A Comparative Study Of The Distribution Of Dissolved Methane And Its Sources And Sinks In The Bohai Sea And Western North Yellow Sea

Methane(CH4)is an important greenhouse gas and plays an important role in global climate change.Although coastal seas account for a small proportion of the global ocean area,CH4 emissions from coastal seas account for 75%of global oceanic fluxes.Sea-to-air CH4 fluxes are controlled by different biogeochemical processes,with significant seasonal and regional variations.So far,distribution and sea-to-air flux of CH4 have been entensively investigated in the Bohai Sea and North Yellow Sea,however,their biogeochemical controls have been scarcely studied.To address this question,we investigated the central Bohai Sea from August 25th to September 1st,2018 aboard the R/V Haili research vessel,and investigated the Bohai Sea and western North Yellow Sea from July 26th to August 1st aboard R/V Beidou and from December 10th to 18th,2019 aboard R/V Dongfanghong III research vessel.We investigated CH4 concentration([CH4]),dissolved oxygen(DO)content,and related hydrological and meteorological parameters in the Bohai Sea and western North Yellow Sea.Then we studied seasonal variation between summer and winter and regional difference between the Bohai Sea and western North Yellow Sea.Finally,we estimated CH4 release from Bohai seafloor and interpreted the budget of dissolved CH4 in the Bohai Sea.The main results of this article are as follows:1.In summer,the Bohai Sea and western North Yellow Sea are stratified,and[CH4]accumulates in the bottom of the Bohai Sea,the vertical distribution of CH4 in the western North Yellow Sea is relatively uniform;In the winter,CH4 showed little vertical variation due to well-mixed water column in the Bohai Sea and western North Yellow Sea.The[CH4]in the Bohai Sea and western North Yellow Sea in summer is higher than that in winter,and the[CH4](12.75±5.91 nmol kg-1)in the bottom water of the Bohai Sea is about twice that of the bottom water in winter(5.80±2.19 nmol kg-1),seasonal evolution of the hydrodynamic environment was the main factor affecting the spatial and temporal changes of[CH4]distribution in the Bohai Sea,but it has no obvious influence on the western North Yellow Sea.2.In summer,the subsurface layer[CH4]was the largest in the oxygen-depleted sea area of the Bohai Sea,which was speculated to in situ biological production.In summer,the distribution of[CH4]in the Bohai Sea water was higher than that in the surface,which indicates that seafloor release of CH4 is an important source of dissolved CH4 in the Bohai Sea.In contrast,the vertical distribution of CH4 in the western North Yellow Sea in summer is relatively uniform,while the release of CH4 from the seafloor of the western North Yellow Sea is not obvious.3.In the Bohai Sea and western North Yellow Sea,CH4 was supersaturated during the investigation and sea-to-air CH4 fulx was estimated to be 0.43×10-2 Tg yr-1,accounting for 0.03%of the global annual oceanic CH4 fluxes.Since the total area of the Bohai Sea and western North Yellow Sea also accounts for 0.03%of the global ocean area,the western North Yellow Sea and Bohai Sea has a minor contribution to global sea-to-air CH4 flux.4.In the Bohai Sea,three types of CH4 sources in the seafloor were identified according to the relationship between the saturation ratio of CH4 and DO.They were diagenesis of sediment organic matter and subsequent sedimentary release,oil/gas leakage,and natural leakage from seabed.Incubation experiments carried out at the oxygen-depleted area in summer showed that CH4 decreased with prolonged incubation time with an initial decreasing rate of 0.124?0.153 nmol kg-1 h-1.Based on a steady box model,sedimentary CH4 release rate was calculated to be 52.2?68.6?mol m-2d-1 and approximately 93%of the CH4 released into the water column is consumed by micribial consuption.