Changes in the Bathymetry and Volume of Glacial Lake Agassiz Between 11,000 and 9300 ¹⁴C yr B.P.

Abstract

Computer reconstructions of the bathymetry of the lake were used to quantify variations in the size and form of Lake Agassiz during its final two phases (the Nipigon and Ojibway phases), between about 9200 and 7700 14 C yr B.P. (ca. 10,300-8400 cal yr B.P.). New bathymetric models for four Nipigon Phase stages (corresponding to the McCauleyville, Hillsboro, Burnside, and The Pas strandlines) indicate that Lake Agassiz ranged between about 19,200 and 4600 km 3 in volume and 254,000 and 151,000 km 2 in areal extent at those times. A bathymetric model of the last (Ponton) stage of the lake, corresponding to the period in which Lake Agassiz was combined with glacial Lake Ojbway to the east, shows that Lake AgassizOjibway was about 163,000 km 3 in volume and 841,000 km 2 in areal extent prior to the final release of lake waters into the Tyrrell Sea. During the Nipigon Phase, a number of catastrophic releases of water from Lake Agassiz occurred as more northerly (lower) outlets were made available by the retreating southern margin of the Laurentide Ice Sheet; we estimate that each of the four newly investigated Nipigon Phase releases involved water volumes of between 1600 and 2300 km 3 . The final release of Lake Agassiz waters into the Tyrrell Sea at about 7700 14 C yr B.P. is estimated to have been about 163,000 km 3 in volume. C 2002 University of Washington.

Impact of floods versus routing events on the thermohaline circulation

“…This result is not very surprising if one considers the total Freshwater fluxes used in our experiments, with initial 1-yr flood followed by increased flux associated with routing. Blue: fluxes used for Younger Dryas (YD) simulation [Licciardi et al, 1999;Leverington et al, 2000]. Red: fluxes used for Preboreal Oscillation (PBO) simulation [Licciardi et al, 1999;Fisher et al, 2002].…”

“…[7] Three different meltwater events are simulated using reconstructed estimates of freshwater fluxes (floods and routing events) for the Younger Dryas (YD), the Preboreal Oscillation (PBO) and the 8.2-ka event [Licciardi et al, 1999;Leverington et al, 2000Leverington et al, , 2002Fisher et al, 2002;Clarke et al, 2004]. In each case, we integrate the model for over 5000 years into equilibrium under corresponding boundary conditions (orbital parameters [Berger, 1978] and atmospheric CO 2 of 240 ppm, 260 ppm and 260 ppm for the YD, PBO and 8.2-ka event, respectively).…”

“…Accordingly, increases in the freshwater flux associated with the opening of new outlets for proglacial lakes such as Lake Agassiz are comprised of two components; a short-lived flood (<1 yr) and a routing event that is sustained as long as the lower outlet remains open (10 2 -10 3 yr). Using the opening of an eastern outlet for Lake Agassiz drainage associated with the Younger Dryas as an example, Lake Agassiz lake level lowered by 110 m, with a corresponding volume loss of 9500 km 3 [Leverington et al, 2000], while at the same time, runoff from the Lake Agassiz drainage basin, with an area of $1.4 Â 10 6 km 2 [Licciardi et al, 1999], was diverted from the Mississippi basin to the St. Lawrence basin by way of the newly opened eastern outlet. The increase in freshwater flux to the St. Lawrence River associated with a loss of 9500 km 3 from the lake will depend on the duration of the lake-level lowering, for example, 0.3 Sv if a duration of one year, 0.06 Sv if a duration of five years, etc.…”

“…In doing so, it is necessary to assume that the lake-level lowering occurred in months to a year. Otherwise, if the volume of most known lake-level lowerings [Leverington et al, 2000 is distributed over any longer interval, the attendant freshwater fluxes become similar to or less than the freshwater fluxes associated with the diversion of the drainage basin, as shown by the Younger Dryas example above.…”

On the reconstruction of palaeo-ice sheets: Recent advances and future challenges

“…Teller et al, 2000;Lepper et al, 2013); (3) data-calibrated numerical ice sheet modelling (Tarasov and Peltier, 2006); (4) the palaeo-topography of the basin inferred from isostatic rebound (e.g. Leverington et al, 2000;2002;Rayburn and Teller, 2007); and (5) the dating of meltwater events in the different outlet channels, and in lakes and oceans beyond the channel mouths (Section 2.6), including the use of deep sea oxygen isotope records and distinctive sediment discharge to identify meltwater pulses (Fisher, 2007;Hillaire-Marcel et al, 2008;Lowell et al, 2009;Lewis et al, 2012;Fisher and Lowell, 2012;Teller, 2013).…”