Journal
JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES
Volume 122, Issue 11, Pages 2817-2830Publisher
AMER GEOPHYSICAL UNION
DOI: 10.1002/2017JG004014
Keywords
dissolved black carbon; South China Sea; photodegradation
Funding
- National Key Basic Research Special Foundation Program of China [2015CB452902, 2015CB452903]
- NSFC [41476061]
- State Oceanic Administration Program [GASI-03-01-02-02]
- Program for New Century Excellent Talents in Fujian Province Program
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Dissolved black carbon (DBC) is of importance for understanding the marine carbon cycle especially on long time scales owing to its refractory nature in the dissolved organic carbon pool. However, its geochemical behavior is poorly understood in the open oceans due to limited DBC data. Here 86 seawater samples were determined using the benzene-polycarboxylic acid method to investigate the source and fate of DBC in the western South China Sea. The DBC concentration varied from 0.49 to 1.60molL(-1), averaging 0.95molL(-1). Spatially, the Mekong River plume (i.e., MR plume) showed higher DBC concentrations and (B6CA+B5CA)/(B4CA+B3CA) ratios (i.e., R-H/L) in the mixed layer than the distal regions. In addition, the DBC concentration positively correlated with salinity in the mixed layer within the MR plume, indicating the important DBC input from the Mekong River. In intermediate water (500-1500m), DBC varied from 0.70 to 0.85molL(-1) and the R-H/L value continually increased with depth, implying another DBC source with distinct R-H/L ratio comparing with the euphotic zone. Based on a conservative mixing model, the Mekong River and atmospheric deposition collectively input 38-100Gg DBC each year, and over one third of DBC were removed within the euphotic zone, probably via photodegradation and/or adsorption on particles. These results highlighted the different DBC behavior in the euphotic and intermediate zones and lent support to DBC as a proxy for tracing water mixing in the intermediate and deep oceans.
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