Distributions, influencing factors and fluxes of dissolved methane in the North Yellow Sea, near the Yalu River estuary, China

Abstract

Distributions, influencing factors and sea-to-air flux of dissolved methane (CH4) in the North Yellow Sea near the Yalu River estuary were investigated using a field survey conducted in October 2022. The dissolved CH4 concentration and degree saturation ranged from 3.4 to 34.2 nmol/L and 120%–1059%, respectively. The highest dissolved CH4 concentrations were observed in the northeast inshore areas, especially the area near the Yalu River estuary, and the CH4 concentrations were seen to decrease sharply from inshore to offshore areas. The processes controlling the variability of dissolved CH4 were identified using the two-end-member mixing model and factor analysis. The high dissolved CH4 concentrations in the North Yellow Sea, especially the northeast inshore areas, are ascribed to the input of river-diluted water, while the invasions of the North Yellow Sea Cold Water Mass and Warm Current are important sources of the dissolved CH4 in the bottom water in the southwest offshore area. Overall, the study area is a net source of atmospheric CH4, at a rate of 2.9–133.8 μmol/(m2·day), while the flux in the inshore area are significantly higher than in the offshore area. It is, therefore, the CH4 release from the estuary and marginal seas, which are impacted by human activities, that are a target hot-spot for CH4 emission reduction.