Abstracts
Abstract
Post-glacial pollen spectra over a wide area of southeastern Canada have been interpreted as showing that the general warmth-adapted trend of regional vegetation change was interrupted between 10 000 and 8000 14C yr BP, reverting to conditions associated with a markedly cooler climate. This biotic reversal has been attributed to a climatic cooling caused by discharge of frigid water from glacial Lake Agassiz, through the Great Lakes to the Goldthwait Sea in the Gulf of St. Lawrence basin. Here, we assemble geological evidence from widely scattered localities in and around the Gulf of St. Lawrence, all previously reported, a majority unexplained, and ascribe it collectively to the same climatic cooling. We interpret the marine diamicts and other faunal reversals as the products of cooling, which intensified sea-ice conditions during the interval 10 000-8000 14C yr BP, specifically at 9300 14C yr (9170 yr, reservoir corrected), conditions resulting from that meltwater influx. This age was several centuries too young to be correlated with the Preboreal Oscillation, but several easterly overflows of Lake Agassiz occurred before and after this date, and may have increased sea-ice in the Goldthwait Sea, singly or by hysteresis. Other truly glacial features in southeastern Canada, such as moraine systems and diamictons, are also referred to this cooling.
Résumé
Les spectres polliniques post-glaciaires d’une vaste région du sud-est du Canada montrent que la tendance générale au réchauffement qu’impliquent les changements de végétation régionaux a été interrompue entre 10 000 et 8000 ans 14C BP, revenant à des conditions typiques d’un climat plus froid. Cette inversion biotique a été attribuée à un refroidissement climatique causé par la décharge des eaux froides du lac glaciaire Agassiz, via les Grands Lacs jusqu’à la mer de Goldthwait en passant dans le Golfe du Saint-Laurent. Nous examinons ici plusieurs indices géologiques aux alentours du Golfe du Saint-Laurent, tous déjà publiés, la plupart inexpliqués, et les attribuons collectivement au même refroidissement. Nous interprétons les diamictons marins et les inversions biotiques comme étant le produit d’un refroidissement qui a intensifié les conditions de glace de mers entre 10 000 et 8000 ans BP, plus particulièrement il y a 9300 ans BP (9170 ans, avec correction du réservoir marin), ces conditions résultant des apports d’eau de fonte. Cet âge est trop jeune de plusieurs siècles pour être corrélé à l’oscillation du Préboréal, mais plusieurs épisodes de décharge du lac Agassiz se sont produites avant et après cette date, et peuvent avoir favorisé un plus grand couvert de glace dans la mer de Goldthwait, chacun ou par hystérésis. D’autres traits typiquement glaciaires, tels les systèmes morainiques, sont également associés à ce refroidissement dans le sud-est du Canada.
Appendices
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