Tracing the sources of excess methane in subsurface seawater using both stable carbon and hydrogen isotope ratios as tracers

Abstract

Methane (CH₄) is typically generated in anaerobic environments and is known to be oxidatively decomposed in aerobic environments. However, seawater, particularly surface water, often contains excess dissolved CH₄ compared to atmospheric equilibrium. This phenomenon is known as the 'methane paradox.'This study aimed to elucidate the sources of excess CH₄ in seawater by measuring d¹³C and d²H. It was confirmed that many samples from subsurface seawater exhibited CH₄ supersaturation compared to atmospheric equilibrium. The d¹³C value of excess CH₄in seawater was determined to be an average of -54.7(per mille). Furthermore, this study revealed the d²H values of excess CH₄, averaging -29.4(per mille) for the first time in the world.The high d²H values observed in this study are believed to result from residual effects of oxidative decomposition during the release of methane generated under reduction environments within sinking particles into oxidative seawater.