Black Carbon in Estuarine and Coastal Ocean Dissolved Organic Matter

We measured black carbon (BC) in ultrafiltered, high–molecular weight dissolved organic matter (UDOM) in surface waters of Delaware Bay, Chesapeake Bay, and the adjacent Atlantic Ocean (U.S.A.) to investigate the importance of riverine and estuarine dissolved organic matter (DOM) as a source of BC to the ocean. BC was 5– 72% of UDOM-C (27 6 17%), which corresponds to 8.9 6 6.5% of dissolved organic carbon (DOC), with higher values in the heavily urbanized midbay region of the Delaware Estuary and lower yields in the river and coastal ocean. The spatial and seasonal distributions of BC along the salinity gradient of Delaware Bay suggest that the elevated levels of BC in surface water UDOM originate from localized sources, possibly from atmospheric deposition or release from resuspended sediments. BC in UDOM made up 4–7% of the DOC in the coastal Atlantic Ocean, revealing that river–estuary systems are important exporters of colloidal BC to the ocean. The annual flux of BC from Delaware Bay UDOM to the Atlantic Ocean was estimated at 2.4 3 1010 g BC yr21, supporting the hypothesis that the DOC pool is an important contributor of BC to the ocean. Dissolved organic matter (DOM) in the ocean constitutes one of the largest pools of organic carbon in the biosphere, yet much of its composition remains uncharacterized. In coastal waters, inputs of marine, terrestrial, and anthropogenic organic matter contribute to the complexity of DOM composition. Observations of black carbon (BC) particles in the atmosphere, ice, rivers, soils, and marine sediments suggest that this material is ubiquitous in the environment (Goldberg 1985; Schmidt and Noack 2000; Masiello and Druffel 2001; Mitra et al. 2002). The contribution of BC to the ocean’s DOM pool, however, is not known. BC represents a refractory and chemically complex product of incomplete combustion of fossil fuels and biomass, including vegetation burns and forest fires. BC is defined as a ‘‘continuum of partly charred plant material through char and charcoal to graphite and soot particles recondensed from the gas phase’’ (Seiler and Crutzen 1980, p. 207). Multiple mechanisms introduce BC to the ocean, including atmospheric deposition of land-derived BC aerosols, material transported by river–estuary systems, and marine diesel engine exhaust. Although most BC is deposited near the site of production, long-range transport of BC through rivers and the atmosphere is also possible (Goldberg 1985). BC accumulates in sediments, making up 10–50% of sedimentary organic carbon (Gustafsson et al. 1997; Middelburg et al. 1999) and is much older (2,400–13,900 yr older) than nonTo whom correspondence should be addressed. Present address: NASA Goddard Space Flight Center, Laboratory for Hydrospheric Processes, Mail Stop 971.1, Greenbelt, Maryland 20771 ([email protected]).