Abstract
Pyrogenic carbon, also called black carbon (BC), is an important component in the global carbon cycle. BC produced by biomass burning or fossil fuel combustion is transported to oceans by the atmosphere or rivers. However, environmental dynamics (i.e., major sources and sinks) of BC in marine environments have not been well documented. In this study, dissolved BC (DBC) collected from surface waters of the Chukchi Sea, the Bering Sea, and the subarctic and subtropical North Pacific were analyzed using the benzene polycarboxylic acid (BPCA) method. The DBC concentration and the ratio of B5CA and B6CA to all BPCAs (an index of the DBC condensation degree) ranged from 4.8 to 15.5 µg-C L-1 and from 0.20 to 0.43, respectively, in surface waters of the Chukchi/Bering Seas and the North Pacific Ocean. The concentration and condensation degree of DBC in the Chukchi/Bering Seas were higher and more variable than those in the subarctic and subtropical North Pacific, which implies that the major factors controlling DBC distribution were different in these marine provinces. In the Chukchi/Bering Seas, the DBC concentration was negatively correlated to salinity but positively correlated to chromophoric dissolved organic matter (CDOM) quantity and total dissolved lignin phenol concentration estimated by CDOM parameters. These correlations indicated that the possible major source of DBC in the Chukchi/Bering Seas was Arctic rivers. However, in the North Pacific, where riverine inputs are negligible for most sampling sites, DBC was possibly derived from the atmosphere. Although spectral slopes of CDOM at 275-295 nm (an index of the photodegradation degree of CDOM) differed widely between the subarctic and subtropical North Pacific, the concentration and condensation degrees of DBC were similar between the subarctic and subtropical North Pacific, which suggests that photodegradation was not the only major factor controlling DBC distribution. Therefore, DBC distributions of the North Pacific Ocean were considered to be mainly controlled by atmospheric deposition of BC and subsequent losses by photodegradation and adsorption onto sinking particles. This study implies that the main influence on DBC distribution in the open ocean and the coastal ocean are atmospheric deposition and fluvial inputs, respectively.
Highlights
Pyrogenic carbon (PyC), including soot, char, black carbon, and biochar, is produced by incomplete combustion of organic matter by biomass burning or fossil fuel combustion (Masiello, 2004)
The range of dissolved BC (DBC) concentration in marine environments observed in this study is one to three orders of magnitude lower than those observed in terrestrial aquatic environments (e.g., Jaffé et al, 2013; Stubbins et al, 2015), which indicates that DBC in surface waters of coastal as well as oceanic environments is possibly derived from terrestrial environments by riverine inputs and/or atmospheric deposition, as previously reported (Dittmar, 2008; Dittmar and Paeng, 2009)
The DBC concentrations in the Chukchi/Bering Seas (4.8–15.5 μg-C L−1) were similar to a value reported for the open Gulf of Mexico (14.4 μg-C L−1; Dittmar, 2008), and the DBC concentration in the North Pacific Ocean (4.9–8.6 μg-C L−1) was slightly lower than those observed in the Southern Ocean (7.2–9.6 μg-C L−1; Dittmar and Paeng, 2009) and in the North Atlantic Deep Water (12.5 ± 1.9 μg-C L−1; Stubbins et al, 2012)
Summary
Pyrogenic carbon (PyC), including soot, char, black carbon, and biochar, is produced by incomplete combustion of organic matter by biomass burning or fossil fuel combustion (Masiello, 2004). Less aromatic PyC (e.g., char) produced at lower temperature is considered to be biologically degradable (Bruun et al, 2008); PyC produced at higher temperature forms more condensed polyaromatic compounds (e.g., soot and elemental carbon) and is considered to be stable in reduced carbon pools for at least thousands of years (e.g., Masiello and Druffel, 1998). The values are largely variable and depend on quantification methods, PyC content has been determined to be
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