Abstract
Abstract. 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 or a decrease in Southern Ocean overturning. 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 remineralization offset of −0.5‰. Thus, a change in PDW ventilation cannot be inferred solely on the basis of changes in the Pacific-South Atlantic benthic δ13C gradient.
Highlights
An important unanswered question about glacial ocean circulation is whether the shift of minimum δ13C values from the North Pacific to the South Atlantic at the Last Glacial Maximum (LGM) implies a major change in Pacific circulation
Because southward flowing Pacific Deep Water (PDW) progressively mixes with younger water during its return trip, today the oldest deep water with the most negative δ13C value is found at mid-depth in the North Pacific (Kroopnick, 1985; Matsumoto et al, 2002)
Does this shift imply that at the LGM Pacific deep water was younger than South Atlantic deep water? Previous studies have suggested that the change in benthic δ13C gradients was caused by additional mixing between PDW and North Pacific Intermediate Water (NPIW) (Duplessy et al, 1988; Herguera et al, 1992; Keigwin, 1998; Oppo and Horowitz, 2000)
Summary
Pacific at depth and returns southward at mid-depths in the form of Pacific Deep Water (PDW) (Ganachaud et al, 2003; Talley et al, 2003). Because southward flowing PDW progressively mixes with younger water during its return trip, today the oldest deep water with the most negative δ13C value is found at mid-depth in the North Pacific (Kroopnick, 1985; Matsumoto et al, 2002). In this study a simple mixing model is used to test the null hypothesis that PDW δ13C values result from a constant mixing ratio of water from the North and South Atlantic throughout the Late Pleistocene. This model reproduces most of the changes in PDW δ13C, including glacial reversals of the Pacific-South Atlantic δ13C gradient.
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