Methane in water columns and sediments of the north western Sea of Japan

Abstract

This paper presents the results of methane measurements in water and sediments, first performed along the north western continental slope and abyssal plain of the Sea of Japan. Methane concentrations in the study area were very low. However, some features of its distribution are revealed. The highest dissolved methane concentrations (10–14nmolkg−1) are characteristic of the pycnocline layer at a depth of 30–50m in the northern shallow stations. With increasing depth, the methane is reduced to minimum values (0.5–1.0nmolkg−1). The greatest variability in methane concentrations was observed in the layers at 0–500m, which can be explained by the hydrodynamic conditions of the environment on the slope. Methane plumes (1.7 and 1.3nmolkg−1) on the northern section were recorded at the depth of 1250 and 1495m, respectively. Plumes (1.2nmolkg−1) are also observed on near bottom layers at the deepest (more than 3000m) stations.CH4 concentration in bottom sediments is also low (from 1nmolkg−1 at 7cm level to 752nmolkg−1 at the 53cm level of the core sediment in the northern part). Reduced sediments in the southern part of the study region have maximal methane concentration for sediment (2549nmolkg−1) at the horizon 44cm bsf (below sea floor) with a smell of H2S. These results assume a close relation of CH4 with sediment properties. A few stations with maximum methane (86–101nmolkg−1) in the surface sediment layer are at the foot of a steep slope. Herewith, the highest abundance of some pericarid species was observed at the points with the highest values of methane concentrations in the surface sediment layer. Weak methane seepage can cause anoxic marine waters.Methane emission from water to the atmosphere is low because its concentration is close to equilibrium in surface water. An improved formula for calculating the methane flux of water into the atmosphere, taking into account high wind speeds, is presented in the paper.