28. RESIDUAL HYDROCARBON GASES IN CANNED CORE MATERIAL FROM HOLES 379A AND 380A, LEG 42B

Nine samples, eight from Hole 379A and one from Hole 380A, of sediment specifically canned for determination of residual gases contained the moderate amounts of hydrocarbon gas, virtually all of which was methane. These quantities are consistent with values detained from previ-ous analysis of samples from earlier legs. Four additional samples from Holes 380 and 380A, collected and frozen for organic geochemistry, had moderate amounts of predominantly amorphous kerogen and were at a low state of thermal alteration (about 2) in the 1 to 5 alteration index scale, which was also evidenced by the low hydrocarbon to total bitumen ratio of the solvent extractables. BACKGROUND Shipboard personnel have frequently noted gas in the deep-sea muds in freshly recovered cores because it causes substantial expansion of the sediment and formation of gas gaps up to 15 cm long along the length of some cores. This gas is of great interest both as to its origin as well as to the hazard it may represent. The composition of the gas is now carefully monitored to determine the ratio of methane to heavier gases; if the ratio of methane to the ethane-and-heavier fraction falls below 500-600, it is judged that the organic matter in the sediments is experiencing the very onset of thermal cracking and gas volumes could increase rapidly. Thus the hazards of further drilling at the particular site must be weighed carefully. The gas is mostly methane, probably produced microbially (Claypool et al., 1973), but quantitative estimates of the amounts per unit volume of sediment or the amount related to solubilities in water under down-hole conditions have not been made. The observations "gassy " and "very gassy " are sometimes recorded. In one case, anomalously high seismic velocities in a gassy section of the Blake-Bahama Outer Ridge and a strong-bottom paralleling reflector that cuts bedding plane reflections, together with mineralogical evidence reported by Lancelot and Ewing (1972), led to recognition that gas in the sediment immediately below the sea floor might occur as hydrate, a solid water-gas clathrate, that is theoretically stable under high pressures but above the freezing point of water (Katz, 1971). In order to find out more about the gases, the DSD