Regulation of anaerobic methane oxidation in sediments of the Black Sea

N J Knab,B A Cragg,C Borowski,B B Jørgensen,E R C Hornibrook,R J Parkes,R D Pancost,L Holmkvist

doi:10.5194/bg-6-1505-2009

Abstract

Abstract. Anaerobic oxidation of methane (AOM) and sulfate reduction (SRR) were investigated in sediments of the western Black Sea, where upward methane transport is controlled by diffusion. To understand the regulation and dynamics of methane production and oxidation in the Black Sea, rates of methanogenesis, AOM, and SRR were determined using radiotracers in combination with pore water chemistry and stable isotopes. In the Danube Canyon and the Dnjepr palaeo-delta AOM did not consume methane effectively and upwards diffusing methane created an extended sulfate-methane transition zone (SMTZ) that spread over more than 2.5 m and was located in brackish and limnic sediment. Measurable AOM rates occurred mainly in the lower part of the SMTZ, sometimes even at depths where sulfate seemed to be unavailable. The inefficiency of methane oxidation appears to be linked to the paleoceanographic history of the sediment, since in all cores methane was completely oxidized at the transition from the formerly oxic brackish clays to marine anoxic sediments. The upward tailing of methane was less pronounced in a core from the deep sea in the area of the Dnjepr Canyon, the only station with a SMTZ close to the marine deposits. Sub-surface sulfate reduction rates were mostly extremely low, and in the SMTZ were even lower than AOM rates. Rates of bicarbonate-based methanogenesis were below detection limit in two of the cores, but δ13C values of methane indicate a biogenic origin. The most δ13C- depleted isotopic signal of methane was found in the SMTZ of the core from the deep sea, most likely as a result of carbon recycling between AOM and methanogenesis.

Highlights

IntroductionThe process of anaerobic oxidation of methane (AOM) is widespread in continental margin sediments and occurs in a variety of different environments, including diffusion controlled sediments (Iversen and Jørgensen, 1985), sediments containing shallow gas accumulations (Niemann et al, 2005; Treude et al, 2005b), gas-hydrate bearing sediments (Treude et al, 2003; Joye et al, 2004; Orcutt et al, 2004), and mud volcanoes (Niemann et al, 2006), In the western part of the Black Sea, which is dominated by an extensive shelf in the northwest and by river deltas of the Danube and Dnjepr rivers, the sediment contains large amounts of methane, and numerous active methane seeps (Popescu et al, 2001)
Tailing of methane was less pronounced in core P824GC, where the sulfate-methane transition zone (SMTZ) was located close to the sapropel and where the limnic sediment was covered by a thick layer of marine deposits
In contrast to the typical increase in δ13C-CH4 due to δ12C CH4 utilization in the Anaerobic oxidation of methane (AOM) zone the stable isotope composition became lighter in core P824GC suggesting that recycling of δ13C-depleted CO2, derived from AOM, occurred in this core in contrast to P771GC and P806GC

Summary

Introduction

The process of anaerobic oxidation of methane (AOM) is widespread in continental margin sediments and occurs in a variety of different environments, including diffusion controlled sediments (Iversen and Jørgensen, 1985), sediments containing shallow gas accumulations (Niemann et al, 2005; Treude et al, 2005b), gas-hydrate bearing sediments (Treude et al, 2003; Joye et al, 2004; Orcutt et al, 2004), and mud volcanoes (Niemann et al, 2006), In the western part of the Black Sea, which is dominated by an extensive shelf in the northwest and by river deltas of the Danube and Dnjepr rivers, the sediment contains large amounts of methane, and numerous active methane seeps (Popescu et al, 2001). Knab et al.: Anaerobic methane oxidation in Black Sea sediments

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Conclusion