NOAA/WDS Paleoclimatology - Curry et al 1999 North Atlantic Isotopic and Trace Element Data

Changes in North Atlantic thermohaline circulation on millennial time scales were analogous to those observed on Milankovitch time scales: production of glacial intermediate water increased at the expense of the deep water. During the first half of isotope stage 3 (55 to 30 ka), NADW production was operating at nearly full strength when its production was interrupted by the iceberg discharges associated with Heinrich events. Reduced production of NADW began before each Heinrich event and continued well after the event, suggesting that the iceberg discharges were not the immediate cause of the change in thermohaline overturning. After 30 ka, when Milankovitch-forced reductions in NADW raised the water mass boundary to above 3200 m, reduced benthic foraminiferal d13C associated with Heinrich events was more pronounced in shallower and more northerly cores. In the western subtropical gyre, high values of benthic foraminiferal d13C and low values of Cd/Ca imply that the upper water column was well ventilated throughout marine isotope stages 3 and 2 with no evidence for decreased ventilation during Heinrich events. Intermediate waters (1849 m) in the subpolar North Atlantic were well ventilated during most of the Heinrich events, although mixing with southern ocean waters is evident for Heinrich events 1 and 2. Enhanced intermediate water production during the last glacial period was associated with oscillating temperatures within the subtropical gyre. Heinrich events H2, H3 and H4 were local warmings of the thermocline, while H1 and H5 were cold events. Two factors compete to cause these oscillations: 1) a redistribution of heat caused by variations in the intensity of thermohaline circulation in the North Atlantic [Manabe and Stouffer, 1997] and 2) variations in the temperature of water advected along isopycnals in the ventilated portion of the thermocline [Gu and Philander, 1997; Bush and Philander, 1998].