Potential origins of 400-500 kyr periodicities in the ocean carbon cycle: A box model approach

Cenozoic proxy records of both the isotopic composition of dissolved inorganic carbon in the oceans (delta C-13(DIC)) and deep ocean carbonate preservation show significant periodicities in the range 400-500 kyr. Sensitivity analysis of the ocean carbon cycle to potential variability on this timescale in patterns of global oceanic primary productivity and/or continental weathering fluxes is performed using a 7 box ocean-atmosphere model. The data constraints imposed by Plio-Pleistocene proxy records of delta C-13(DIC), carbonate preservation, and atmospheric pCO(2) variability allow different scenarios to be evaluated. Forcing with the global oceanic primary production ratio of inorganic and organic carbon leads to the relative phases of response most consistent with the proxy data. However, only when changes also occur in total marine primary production can the observed relative amplitudes of delta C-13(DIC) and pCO(2) variability also be reproduced. This scenario is consistent with oscillations between a highly productive, coccolithophore-rich global ocean and a less productive, diatom-rich one. Such oscillations have been proposed to originate from the orbital eccentricity cycle leading to changes in seasonality and thus silica utilization in the Southern Ocean. However, the period of response in model delta C-13(DIC) is always close to that of the forcing function used, and thus a significant discrepancy remains between the orbital eccentricity cycle period of 413 kyr and the observed similar to 500 kyr delta C-13(DIC) periodicity seen in the Pleistocene.