Biology and air-sea gas exchange controls on the distribution of carbon isotope ratios ([delta]13C) 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 δ¹³ C of dissolved inorganic carbon (DIC) in the contemporary and preindustrial ocean. Biological fractionation and the sinking of isotopically light δ¹³ C organic matter from the surface into the interior ocean leads to low δ¹³ C_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) δ¹³ C_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 δ¹³ C_DIC distributions. However, since air-sea gas exchange is slow in the modern ocean, the biological effect dominates spatial δ¹³ C_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 δ¹³ C_DIC . Accumulation of isotopically light carbon from anthropogenic fossil fuel burning has decreased the spatial variability of surface and deep δ¹³ C_DIC since the industrial revolution in our model simulations. Analysis of a new synthesis of δ¹³ C_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 δ¹³ C_DIC as well as the individual contributions and effects. Residual misfits are documented and analyzed. Simulated surface and subsurface δ¹³ C_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 δ¹³ C_DIC observations and satellite estimates of phytoplankton growth rates and biomass, suggesting that δ¹³ C can also be a useful test of ecosystem models.

[A & I plus PDF only]

COPYRIGHT: © Author(s) 2013. This work is distributed under the Creative Commons Attribution 3.0 License.