Depth Distributions of Gases in Shallow Water Sediments
Depth distributions of Ar, N2, CH4, total CO2 and total H2S have been obtained in sediments from Chesapeake Bay, as well as fjords, coastal lagoons and lakes in Alaska. Interstitial water was separated from samples of sediment cores with a filter-press type sediment squeezer and transferred to a specially designed sampler-stripper, where the gases were stripped from solution and analyzed by gas chromatography. The technique permits repeated sampling of interstitial water from a given sediment sample, and yields unambiguous measurements of gas concentrations that are precise to about 5%.
In Chesapeake Bay, CH4 increased from undetectable quantities at the sediment surface to concentrations of 150 and 85 ml/l in water depths of 30.4 and 15.2 meters. The observed maximum concentrations agree well with values calculated, assuming that the CH4 concentration is controlled by ebullition from the sediment. The absence of CH4 in the upper 25 cm of these sediments and the occurrence of Ar and N2 in concentrations similar to the overlying water in this zone indicated mixing to at least this depth. Total CO2 increased with depth to values of 1200 ml/l, pH was uniform with depth, and alkalinity increases to as high as 50 meq/l were observed. Low contents of total H2S and an abundance of acid-labile sulfides in the sediments indicated that the concentration of H2S is controlled by formation of solid phases.
In Alaska, methane was observed only in freshwater environments in concentrations far below those necessary for ebullition. Active denitrification was observed in Ace Lake sediments, where the N2/Ar ratio was 65 for one sample. Argon was constant with depth in all environments. Large quantities of H2S were observed in the eelgrass beds at Izembek Lagoon. Total CO2 distribution was similar in all of the fjords studied, showing an increase from about 50 ml/l at the sediment surface to relatively constant concentrations greater than 200 ml/l below about 30 cm.
KeywordsOverlie Water Depth Distribution Interstitial Water Zostera Marina Solubility Coefficient
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