James T. Teller

Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean

The melting Laurentide Ice Sheet discharged thousands of cubic kilometres of fresh water each year into surrounding oceans, at times suppressing the Atlantic meridional overturning circulation and triggering abrupt climate change. Understanding the physical mechanisms leading to events such as the Younger Dryas cold interval requires identification of the paths and timing of the freshwater discharges. Although Broecker et al. hypothesized in 1989 that an outburst from glacial Lake Agassiz triggered the Younger Dryas, specific evidence has so far proved elusive, leading Broecker to conclude in 2006 that “our inability to identify the path taken by the flood is disconcerting”. Here we identify the missing flood path—evident from gravels and a regional erosion surface—running through the Mackenzie River system in the Canadian Arctic Coastal Plain. Our modelling of the isostatically adjusted surface in the upstream Fort McMurray region, and a slight revision of the ice margin at this time, allows Lake Agassiz to spill into the Mackenzie drainage basin. From optically stimulated luminescence dating we have determined the approximate age of this Mackenzie 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 North Atlantic Ocean.

Glacial Lake Agassiz: A 5000 yr history of change and its relationship to the δ18O record of Greenland

Lake Agassiz was the largest lake in North America during the last period of deglaciation; the lake extended over a total of 1.5 × 10 6 km 2 before it drained at ca. 7.7 14 C ka (8.4 cal. [calendar] ka). New computer reconstructions—controlled by beaches, isostatic rebound data, the margin of the Laurentide Ice Sheet, outlet elevations, and a digital elevation model (DEM) of modern topographic data—show how variable the size and depth of this lake were during its 4000 14 C yr (5000 cal. yr) history. Abrupt reductions in lake level, ranging from 8 to 110 m, occurred on at least 18 occasions when new outlets were opened, reducing the extent of the lake and sending large outbursts of water to the oceans. Three of the largest outbursts correlate closely in time with the start of large δ 18 O excursions in the isotopic records of the Greenland ice cap, suggesting that those freshwaters may have had an impact on thermohaline circulation and, in turn, on climate.

Volume and routing of late-glacial runoff from the southern Laurentide Ice Sheet

Melting of the Laurentide Ice Sheet during the last deglaciation added large volumes of water to many rivers and lakes of North America and to the worlds oceans. The volume and routing of this meltwater not only helped shape the lands surface but also played a role in the evolution of late-glacial climate. A computerized model was prepared to quantify meltwater generation from seven drainage areas along the southern side of the Laurentide Ice Sheet at 500-yr time slices between 14,000 and 8000 yr B.P. Nearly all waters reaching the oceans flowed through the St. Lawrence, Hudson, or Mississippi River valleys. Discharge through the Mississippi River to the Gulf of Mexico during late-glacial time varied by more than a factor of 5, ranging between 17,400 m3 sec−1 (550 km3 yr−1) and 98,200 m3 sec−1 (3200 km3 yr−1). Discharge entering the North Atlantic Ocean through the St. Lawrence and Hudson valleys ranged between 20,300 m3 sec−1 (640 km3 yr−1) and 65,300 m3 sec−1 (2060 km3 yr−1), with very abrupt, twofold changes at about 11,000, 10,000, and 9500 yr B.P. as a result of the rerouting of water from the Lake Agassiz basin. As the areal extent and mass of the Laurentide Ice Sheet diminished, the total volume of meltwater plus runoff due to precipitation from its southern side declined from 3800 km3 yr−1 at about 14,000 yr B.P. to 2100 to 2600 km3 yr−1 between 11,500 and 8000 yr B.P. No meltwater entered the Gulf of Mexico after 9500 yr B.P. After the demise of the ice sheet over Hudson Bay about 8000 yr B.P., the modern continental drainage network was established and flows through the St. Lawrence declined to modern values of about 320 km3 yr−1.

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

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 14C 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 km3 in volume and 254,000 and 151,000 km2 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 Agassiz– Ojibway was about 163,000 km3 in volume and 841,000 km2 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 km3. The final release of Lake Agassiz waters into the Tyrrell Sea at about 7700 14C yr B.P. is estimated to have been about 163,000 km3 in volume. C

Testing the Lake Agassiz meltwater trigger for the Younger Dryas

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 lakes 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). All preliminary 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.

