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Identification of Younger Dryas Outburst Flood Path from Lake Agassiz to the Arctic Ocean

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Identification of Younger Dryas Outburst Flood Path from Lake Agassiz to the Arctic Ocean Vol 464 | 1 April 2010 | doi:10.1038/nature08954 LETTERS Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean Julian B. Murton1, Mark D. Bateman2, Scott R. Dallimore3, James T. Teller4 & Zhirong Yang4 The melting Laurentide Ice Sheet discharged thousands of cubic proglacial drainage delivered water to the Beaufort Sea coast10. Later, kilometres of fresh water each year into surrounding oceans, at times the Holocene marine transgression trimmed and buried part of the suppressing the Atlantic meridional overturning circulation and unconformity7. triggering abrupt climate change1–4. Understanding the physical We determined the age of intervalley erosion and gravel deposition mechanisms leading to events such as the Younger Dryas cold inter- by optically stimulated luminescence dating of eleven samples above val requires identification of the paths and timing of the freshwater and below the erosion surface at six locations (Table 1; Fig. 2). discharges. Although Broecker et al. hypothesized in 1989 that an Aeolian sand from 0.2–5.0 m below the erosion surface provides outburst from glacial Lake Agassiz triggered the Younger Dryas1, seven ages between 13.4 6 0.9 kyr and 12.7 6 0.9 kyr (weighted mean specific evidence has so far proved elusive, leading Broecker to con- by inverse variance is 13.0 6 0.2 kyr). Fluvial and aeolian sand from clude in 2006 that ‘‘our inability to identify the path taken by the above the gravel provides ages of 11.8 6 1.0 kyr, 11.9 6 1.0 kyr and flood is disconcerting’’2. Here we identify the missing flood path— 11.5 6 0.7 kyr (mean 5 11.7 6 0.1 kyr), the latter two separating two evident from gravels and a regional erosion surface—running gravelly layers. Aeolian sand that post-dates all fluvial gravelly sedi- through the Mackenzie River system in the Canadian Arctic ments and channels that we have observed provides an age of Coastal Plain. Our modelling of the isostatically adjusted surface 9.3 6 0.7 kyr. From these data, we identify two major high-energy in the upstream Fort McMurray region, and a slight revision of the fluvial episodes, indicated by the two gravelly beds shown in ice margin at this time, allows Lake Agassiz to spill into the Supplementary Fig. 3A and C. The first was deposited between Mackenzie drainage basin. From optically stimulated luminescence 13.0 6 0.2 kyr and 11.7 6 0.1 kyr ago, and the second between dating we have determined the approximate age of this Mackenzie 11.7 6 0.1 kyr and 9.3 6 0.7 kyr ago. River flood into the Arctic Ocean to be shortly after 13,000 years ago, near the start of the Younger Dryas. We attribute to this flood a boulder terrace near Fort McMurray with calibrated radiocarbon dates of over 11,500 years ago. A large flood into the Arctic Ocean at the start of the Younger Dryas leads us to reject the widespread view that Agassiz overflow at this time was solely eastward into the Arctic Ocean North Atlantic Ocean. Widespread fluvial erosion and deposition punctuated the last glacial–interglacial transition near the mouth of the Mackenzie Fig. 2 Mackenzie River on the Canadian Arctic Coastal Plain (Fig. 1 and Supplemen- River Fort tary Figs 1 and 2). Onshore, we have observed a regional erosion Good 55o N Hope surface and gravel to elevations of 30 metres above sea level Lake 5 McConnell North Atlantic (m.a.s.l.) in the northeast Richards Island area —an intervalley region Lake Ocean bordered by the East Channel–Kugmallit Trough valley system to the Mackenzie 50o N Laurentide east and the Middle Channel–Mackenzie Trough system to the west Fig. 3 Ice Sheet (Fig. 2). Our field investigations on northeastern Richards Island o Fort have revealed that fluvial erosion has stripped off much of the pre- 45 N McMurray existing till and terminated the building of large aeolian dunes and Lake St Lawrence sand sheets that characterized non-glacial periods of Marine Isotope Agassiz River Stage 2 (ref. 6). The erosion produced an unconformity that truncates remnant till patches and aeolian sand deposits, and underlies a pebble–boulder lag or gravel reworked from the till5. Mississippi River o Offshore, beneath the eastern Beaufort Shelf, the erosion surface 120o W 100o W 80o W 60 W occurs as a regional unconformity7 and forms part of an undulating Figure 1 | Large proglacial lakes along the