Black Sea methane geochemistry

William S Reeburgh,Bess B Ward,Stephen C Whalen,Kenneth A Sandbeck,Katherine A Kilpatrickt,Lee J Kerkhof

doi:10.1016/s0198-0149(10)80030-5

Abstract

Methane concentrations and oxidation rates were measured in the water column and sediments of the Black Sea at a central station during leg 5 of the 1988 U.S.—Turkey Black Sea Expedition. Methane concentrations were 10 nM in the upper 100 m, increased to 11 μM at 550 m, and were uniform to the bottom. Water column methane oxidation rates were measured using two independent radiotracer techniques: tracer level additions of 3H−CH 4, and non-tracer level additions of 14C−CH 4. The methods agree within a factor of two. Methane oxidation rates were low in the surface 100 m and increased to relatively uniform values of 0.6 μM y −1 below 500 m. Sediment methane concentration and oxidation rate distributions showed that shelf and slope sediments were methane sources, while deep basin sediments were methane sinks. These measurements were used to construct a methane budget for Black Sea waters. Microbially mediated anaerobic methane oxidation is the dominant water column methane sink, followed by evasion to the atmosphere, abyssal plain sediment consumption and outflow at the Bosporus. The source of methane appears to be anoxic, high deposition rate shelf and slope sediments. The water column oxidation rate measurements suggest a short (5–20 year) residence time for methane in the Black Sea, indicating a higher geochemical activity than previously believed. The quantity of carbon participating in the Black Sea methane cycle is equivalent to about 0.5% of the primary production.

Highlights

METHANE is a radiatively active trace gas that is presently undergoing an atmospheric concentration increase of about 1% per year (KHALIL and RASMUSSEN, 1983; STEELE et al, 1987; BLAKE and ROWLAND, 1988)
Large inventories of methane are present in anoxic marine sediments and anoxic water columns, the ocean is a minor source (1.9%) in the global methane budget
Anaerobic methane oxidation has been studied in the water columns of the Cariaco Trench (REEBURGH, 1976; WARD et al, 1987; SCRANTON, 1988) and Saanich Inlet (WARD et al, 1989)

Summary

INTRODUCTION

METHANE is a radiatively active trace gas that is presently undergoing an atmospheric concentration increase of about 1% per year (KHALIL and RASMUSSEN, 1983; STEELE et al, 1987; BLAKE and ROWLAND, 1988). Black Sea sediment hydrocarbon measurements (including methane) were reported by HUNT (1974) and HUNT and WHELAN (1978), but limited information on methane depth distributions is available. Anaerobic methane oxidation has been studied in the water columns of the Cariaco Trench (REEBURGH, 1976; WARD et al, 1987; SCRANTON , 1988) and Saanich Inlet (WARD et al, 1989). A strong geochemical case has been advanced for anaerobic methane oxidation, involving model (REEBURGH, 1976; ALPERIN, 1988), rate (REEBURGH, 1980; ALPERIN and REEBURGH, 1984, 1985) and stable isotope (ALPERIN et al, 1988) studies in anoxic marine sediments. Anaerobic methane oxidation in anoxic marine sediments is an important sink in the global methane budget, consuming between 20 Tg y-l (HENRICHS and REEBURGH, 1987) and 100 Tg Y-I (REEBURGH, 1989), and limiting net transport of methane to the water column and to the atmosphere. We performed these analyses at a central station with a view toward assembling a methane budget for the Black Sea and comparing Black Sea methane geochemistry with that of other anoxic basins

METHODS AND MATERIALS

RESULTS

DISCUSSION

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CONCLUSIONS

Future work