Unfortunately, the hydrocarbon gas concentrations of the headspace gas samples obtained in the shore-based laboratory were lower than those measured in the shipboard laboratory (Shipboard Scientific Party, 2000b), presumably because gas escaped from the sediments during sampling, transportation, and storage. The results of shipboard analyses are, therefore, used to discuss the hydrocarbon anomalies at Site 1146. Methane concentrations are relatively low (<30 parts per million by volume [ppmv]) and constant above 217.2 mbsf in Hole 1146A. Below this depth, the concentrations rise sharply, reaching 1,000 ppmv at 284.7 mbsf; 10,000 ppmv at 390.6 mbsf; and a maximum of 85,000 ppmv at 563.2 mbsf and then decrease slightly down to the base of the hole (Fig. F3). C2H6 was first detected at 505.7 mbsf and reached a maximum concentration (155 ppmv) at 572.8 mbsf (Fig. F3). C3H8 was first detected at 534.4 mbsf and reached a maximum concentration (7 ppmv) at 572.8 mbsf (Fig. F3). The resulting C1/(C2+C3) ratio rapidly decreased from 2460 ppmv at 505.7 mbsf to 345 ppmv at the bottom of the hole (Fig. F3). A very similar pattern was observed in the lower 100 m at Hole 1146C, and the concentrations of methane, ethane, and propane reached the maximum values of 56,987, 130, and 9 ppmv at 588.7 mbsf, respectively (Shipboard Scientific Party, 2000b).
The 
13C values of CH4 from nine headspace gas samples range from -37.8
to -24.0, relative to PDB. The two values from Hole 1146A are significantly higher (-24.0 and -26.4) than all the values from Hole 1146C (-31.3 to -37.8 (Table T2), which is probably closer to the faults and where more thermogenic gas was trapped.
