Temperature-dependent remineralization and carbon cycling in the warm Eocene oceans

Metabolic rates in heterotrophic bacteria are more sensitive to temperature than rates of primary production (e.g., Lopez-Urrutia et al., 2006; Regaudie-de-Gioux and Duarte, 2012). Consequently, faster bacterial respiration rates in a warmer ocean may result in more efficient remineralization of sinking organic matter higher in the water column, with implications for carbon and nutrient cycling during warm climate states. This is supported by a series of reconstructed delta C-13(DIC): depth profiles based on well-preserved planktonic foraminifera assemblages from Tanzania from the warm Eocene epoch (55.5-33.7 Ma) when global surface and deep ocean temperatures exceeded those of the modern day (John et al., 2013). These results indicate relatively sharp delta C-13(DIC) gradients in the upper water column which supports the hypothesis that high metabolic rates in warm Eocene oceans led to more efficient recycling of organic matter and reduced burial rates of organic carbon (Olivarez Lyle and Lyle, 2006). Shallower remineralization depths would also cause an upward displacement and intensification of the oxygen minimum zone which is consistent with evidence for a pelagic ecosystem that was focused in a narrow depth range near the surface during the warm early and middle Eocene. Here we use the Earth System model, cGENIE, that incorporates a new temperature dependence of remineralization rates to illustrate the potential effects of temperature on particulate organic carbon fluxes and hence vertical delta C-13(DIC) gradients. Modeled delta C-13(DIC) vertical profiles off the coast of Tanzania for the Eocene agree well with the reconstructed delta C-13(DIC) profiles, supporting our interpretations based on temperature-dependent remineralization. (C) 2014 Published by Elsevier B.V.