Ocean circulation, ice sheet growth and interhemispheric coupling of millennial climate variability during the mid-Pleistocene (ca 800-400 ka)

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 800-400 ka). 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 O-18 values and high benthic delta C-13 values. The continuous increase in benthic delta O-18 suggests a continuous ice sheet growth whereas the positive benthic delta C-13 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 parts per thousand threshold in the benthic delta O-18 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 C-13 values dropped 0.5-1 parts per thousand) 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 similar to 3 kyr). Subsequent millennial-scale warm events were cooler probably because the AMOC was rather reduced as suggested by the low benthic delta C-13 values. These two abrupt warming events that occurred at early glacial periods were also observed in the Antarctic temperature and CO2 records, suggesting a close correlation between both Hemispheres. The comparison of the sea surface proxies with the benthic delta O-18 record (as the Southern sign) indicates the presence of a millennial-scale seesaw pattern similar to that seen during the Last Glacial period. (C) 2011 Elsevier Ltd. All rights reserved.