Oxygen-carbon ratios, marine organic

Whole-rock samples of mudrock from both shallowly and deeply buried rocks have similar carbon isotopic ratios, but distinctly different oxygen isotopic ratios (Fig. 8). 5 C values for most samples are from -2 to +l%o, values typical of marine biogenic carbonate or detrital clasts of marine limestone. One anomalously light sample (-8%o) in the Antog-nola Formation, and other samples with 5 C values less than -2%o, contain some authigenic carbon derived from organic material. 

The scatter diagram of TS vs. TOC illustrates the S/C ratios of our samples (Fig. 7). Although the study area is a normal marine environment Le. clastic sediments overlain by oxic waters of typical oceanic salinity), most of the S/C ratios in the sediments are considerably lower than the average ratio of 1/2.8 obtained for reduced sulfur and organic carbon in sediments beneath oxygenated seawater (Berner, 1982). By contrast, the S/C ratios in our samples are mostly lower than 1/10, typically occurring in fresh water environments. Similarly, low S/C ratios have been observed in Amazon inner shelf muds (Alter et al, 1986 Alter and Blair, 1996). The authors attribute the low ratios to the oxidation power of iron oxides and reworking of sediments. Unlike the Amazon case, our samples are mostly from the slope and submarine canyons rather than the shelf and, therefore, deserve further discussion. 

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