Late Quaternary paleoclimates and biotic responses in eastern North America and the western North Atlantic Ocean
Abstract Broad-scale late Quaternary paleoclimatic changes are interpreted from palynological records across eastern North America and from assemblages of marine Foraminifera in the western North Atlantic Ocean. Quantitative reconstructions of past dominance for major tree taxa at eight fossil-pollen sites, located south of the maximum glacial extent in the southeastern United States, illustrate dramatic responses of tree populations to climatic change during the transition from Pleistocene to Holocene conditions. A series of maps depicting changes in inferred boundaries of predominant airmasses during the late Quaternary are based upon paleoclimatic interpretation of paleovegetation maps for eastern North America, and CLIMAP maps of winter sea-surface temperatures for the western North Atlantic Ocean. In the eastern United States, at 18, 000 B.P. strong zonal circulation resulted in a fixed position of the atmospheric Polar Frontal Zone along 33°N latitude. The Pacific Airmass extended from 33°N latitude north to the southern margin of the Laurentide Ice Sheet, with the Maritime Tropical Airmass dominating south of 33°N across the Gulf Coastal Plain, the Gulf of Mexico, and the equatorial Atlantic Ocean. The oceanic Polar Frontal Zone extended northeastward from Cape Hatteras to 37°N in the western North Atlantic Ocean. Late glacial climatic amelioration was first reflected in vegetational changes south of the Laurentide Ice Sheet beginning at 16,500 B.P. Late glacial weakening in zonal atmospheric flow resulted in the penetration of the Maritime Tropical Airmass farther to the north during the summer season. Increased summer warmth and rain produced substantial melting of the southern flank of the Laurentide Ice Sheet. The early Holocene interval, from 12,500 to 8,000 B.P., bracketed the time of maximum seasonality of solar radiation, ca. 11,000 B.P. During the early Holocene, enhanced meridional air flows resulted in increased seasonality of temperatures. By 10,000 B.P., the oceanic Polar Frontal Zone was displaced northward from the Gulf Stream. Anomalously cold, maritime climatic conditions persisted along the central Atlantic coast, reflecting the position of the cold-marine Labrador Current between the Atlantic Seaboard and the Gulf Stream. During the mid-Holocene, or Hypsithermal, interval, from 8,000 to 4,000 B.P., prevailing westerlies expanded the midcontinental region of warmth and aridity. For the last 4,000 years, the modern climatic regime has been characterized by increased meridional flow.