INTRODUCTION

Methane (CH4) is a major product of subseafloor microbial metabolic activity, and quantification of its abundance in sediments is necessary to understand subseafloor biology and biogeochemistry (Martens and Berner, 1974). However, often it is not possible to accurately determine in situ abundances by the standard technique of sediment headspace analysis (Kvenvolden and McDonald, 1986). This problem occurs when gas voids form during core retrieval and sampling, stripping CH4 from the pore fluid. The formation of gas voids can occur when the total gas pressure exceeds the confining pressure within the core liner. Thus, as confining pressure decreases during core retrieval and sampling, gas voids and bubbles commonly collect within the liners of cores recovered from sediments with high CH4 concentrations. The pressure core sampler (PCS) was designed to overcome this problem and quantify the abundance of methane hydrates by recovering sediments under in situ pressure (Pettigrew, 1992).

A principal aim of Ocean Drilling Program (ODP) Leg 201 was the examination of microbial activity in sediments that included those with high methane abundances and generated gas voids on recovery (D'Hondt, Jørgensen, Miller, et al., 2003). The PCS was deployed successfully during Leg 201; however, its use requires significant drilling and processing time (Dickens et al., 2003). Here we present the basis for and preliminary tests of an alternative method for the quantification of CH4 in gas-rich sediments that does not require significant additional drilling or processing time.

NEXT