Cooling in the Gulf of St. Lawrence and estuary region at 9.7 to 7.2 C-14 ka (11.2-8.0 cal ka): Palynological response to the P130 and 8.2 cal ka cold events, Laurentide Ice Sheet air-mass circulation and enhanced freshwater runoff

摘要

An examination of post-Younger Dryas (YD) pollen stratigraphies in the Gulf of St. Lawrence and St. Lawrence estuary region reveals features in the pollen records that represent breaks in the normal vegetation succession, widespread vegetation suppression, and a delay in migration of plant taxa between 9.7 and 7.2 C-14 ka (11.2 and 8.0 cat ka). The domination of Alnus crispa at sites bordering the St. Lawrence estuary-Gutf region in Gaspesie and northern New Brunswick within this timeframe represents a diversion from the typical vegetation progression from Picea and/or Populus or Picea/Betula to Pinus and/or Betula, and signifies a shift to a cooler, drier climate. Coinciding with the A. crispa expansion and domination in that region was the contraction of Picea populations in other areas. In southwestern New Brunswick and eastern and southeastern Nova Scotia, Picea was replaced by the first appearance of tree birch, B. whereas in western and southwestern Newfoundland, Picea gave way to a resurgence of shrub birch, Betula glandulosa. The Picea contraction and immediate resurgence of Betula represents Cooling, and is reliably dated at 9720 +/- 110 C-14 BP (10,800-11,240 cat BP) in southwest Newfoundland. This first post-YD episode of widespread cooling is correlated with the North Atlantic Preboreal Oscillation (PBO) centered around 9650 +/- C-14 BP (10,900 - 11,180 cat BP) in the adjacent Great Lakes region. Sites exposed to winds from the Gulf of St. Lawrence in eastern New Brunswick, Prince Edward Island, and northern Nova Scotia show a lingering persistence of Picea and delay in arrival of Pinus to 8.0 and 7.7 C-14 ka (9.0 and 8.4 cat ka), yet Pinus was dominant as early as 9.4 C-14 ka (10.6 cat ka) in southwestem New Brunswick. At the same time, tundra vegetation persisted at high elevations in western and southwestern Newfoundland only to be replaced by upslope migration of shrub-birch heath by 8 C-14 ka. Prolonged broad-scale cooling to 8 C-14 ka and to as late as 7.7 C-14 ka extended up to 200 km inland in areas exposed to the St. Lawrence estuary and Gulf region and was in response to strong, cold, dry anticyclonic winds coming off the retreating Laurentide Ice Sheet in combination with enhanced freshwater runoff through the Gulf of St. Lawrence. The end of the period of prolonged cooling and onset of regional warming coincided with the diversion of western Canada runoff and Agassiz-Ojibway drainage to Hudson Bay and reduced effect or final break-up of the Laurentide Ice Sheet. Several sites document a subsequent cold shift, that interrupted regional warming at 7650 to 7200 C-14 BP (8400 to 8000 cat BP), and which is variously represented by the suppression of Pinus and resurgence of Picea, sometimes with A. crispa (Quebec-Maritime region), or by an abrupt decrease of Picea and resurgence of Betula (western Newfoundland). This second post-YD cool interval is equated with the 8200 cat BP cold event registered in the Greenland ice isotopic record. (c) 2006 Elsevier B.V. All rights reserved.