Using chemical compositions of sediments to constrain methane seepage dynamics: A case study from Haima cold seeps of the South China Sea

Abstract

Cold seeps frequently occur at the seafloor along continental margins. The dominant biogeochemical processes at cold seeps are the combined anaerobic oxidation of methane and sulfate reduction, which can significantly impact the global carbon and sulfur cycles. The circulation of methane-rich fluids at margins is highly variable in time and space, and assessing past seepage activity requires the use of specific geochemical markers. In this study, we report multiple sedimentary proxy records for three piston gravity cores (QDN-14A, QDN-14B, and QDN-31) from the Haima seep of the South China Sea (SCS). By combining total organic carbon (TOC), total inorganic carbon (TIC), total nitrogen (TN), total sulfur (TS), acid insoluble carbon and sulfur isotope (δ13Corganic carbon and δ34Sacid-insoluble), and δ34S values of chromium reducibility sulfur (δ34SCRS), as well as carbon isotopes of TIC (δ13CTIC) in sediments, our aim was to provide constraints on methane seepage dynamics in this area. We identified three sediment layers at about 260-300 cm, 380-420 cm and 480-520 cm sediment depth, characterized by particular anomalies of low δ13CTIC values and high TS content, high TS and CRS contents, and high δ34Sacid-insoluble and δ34SCRS values, respectively. On this basis, we propose that these sediment horizons correspond to distinct methane release events preserved in the sediment record. While the exact mechanisms accounting for the presence (or absence) of these particular geochemical signals in the sediment are not known, we propose that they correspond to variations in methane flux and their duration through time. Overall, our results suggest that sedimentary carbon and sulfur and their isotopes are useful tracers for better understanding of methane seepage dynamics over time. Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.

An Areal Assessment of Subseafloor Carbon Cycling in Cold Seeps and Hydrate-Bearing Areas in the Northern South China Sea

“…Since the 1990's, high-resolution seismic surveys have been carried out to delineate the distribution of bottom-simulating reflectors (BSR) and determine the prospecting area of gas hydrates. Through several hydrate survey expeditions, six promising hydrate-bearing areas including Dongsha, SW Taiwan, Xisha, Qiongdongnan, Shenhu, and Beikang have been confirmed to date [13][14][15][16][17][18][19][20]. Besides, more than 40 cold seepage sites have been indicated by the occurrence of 13 C-depleted authigenic carbonate and seep-associated fauna and anomalous pore water and sediment geochemistry influenced by fluid seepage [6,[21][22][23][24][25].…”

Methane Source and Turnover in the Shallow Sediments to the West of Haima Cold Seeps on the Northwestern Slope of the South China Sea

“…The difference of estimated duration of constant methane flux between these two approaches may suggest that the barite front was not a result of the recent increase in methane flux inducing the kink-type sulfate profile. Instead, it is more likely that the SMTZ has experienced several fluctuations in depth, considering the episodic pulses of upward methane flux which have occurred in this area as shown by previous studies [25,32,33]. However, this decoupled record between sediments and porewaters is commonly observed at cold seeps [74][75][76][77] and is considered to reflect the variations of methane fluxes and the resulting SMTZ in the sedimentary column [74].…”

“…Previous studies have suggested that microbial methane was the main hydrocarbon source with minor contributions of oil-derived compounds and pyrolysis gas at the Haima cold seeps [32,33,89,90]. Gas chimney structures, which are well developed around the Haima cold seeps, might serve as conduits for the upward migration of biogenic gas from the underlying free gas reservoir beneath the GHSZ to shallow sediments [29,42].…”

“…Several sites with gas bubbling identified by hydroacoustic anomalies, and shallow gas hydrates were found around this area [27][28][29][30][31]. Recent studies have shown a pronounced temporal change in methane seepages and a potential lateral migration of methane-bearing fluid along more permeable sand-bearing layer at Haima cold seeps [25,32,33]. Nevertheless, our quantitative understanding of the methane dynamics in this area remains scarce.…”

Conjugated enrichments in arsenic and antimony in marine deposits used as paleoenvironmental proxies: preliminary results

“…As and Sb are occasionally reported for sediments that underwent the influence of hydrothermal-fluid circulation or exhalation (plumes; Zeng et al, 2018 and references therein); however, associations of these two elements have been seldom mentioned for sediments impacted by cold (hydrocarbon) seepage (Tribovillard et al, 2013;Hu et al, 2014;Chen et al, 2016;Wang et al, 2018Wang et al, , 2019aWang et al, , 2019bZwicker et al, 2018). Regarding cold seeps, most often, analyses are performed on associated authigenic minerals such as carbonates and/or sulfides, but only rarely on bulk sediments as is the case in the present study.…”