BGC-06\INT-04 Ocean Health and Biological Carbon Pump with BGC-Argo
Constraining the twilight zone remineralization rate in South China Sea basin: insights from a multi-method intercomparison
Chao Xu* , State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China
Mingwang Xiang, State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China
Bingzhang Chen, Department of Mathematics and Statistics, University of Strathclyde, Glasgow, UK
Yibin Huang, NOAA/OAR Pacific Marine Environmental Laboratory, Seattle, WA, USA
Guoqiang Qiu, State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China
Haili Wang, State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China
Bangqin Huang, State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China

Over 70% of organic carbon exported from the surface ocean was consumed by the biotic remineralization processes in the twilight zone (typically defined as the depth between the bottom of the euphotic zone and 1000 meters). The remineralization rate in the twilight zone determines the efficiency of carbon sequestration by biological carbon pump (BCP) and thus significantly affects the atmospheric CO2 concentration. However, we have not been able to measure the remineralization rate accurately in the twilight zone, and fail to satisfy the constraint that the upper carbon supply should meet the carbon demand at depth. Here we quantified the remineralization rate in the oligotrophic South China Sea (SCS) basin by employing multiple methods including the Biogeochemical Argo (BGC-Argo) float observations, in vivo electron transport system (ETS) activity measurement, synthesis of bacterial and zooplankton respiration (BR+ZR). The 100-1000 m depth-integrated remineralization of BGC-Argo floats and BR+ZR were 5.1 ± 0.5 and 6.4 ± 3.0 mol C m-2 yr-1, respectively. However, the depth-integrated remineralization of the ETS was nearly an order of magnitude higher than the other two methods, reaching 44.8 ± 19.6 mol C m-2 yr-1?the ETS method may overestimated the respiration of the twilight zone compared with the other methods. However, the sinking particles can only supply 43.3% of the depth-integrated remineralization (the average of the BGC-Argo floats and BR+ZR) in the twilight zone. Taking into account other carbon sources, downward mixing of dissolved organic carbon (DOC) export (1.1 ± 0.3 mol C m-2 yr-1, 18.3%), dark carbon fixation (DCF) by chemolithoautotrophy prokaryotes (0.7 ± 0.4 mol C m-2 yr-1, 11.7%), lateral transport of organic matter (1.6 ± 0.4 mol C m-2 yr-1, 26.7%) and active transport flux via diel vertical migration of zooplankton (0.01 ± 0.02 mol C m-2 yr-1, <1%), we closed the carbon budget at a basin scale. The comprehensive consideration of organic carbon supply allows us to balance the carbon sources and sinks at the basin scale. Our study highlights the importance of multi-disciplinary and integrated process studies for constraining the biogeochemical processes.