QUATERNARY SEA-ICE REGIME CHANGE IN THE WESTERN ARCTIC OCEAN BASED ON BENTHIC FORAMINIFERS

Benthic foraminiferal assemblages from sediment cores of the Northwind, Mendeleev, and Lomonosov Ridges (western Arctic Ocean) were investigated to better understand Quaternary paleoceanographic and sea-ice conditions. Calcareous foraminiferal abundances and assemblage composition show stratigraphically consistent changes, with major transitions at the end of the Mid-Pleistocene Transition (MPT; estimated ca. 0.7-0.8 Ma) and later near the bottom of Marine Isotope Stage 7 (ca. 0.25 Ma). These records of downcore foraminiferal distribution interpreted in terms of sea-ice conditions indicate an overall expansion of year-round Arctic ice cover during the Pleistocene. The climatic shift at the MPT from mostly seasonal to perennial sea ice occurred alongside the growth of North American ice sheets, leading to the beginning of glacially-controlled sedimentation in the western Arctic Ocean. The ca. 0.25 Ma transition also appears to co-occur with signs of ice-sheet expansion in both North America and Eurasia. The Holocene part of the record is not yet adequately resolved due to mostly insufficient sedimentation rates. A more expanded Holocene unit from the central Lomonosov Ridge shows assemblages differing from those in the Pleistocene sediments from the western Arctic. Whether this difference is of stratigraphic or geographic nature is yet to be investigated.