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TitleTracking Agassiz floodwaters beyond Hudson Strait: correlation to the onset of the 8.2 cal ka cold event
AuthorLewis, C F MORCID logo; Miller, A A L; Levac, E; Piper, D J; Sonnichsen, G V
SourceEos, Transactions of the American Geophysical Union vol. 90, no. 22, 2009, 1 pages
Alt SeriesEarth Sciences Sector, Contribution Series 20080746
MeetingAmerican Geophysical Union, Geological Association of Canada, Joint Assembly; Toronto; CA; May 25-29, 2009
Mediapaper; on-line; digital
AreaHudson Bay; Labrador Sea
Subjectssedimentology; surficial geology/geomorphology; geochronology; Nature and Environment; sedimentation; carbonates; climate; climatic fluctuations; climate effects; floods; glacial lakes; glacial lake deposits; Lake Agassiz; Climate change
ProgramGeoscience for Oceans Management Geohazards and Constraints to Offshore Development
AbstractBarber et al. (1999, Nature 400: 344-348) originally hypothesized that floodwaters from glacial Lake Agassiz flowed from Hudson Strait directly into the Labrador Sea about 8.47 cal ka and suppressed thermohaline circulation there to initiate the 8.2 cal ka cold event. Keigwin et al. (2005, Paleoocean. 20: PA2003, doi:10.1029/2004PA001074) and Hillaire-Marcel et al. (2007, Geophys. Res. Let. 34: doi:1029/2007GL030396) did not find evidence for the floodwaters in the Labrador Sea. It is now known that Labrador Sea convection began about 1000 years later. Also, the original radiocarbon chronology offset the Agassiz outflow from the cold event by about 200-300 years. Energetic drainages of glacial lakes from the limestone- and dolomite-rich Hudson Bay and Hudson Strait regions carried suspended sediment which subsequently rained out over the floodwater trajectory to produce distinct sedimentary beds of enhanced detrital carbonate (DC) content. Using the DC beds as a proxy for floods emanating from Hudson Strait, our studies of sediment cores show that the Lake Agassiz and most other floods turned south after leaving the Strait and flowed over the Labrador and Newfoundland shelves and upper continental slopes, rather than into the Labrador Sea. DC beds in cores collected south of the Grand Banks of Newfoundland show that the Agassiz waters reached the Gulf Stream and were likely transported northeastward by the Atlantic Current into the Nordic seas, where they could have suppressed North Atlantic deepwater production. Bard et al. (1994, Earth and Planetary Sci. Let. 126: 275-287) have shown that corrections to North Atlantic 14C dates on biogenic carbonate depend mainly on the presence of Gulf Stream subtropical water and the duration of annual sea-ice cover that suppresses air-water CO2 exchange. Along the Labrador and Newfoundland margins Gulf Stream water is not a factor, but sea ice has a significant presence. Reservoir corrections applied to 14C dates on biogenic carbonate are based on the age of modern (pre-bomb) shells, and incorporate the effects of current sea-ice cover duration (5-6 months), but not necessarily the duration of former ice cover. Transfer function analysis of dinoflagellate assemblage data show that sea-ice durations at the time of the Agassiz floods ranged up to 11 months; this difference translates to increased corrections of up to -200 years for radiocarbon dates on foraminifera and mollusk shells. An additional -100 year correction allows for the likely presence of dissolved inorganic `old' carbon in oceanic waters, indicated by relatively high (5 to 50%) contents of Paleozoic-aged detrital carbonate in early Holocene sediments. These adjustments, when applied to new and previous Labrador and Newfoundland offshore 14C dates, show that the Agassiz floods were coeval with the onset of the 8.2 cal ka event oxygen isotope excursion in Greenland ice records. These findings raise confidence in the conclusion that ice-dam failure and rapid flooding of glacial Lake Agassiz into the North Atlantic Ocean played a significant role in causing abrupt climate change at 8.2 cal ka.

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