ABSTRACT

Marine organic matter in seawater is one of the most active carbon reservoirs on the earth’s surface and plays an important role in earth’s climate system. Marine organic matter is also a key component in the exchange among the biosphere, hydrosphere, and geosphere (Hedges, 1992). Therefore, knowledge of the cycling of organic matter in the marine environments is indispensable for understanding of the biogeochemistry of a variety of elements, function of ecosystems, and impact of human activities on global climate changes. Isotopic signatures, such as those of stable isotope ratios of C, N, and S, as well as radiocarbon, have been widely applied as powerful tools to study biogeochemical cycling of organic matter in marine environments (Eadie et al., 1978; Sigleo and Macko, 1985; Peterson and Fry, 1987; Altabet, 1988; Altabet et al., 1991; Cifuentes et al., 1988; Sackett, 1989; Rau et al., 1990, 1991a; Druffel and Williams, 1992; Druffel et al., 1992; Benner et al., 1997; Raymond and Bauer, 2001a; Tanaka et al., 2004; Knapp et al., 2005; Chen et al., 2006; Tagliabue and Bopp, 2008). Interactions of organic carbon with inorganic carbon in marine environments and their interchange with atmospheric CO2 are always accompanied by variations in carbon isotope signals (Tagliabue and Bopp, 2008). Therefore, isotopic measurements provide a baseline for assessment of sources, sinks, transport, and transformation of marine organic matter, as well as the geochemical processes controlling the distribution of organic matter in the oceans (Druffel and Williams, 1992; Tumbore and Druffel, 1995; Raymond and Bauer, 2001a).