THE DEEP-WATER TEMPERATURE COMPONENT OF THE NORTH ATLANTIC BENTHIC d18O RECORD ACROSS TERMINATIONS I AND II: THE EFFECT OF NADW - AABW EXCHANGE ON THE APPARENT TIMING OF MARINE ISOTOPE STAGE BOUNDARIES

Paired measurements of Mg/Ca and d¹⁸O in benthic foraminifera from two well-dated deep-sea cores (MD99-2334K, 3146m; MD01-2444, 2637m) recovered on the Iberian Margin (37^oN, 10^oW) permit the estimation of the deep-water temperature (DWT) component of the benthic d¹⁸O record across the last two glacial terminations.  The glacio-eustatic component since ~ 20 ka BP is well constrained by sea-level estimates, allowing the marine d¹⁸O signal to be completely de-convolved across Termination I.  An abrupt drop in DWT during the Younger Dryas indicates a collapse in North Atlantic Deep Water (NADW) supply to the North Atlantic at this time.  Hence a significant component of the benthic d¹⁸O record of Termination I is in fact an expression of local deep water-mass exchange, probably between high-d¹⁸O NADW and low-d¹⁸O Antarctic Bottom Water (AABW).  This interpretation is corroborated by benthic d¹³C values and ¹⁴C-ventilation ages, which indicate the incursion of a highly ‘aged’ high-nutrient (cold) water-mass during the Younger Dryas.  The inferred NADW collapse and associated AABW incursion during the Younger Dryas results in a lead of North Atlantic benthic d¹⁸O over both ice-volume change and deep Pacific benthic d¹⁸O, that is due to local deep water-mass effects only.  A similar pattern of events has also been reconstructed for the penultimate glacial termination.  Deep-water temperature fluctuations across Termination II imply a significant component in the benthic d¹⁸O record that is not attributable to DWT change and, by analogy with Termination I, probably represents the effect of local water-mass exchange.  This has a direct bearing on the discrepancy previously identified between the beginning of the Eemian interglacial (based on pollen evidence for terrestrial climate) and the base of Marine Isotope Stage 5e (based on benthic d¹⁸O).  The observed lead of benthic d¹⁸O with respect to terrestrial climate may thus be due to local water mass effects rather than a delayed surface-climate response with respect to ice-volume drop.  These results demonstrate the important role of thermohaline circulation in millennial-scale climate change and its impact on marine isotope stratigraphy.