Fluvial Discharge of Black Carbon and Its Role in the Global Carbon Cycle
Dec. 2001 / Jan. 2002
With escalating human influence on coastlines, coastal and estuarine environments are exposed to
increasing amounts of combustion byproducts, such as polycyclic aromatic hydrocarbons (PAHs) and
black carbon. Black carbon and PAHs result 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 black carbon 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. For example, the formation of airborne black-carbon particles and their subsequent
deposition into soils and aquatic systems result in organic matter being shunted to the geosphere
rather than to the atmosphere as carbon dioxide, a well-known greenhouse gas. On a global level,
circumvention of greenhouse-gas formation via black-carbon production may be linked to essential
climate issues. The importance of black carbon on a global scale is further evidenced by its presence
in sediment in the central Pacific Ocean, where at one location black carbon was found to compose
12 to 31 percent of the sedimentary organic carbon. In contrast to black carbon, PAHs are typically
detected at trace concentrations in the environment. Even at these trace concentrations, many PAHs
are toxic and carcinogenic and so may have deleterious effects on organisms in coastal areas.
In this study, PAHs are primarily being used to delineate sources of black carbon (see below).
Collecting water samples for analyses of black carbon and polycyclic aromatic hydrocarbons (PAHs).
Water samples were collected from the headwaters of the Eel River just east of Lake Pillsbury. In an
effort to minimize black carbon and PAH sorption onto sampling containers, water samples were
collected by using precleaned 40-L stainless-steel pressure vessels. Water samples were collected
by carefully submersing the pressure vessels under the air-water interface. Water from the Eel was
allowed to flow naturally into each pressure vessel; then, each vessel was capped and transported
to a nearby hotel for water filtration.
Despite several decades of research dedicated to the global cycling of black carbon, the amount and
source(s) of black carbon discharged into the ocean by rivers remain largely unknown. Black carbon
may enter rivers and streams either directly from atmospheric deposition or indirectly from runoff.
Characterizing the river discharge of black carbon (and PAHs) into the oceans will help us to quantify
the abundance and source(s) of combustion byproducts to the river-coast-ocean transition zone (that
is, coastal margins). Quantifying the fluvial discharge of black carbon is an essential step toward
evaluating the role of anthropogenic and natural combustion processes on the global carbon cycle.
For this reason, Siddhartha Mitra, a Mendenhall Postdoctoral Fellow with the Western Coastal and Marine
Geology Team (WCMG), is examining black carbon 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 black carbon and PAH abundance
and (2) to attempt to ascertain sources of these combustion byproducts within each coastal system by
coupling the use of high-molecular-weight PAH isomer ratios with radiocarbon dating.
Filtration of water samples from the Eel River. Water samples were filtered for polycyclic aromatic
hydrocarbons (PAHs) by pressurizing the sample vessels with ultra-high-purity nitrogen. The exit line
of the pressure vessel was connected to a 142-mm-diameter stainless-steel filter holder housing a
precleaned glass-fiber filter. Additional water samples were simultaneously filtered for black carbon.
Typically, the radiocarbon age of black carbon is used to infer its source. Black carbon derived from
biomass burning will have a modern 14C age, whereas black carbon derived from combustion of
fossil fuel will have an ancient 14C age (depleted in radiocarbon). Because black carbon in coastal
environments originates from a mixture of biomass and fossil-fuel combustion processes and is
transported across various distances, applying mixing models of apparent radiocarbon ages to black
carbon in coastal environments may be fraught with uncertainties. These uncertainties can be
reduced by using source-specific markers of natural organic-matter combustion, such as PAHs.
Black carbon and PAHs are cogenerated during combustion of such organic matter as biomass or
fossil fuels. Therefore, PAHs can offer useful information about anthropogenic versus naturally
derived sources of black carbon.
In October 2001, Siddhartha Mitra and Tom Lorenson (WCMG), as part of the Marine Organic
Geochemistry Project, conducted fieldwork in the Eel River of northern California. Suspended
sediment was collected at two stations, one near the headwaters of the river and the other near
its mouth. During water collection, care was taken to avoid the influence of lumbermills and
agricultural areas. Water was filtered in the field to obtain samples for analysis of black carbon
and PAHs. These filter samples were frozen and transported to the WCMG organic-geochemistry
laboratory directed by Keith Kvenvolden and Bob Rosenbauer for subsequent chemical analyses.
Similar field investigations have already been conducted in the Chesapeake Bay with assistance
from Bill Orem's geochemistry research group in Reston (consisting of Margo Corum, Antonio
Mannino, Sarah Kleckner, and Harry Lerch), and in the Mississippi River with assistance
from Pete Swarzenski (St. Petersburg).
On a global scale, few rivers have been sampled for black-carbon discharge. One recent study
estimated that in 1999 the Mississippi River discharged approximately 5 percent of the world's
annual black carbon discharged to the ocean. Furthermore, much of this discharge was derived
from the combustion of fossil fuel (coal). Thus, accurate quantification of the fluvial discharge of
combustion byproducts from a composite of coastal discharge systems, as in this study, will help
to constrain the societal implications of combustion on the global carbon cycle.
Dec. 2001 / Jan. 2002
in this issue:
Honduras Coral Reefs
Miami Canal Surveys
Cape Cod Lakes
African Dust Lecture
Falmouth, MA Public Schools
WHFC Web Site
Sea-Level Rise & Coastal Disasters
Restoring Louisiana's Coastal Ecosystems
Two New Postdocs
Student & Visiting Scientist
Cape Cod Marathon
Dec./Jan. Publications List