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

[摘要] Analysis of observations and sensitivity experiments with a newthree-dimensional global model of stable carbon isotope cycling elucidateprocesses that control the distribution of δ¹³C of dissolvedinorganic 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 lowδ¹³C_DIC values at depths and in high latitude surfacewaters and high values in the upper ocean at low latitudes with maxima in thesubtropics. Air–sea gas exchange has two effects. First, it acts toreduce the spatial gradients created by biology. Second, the associatedtemperature-dependent fractionation tends to increase (decrease) δ¹³C_DIC values of colder (warmer) water, which generatesgradients that oppose those arising from biology. Our model results suggestthat both effects are similarly important in influencing surface and interiorδ¹³C_DIC distributions. However, since air–sea gasexchange is slow in the modern ocean, the biological effect dominates spatialδ¹³C_DIC gradients both in the interior and at thesurface, in contrast to conclusions from some previous studies. Calciumcarbonate cycling, pH dependency of fractionation during air–sea gasexchange, and kinetic fractionation have minor effects onδ¹³C_DIC. Accumulation of isotopically light carbon fromanthropogenic fossil fuel burning has decreased the spatial variability ofsurface and deep δ¹³C_DIC since the industrial revolutionin our model simulations. Analysis of a new synthesis of δ¹³C_DIC measurements from years 1990 to 2005 is used to quantifypreformed and remineralized contributions as well as the effects of biologyand air–sea gas exchange. The model reproduces major features of theobserved large-scale distribution of δ¹³C_DIC as well asthe individual contributions and effects. Residual misfits are documented andanalyzed. Simulated surface and subsurface δ¹³C_DIC areinfluenced by details of the ecosystem model formulation. For example,inclusion of a simple parameterization of iron limitation of phytoplanktongrowth rates and temperature-dependent zooplankton grazing rates improves theagreement with δ¹³C_DIC observations and satelliteestimates of phytoplankton growth rates and biomass, suggesting that δ¹³C can also be a useful test of ecosystem models.

[学科分类] 地球化学与岩石