| Summary: | Past surface ocean circulation changes associated with the mid-Pleistocene transition, 0.9-0.6 Ma, were reconstructed in the northern North Atlantic (ODP 983) and the northwest Pacific (ODP 882), using proxies for subarctic/subpolar water mass distributions (7:4 alkenone) and sea surface temperature (U37K). Both sites experienced a secular expansion of subarctic waters from ∼1.15 Ma, spanning both glacial and interglacial intervals. After 0.9 Ma, low 7:4 at Site 983 records a northward retreat of subarctic waters during interglacials in the Atlantic, while continued high glacial 7:4 indicate extensive subarctic waters during glacial maxima associated with the development of the larger late Pleistocene ice sheets. In contrast, a secular decline in 7:4 occurred at Site 882 from 0.9 to 0.5 Ma, marking a more gradual retreat of subarctic conditions in the Pacific. It is proposed that the expansion of subarctic waters between 1.15 and 0.9 Ma exerted negative feedbacks to the moisture supply to the ice sheet source regions and may account for the apparent delayed ice sheet response to atmosphere-ocean circulation changes associated with the mid-Pleistocene transition that began as early as 1.2 Ma.
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