The stable carbon isotopic composition of particulate organic matter in the ocean, delta C-13(POC), shows characteristic spatial variations with high values in low latitudes and low values in high latitudes. The lowest delta C-13(POC) values (-32 parts per thousand to -35 parts per thousand) have been reported in the Southern Ocean, whereas in arctic and subarctic regions delta C-13(POC) values do not drop below -27 parts per thousand. This interhemispheric asymmetry is still unexplained. Global gradients in delta C-13(POC) are much greater than in delta C-13(DIC), suggesting that variations in isotopic fractionation during organic matter production are primarily responsible for the observed range in delta C-13(POC). Understanding the factors that control isotope variability is a prerequisite when applying delta C-13(POC) to the study of marine carbon biogeochemistry. The present model study attempts to reproduce the delta C-13(POC) distribution pattern in the ocean. The three-dimensional (3D) Hamburg Model of the Oceanic Carbon Cycle version 3.1 (HAMOCC3.1) was combined with two different parametrizations of the biological fractionation of stable carbon isotopes. In the first parametrization, it is assumed that the isotopic fractionation between CO(2)in seawater and the organic material produced by algae, epsilon (p), is a function of the ambient CO2 concentration. The two parameters of this function are derived from observations and are not based on an assumption of any specific mechanism. Thus, this parametrization is purely empirical. The second parametrization is based on fractionation models for microalgae. It is supported by several laboratory experiments. Here the fractionation, E-p, depends on the CO(2)concentration in seawater and on the (instantaneous) growth rates, mu (i) Of the phytoplankton. In the Atlantic Ocean, where most field data are available, both parametrizations reproduce the latitudinal variability of the mean delta C-13(POC) distribution. The interhemispheric asymmetry of delta C-13(POC) can mostly be attributed to the interhemispheric asymmetry of CO(2)concentration in the water. However, the strong seasonal variations of delta C-13(POC) as reported by several authors, can only be explained by a growth rate-dependent fractionation, which reflects variations in the cellular carbon demand. (C) 2000 Elsevier Science B.V. All rights reserved.
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