Only trace amounts of methane (<10 ppmv) and no other hydrocarbon gases were detected in sediments at Site 1147. This mirrors low concentrations found in the upper sediments of the other low-productivity sites drilled in the northern South China Sea.
Sampling and analysis for inorganic carbon (IC), total carbon (TC), nitrogen, and sulfur in Hole 1147A were conducted as described in "Organic Geochemistry" in the "Explanatory Notes" chapter. Carbonate varies from 4.4 to 30.5 wt% (average [AV] = 12.8 wt%; standard deviation [SD] = 6.0), with all values <20 wt% with the exception of Sample 184-1147A-4H-6, 107-108 cm. No downhole trend is observed (Table T12; Fig. F12A). Total organic carbon (TOC) concentration by difference (TC - IC) declines markedly from 1.21 wt% at the top of the hole to 0.58 wt% at 12 mcd. A more gradual decline is noted downhole, with values of <0.4 wt% (AV = 0.56 wt%; SD = 0.33; Table T12; Fig. F12B). The sharp decline in TOC in the upper two cores is characteristic of oxidation and microbial diagenesis of organic matter (OM). Calculated C/N ratios similarly decrease downhole from 5 to 8 above 40 mcd to unreasonably low values (Table T12; Fig. F12C), indicating nitrogen presence in clay matrices (see "Organic Geochemistry" in the "Site 1145" chapter). We conclude that the low levels of OM in these sediments are dominated by degraded marine matter.