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Limited Stability
Deep ocean circulation is thought to be stable during warm, interglacial periods. Galaasen et al. (p. 1129, published online 20 February) constructed a highly resolved record of North Atlantic Deep Water production during the last interglacial period, around 128,000 to 116,000 years ago. The findings reveal large, centennial-scale reductions—in contrast to the prevailing paradigm. These changes occurred in an ocean warmer than that of today, but in a temperature regime similar to that expected because of global warming, raising the possibility that future ocean circulation, regional climate, and CO2 sequestration pathways could be impacted.
Abstract
Deep ocean circulation has been considered relatively stable during interglacial periods, yet little is known about its behavior on submillennial time scales. Using a subcentennially resolved epibenthic foraminiferal δ13C record, we show that the influence of North Atlantic Deep Water (NADW) was strong at the onset of the last interglacial period and was then interrupted by several prominent centennial-scale reductions. These NADW transients occurred during periods of increased ice rafting and southward expansions of polar water influence, suggesting that a buoyancy threshold for convective instability was triggered by freshwater and circum-Arctic cryosphere changes. The deep Atlantic chemical changes were similar in magnitude to those associated with glaciations, implying that the canonical view of a relatively stable interglacial circulation may not hold for conditions warmer and fresher than at present.
