SPECULATIONS ABOUT THE UPPER MIOCENE CHANGE IN ABYSSAL PACIFIC DISSOLVED BICARBONATE DELTA-C-13

New data from three Tasman Sea cores support Keigwin's [1] observation that the δ13C of Pacific benthic foraminifera (and by inference bottom-water TCO2) decreased by 0.7‰ at about 6.5 Myr B.P. Simple box models are developed and used to test several hypotheses about the cause of the δ13C decrease. We favor the idea that the δ13C shift was due to a rapid change in TCO2 cycling within the oceans (such as would result from either a decrease in upwelling rate, or an increase in the fraction of PO43- reaching the deep oceans in particulate organic matter and a corresponding drop in the preformed PO43- concentration). The δ13C decrease across the shift might reflect either a global decrease in upwelling rate, or a different abyssal circulation pattern before the shift. © 1979.