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

Measurements of dissolved Na, Ar, CH,, and SO,‘and salinity and temperature are used to examine the processes controlling their distributions in the interstitial waters of nearshore sediments. Whcrc sulfate reduction is incomplete, dissolved N:! and Ar concentrations in the upper lo-30 cm appear to be controlled by macroinfaunal irrigation activities and vary in accordance with overlying water conditions. Below 30 cm, Ns and Ar concentrations appear to have been last affected by August through October irrigation. In harbor sediments where sulfate reduction is complete at shallow clepths, low Ns and Ar concentrations result from in situ stripping by methane bubbIes. Methane concentrations remain below about 0.1 mM until about 90% of seawater sulfate is removed. As sulfate concentrations approach zero, mcthanc 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 cluring sulfate reduction, in agrecment 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 mainIy after sulfate is depleted but is consumed within the sulfate reduction zone as it diffus& upward. We present here measurements of dis-solved gases and some major ions in interstitial waters squeezed from anoxic sediments collected at three nearshore Long Island Sound coring sites and discuss the general problems of dissolved CIIA, N2, and Ar distributions in anoxic marine sediments. A series of laboratory experiments consisting of similar measurements on natural sediments sealed in jars and opened scquentially is also discussed. Dissolved N2 and Ar were initially determined to study possible chemical reactions involving tither N2 production or reduction; studies by Thorstenson and Mackenzie ( 1974)) IIammond (1974), and Barnes ct al. (1975) suggested that both of these processes may be important in various sedimentary environments. We have used N2 and Ar concentrations as tracers for studying mixing 1 Financial support was provided by American Chemical Society Petroleum Research Fund grant 7002-AC2 (R. A. Berner) and by National Science Foundation Oceanography Section grants GA 30288X (R. A. Berner) and DES 75-06199 (C. S. Martens ) . ’ Present address : Curriculum in Marine Sciences and Department of Geology, University of North Carolina, Chapel Hill 27514. processes between overlying waters and interstitial waters. Methane measurements were made to study factors controlling its production and decomposition in coastal sediments. Particular emphasis was placed on examining the relationship between sulfate reduction and methane production. We thank members of the FOAM (friends of anoxic mud) group including M. Goldhaber, R. Aller, K. Cochran, and J. Rosenfeld for comments and cliscussions concerning this work. Additional discussions wit< W. S. Reeburgh and G. Claypool concerning methane production and consumption were beneficial. A. Ruggiero provided analytical chemical assistance. M. Reed’s assistance, particularly with rainsoaked winter coring operations, was indispcnsable. A. Goodhue fabricated ment squeezing system.