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Abstract
The precise reason for the differences and out-of-phase relationship between the abrupt Dansgaard-Oeschger warmings in the Nordic seas and Greenland ice cores and the gradual warmings in the south-central Atlantic and Antarctic ice cores is poorly understood. Termed the bipolar seesaw, the differences are apparently linked to perturbations in the ocean circulation pattern. Here we show that surface and intermediate-depth water south of Iceland warmed gradually synchronously with the Antarctic warming and out of phase with the abrupt warming of the Nordic seas and over Greenland. The hinge line between areas showing abrupt and gradual warming was close to the Greenland-Scotland Ridge and the marine system appears to be a ‘push-and-pull’ system rather than a seesaw system. ‘Pull’ during the warm interstadials, when convection in the Nordic seas was active; ‘push’ during the cold stadials, when convection stopped and warm water from the south-central Atlantic pushed northward gradually warming the North Atlantic and Nordic seas.
Highlights
The climate of last glacial period was extremely unstable and interrupted by about 24 distinct warming and cooling events
In most Dansgaard-Oeschger events (D-O) events, maximum temperature occurs in the early part of the interstadials (Fig. 4g)
The peaks are marked by low percentages of the polar planktonic foraminiferal species Neogloboquadrina pachyderma s, high percentages of subpolar species and high abundance of planktonic foraminifera (Fig. 4a–d)
Summary
The climate of last glacial period was extremely unstable and interrupted by about 24 distinct warming and cooling events. Portugal continued all the way to the southern edge of the so-called IRD-belt[20] This belt, which stretches across the Atlantic from Newfoundland to Ireland and Portugal, is characterized by glacial sediments containing distinct layers with abundant IRD reflecting periodical releases of huge numbers of icebergs from the Laurentide ice sheet[6,21]. These outbreaks, which are termed Heinrich events, are generally considered to be in phase with the larger and longer-lasting stadials in the Greenland ice cores[6], in the central northernmost Atlantic the arrival of icebergs may have lagged the beginning of the cold phase by several hundred years[13]. Numerous factors have been suggested including changes in the strength[6,22,23,24] and location of the deep convection[25], changes in the continental ice sheets[26], melt water release[9], variability of sea ice cover[27], heat exchange between the ocean and the atmosphere, and atmospheric heat transport[28]
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