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TitleThe last 9,000 years in lake records from the NWT: time constraints from 14C dating, age-depth modeling, and a new occurrence of the White River Ash
AuthorCrann, C; Patterson, R T; Macumber, A L; Galloway, J M; Roe, H M; Falck, H
SourceNorthwest Territories Geoscience Office, Yellowknife Geoscience Forum Abstracts Volume 2013, 2013 p. 1
Alt SeriesEarth Sciences Sector, Contribution Series 20130444
Mediapaper; on-line; digital
File formatpdf
ProvinceNorthwest Territories
AreaYellowknife; Lac de Gras
Subjectspaleontology; geochronology; paleoclimates; Holocene; lake sediments; lake sediment cores; carbon-14 dates; carbon isotopes; Cenozoic; Quaternary
Programenvironmental impacts and adaptation in the northern environment, Environmental Geoscience
LinksOnline - En ligne
AbstractAs part of a multidisciplinary paleoclimate project aimed at assessing the future viability of the Tibbitt to Contwoyto Winter Road (TCWR) in the central Northwest Territories, we examined late Holocene sedimentary records from lake cores transecting boreal forest, treeline, and tundra zones. Before undertaking paleoclimate and paleoenvironmental studies on the cores, radiocarbon based age-depth models were constructed to ensure the sedimentary record covers at least the last 3,500 years and there are no major hiatuses in deposition.

Chronologies are fundamental to interpreting the timing and rates of past environmental changes. Significant advancements have been made during the last decade in the methods of age-depth modeling and the calibration of radiocarbon dates. Traditionally, linear regression linear interpolation models were used to estimate the ages of depths between a few dated horizons. While the classic approach is sufficient at a coarse scale, our study requires much higher precision in order to resolve paleoclimate shifts on a multi-decadal scale. In addition to improving the dating resolution by adding more 14C dates than have been traditionally used for cores in the region, the age-depth models for sedimentary successions retrieved as part of this study are constructed using a probabilistic/Bayesian approach involving multiple simulations. Such chronologies are essential in high-resolution paleoclimate and paleoenvironmental studies.

Thus far, over 130 14C dates have been obtained, primarily on bulk sediment, from 19 sediment cores obtained from 11 lakes along a latitudinal gradient spanning from boreal forest near Yellowknife to tundra sites near Lac de Gras. There are between four to twenty-five 14C dates per core, some of which extend back as far as 9,000 years before present. In two sediment cores obtained from Pocket Lake near Yellowknife, a visible tephra is geochemically fingerprinted as the White River Ash (WRA). The WRA is from a Plinian-type eruption of Mount Churchill in Alaska that occurred ca. 1,200 years ago. The timing of the eruption is best constrained by radiocarbon dates from the outer rings of trees that were buried in coarse tephra and a pyroclastic flow near the source vents (Clague et al., 1995). The occurrence of the WRA in the Pocket Lake cores is, to the best of our knowledge, the furthest east it is recognized as a visible tephra. Because the timing of the WRA is well constrained, its occurrence in Pocket Lake is incorporated into the age-depth models for cores obtained from this site. This is important validation of the 14C based age-depth models.
Summary(Plain Language Summary, not published)
This research is part of a larger, multi-disciplinary project aimed at assessing the future viability of the Tibbitt to Contwoyto Winter Road, the sole overland route servicing mines in the central Northwest Territories. High-resolution analysis of sediments from lakes spanning from near Yellowknife to Lac de Gras will be used to better understand climate variability in this region. Central to paleoclimate reconstruction are age-depth relationships. This contribution establishes chronologies for the studied lake sediments using radiocarbon geochronology, age-depth modeling, and tephra analysis. Over 130 radiocarbon dates are obtained from 19 lake sediment cores. Four to 25 radiocarbon dates occur per core, mainly on bulk sediments. Two sediment cores obtained from Pocket Lake, near Yellowknife, contain a visible tephra geochemically identified as the White River Ash, which is belived to have originated from the eruption of Mount Churchill in Alaska approximately 1200 years ago. This occurrence is the furthest east the tephra has been recognized.