SUMMARY

The amounts and types of organic matter and concentrations of dissolved sulfate and headspace methane have been determined in sediments of a Pliocene-Pleistocene distal turbidite sequence that was recovered during Leg 149 in four closely spaced drill sites on the landward edge of the Iberia Abyssal Plain (Sawyer, Whitmarsh, Klaus, et al., 1994). Organic carbon concentrations average 0.7% in this sequence at Sites 897 and 898 and 0.4% at Sites 899 and 900. Atomic C/N ratios are generally between 3 and 10 and indicate an algal origin (Meyers, 1994). Organic ¹³C values vary little from -23%o, which is typical of marine algae (Meyers, 1994). Rock-Eval results reveal that the marine organic matter in the sediments is heavily oxidized. 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. Partially preserved marine organic matter is present in the sediments of Sites 897 and 898, whereas poorly preserved, detrital marine material is found at the other two sites. Moreover, the degree of preservation of organic matter in Site 897 and 898 sediments fluctuates with depth. A correlation between low organic matter C/N ratios and low concentrations of organic carbon indicates that postdepositional diagenesis of organic matter controls TOC concentrations. The amount of in situ oxidation of organic matter has been controlled by rates of turbidite sedimentation, which are high at Sites 897 and 898 and low at Sites 899 and 900. An additional 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 contribute to heterogeneity of the types and amounts of organic matter in these deep-sea sediments, and this variability has influenced subsequent sulfate reduction and methanogenesis. 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.