Abstract
Periodic ~400 kyr orbital scale variations in the ocean carbon cycle, manifest in indicators of deep sea dissolution and benthic 13C, have been observed throughout the Cenozoic but the driving mechanisms remain under debate. Changes in coccolithophore productivity may change the global rain ratio (Corganic: Cinorganic fluxes from ocean into sediment) and the balance of ocean carbonate system and thereby, potentially contributing to the ~400 kyr oscillation of the marine carbon cycle. Some evidence suggests that Pleistocene coccolithophore productivity was characterized by “bloom” events of high productivity coincident with the orbital benthic 13C signal. However, there is no consensus on the mechanism responsible for bloom events nor whether they were regional or global phenomena. In this study, we investigate the timing and spatial pattern of the most recent purported coccolithophore bloom event, which occurred during the Mid-Brunhes period. We find that maximum coccolithophore productivity is diachronous, peaking in the Southern Ocean sub-Antarctic zone with eccentricity minimum (~430 ka), peaking in upwelling zones some ~28 kyr later, and finally peaking in the western tropical Pacific occurred some ~80 kyr later. Simple globally homogeneous mechanisms of driving productivity such as temperature or light duration are not consistent with this pattern. Rather, we propose a dual high and low latitude control on blooms. Coincident with eccentricity minimum, increased high-latitude diatom silica consumption lowers the Si/P, leading to coccolithophorid blooms in the Southern Ocean north of the polar front. Coincident with increasing eccentricity, stronger tropical monsoons deliver higher fluvial nutrients to surface waters, increasing total (diatom and coccolithophore) productivity. Most of the tropical and subtropical locations are influenced by both processes with varying degrees, through the effect of silicic acid leakage on tropical thermocline waters and monsoon-related nutrient supply. Moreover, we propose that the high latitude processes have intensified over the Pleistocene, extending the 405 kyr carbon cycle to about 500 kyr.
Carbon cycle at steady state in the ocean. Simulation using a two box model.
