Subpolar North Atlantic Circulation at 9300 yr BP: Faunal Evidence

Abstract

We have examined the circulation of the subpolar North Atlantic at 9300 yr BP by using a dispersed layer of silicic volcanic ash as a synchronous horizon. At the level of this datum, we have reconstructed from foraminiferal evidence a geologically synoptic view of seasonal variations in sea-surface temperatures and salinities. The reconstruction defines two oceanic fronts at 9300 yr BP: (1) the meridionally oriented Polar Front bordering the axis of deglacial outflow of Arctic and Laurentide ice and meltwater and (2) a zonal portion of the Subarctic Convergence along 48° N, marking a major confluence between the subtropical and subpolar gyres. The 9300-yr configuration primarily differed from the modern pattern in the more easterly position (by 3°) of the Polar Front and the more southerly (3°) and easterly (5°) position of the Subarctic Convergene. Both fronts had been merged at 18,000 yr BP into the full-glacial Polar Front; at 9300 yr BP, they were approaching the end of a northwestward deglacial retreat toward the modern interglacial positions. There were two dominant departures at 9300 yr BP from the Earth's modern configuration, both related to deglaciation: the very large Laurentide Ice Sheet still covering eastern North America to 48° N, and the region of cold Arctic/Laurentide deglacial outflow. These two features caused: a more easterly position than now of the region of upper air divergence and lower air convergence downstream from the Ice Sheet and meltwater outflow; a more intense expression of this upper air divergence and lower air convergence over the central portion of the subpolar North Atlantic; and a latitudinally more stable axis of convergence of surface westerlies over this region. These factors apparently caused the stronger oceanic convergence along 48°N than at present. They also created a stronger, southeastward-directed wind drift current, which opposed the meridional (northward) flow typical of the present interglaciation.