Late quaternary paleohydrology of the Gulf of St. Lawrence (Quebec, Canada) based on diatom analysis

Postglacial sediments from three cores raised from the Gulf of St. Lawrence along the path of the Laurentian Channel, were analyzed for diatoms. A sequence of four distinct zones was defined from oldest to youngest: 1. Characterized by low diatom concentrations, this zone is mainly represented by heavily silicified species Delphineis surirella, Stephanopyxis turris, Coscinodiscus radiatus and Paralia ornata that were probably deposited by glacial drift in a glaciomarine environment from about 18 000 to 14 000 years BP. 2. This postglacial sedimentation zone deposited around 13 000 years BP, is marked by high diatom concentrations with cold-water marine species, Thalassiosira antarctica resting spores and Porosira glacialis. The upper part of the zone suggests a gradual warming with the appearance of temperate water species, Bacterosira bathyomphala and Chaetoceros spp., both resting spores and vegetative cells. 3. An important cooling of surface temperatures is noticed by the resurgence of cold species, Porosira glacialis, Thalassiosira antarctica resting spores and Thalassiosira hyalina. This zone is associated with the Younger Dryas chronozone dated from 10 800 to 10 300 years BP. This interval is also characterized by two marked freshwater pulses, coming from the outflow of Laurentide Ice Sheet meltwater, on the surface layer of the Goldthwait Sea. Although well-defined in the northwestern Gulf area (core 90-031-015), the influence of this meltwater decreases considerably or even disappears in Cabot Strait (core 89-007-111) and offshore (core 90-031-044). Evidence of reduced postglacial glacial runoff on surface water supports the conclusion that it could not have generated the Younger Dryas cooling episode. 4. The diatom assemblages from 10 000 years BP through Recent correspond to the progressive establishment of modern-like surface water conditions. (C) 2000 Elsevier Science B.V. All rights reserved.