Geochemical record of methane seepage in authigenic carbonates and surrounding host sediments: A case study from the South China Sea

Abstract

Sediments at marine methane seep sites provide potential archives of past fluid flow that serve to explore seepage activities over time. Three gravity cores (D-8, D-F, and D-7) were collected from seep sites on the northern slope of the South China Sea where gas hydrates were drilled in the subsurface. Various carbon and sulfur contents, δ 13 C values of total inorganic carbon (δ 13 C TIC), δ 34 S values of chromium reducible sulfur (δ 34 S CRS), trace element contents, grain size, and AMS 14 C dating of planktonic Foraminifera in the sediments were determined to explore the availability of related proxies at seeps and to trace past methane seepage activities. Evidence for the presence of methane seepage and consequently anaerobic oxidation of methane comes from the occurrence of 13 C-depleted authigenic carbonate nodules (δ 13 C values as low as-49‰) discovered at an interval of 150 cm-200 cm in core D-7. This finding is supported by high S/C ratios and molybdenum enrichment in the same interval. However, low contents of CRS and negative δ 34 S CRS values are present. It is suggested to reflect a transient methane seepage event, which continued for about 1 ka based on the 14 C ages. Cores D-8 and D-F have δ 13 C TIC values close to zero, low S/C ratios and CRS contents, negative δ 34 S CRS values, and no trace element enrichment, suggesting a negligible impact of methane-seepage on the sediments. The negative δ 34 S CRS values of the studied seep-impacted and background sediments suggest that the application of δ 34 S CRS alone as a proxy to identify AOM-related process may be insufficient. Sediment carbon-sulfur-trace element systematics and 14 C ages used here have the potential to be a promising tool to recognize transient methane seepages and constrain their timescales.

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

“…In this model, phosphate concentration profiles were used to constrain the initial age of POC. Due to the lack of phosphate concentration profiles in the cores of Dongsha, initial ages in Dongsha (5000 yr) were adopted from Hu et al [54]. For the same reason, the initial ages of 2015XS-44 and 2015XS-50 in Xisha were assumed to be the same as that of 2015XS-R2 derived from a measured phosphate concentration profile.…”

Magnetic Mineralogical Approach for the Exploration of Gas Hydrates in the Bay of Bengal

“…Recent studies have demonstrated that in rapidly depositing methane‐rich marine sedimentary environments, enrichment of CRS (δ 34 S CRS ) and molybdenum (Mo) can be used as proxies to identify paleo‐SMTZs and methane seepage events in marine sediments (Chen et al, ; Hu et al, ; Li et al, ; Lin et al, ; Peketi et al, , ). Negative correlation between CRS content and χ lf in sediment cores MD161/Stn8, MD161/Stn13, and NGHP‐01‐10D (Figure e; Badesab et al, ) suggests that magnetic tracking in combination with geochemical proxies (CRS, δ 34 S CRS , Mo) can be used to constrain paleo‐SMTZ and methane seepage events in marine sediments.…”

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

“…Moreover, by comparing the depth profiles of CaCO 3 content in CL44 and CL47 with that in an adjacent core (SO49-37KL) with established Marine Isotope Stage, we found that the upper~2 m sediments of CL44 and CL47 represent normal hemipelagic background deposition during the Holocene (Figure S3) 8 Geofluids Figure S4). In contrast, the layers at the interval of~1.4 to~4.2 mbsf in CL44 and~1.8 to~5 mbsf in CL47 exhibiting high Si, Ti, and Zr/Rb contents and coarser grain sizes ( Figure S4) suggest elevated input of detrital fraction during sea-level lowstands [71,72]. In addition, the flat seafloor topography in the study area also precludes the occurrence of abrupt depositional event such as landslide (Figure 1).…”