Biology and air–sea gas exchange controls on the distribution of carbon isotope ratios (δ¹³C) in the ocean

Abstract. Analysis of observations and sensitivity experiments with a new three-dimensional global model of stable carbon isotope cycling elucidate processes that control the distribution of δ¹³C of dissolved inorganic carbon (DIC) in the contemporary and preindustrial ocean. Biological fractionation and the sinking of isotopically light δ¹³C organic matter from the surface into the interior ocean leads to low δ¹³C_DIC values at depths and in high latitude surface waters and high values in the upper ocean at low latitudes with maxima in the subtropics. Air–sea gas exchange has two effects. First, it acts to reduce the spatial gradients created by biology. Second, the associated temperature-dependent fractionation tends to increase (decrease) δ¹³C_DIC values of colder (warmer) water, which generates gradients that oppose those arising from biology. Our model results suggest that both effects are similarly important in influencing surface and interior δ¹³C_DIC distributions. However, since air–sea gas exchange is slow in the modern ocean, the biological effect dominates spatial δ¹³C_DIC gradients both in the interior and at the surface, in contrast to conclusions from some previous studies. Calcium carbonate cycling, pH dependency of fractionation during air–sea gas exchange, and kinetic fractionation have minor effects on δ¹³C_DIC. Accumulation of isotopically light carbon from anthropogenic fossil fuel burning has decreased the spatial variability of surface and deep δ¹³C_DIC since the industrial revolution in our model simulations. Analysis of a new synthesis of δ¹³C_DIC measurements from years 1990 to 2005 is used to quantify preformed and remineralized contributions as well as the effects of biology and air–sea gas exchange. The model reproduces major features of the observed large-scale distribution of δ¹³C_DIC as well as the individual contributions and effects. Residual misfits are documented and analyzed. Simulated surface and subsurface δ¹³C_DIC are influenced by details of the ecosystem model formulation. For example, inclusion of a simple parameterization of iron limitation of phytoplankton growth rates and temperature-dependent zooplankton grazing rates improves the agreement with δ¹³C_DIC observations and satellite estimates of phytoplankton growth rates and biomass, suggesting that δ¹³C can also be a useful test of ecosystem models.