The stable carbon isotopic composition of particulate organic matter in the ocean,
δ13C
POC, shows characteristic spatial variations with high values in low latitudes and low values in high latitudes. The lowest
δ13C
POC values (−32‰ to −35‰) have been reported in the Southern Ocean, whereas in arctic and subarctic regions
δ13C
POC values do not drop below −27‰. This interhemispheric asymmetry is still unexplained. Global gradients in
δ13C
POC are much greater than in
δ13C
DIC, suggesting that variations in isotopic fractionation during organic matter production are primarily responsible for the observed range in
δ13C
POC. Understanding the factors that control isotope variability is a prerequisite when applying
δ13C
POC to the study of marine carbon biogeochemistry. The present model study attempts to reproduce the
δ13C
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,
P, is a function of the ambient CO
2 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,
P, depends on the CO
2 concentration in seawater and on the (instantaneous) growth rates,
μi, of the phytoplankton. In the Atlantic Ocean, where most field data are available, both parametrizations reproduce the latitudinal variability of the mean
δ13C
POC distribution. The interhemispheric asymmetry of
δ13C
POC can mostly be attributed to the interhemispheric asymmetry of CO
2 concentration in the water. However, the strong seasonal variations of
δ13C
POC as reported by several authors, can only be explained by a growth rate-dependent fractionation, which reflects variations in the cellular carbon demand.
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