Interstitial water chemistry of anoxic Long Island Sound sediments. 1. Dissolved gases1

Measurements of dissolved N 2 , Ar, CH 4 , and SO 4 2− and salinity and temperature are used to examine the processes controlling their distributions in the interstitial waters of nearshore sediments. Where sulfate reduction is incomplete, dissolved N 2 and Ar concentrations in the upper 10–30 cm appear to be controlled by macroinfaunal irrigation activities and vary in accordance with overlying water conditions. Below 30 cm, N 2 and Ar concentrations appear to have been last affected by August through October irrigation. In harbor sediments where sulfate reduction is complete at shallow depths, low N 2 and Ar concentrations result from in situ stripping by methane bubbles. Methane concentrations remain below about 0.1 mM until about 90% of seawater sulfate is removed. As sulfate concentrations approach zero, methane builds up to bubble saturation. Concave‐up methane depth distributions can be explained by methane consumption following first‐order kinetics in the zone of sulfate reduction. Laboratory jar experiments with natural sediments indicate no methane production during sulfate reduction, in agreement with previous models and distribution studies of sulfate reducing and methanogenic bacteria in lake sediments. Combined laboratory and field results support the hypothesis that methane is produced mainly after sulfate is depleted but is consumed within the sulfate reduction zone as it diffuses upward.