University of Barcelona, Spain

Stable carbon and oxygen isotopes from benthic and planktic foraminifers, planktic foraminifer assemblages and ice rafted debris from the North Atlantic Site U1314 (Integrated Ocean Drilling Program Expedition 306) were examined to investigate orbital and millennial-scale climate variability in the North Atlantic and its impact on global circulation focusing on the development of glacial periods during the mid-Pleistocene (ca 800-400ka). Glacial initiations were characterized by a rapid cooling (6-10 degrees C in less than 7 kyr) in the mean annual sea surface temperature (SST), increasing benthic delta (super 18) O values and high benthic delta (super 13) C values. The continuous increase in benthic delta (super 18) O suggests a continuous ice sheet growth whereas the positive benthic delta (super 13) C values indicate that the flow of the Iceland Scotland Overflow water (ISOW) was vigorous. Strong deep water formation in the Norwegian Greenland Sea promoted a high transfer of freshwater from the ocean to the continents. However, low SSTs at Site U1314 suggest a subpolar gyre cooling and freshening that may have reduced deep water formation in the Labrador Sea during glacial initiations. Once the 3.5 ppm threshold in the benthic delta (super 18) O record was exceeded, ice rafting started and ice sheet growth was punctuated by millennial-scale waning events which returned to the ocean part of the freshwater accumulated on the continents. Ice-rafting events were associated with a rapid reduction in the ISOW (benthic delta (super 13) C values dropped 0.5-1 ppm) and followed by millennial-scale warmings. The first two millennial-scale warm intervals of each glacial period reached interglacial temperatures and were particularly abrupt (6-10 degrees C in approximately 3 kyr). Subsequent millennial-scale warm events were cooler probably because the AMOC was rather reduced as suggested by the low benthic delta (super 13) C values. These two abrupt warming events that occurred at early glacial periods were also observed in the Antarctic temperature and CO (sub 2) records, suggesting a close correlation between both Hemispheres. The comparison of the sea surface proxies with the benthic delta (super 18) O record (as the Southern sign) indicates the presence of a millennial-scale seesaw pattern similar to that seen during the Last Glacial period. Abstract Copyright (2011) Elsevier, B.V.