ABSTRACT: Testing the Lake Agassiz Meltwater Trigger for the Younger Dryas.

CGRG Bibliography of Canadian Geomorphology
Search Results

Author :

Date :

Title :

Publication :

Issue :

Page(s) :

Abstract

Meltwater drainage from glacial Lake Agassiz has been implicated for nearly 15 years as a trigger for thermohaline circulation changes producing the abrupt cold period known as the Younger Dryas. On the basis of initial field reconnaissance to the lake’s proposed outlets, regional geomorphic mapping, and preliminary chronological data, an alternative hypothesis may be warranted. Should ongoing data collection continue to support preliminary results, it could be concluded that Lake Agassiz did not flood catastrophically into the Lake Superior basin preceding the Younger Dryas (Figure 1).Allpreliminary findings imply a retreating ice sheet margin approximately 1000 years younger than previously thought, which would have blocked key meltwater corridors at the start of the Younger Dryas. If Lake Agassiz meltwater passing into the North Atlantic is not the trigger for the Younger Dryas, then perhaps there were different sources of water or triggers.At this point, it seems prudent to carefully examine the role of glacial Lake Agassiz in any abrupt climate change scenario. The current paradigm for driving abrupt climate change is the modification of thermohaline circulation by the addition of external freshwater to the North Atlantic Ocean. Numerous modeling experiments have demonstrated the extreme sensitivity of this system, and attributing the source of that freshwater to glacial Lake Agassiz has evolved with numerous investigations. In the mid-1970s, Kennett and Shackleton[1975] noted that the isotopic composition of seawater in the Gulf of Mexico fl uctuated substantially during deglacial time, and they attributed the fluctuation to changing sources of meltwater from the Laurentide Ice Sheet. Approximately coeval with the isotope changes, the Laurentide Ice Sheet retreated northwardinto an isostatically depressed basin behind the subcontinental drainage divide. Researchers in the Great Lakes reconstructed lake-level history, and they recognized variations in meltwater routing either through the MississippiRiver to the Gulf of Mexico or through the St. Lawrence River to the North Atlantic. By the late 1980s, Broecker et al. [1989] had honed the concept of a freshwater trigger upsetting thermohaline circulation, and modelingexperiments defined the necessary meltwater fluxes.The climate connection to glacial Lake Agassiz arose because organic sediments (10,960–9900 radiocarbon years [14C] B.P.) deposited between two sequences of deepwaterclays would require a major drop in lake level, i.e. a meltwater releasing event coeval with the Younger Dryas. Subsequently,Teller and colleagues [e.g.,Teller and Leverington, 2004] employed rebound models (delayed glacio-isostatic uplift of the Earth’s crust from ice-sheet loading following deglaciation), lake-level histories, and ice-retreat patterns to calculate meltwater volumes reaching the North Atlantic via an eastern route.These calculations were compatible with modeling estimates needed to affectocean circulation. Thus, a terrestrial meltwater drainage reconstruction for triggering the Younger Dryas existed that was compatible with ocean records.An initial aerial survey on 4–7 May 2003 of areas north and west of Thunder Bay, Ontario, referred to as the eastern outlets, was followed by a second survey on 20–22 September 2003 north and east of Fort McMurray, Alberta, along the Clearwater River. This area is referred to as the northwestern outlet. Boulders from moraine crests and flood channels were collected in each areafor cosmic ray exposure dating.The results of these surveys, and dates obtained, have directed further ongoing research activities. In the Thunder Bay region, channels and dry waterfalls in bedrock west and south (Ouimet Canyon) of Lake Nipigon were examined, and they were considered by all researchers as unconnected to discharge coeval with the onset of the Younger Dryas. Possible older outlets had been proposed west, rather than north, of Thunder Bay, but neither maps nor aerial survey revealed any large continuous channels, dry waterfalls, or spillways cut into bedrock, as is the case west of Lake Nipigon or in the southern outlet of Lake Agassiz. This was surprising, given the proposed catastrophic nature of the flood necessaryto trigger a climate change. Alternatively, the resistant bedrock of the Canadian Shield prevented formation of a well-developed spillway, resulting in non-catastrophic flow instead. In contrast to the uncertain flood routing west of Thunder Bay, the Clearwater spillway served as a route for meltwaterflows. The geomorphic evidence is stunning: a wide, linear channel with numerous feederchannels at its eastern end, and a large delta at its downstream end. Catastrophic flood deposits contain wood giving a maximum age of 9860 ± 230 14C years B. P. [Fisher et al., 2002]. Strandlines (water-plane indicators such as beaches, spits, or escarpments) near the head of this system are discontinuous and are covered by boreal forest, with the only known Agassiz strandline projecting to the base of the spillway. Evidence that Lake Agassiz existed at the head of the spillway is based on the distribution of scattered high-elevation strandlines, lacustrine sediment, and radiocarbon age-datedflood gravels. Given the importance of unraveling Lake Agassiz’s drainage history as a trigger for abrupt climate change, and given that the nature of the field evidence for Younger Dryas aged drainage at either the eastern or northwestern outlets is ambiguous, it was decided to construct and apply a chronological test. Did the ice sheet margin, either at Thunder Bay or Fort McMurray, withdraw enough to allow passage of meltwater at the start of theYounger Dryas? An affirmative response allows, but does not prove, an Agassiz meltwater trigger; a negative response rules out that trigger, forcing other explanations for the cause of the Younger Dryas. Preliminary results indicate that ice recession at both outlet areas is later than supposed, and that large volumes of meltwater were not catastrophically released from LakeAgassiz at the beginning of the Younger Dryas. Thus, if the Lake Agassiz floods did not upset the circulation pattern, the question becomes: What did? Could other pathways of the hydrological cycle alter the thermohaline circulationpattern at the beginning of the Younger Dryas, or alter other climate fluctuations that preceded Lake Agassiz? These investigations indicate that the geologicalunderstanding of past abrupt climate changes is only preliminary.This does not bode well for predicting future, abrupt climate changes.

Bibliography of Canadian Geomorphology