Isotopic Biogeochemistry of Marine Organic Carbon

Abstract

The ocean accounts for over 90% of the active pools of carbon on the Earth’s surface, with over 95% of marine carbon in the form of dissolved inorganic carbon (DIC) (Hedges and Keil 1995). Organic carbon dissolved in the ocean, suspended as particles or cells, and accumulating in sediments together constitute the other significant fractions of marine carbon, with organic carbon in the water column similar in quantity to the current atmospheric inventory of carbon dioxide. Isotopic partitioning among various inorganic and organic carbon phases reflects biological, physical and chemical processes, and the resulting fractionations are important tools in the study of modern and ancient carbon cycling. The focus of this review is on the isotopic geochemistry of marine organic carbon. It will begin by setting the stage with the isotopic patterns of DIC in the modern oceans. As will be discussed below, the distribution of inorganic carbon and related nutrient concentrations as well as DIC isotopic compositions are important influences on the quantity and isotopic character of organic carbon produced in marine surface waters. The remainder of the review will discuss isotope fractionation associated with the production and preservation of marine organic carbon. The combination of organic matter composition and 13C content is a potentially powerful approach for addressing the nature and pace of ecological and environmental change both in the modern and ancient ocean. This work reviews biogeochemical processes that generate, transform and ultimately preserve such signatures in marine sediments. The first truly global measurements of DIC δ13C values were produced as a result of the Geochemical Ocean Sections Study (GEOSECS). The resulting data were published in a series of articles by Kroopnick and colleagues (Kroopnick et al. 1977; Kroopnick 1980, 1985) and summarized by Takahashi et al. (1980, 1981). A striking outcome of this …