Natural abundances of carbon isotopes in acetate from a coastal marine sediment.

Measurements of the natural abundances of carbon isotopes were made in acetate samples isolated from the anoxic marine sediment of Cape Lookout Bight, North Carolina. The typical value of the total acetate carbon isotope ratio (delta 13C) was -16.1 +/- 0.2 per mil. The methyl and carboxyl groups were determined to be -26.4 +/- 0.3 and -6.0 +/- 0.3 per mil, respectively, for one sample. The isotopic composition of the acetate is thought to have resulted from isotopic discriminations that occurred during the cycling of that molecule. Measurements of this type, which have not been made previously in the natural environment, may provide information about the dominant microbial pathways in anoxic sediments as well as the processes that influence the carbon isotopic composition of biogenic methane from many sources.

Seasonal variations in the stable carbon isotopic signature of biogenic methane in a coastal sediment.

Systematic seasonal variations in the stable carbon isotopic signature of methane gas occur in the anoxic sediments of Cape Lookout Bight, a lagoonal basin on North Carolina's Outer Banks. Values for the carbon isotope ratio (delta 13C) of methane range from -57.3 per mil during summer to -68.5 per mil during winter in gas bubbles with an average methane content of 95%. The variations are hypothesized to result from changes in the pathways of microbial methane production and cycling of key substrates including acetate and hydrogen. The use of stable isotopic signatures to investigate the global methane cycle through mass balance calculations, involving various sediment and soil biogenic sources, appears to require seasonally averaged data from individual sites.

Methane production from acetate and associated methane fluxes from anoxic coastal sediments.

The apparent microbial conversion of acetate to methane ranges seasonally from 0.7 to 88 micromoles per liter of whole wet sediment per hour in the top 5 centimeters of methane-producing sediments underlying sulfate-reducing sediments in Cape Lookout Bight, North Carolina. The associated methane flux across the sediment-water interface into overlying waters exhibits the same seasonal pattern. Significant methane production from acetate is observed only in sulfate-depleted sediments.

Sediment-Water Chemical Exchange in the Coastal Zone Traced by in situ Radon-222 Flux Measurements.

In situ radon-222 flux experiments conducted in benthic chambers in Cape Lookout Bight, a small marine basin on the North Carolina coast, reveal that enhanced chemical transport across the sediment-water interface during summer months is caused by abiogenic bubble tube structures. Transport rates for dissolved radon, methane, and ammonium more than three times greater than those predicted on the basis of molecular diffusion occur when open tubes are maintained by semi-diurnal low-tide bubbling.

Control of methane sediment-water bubble transport by macroinfaunal irrigation in cape lookout bight, north Carolina.

Methane transport by bubble ebullition through bubble tubes from sediments to overlying waters in Cape Lookout Bight, Outer Banks of North Carolina, occurs only in the absence of burrowing macroinfauna, which indirectly prevent saturation methane concentrations by irrigating surface sediments with dissolved sulfate. Distribution of macroinfauna in the bight is limited to bottom areas not subjected to periodic anoxic conditions.

Methane production in the interstitial waters of sulfate-depleted marine sediments.

Methane in the interstitial waters of anoxic Long Island Sound sediments does not reach appreciable concentrations until about 90 percent of seawater sulfate is removed by sulfate-reducing bacteria. This is in agreement with laboratory studies of anoxic marine sediments sealed in jars, which indicate that methane production does not occur until dissolved sulfate is totally exhausted. Upward diffusion of methane or its production in sulfate-free microenvironments, or both, can explain the observed coexistence of measurable concentrations of methane and sulfate in the upper portions of anoxic sediments.