Southward penetrations of the North Atlantic polar front: faunal and floral evidence of large-scale surface water mass movements over the last 225,000 years

Abstract

Eight cores taken on the eastern flank of the Mid-Atlantic Ridge from 3°N to 60°N document climatic shifts over the last 225,000 years. Southward penetration of polar water elimiontated the coccolith carbonate, heightened the amount of ice-rafted mineral detritus, and drastically altered the foraminiferal assemblages. Only a single polar foraminiferan, left-coiled Globigerina pachyderma, remained to dominate the cold waters, whereas diverse subpolar species and abundant Coccolithophorida fluorished in warmer times. The major glacial pulses occured from roughly 30,000–15,000 Y.B.P. (the late Wisconsin maximum) and from 160,000–135,000 Y.B.P. Shorter and less severe cold periods are present on at least four occasions in the last 225,000 years. Conversely, marked northward incursions of species typical of the present subtropical gyre happened at least six times, the three strongest being: slightly post Terminations I (8000 Y.B.P.) and the oldest approximately 175,000 Y.B.P. The continuing similarity through late Pleistocene time for foraminiferal and coccolithophorid assemblages in the six northernmost cores suggests there has existed a uniform water mass with minimal variations from 45°N to above 55°N. The southern margin (42–45°N) of this faunally-defined unit in the eastern Atlantic coincides with the southward limit of late Pleistocence polar water penetration.