organic matter accumulation, sulfate reduction, and methane generation in deep-sea sediments on the Iberia Abyssal Plain

Non-steady-state sedimentation has affected preservation of organic matter in a Pliocene-Pleistocene turbidite sequence sampled by the Ocean Drilling Program at four closely spaced drill sites on the Iberia Abyssal Plain. Organic carbon concentrations average ca. 0.7% in sediments from Sites 897 and 898 and ca. 0.4% at Sites 899 and 900. Headspace concentrations of interstitial methane exceed 100,000 ppm in sediments from Sites 897 and 898 but are essentially zero in those from Sites 899 and 900. Methane concentrations do not rise until interstitial sulfate concentrations are virtually depleted, suggesting the presence of deep in situ methanogenic bacterial activity at Sites 897 and 898 and its absence at Sites 899 and 900. Two factors associated with the turbidity flows that created the sedimentary sequence evidently influenced post-depositional diagenesis at these sites. The principal factor is that the rapidly deposited turbidite sequences at Sites 897 and 898 protected organic matter from early oxic degradation and thereby permitted subsequent anoxic degradation to proceed. In contrast, organic matter in the more slowly deposited turbidites at Sites 899 and 900 was oxidized soon after deposition and was therefore not available for later microbial utilization. A lesser factor is that the turbidity flows may have obtained their entrained organic matter from different environments and consequently delivered organic matter with different characteristics. Both factors contributed to heterogeneity of the types and amounts of organic matter that accumulated in these deep-sea sediments, and this variability has influenced subsequent sulfate reduction and methanogenesis. Copyright (C) 1996 Elsevier Science Ltd.