Exploring Sources and Biogeochemical Dynamics of Dissolved Methane in the Central Bohai Sea in Summer

Abstract

To clarify the sources and biogeochemical dynamics of dissolved methane (CH 4 ) in shallow-water coastal oceans, we investigated the concentrations, sea-to-air fluxes, and net cycling rate of CH 4 in the central Bohai Sea in summer 2018. During the survey, both summertime stratification and dissolved oxygen (DO) deficit were observed. In the surface layer, CH 4 concentration ([CH 4 ]) ranged from 3.08 to 10.27 nmol kg −1 . The average sea-to-air flux was estimated at 6.46 ± 3.32 µmol m −2 d −1 , indicating that the Bohai Sea serves as a source of atmospheric CH 4 . In the bottom layer, [CH 4 ] ranged from 4.29 to 31.04 nmol kg −1 . The downward increase in [CH 4 ] within the water column indicated substantial sources of CH 4 in the seafloor. Based on the complex relationship between the saturation ratio of CH 4 and DO, three types of CH 4 release from the seafloor were identified, including diagenesis of buried organic matter, natural leakage from geological settings, and anthropogenic release from offshore oil/gas development. The saturation ratios of CH 4 in bottom waters were positively correlated with the degree of stratification of the water column. Using a two-layer box model, sedimentary CH 4 release rates at two oxygen-deficient stations were estimated to be 56.0-60.8 µmol m −2 d −1 , which was much higher than the sea-to-air fluxes. The modeling results also indicated that the majority of seafloor released CH 4 (>90%) was consumed in the water column of this shallow-water coastal sea. This is the first comprehensive study on CH 4 cycling in the Bohai Sea. This work shows the indicative role of DO in identifying multiple CH 4 sources and highlights the dominance of water stratification and microbial consumption in local CH 4 dynamics. KEY POINTS -Dissolved CH 4 exhibited complex relations with oxygen in the Bohai Sea. -Three types of CH 4 sources from seafloor were identified. -Stratification hampered the upward transport of dissolved CH 4 . -Strengths of CH 4 cycling terms are quantified.

A Three-Dimensional Spatial Interpolation Method and Its Application to the Analysis of Oxygen Deficit in the Bohai Sea in Summer

“…This pattern may be attributable to the weak stratification and anticyclone circulation of the central ridge (the lighter-colored part of the central Bohai Sea in Figure 1) weakening the connectivity between the north and south oxygen-deficit regions, eventually leading to the emergence of two core oxygen-deficit regions [19]. At the same time, it is worth noting that the oxygen deficit area in 2018 was more northerly [27]; thus, the mechanism is worthy of further investigation. To further investigate the relationship among DO, temperature, and salinity in the bottom of the Bohai Sea, we selected a mid-transect (Figure 1b) that traversed the oxygen deficient region.…”

“…Furthermore, current research on hypoxia primarily focuses on the impact of water stratification and biochemical processes on the extent of hypoxia within a specific area [24][25][26]. This emphasis is particularly evident in studies conducted in the Bohai Sea region [16,20,22,27]. Nevertheless, there exists a paucity of analysis regarding other physical mechanisms and the spatial distribution and impact of oxygen-deficient areas.…”

High photoreactivity of chromophoric dissolved organic matter derived from Ulva prolifera and Sargassum

“…CH 4 and CO were measured using a static headspace method as described by Zhang et al (2020). Briefly, water samples were transferred to a 50-mL glass syringe fitted with three-way valves (glass fiber-reinforced polypropylene).…”

Distribution and geological controls of the seabed fluid flow system, the central‐western Bohai Sea: A general overview

“…The Bohai Sea is a semi‐closed marginal sea in eastern China, with an average depth of 18.7 m and a relatively flat topography (Zhang et al, 2020). The study area lies in the central‐west Bohai Sea (Figure 1c), which belongs to the offshore portion of the BBB, principally consisting of a part of the Bozhong, Huanghua, Jiyang depressions and the Chengning Uplift (Figure 1b).…”

“…The Bohai Sea is heavily explored and hosts widespread sandstone reservoirs (Xu et al, 2016), which in turn may offer a potential location for CO 2 geological storage, like the Sleipner, North Sea. Both thermogenic and biogenic gases are detected in surface sediments and bottom water analyses (Hu et al, 2009; Zhang et al, 2016, 2020), suggesting significant hydrocarbon leakages in the Bohai Sea. However, fluid flow system in the Bohai Sea is relatively poorly understood until now.…”