CONCLUSIONS

Organic matter is not abundant in Leg 184 samples from the SCS, ranging from <0.1 to 0.5 wt%, with a few samples between 0.6 and 1.1 wt% TOC. Organic matter from five cores/sites from Leg 184 was analyzed for stable carbon isotope content. Disseminated OM generally shows ¹³C values between -22 and -24, indicating that it is a mixture of marine- and land-sourced OM. Samples of centimeter-size woody material and plant fragments give ¹³C isotope values generally between -24 and -29. The ¹³C values lighter than -26 are unusual for wood and lignite but are typical of twigs and lignin. Using -20 as the marine OM and -26 as the land OM end-members, the proportion of OM measured in the Leg 184 sediments ranges from 40%-60% marine sourced. This seems rather low for deep-sea sediments but actually is not unreasonable for two reasons. First, these sediments, while deep, are ~100 km from land but were probably closer to land at the time of deposition (and during ice ages the sea level was lower). Second, marine OM is preferentially metabolized (over more refractory land-sourced OM) by microbiological sulfate reduction to form iron monosulfide, pyrite, and marcasite. If the S content of the sediments is factored into the OM budget, the original amount of OM deposited with the sediments was 60%-90% marine sourced.