retreating Laurentide Ice Sheet surface that includes submerged valleys 30–50 m deep that cross the 8 at 12.65–12.75 cal. kyr BP, near the start of the Younger Dryas. Three shelf to a depth of ,70 m below the present sea level . Valley incision outlets are indicated: (1) northwest along the Mackenzie River to the Arctic has been attributed to a major increase in discharge across the emergent Ocean, (2) east along the St Lawrence River to the North Atlantic Ocean, and 9 shelf during the Sitidgi glacial stade in the Mackenzie Trough at (3) south along the Mississippi River to the Gulf of Mexico. The distribution ,15,000 calibrated years before present (15 cal. kyr BP), when relative of glacial ice (white), proglacial lakes (dark blue) and land (grey) is from ref. sea level was ,70 m or more below the present level9 and extensive 11. Purple boxes mark the location of Figs 2 and 3. 1Permafrost Laboratory, Department of Geography, University of Sussex, Brighton BN1 9QJ, UK. 2Sheffield Centre for International Drylands Research, Department of Geography, Winter Street, University of Sheffield, Sheffield S10 2TN, UK. 3Geological Survey of Canada, 9860 West Saanich Road, Sidney, British Columbia V8L 4B2, Canada. 4Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada. 740 ©2010 Macmillan Publishers Limited. All rights reserved NATURE | Vol 464 | 1 April 2010 LETTERS 135o W 134o W Figure 2 | Digital elevation model of Richards Island region. North Head Shown 9 Eastern Shelf are luminescence dates (red 14 Reindeer 0 10 20 30 km numbers, in kyr with 1s Mackenzie Bay Islands uncertainties) constraining the age 10 Had 3.1 CP3.11 6 >11.5±0.7 kyr of a regional erosion surface and >9.3±0.7 kyr Elevation (m) MB05-02 <12.8±0.8 kyr <13.2±0.9 kyr Kugmallit 0.905 – 1.24 overlying gravelly deposits (yellow (lower gravel) <12.7±0.8 kyr Bay 1.25 – 1.57 >11.9 ± 1.0 kyr 14 1.58 – 1.91 squares) relating to the first flood 8 16 1.92 – 2.25 <12.9 ± 1.1 kyr 17 7–10 6 2.26 – 2.58 8 Summer Island 2.59 – 3.25 event. Numbers in white next to 3.26 – 4.26 4.27 – 5.93 yellow squares are approximate Hadwen Island 5.94 – 7.28 SB05-01 7.29 – 8.62 SB08-12 Mason 8.63 – 9.96 elevations of fluvial features in 11 <12.9 ± 0.8 kyr 9.97 – 11.3 <12.7 ± 0.9 kyr Bay 11.4 – 12.6 m.a.s.l. Blue half-moons indicate 12 30 12.7 – 14 11–14 3 14.1 – 15.3 channelled erosion surfaces. See Beluga Bay 18 17 15.4 – 16.7 5 16.8 – 18 18.1 – 19.3 o ′ Supplementary Discussion for 17 1 19.4 – 20.7 69 30 N 20.8 – 22.4 22.5 – 24 details about fluvial sediments and Mallik 24.1 – 25.7 MB05-01 25.8 – 27.7 stratigraphy. Bay >11.8 ± 1.0 kyr 27.8 – 29.7 29.8 – 32.1 <13.4 ± 0.9 kyr 32.2 – 34.8 Richards Island 34.9 – 37.8 3.5–7.5 37.9 – 42.1 42.2 – 48.9 49 – 85.1 10 Kittigazuit Bay 69o 15′ N Middle Chann el East Channel Mackenzie Delta YaYa Lake The distribution of the coarse gravel and the spatially extensive yet a time when the nearest actively retreating ice margin—according to narrowly timed fluvial erosion are exceptional. We discount non- the recent reconstruction of Laurentide deglaciation11—was in the glacial fluvial activity because the post-glacial Mackenzie River delivers vicinity of Great Bear Lake, ,600 km southeast of Richards Island mostly silt and clay to its low-lying delta, rather than the pebble– (Fig. 1). We infer two vast, yet rapid fluvial events, with the most likely boulder gravel that we observe to elevations up to 30 m.a.s.l. in a region water source being a distant glacial lake such as Agassiz or McConnell. lacking evidence for glacio-isostatic uplift. Instead, the water source Outburst floods from such massive lakes could reasonably explain the must have been glacial meltwater. A local glaciofluvial source from the age, wide spatial extent and elevation of the erosion surface and gravel. Sitidgi ice margin is also unlikely. First, meltwater issuing from glacier We envisage that the flooding events significantly affected the margins tends to flow along the lowest parts of landscapes, rather than palaeogeography of the Mackenzie Delta area and the Beaufort over topographic highs such as northeast Richards Island. To sub- Shelf. While the active glacial ice margin moved south, cold per- merge it and transport cobbles and boulders over it would have mafrost conditions on the coastal plain preserved large amounts of required a huge amount of water washing over and inundating the stagnant glacial ice12. Thus initial flow was probably affected by glacial entire area. Second, the erosion post-dates 13.0 6 0.2 kyr ago (Table 1), drainage systems.
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