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

Abstract

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 °C in less than 7 kyr) in the mean annual sea surface temperature (SST), increasing benthic δ 18O values and high benthic δ 13C values. The continuous increase in benthic δ 18O suggests a continuous ice sheet growth whereas the positive benthic δ 13C 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‰ threshold in the benthic δ 18O 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 δ 13C values dropped 0.5–1‰) 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 °C in ∼3 kyr). Subsequent millennial-scale warm events were cooler probably because the AMOC was rather reduced as suggested by the low benthic δ 13C values. These two abrupt warming events that occurred at early glacial periods were also observed in the Antarctic temperature and CO 2 records, suggesting a close correlation between both Hemispheres. The comparison of the sea surface proxies with the benthic δ 18O record (as the Southern sign) indicates the presence of a millennial-scale seesaw pattern similar to that seen during the Last Glacial period.