HR: 0800h
AN: PP11A-0221    [Abstracts]
TI: Age Constraints On Interglacial Sediments Beneath The Former Center Of The LIS Using OSL And U-Th Dating
AU: * Allard, G
EM: allard.guillaume@courrier.uqam.ca
AF: Université du Québec à Montréal, 201 President-Kennedy, Montreal, QC H3C3P8, Canada
AU: Dubé-Loubert, H
EM: dube-loubert.hugo@courrier.uqam.ca
AF: Université du Québec à Montréal, 201 President-Kennedy, Montreal, QC H3C3P8, Canada
AU: Roy, M
EM: roy.martin@uqam.ca
AF: Université du Québec à Montréal, 201 President-Kennedy, Montreal, QC H3C3P8, Canada
AU: Lamothe, M
EM: lamothe.michel@uqam.ca
AF: Université du Québec à Montréal, 201 President-Kennedy, Montreal, QC H3C3P8, Canada
AU: Ghaleb, B
EM: ghaleb.bassam@uqam.ca
AF: Université du Québec à Montréal, 201 President-Kennedy, Montreal, QC H3C3P8, Canada
AU: Veillette, J
EM: JVeillet@nrcan.gc.ca
AF: Geological Survey of Canada, 601 Booth, Ottawa, ON K1A0E8, Canada
AU: Parent, M
EM: miparent@nrcan.gc.ca
AF: Geological Survey of Canada, 490 de la Couronne, Quebec, QC G1S2L2, Canada
AB: The main rivers of the Hudson and James Bay lowlands expose an extensive sedimentary record consisting of several glacial units locally interbedded with pre-Holocene nonglacial sediments. The presence of nonglacial sediments near the former geographic center of the Laurentide Ice Sheet (LIS) is significant because it implies a large reduction of ice volume or a completely ice-free episode (i.e. interglacial). Distinguishing between these two scenarios has been a recurrent problem in the lowlands because of the lack of reliable ages on these deposits. Radiocarbon dating of these deposits routinely yielded infinite ages while thermoluminescence dating gave ages requiring unrealistic paleogeographic reconstructions. Here we use three different methods to date an extensive pre-Holocene nonglacial unit that crops out along the Nottaway River and Harricana River in northwestern Québec. This unit consists primarily of an accumulation of abundant compressed wood fragments and organic-rich clays of about 5 m in thickness along the Nottaway River, while fluvial sands comprising abundant wood fragments and organic matter of about 3 m in thickness are exposed along the Harricana. Along both rivers, these sediments lie below several glacial and glaciolacustrine units. Radiocarbon dating of two wood fragments yielded infinite ages of 55.2 ka and 54.8 ka on the Nottaway and Harricana rivers, respectively. Uranium-Thorium (U-Th) dating was carried out on four samples of compressed wood logs of the Nottaway River. Because living wood does not contain significant Uranium concentrations in its structure, the Uranium measured in fossil wood samples likely originates from diagenetic uptake. The calculated U-Th age can thus be considered as a minimal age, given that the system remains closed after this uptake phase. Reproducible results were obtained from the four samples which yielded U-Th ages ranging between 100 and 110 kyr, with an average age of 106.3 ± 2 ka, thereby suggesting a single phase of Uranium uptake during burial or early diagenesis. Optical luminescence dating of the wood-bearing fluvial sands on the Harricana River also support the antiquity of the nonglacial unit, and preliminary measurements suggest and an IRSL age of 131 ± 2 ka. Additional U-Th and OSL analyses are currently being processed on other samples in order to reinforce correlations between the two sectors investigated. Consequently, the application of different dating methods suggests that pre-Holocene organic- bearing sediments in the study area belong to the last interglacial (Sangamonian; marine isotopic stage 5e). This is consistent with the paleoecological content of this unit, which suggests deposition under climate conditions roughly similar to today. Our results suggests that only one ice-free interval is recorded in the sequences of the James Bay lowlands of Québec, indicating that the following glacial advances and partial ice retreat events took place throughout the course of the last glacial cycle. This represents the first time that pre-Holocene organic- bearing sediments are dated reliably in the Hudson-James Bay lowlands, thereby providing important constraints for paleogeographic reconstructions of the LIS.
DE: 0726 Ice sheets
DE: 1040 Radiogenic isotope geochemistry
DE: 1105 Quaternary geochronology
DE: 4914 Continental climate records
DE: 4936 Interglacial
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting