A clathrate source for Black Sea Basin methane—?

William S. Reeburgh, Ph.D.
University of California, Irvine

Wednesday, January 13, 1999
3:00 p.m.—Pacific Forum

Following a brief summary of methane biogeochemistry, this seminar focuses on methane data obtained during the 1988 U.S.-Turkish Black Sea Expedition. The water column data consists of methane concentration profiles, oxidation rate profiles measured using two independent radioactive tracers, C³H₄ and ¹⁴CH₄, and a recently measured stable isotope (d ²H-CH₄ and d ¹³C-CH₄) profile from a central station. Sediment methane profiles were measured on cores collected at a shelf and deep-basin station.

The water column profiles show uniform and high methane concentrations and oxidation rates at depths below 500 m, suggesting a methane residence time on the order of decades. Although shelf sediments are a source of methane, the deep basin sediments consume methane and cannot supply the deep-water column. Because there is no way to maintain the uniform deep-water column methane distribution with a shelf source, our problem is to find a deep methane source.

The recent carbon and hydrogen stable isotope measurements offer insights into the "missing" methane source. The deep basin methane is isotopically heavy below 500 m, indicating that it is either the most extensively oxidized biogenic methane observed to date, or that dissociating clathrates might be the source. A clathrate source appears to be the most reasonable possibility for maintaining the uniform deep methane concentration and isotope distributions.

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