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

A Quantitative Assessment of Methane-Derived Carbon Cycling at the Cold Seeps in the Northwestern South China Sea

by Junxi Feng 1,2,†, Niu Li 3,†, Min Luo 4,*, Jinqiang Liang 1,*, Shengxiong Yang 1, Hongbin Wang 1 and Duofu Chen 4,5
1
MLR Key Laboratory of Marine Mineral Resources, Guangzhou Marine Geological Survey, Guangzhou 510075, China
2
School of Marine Sciences, Sun Yat-sen University, Guangzhou 510006, China
3
CAS Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China
4
Shanghai Engineering Research Center of Hadal Science and Technology, College of Marine Sciences, Shanghai Ocean University, Shanghai 201306, China
5
Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266061, China
*
Authors to whom correspondence should be addressed.
The authors contributed equally to this work.
Minerals 2020, 10(3), 256; https://doi.org/10.3390/min10030256
Received: 31 January 2020 / Revised: 29 February 2020 / Accepted: 3 March 2020 / Published: 12 March 2020
(This article belongs to the Special Issue Marine Geology and Minerals)
Widespread cold seeps along continental margins are significant sources of dissolved carbon to the ocean water. However, little is known about the methane turnovers and possible impact of seepage on the bottom seawater at the cold seeps in the South China Sea (SCS). We present seafloor observation and porewater data of six push cores, one piston core and three boreholes as well as fifteen bottom-water samples collected from four cold seep areas in the northwestern SCS. The depths of the sulfate–methane transition zone (SMTZ) are generally shallow, ranging from ~7 to <0.5 mbsf (meters below seafloor). Reaction-transport modelling results show that methane dynamics were highly variable due to the transport and dissolution of ascending gas. Dissolved methane is predominantly consumed by anaerobic oxidation of methane (AOM) at the SMTZ and trapped by gas hydrate formation below it, with depth-integrated AOM rates ranging from 59.0 and 591 mmol m−2 yr−1. The δ13C and Δ14C values of bottom-water dissolved inorganic carbon (DIC) suggest discharge of 13C- and 14C-depleted fossil carbon to the bottom water at the cold seep areas. Based on a two-endmember estimate, cold seeps fluids likely contribute 16–26% of the bottom seawater DIC and may have an impact on the long-term deep-sea carbon cycle. Our results reveal the methane-related carbon inventories are highly heterogeneous in the cold seep systems, which are probably dependent on the distances of the sampling sites to the seepage center. To our knowledge, this is the first quantitative study on the contribution of cold seep fluids to the bottom-water carbon reservoir of the SCS, and might help to understand the dynamics and the environmental impact of hydrocarbon seep in the SCS. View Full-Text
Keywords: porewater geochemistry; bottom seawater; methane-derived carbon cycling; cold seeps; South China Sea porewater geochemistry; bottom seawater; methane-derived carbon cycling; cold seeps; South China Sea
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Feng, J.; Li, N.; Luo, M.; Liang, J.; Yang, S.; Wang, H.; Chen, D. A Quantitative Assessment of Methane-Derived Carbon Cycling at the Cold Seeps in the Northwestern South China Sea. Minerals 2020, 10, 256.

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