RIVERINE DISCHARGE OF BLACK CARBON FROM THE US

With escalating human influence on coastlines, coastal and estuarine environments are exposed to increasing amounts of combustion byproducts, such as black carbon (BC). BC results from the incomplete combustion of fossil fuels (for example, coal and petroleum) and biomass (for example, vegetation burned in forest fires and slash-and-burn agriculture). The impetus for studying the environmental cycling of BC results from its importance as (1) a "sink" for atmospheric carbon, (2) a tracer for recent and historical combustion processes, (3) a mediator of the Earth's radiative heat balance, and (4) a carrier of inorganic and organic pollutants. BC abundance and isotopic signatures in suspended and surface sediments from the coastal zones of three watersheds in the US were quantified in this study. The three watersheds studied were (1) the Eel River, (2) the Mississippi/Atchafalaya Rivers, and (3) the Chesapeake Bay estuary. These three watersheds, if taken together, are responsible for a significant portion of carbon fluxes from the continental US to the oceans.

Sediments were extracted for BC using a hydrofluoric acid demineralization-potassium dichromate oxidation procedure. At all sites in this study, the BC extraction procedure yielded a lower total % mass recovery in fluvial suspended sediments in contrast to coastal surface sediments, possibly implicating aerosol rather than fluvial transport and deposition, as a relatively greater contributor of sedimentary BC to offshore regions. In this study, carbon stable isotope ratios and natural abundances of radiocarbon were used to delineate sources of BC. Whereas, isotopic analyses of BC for all sites are currently being completed, preliminary radiocarbon abundances from the Mississippi River/Atchafalya River watershed (draining 41% of the US), suggest a predominance of age depleted and possibly fossil-fuel derived BC from that watershed. Ongoing analyses of the Eel River and Chesapeake Bay will complete this snapshot assessment of BC flux from the US into the surrounding coastal oceans.