Water column methane oxidation adjacent to an area of active hydrate dissociation, Eel river Basin

David L Valentine,Douglas C Blanton,William S Reeburgh,Miriam Kastner

doi:10.1016/s0016-7037(01)00625-1

Abstract

The role of methane clathrate hydrates in the global methane budget is poorly understood because little is known about how much methane from decomposing hydrates actually reaches the atmosphere. In an attempt to quantify the role of water column methanotrophy (microbial methane oxidation) as a control on methane release, we measured water column methane profiles (concentration and δ13C) and oxidation rates at eight stations in an area of active methane venting in the Eel River Basin, off the coast of northern California. The oxidation rate measurements were made with tracer additions of 3H-CH4.Small numbers of instantaneous rate measurements are difficult to interpret in a dynamic, advecting coastal environment, but combined with the concentration and stable isotope measurements, they do offer insights into the importance of methanotrophy as a control on methane release. Fractional oxidation rates ranged from 0.2 to 0.4% of ambient methane per day in the deep water (depths >370 m), where methane concentration was high (20–300 nM), to near-undetectable rates in the upper portion of the water column (depths <370 m), where methane concentration was low (3–10 nM). Methane turnover time averaged 1.5 yr in the deep water but was on the order of decades in the upper portion of the water column. The depth-integrated water column methane oxidation rates for the deep water averaged 5.2 mmol CH4 m−2 yr−1, whereas the upper portion of the water column averaged only 0.14 mmol CH4 m−2 yr−1; the depth-integrated oxidation rate for deep water in the 25-km2 area encompassing the venting field averaged 2 × 106 g CH4 yr−1. Stable isotope values (δ13C-CH4) for individual samples ranged from −34 to −52‰ (vs. PDB, Peedee belemnite standard) in the region. These values are isotopically enriched relative to hydrates in the region (δ13C-CH4 about −57 to −69‰), further supporting our observations of extensive methane oxidation in this environment.

Highlights

Methane (CH4) is a radiatively important trace gas in the atmosphere, and its concentration has increased dramatically because of anthropogenic activity
Methane hydrates are only stable under limited conditions and are thought to be especially abundant in sediments along the world’s continental margins (Kvenvolden et al, 1993)
All samples were taken during an August 1999 cruise of the R/V Melville to the Eel River Basin, over the continental margin off the coast of northern California (Fig. 1)

Summary

Introduction

Methane (CH4) is a radiatively important trace gas in the atmosphere, and its concentration has increased dramatically because of anthropogenic activity. Past changes in CH4 concentration from glacials to interglacials have contributed to the idea that CH4 may play an important role in modulating climate (Cicerone and Oremland, 1988). The oceans constitute a vast CH4 reservoir, mainly in the form of CH4 hydrates, they are thought to play only a small role in the modern CH4 cycle (Cicerone and Oremland, 1988). Little is known about the magnitude and fate of CH4 emissions from continuous venting systems (Hovland et al, 1993). Methane hydrates are only stable under limited conditions and are thought to be especially abundant in sediments along the world’s continental margins (Kvenvolden et al, 1993).

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Conclusion