No abstract

Thirty-five samples of deep sea sediments were collected on DSDP Leg 15 and stored in sealed kettles for over a year. The PCO2 of these kettles was monitored as a function of time and temperature. When CO2 is removed for analysis, the decrease observed in kettle PCO2 is consistent with the assumption that equilibration of water and calcite controls the alkalinity and pH of the interstitial water. An empirical relation between ΣCC>2 and PCO2 is selected which allows correction for CO2 removal and comparison of PCO2 to other carbonate parameters (i.e., pH) measured on Leg 15. The resolution of the data is limited, but pH and PCO2 appear to be consistent within 0.1 pH units. Calculation of the degree of calcite saturation is used to suggest that pressure may significantly influence ionic equilibria between clay and pore waters, causing samples from Site 149 to appear understaurated with calcite. Sediment samples from regions with strong [SC 4 = ] gradients show increases in PCO2 with time, indicating active sulfate reduction continuing in the kettles. Comparison of the CO2 generation rate in the laboratory with that in situ at Sites 147 and 148 suggests that the process may be limited by the availability of suitable organic substrates, which is a function of temperature.

Ten gas pocket samples were collected at DSDP Site 147 in the Cariaco Trench. Gas chromatographic analysis showed them to range in concentration as follows: 85 to 95% CH4; 1 to 13% CO 2 ; 1 to 2% H 2 O; 0 to 3% N 2 ; and 0.004 to 0.05% C 2 H 6. Gas pocket PCO2 is consistent with punch-in pH measurements within 0.2 pH units. Conversion of gas pocket data to in situ concentrations is hindered by the difficulty in determining the ratio of gas pocket volume to interstitial water volume. Development of a device for collecting samples at the in situ pressure would eliminate this problem. Visual estimates of gas content do not resolve the anticipated methane increase with depth. The best estimate of in situ concentrations in the interstitial water is [CH4] 0 = 20 mmol ±50%, [C 2 H 6 ]° = 1 to 10 mmol/1 ±100%, and [N 2 ]° = 0.7 to 0.0 mmol/1 ±50%. This corresponds to the conversion of about 1.5% of the organic carbon into methane. Zobell (1947) has shown that H 2 is a likely product in the anaerobic decomposition of organic matter by bacteria, but that it is rapidly utilized by other bacteria as an electron source. Assuming this to be true, the reactions N 2 + 3H 2 • 2NH3 and Ca ++ + 2 HCO3 + 4H 2 CaCO 3 + 3H 2 O + CH 4 should occur, accounting for observed decreases in alkalinity and dissolved nitrogen. It is possible that the dissolved nitrogen decrease may be due to extensive gas loss and consequently the best estimates of [C 2 HÖ] and [CH4] 0 are each an order of magnitude low. The ethane/methane ratio in the sediment is lower than in the overlying water column but increases with depth. This is attributed to different mechanisms of production in the two regions.