Complex gas hydrate system in a gas chimney, South China Sea

Ye, Jian‐Liang; Wei, Jiangong; Jiang, Liang; Lu, Jing; Lu, Hailong; Zhang, Wei

doi:10.1016/j.marpetgeo.2019.03.023

Cited by 136 publications

(90 citation statements)

References 57 publications

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“…The dating of bivalve shells and seep carbonates revealed episodic changes in seepage activity [25]. Other features of methane seeps, such as acoustic plume, acoustic void, chimney structures, and pockmarks, were also reported at the Haima cold seeps and its surrounding area [27][28][29][30][31]42]. The sampling sites are ca.…”

Section: Geological Backgroundmentioning

confidence: 99%

“…The Haima cold seeps were recently discovered on the northwestern slope of SCS [25]. 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].…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2019

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The Haima cold seeps are active cold seep areas that were recently discovered on the northwestern slope of the South China Sea (SCS). Three piston cores (CL30, CL44, and CL47) were collected within an area characterized by bottom simulating reflectors to the west of Haima cold seeps. Porewater profiles of the three cores exhibit typical kink-type feature, which is attributed to elevated methane flux (CL30) and bubble irrigation (CL44 and CL47). By simulating the porewater profiles of SO42-, CH4, PO43-, Ca2+, Mg2+, and dissolved inorganic carbon (DIC) in CL44 and CL47 using a steady-state reaction-transport model, we estimated that the dissolved SO42- was predominantly consumed by anaerobic oxidation of methane (AOM) at rates of 74.3 mmol m−2 yr−1 in CL44 and 85.0 mmol m−2 yr−1 in CL47. The relatively high AOM rates were sustained by free gas dissolution rather than local methanogenesis. Based on the diffusive Ba2+ fluxes and the excess barium contents in the sediments slightly above the current SMTZ, we estimated that methane fluxes at core CL44 and CL47 have persisted for ca. 3 kyr and 0.8-1.6 kyr, respectively. The non-steady-state modeling for CL30 predicted that a recent increase in upward dissolved methane flux was initiated ca. 85 yr ago. However, the required time for the formation of the barium front above the SMTZ at this core is much longer (ca. 2.2-4.2 kyr), which suggests that the depth of SMTZ possibly has fluctuated due to episodic changes in methane flux. Furthermore, using the model-derived fractions of different DIC sources and the δ13CDIC mass balance calculation, we estimated that the δ13C values of the external methane in cores CL30, CL44, and CL47 are -74.1‰, -75.4‰, and -66.7‰, respectively, indicating the microbial origin of methane. Our results suggest that methane seepage in the broader area surrounding the Haima cold seeps probably has persisted at least hundreds to thousands of years with changing methane fluxes.

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Section: Geological Backgroundmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

et al. 2019

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“…Seismic profiles show that a large number of gas chimneys and bottom simulating reflectors, indicators of gas migration and gas hydrate, were developed in the QDNB (Fang et al, ; Liang et al, ; Zhang et al, ). The fifth gas hydrate drilling expedition conducted in 2018 recovered natural gas hydrates within a gas chimney system (Wei, Liang, Lu, Zhang, & He, ; Ye et al, ).…”

Section: Geological Settingmentioning

confidence: 99%

Geologically controlled intermittent gas eruption and its impact on bottom water temperature and chemosynthetic communities—A case study in the “HaiMa” cold seeps, South China Sea

