Sediments in the Pliocene-Pleistocene turbidite sequence sampled at Sites 897, 898, 899, and 900 illustrate the interplay of sea-bottom morphology, sedimentation rate, and preservation of organic matter in influencing non-steady-state diagenesis in deep-sea sediments. Partially preserved algal organic matter is generally more reactive and therefore more suitable for microbial utilization than is detrital or continental organic matter (Lallier-Verges et al., 1993), and the availability of metabolizable organic matter has evidently impacted microbial processes in these sediments. Sulfate reduction proceeds with depth at Sites 897 and 898 where adequate amounts of reactive organic matter are available to support bacterial activity. In contrast, little sulfate reduction occurs at Sites 899 and 900. Headspace concentrations of interstitial methane are high in sediments from Sites 897 and 898 but are essentially zero in those from Sites 899 and 900. Because methane concentrations do not rise until interstitial sulfate concentrations are virtually depleted, the existence of in situ methanogenic bacterial activity is indicated at sub-bottom depths of several hundred meters at Sites 897 and 898 (Claypool and Kvenvolden, 1983). Similar methanogenic activity is evidently absent at Sites 899 and 900. Heterogeneity in delivery and preservation of organic matter has controlled sulfate reduction and methanogenesis in these deep-sea sediments.
The ratio of carbon degraded under oxic versus anoxic conditions will impact the character of the buried carbon. In the more rapidly accumulated turbidites of Sites 897 and 898, much of the carbon degradation occurred under anoxic conditions (Shaw and Meyers, this volume). At depths greater than 10 cm into any one turbidite sequence, the maximum oxygen flux into the remaining interval can be calculated to be on the order of 0.02 moles/m2/yr, assuming a bottom water oxygen concentration of 200 µM. For thick turbidite sequences such as those in Sites 897 and 898, sulfate reduction was consequently the predominant mode of carbon degradation. The character of the surviving carbon in deep-sea turbidite sequences in general may therefore be a function of the rate of burial, resulting in diagenetic differences from location to location and from depth to depth.
The turbidite layers in the sequences at Sites 899 and 900 are thinner and were deposited less frequently than at Sites 897 and 898. The generally thicker intervals of interlayered nannofossil ooze suggest longer periods of slow sedimentation. In thin turbidite sequences, oxic degradation of the carbon probably occurred via the "burn down" mechanism described by Wilson et al. (1986), Thomson et al. (1984; 1993), and Pruysers et al. (1993). Oxygen would slowly diffuse into the carbon-enriched turbidite layer during the slow deposition of the pelagic oozes that separate turbidite layers at these two sites. The slow sedimentation and thin turbidite emplacement would result in early diagenetic oxic degradation being the predominant mode of organic carbon degradation, and little oxidizable, or microbially metabolizable, organic matter would survive in deeper sediments. Sulfate reduction and subsequent methane production deeper in the sediments would therefore not be significant.
If the origins of the turbidite layers on the Iberia Abyssal Plain are assumed to be similar settings on the Iberia Margin, then the turbidite sequences encountered at the four drill sites can be considered compositionally uniform deposits that originally differed only in their thickness. In other words, the presence or absence of basement highs controlled the emplacement of thinner or thicker turbidite deposits, respectively. Subsequent oxidation of organic matter, however, would proceed by different geochemical processes depending on the thickness of each layer. The thinner deposits would have a greater proportion of organic carbon oxidized under oxic conditions, whereas anoxic conditions would dominate in the thicker deposits. Interstitial sulfate concentrations support this concept (Shaw and Meyers, this volume). The presence of relatively high sulfate concentrations in sediments from Sites 899 and 900 indicates that little organic carbon survived oxic degradation in these slowly accumulated turbidites to be available to consume sulfate. In contrast, sulfate consumption during organic matter oxidation has removed essentially all of the dissolved sulfate from the sediments of Sites 897 and 898.