Source and Fate of Dissolved Black Carbon in the Western South China Sea During the Southwest Monsoon Prevailing Season

Abstract

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 1.60 μmol L−1, averaging 0.95 μmol L−1. Spatially, the Mekong River plume (i.e., MR plume) showed higher DBC concentrations and (B6CA + B5CA)/(B4CA + B3CA) ratios (i.e., RH/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 (500–1500 m), DBC varied from 0.70 to 0.85 μmol L−1 and the RH/L value continually increased with depth, implying another DBC source with distinct RH/L ratio comparing with the euphotic zone. Based on a conservative mixing model, the Mekong River and atmospheric deposition collectively input 38–100 Gg 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.

Transport of dissolved black carbon from the Prydz Bay Shelf, Antarctica to the deep Southern Ocean

“…a). These values are higher than those observed in the Southern Ocean (0.61–0.80 μ mol L −1 , Dittmar and Paeng ) and Pacific and Atlantic Oceans (0.09–0.33 μ mol L −1 , Ziolkowski and Druffel ), but comparable to DBC concentrations reported in the South China Sea (0.49–1.60 μ mol L −1 , Fang et al ), a coastal sea surrounded by BC emission (Bond et al ). Previous studies indicate that marine sediments and pore waters are enriched in DBC (Schmidt et al ; Ziolkowski et al ).…”

“…The dense shelf water (DSW) formed around Antarctica in winter sinks along the continental slope down to the deep ocean, leading to the formation of Antarctic Bottom Water (AABW; Ohshima et al 2013;Williams et al 2016). This DSW is a globally important driver of oceanic deep-water formation (Foldvik et al 2004;Ohshima et al 2013) and ventilation (Jacobs 2004;Gordon et al 2009). The downward transport of DSW also carries solutes, including DOC, from shelf regions to the deep waters of the Southern and Global Ocean (Druffel and Bauer 2000;Follett et al 2014).…”

Cycling and Budgets of Organic and Black Carbon in Coastal Bohai Sea, China: Impacts of Natural and Anthropogenic Perturbations

“…The inputs of both DOC and DBC to BS were predominantly (>83%) from NYS through the water inflow in the northern Bohai Strait, with riverine discharge and atmospheric deposition together accounting for <20% of total influx. The water outflow to NYS through the southern Bohai Strait can be regarded as the exclusive export route for DOC and DBC, although a minor fraction may be adsorbed onto sinking particles that subsequently settle and are finally sequestrated in sediments (Coppola et al, ; Fang et al, ). The dominant import and export pathways for both DOC and DBC in the Bohai Strait result in an exchange that is in a state of relative dynamic equilibrium.…”

“…We quantified both particulate and dissolved phases of OC and BC, including particulate OC (POC), particulate BC (PBC), dissolved OC (DOC), and dissolved BC (DBC). Among these, the DBC flux and cycling remain the most poorly constrained (Bao et al, 2017;Fang et al, 2017;Jaffé et al, 2013;Masiello & Louchouarn, 2013). This study had three major objectives: (1) to calculate each process-based OC and BC…”

“…We quantified both particulate and dissolved phases of OC and BC, including particulate OC (POC), particulate BC (PBC), dissolved OC (DOC), and dissolved BC (DBC). Among these, the DBC flux and cycling remain the most poorly constrained (Bao et al, ; Fang et al, ; Jaffé et al, ; Masiello & Louchouarn, ). This study had three major objectives: (1) to calculate each process‐based OC and BC flux in coastal BS, (2) to identify the impacts of natural and anthropogenic perturbations on the temporal and/or spatial regulation of these fluxes, and (3) to construct coastal BS OC and BC cycling and budgets.…”