Wei

et al. 2020

Geological Journal

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In 2018, the Institute of Deep Sea Science and Engineering investigated the “HaiMa” cold seeps in the northwestern part of the South China Sea using R/V “TanSuoYiHao” with manned submersible “ShenHaiYongShi.” Ten dives were conducted to make seafloor observations at six cold seeps (HM‐1 to HM‐6). By combining multibeam echosounder data and seismic profiles, a systematic study was made to understand fluid migration, accumulation, and eruption and their impact on the ecosystem. Location of gas flares observed in the multibeam echosounder data obtained in 2016 and 2018 is different, indicating a year‐level spatiotemporal variation of the cold seep activities. Seafloor observation shows that massive carbonates occur in HM‐1 and HM‐5, indicating two long‐term existed cold seeps (several thousands of years). Mussels covered large areas of HM‐2, HM‐3, and HM‐6, suggesting young active seeps. Elevated seabed with broken edges, mud cones, clay pools, and sudden increased water turbidity and temperature was encountered in multiple areas, which are inferred to be associated with shallow gas accumulation and eruption. Methane content of the headspace gas from shallow sediment at HM‐2 is higher than 99.5% with δ13C‐CH4 of −71.5‰ to 72.3‰, indicating a biogenic gas origin. It is inferred that the gas is sourced by reservoirs at ~1,280 mbsf, which are characterized by low frequency in the seismic profiles. Gas migrates from the gas reservoir to the top of the Basel uplift along the slope. High‐angle faults are widely developed between 800 and 240 mbsf, facilitating gas migration from the top of the Basel uplift to the shallow sediment. Most of the faults did not reach the seafloor; therefore, gas accumulated at shallow depth exhibits bright spots in the seismic profiles. When the pore pressure overcomes the overburden sediment, fractures will be created for gas entering the water column. Gas emission provides a pressure release mechanism; therefore, most fractures did not reach the seafloor as observed in the seismic profiles. However, it is speculated that gas emission will stop with growth of gas hydrates and authigenic carbonates, which continuously decrease the permeability of the original fractures. Pressure will be built up, and new fractures will be generated, leading to the formation of new seeps. This mechanism is inferred to result in the spatiotemporal variation of cold seep activity and affects the development of the cold seep ecosystem.

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“…In terms of total carbon release, describing methane flow in mole values may lead to an underestimation since higher hydrocarbons are not considered, e.g., Mega Plumes 1 and 2 in [24]. However, because it was assumed that pure methane was released from the Site F and Haima cold seeps [57][58][59], the volume and gas amount (moles) were calculated separately for each bubble size measurement and then averaged for more accurate results. The estimated methane bubble flow rates for seeps 1-3 at Site F were 0.4, 4.4, and 0.1 L h -1 , respectively, and seep 1 of the Haima cold seeps yielded a rate of 12.6 L h -1 .…”

Section: Methane Bubble Fluxmentioning

confidence: 99%

Using Time-Series Videos to Quantify Methane Bubbles Flux from Natural Cold Seeps in the South China Sea

Dong

Tao

et al. 2020

Minerals

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Natural cold seeps are an important source of methane and other greenhouse gases to the ocean and atmosphere in the marine environment. Accurate quantification of methane bubble fluxes from cold seeps is vital for evaluating their influence on the global methane budget and climate change. We quantified the flux of gas bubbles released from two natural cold seep sites in the South China Sea: one seep vent in the Haima cold seeps (1400 m depth) and three seep vents at Site F (1200 m depth). We determined bubble diameter, size distribution, and bubble release rate using image processing techniques and a semiautomatic bubble-counting algorithm. The bubble size distributions fit well to log-normal distribution, with median bubble diameters between 2.54 mm and 6.17 mm. The average bubble diameters and release rates (4.8–26.1 bubbles s−1) in Site F was lower than that in Haima cold seeps (22.6 bubbles s−1), which may be attributed to a variety of factors such as the nature of the gas reservoir, hydrostatic pressure, migration pathways in the sediments, and pore size. The methane fluxes emitted at Haima cold seeps (12.6 L h−1) and at Site F (4.9 L h−1) indicate that the Haima and Site F cold seeps in the South China Sea may be a source of methane to the ocean. However, temporal variations in the bubble release rate and the geochemical characteristics of the seeps were not constrained in this study due to the short observational time interval.

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Complex gas hydrate system in a gas chimney, South China Sea

Cited by 136 publications

References 57 publications

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

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

Geologically controlled intermittent gas eruption and its impact on bottom water temperature and chemosynthetic communities—A case study in the “HaiMa” cold seeps, South China Sea

Using Time-Series Videos to Quantify Methane Bubbles Flux from Natural Cold Seeps in the South China Sea

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