Climate of the Past Discussions www.clim-past-discuss.net 1814-9340 1814-9359 5 6 2009 10.5194/cpd-5-2607-2009 http://www.clim-past-discuss.net/5/2607/2009/ http://www.clim-past-discuss.net/5/2607/2009/cpd-5-2607-2009.html http://www.clim-past-discuss.net/5/2607/2009/cpd-5-2607-2009.pdf 2607 2630 2009-12-23 A simple mixing explanation for late Pleistocene changes in the Pacific-South Atlantic benthic δ¹³C gradient L. E. Lisiecki lisiecki@geol.ucsb.edu Department of Earth Science, University of California, Santa Barbara, USA The fact that the deep-ocean benthic δ¹³C minimum shifted from the North Pacific to the South Atlantic during the Last Glacial Maximum is often interpretted as evidence of a change in deep water circulation, such as the development of deep water ventilation in the North Pacific. This study re-evaluates the implications of changes in benthic δ¹³C gradients by comparing Pacific Deep Water (PDW) δ¹³C measurements with the values expected for the null hypothesis that PDW ventilation sources remained unchanged throughout the Late Pleistocene. The δ¹³C compositions of PDW, Northern Component Water (NCW) and Southern Component Water (SCW) are estimated from regional benthic δ¹³C stacks of 3–6 sites. Changes in PDW δ¹³C and PDW-SCW δ¹³C gradients over the past 800 kyr are found to be well described by a constant mixture of 60% NCW and 40% SCW plus a constant Pacific "age" offset of −0.5‰. Thus, an additional ventilation source for glacial PDW (e.g., in the North Pacific) cannot be inferred solely on the basis of changes in the Pacific-South Atlantic benthic δ¹³C gradient. 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