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Riverine Discharge of Black Carbon and Its Role in the Global Carbon Cycle: Siddhartha Mitra

Project Title: Riverine Discharge of Black Carbon and Its Role in the Global Carbon Cycle
Mendenhall Fellow: Siddhartha Mitra, (650) 329-5466, smitra@usgs.gov
Duty Station: Menlo Park
Start Date: February 25, 2001
Education: Ph.D. (Marine Science), Virginia Institute of Marine Science - the College of William and Mary, 1997
Research Advisor: Keith Kvenvolden, (650) 329-4196, kkvenvolden@usgs.gov
Project Description: Due to the escalating anthropogenic influence exerted on coastlines, coastal and estuarine environments continue to be exposed to increasing amounts of combustion by-products such as polycyclic aromatic hydrocarbons (PAHs) and black carbon (BC) (fig. 1).
 

Figure 1. TEM-image (mag. 100000x) of furnace-derived black carbon.

   
 
Black carbon results from the incomplete combustion of fossil fuels (for example, coal, petroleum) or biomass (for example, forest fires). 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. Despite several decades of research dedicated to the global cycling of BC, the amount and source(s) of riverine BC discharged into the ocean remains largely unquantified.

Black carbon has been suggested to serve as a "sink" for carbon dioxide, a well-known greenhouse gas. The formation of polymeric BC from organic-carbon (OC) oxidation bypasses the organic carbon's complete oxidation to CO2(g) (fig. 2). Thus, the formation of BC is linked to the global carbon cycle, both as a tracer for pyrogenic processes (for example, biomass burning and fossil fuel combustion) and as a sink for an atmospheric greenhouse gas over geologic time scales. Evidence of BC in the sedimentary record serves as an indicator of historical combustion processes such as vegetation fires and the increasing influence of anthropogenic processes in the global carbon cycle such as fossil fuel combustion. For example, BC has been found in Pacific Ocean sediments where in some cases, it constitutes 12 % to 31 % of the sedimentary organic carbon (OC). In this context, quantifying the age and residence time of BC in the environment is important in constraining the role of combustion in the global carbon cycle.

                Figure 2. Cartoon of the stoichiometry of combustion. Black carbon formation results in the product of organic carbon combustion being shunted into the geosphere rather than the atmosphere.

Characterizing the river discharge of black carbon and PAHs into the oceans is an essential component of quantifying the importance of the river-coast-ocean transition zone (that is, coastal margins) to the global carbon cycle. For this reason we are examining BC and PAH discharge from three typical North American coastal discharge systems: (1) a small mountainous west-coast river (the Eel River), which discharges directly into the ocean; (2) a deltaic river (the Mississippi River), which discharges into an active deltaic shelf; and (3) an estuary (Chesapeake Bay) where much of the discharge is stored within the estuary. The objectives of this research are (1) to quantify fluvial BC and PAH abundance and (2) to attempt to ascertain sources of these combustion by-products within each coastal system using geochemical methods and radiocarbon dating. Accurate quantification of the amount of BC discharged into the ocean from different types of rivers and identification of the sources of BC are necessary in order to extrapolate the role of rivers worldwide in discharging BC into the global carbon cycle.

Many of the active ingredients in pharmaceutical and personal care products are referred to as PPCPs and may be deleterious to the environment. Several PPCPs survive conventional wastewater treatment technologies and are released into natural waters via treated wastewater discharge. In conjunction with the BC research described above, analytical procedures are being developed to analyze water and sediment samples for the presence of certain PPCPs. In that context, samples from the Chesapeake Bay, coastal zone of the Mississippi River and the Eel River are also being analyzed for a known endocrine disrupting hormone (17-estradiol), an analgesic (naproxen), and an antibiotic (triclosan). Identifying the presence of PPCPs in natural samples is the first step in recognizing their potential hazard to human and ecosystem health and predicting their fate and transport in the environment.

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