Shallow-ocean methane leakage and degassing to the atmosphere: triggered by offshore oil-gas and methane hydrate explorations

Abstract

Both offshore oil-gas exploration and marine methane hydrate recovery can trigger massive CH 4 release from seafloor. During upward transportation of CH 4 plume through water column, CH 4 is subjected to dissolution and microbial consumption despite the protection of hydrate and oil coating on bubbles surface. The ultimate CH 4 degassing to the atmosphere appears to be water-depth dependent. In shallow oceans with water depth less than 100 m, the natural or human-induced leakages or both lead to significant sea-to-air CH degassing from 3.00 to 1 36 10 5 mol m −2 − × 4. µ d 1. To quantify the human-perturbation induced CH 4 degassing, the combination of top-down modeling and bottom-up calculations is essential due to spatial and temporal variability of diffusion and ebullition at water-air interface.

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

“…Table 2 also shows that only 7% of the CH 4 released from seafloor escaped through the water column into the atmosphere, implying that 93% of the CH 4 released from the seafloor was microbially consumed in the water column. This ratio of integrated oxidation rate to sea-to air flux (93: 7 = 13.3) fell into the range of 2.5-19.0 in the Baltic Sea during the stratified season (Steinle et al, 2017), in line with the previous findings that microbial oxidation usually predominates over gas exchange as sinks of CH 4 in an oxygenated water column (Kessler et al, 2011;Zhang and Zhai, 2015).…”

“…Ex situ processes usually refer to (1) CH 4 release from the seafloor, including release from sediment due to diagenesis of buried organic matter Sun et al, 2018) and release from other natural geological settings such as submarine hydrocarbon seeps (e.g., Boles et al, 2001;Reeburgh, 2007) and (2) CH 4 addition from river runoffs, which is significant in estuarine areas (e.g., Ye et al, 2016;Sun et al, 2018). In addition, anthropogenic eutrophication (e.g., Naqvi et al, 2010;Borges et al, 2016) and episodic oil-gas leakages (e.g., Zhang et al, 2014;Zhang and Zhai, 2015) can enhance the CH 4 concentration in the overlying water column.…”

Highly diverse Asgard archaea participate in organic matter degradation in coastal sediments

“…Due to its higher energetic efficiency compared to fermentative processes 43 , aerobic respiration contributes to 50% or more of the total organic matter decomposition in the offshore marine sediments. The expressed gene set for aerobic respiration in Gerdarchaeota, Heimdallarchaeota-AAG and Heimdallarchaeota-MHVG, including the key transcript of cytochrome c oxidase (belonging to Gerdarchaeota) indicates that these Asgard archaea might participate aerobically in organic matter degradation in surface sediments (Fig.…”

“…Like Gerdarchaeota, other Asgard archaea have the potential to perform anaerobic metabolisms (e.g., acetogenesis) under anoxic conditions (subsurface layers) 42 .Notably, Helarchaeota-like mcrA gene transcripts found in unbinned scaffolds (e.g., SZ_4_scaffold_203331_2, fig. S12) highlight the involvement of Helarchaeota in alkane oxidation in coastal sediments, in which ethane and butane might originate from oil-gas seepage or human activities 43 , are preferentially used as revealed by molecular modelling and dynamics studies (fig. S13 and Supplementary Results).…”

Methane in the South China Sea and the Western Philippine Sea

“…As a result, these high mid-depth CH 4 concentrations and especially the one that was 115 nmol L −1 at a depth of 35 m -most likely originated from gas hydrates (Chen and Tseng, 2006) and oil gas beneath the sediment with subsequent vertical migration and advection of CH 4 . The CH 4 that is released from the seafloor will migrate upward through the water column either as dissolved CH 4 or as bubbles (Zhang and Zhai, 2015). Domain D includes the eastern and southern parts of the SCS, from the southern Luzon Strait southward along the western coast of Luzon, Palawan and Borneo.…”