Quaternary evaporites and hydrological changes, Lake Tyrrell, North‐West Victoria

Abstract Lake Tyrrell, a saline playa in semi‐arid north‐western Victoria, records a long history in which a succession of lacustrine and aeolian environments can be related to past hydrologic variations. Cores through the saline evaporitic facies reveal a vertical pattern reflecting cyclic changes through time. Detrital clastics predominated during deep‐water lacustral phases; evaporites were deposited during drying phases. A model, depicting surface‐groundwater interaction during discrete stages of a typical cycle, relates changes in water depth, salinity and typical depositional facies. On the drying trend, the sequence evolves through carbonate to sulphate deposition. Progressive reduction in water level results in partial drying and production of the groundwater outcrop playa stage. Salt efflorescence and production of detrital pelletal clays provide parent materials for aeolian transport and dune building. Any additional fall in watertable permits downward leaching of salts, plant colonization of th...

Freshwater Outburst from Lake Superior as a Trigger for the Cold Event 9300 Years Ago

Down the Drain A pervasive cooling event affected much of the Northern Hemisphere approximately 9300 years ago. This event was accompanied by changes in ocean circulation in the North Atlantic, forced presumably by a large injection of fresh water produced by melting of the Laurentide Ice Sheet, but the source, magnitude, and routing of the meltwater remain unknown. Yu et al. (p. 1262, published online 29 April) present evidence that the trigger for this cooling episode was an outburst flood from Lake Superior. Reconstructing lake-level changes in the Superior basin suggests that a rapid fall of lake level of about 45 meters occurred 9300 years ago, possibly due to the sudden failure of a drift dam. Rapid drainage through the North Bay–Ottawa River–St. Lawrence River valleys into the North Atlantic should have been sufficient to disturb ocean circulation in line with the geologic record. The trigger for the dramatic North Atlantic cooling event 9300 years ago was an outburst flood from Lake Superior. Paleoclimate proxy records reveal a pervasive cooling event with a Northern Hemispheric extent ~9300 years ago. Coeval changes in the oceanic circulation of the North Atlantic imply freshwater forcing. However, the source, magnitude, and routing of meltwater have remained unknown. Located in central North America, Lake Superior is a key site for regulating the outflow of glacial meltwater to the oceans. Here, we show evidence for an ~45-meter rapid lake-level fall in this basin, centered on 9300 calibrated years before the present, due to the failure of a glacial drift dam on the southeast corner of the lake. We ascribe the widespread climate anomaly ~9300 years ago to this freshwater outburst delivered to the North Atlantic Ocean through the Lake Huron–North Bay–Ottawa River–St. Lawrence River valleys.

Paleohydrological indicators in playas and salt lakes, with examples from Canada, Australia, and Africa

Abstract The sedimentary records of playas and salt lakes provide some of the best evidence for hydrological change in a region. These records are comprised of siliciclastic sediments and chemically-precipitated minerals that typically vary temporally and spatially. In some cases, the interpretation is straightforward, with mineralogical sequences related to changing brine concentrations and hydrology, as well as to climate. In other cases, it is difficult to identify the controlling factors, which include climate, groundwater, geological setting, and basin morphology. In addition, diagenesis commonly affects the sedimentary record. Models of lake level fluctuations are given for basins in the Canadian Prairies and southeastern Australia. The Lake Manitoba model is based mainly on secondary changes associated with pedogenesis that alters the deeper-water sediment during low water stages. Smaller Canadian lakes in more arid regions display a larger variety of diagnostic paleohydrological indicators, such as specific carbonate and salt mineralogy, nature of bedding, grain size, and secondary alterations. Decreasing water levels in shallow lakes tend to show a progression from clastics to carbonates, to salts, and then back to clastics as groundwater levels fall below the lake floor. Similar parameters are used in Australian lakes. The hydrological model for the Lake Tyrrell playa in Victoria, whose present ionic composition is similar to seawater, shows clastics being deposited when average lake levels are high, followed by deposition of primary carbonates, gypsum, and halite; once seasonal drying on the lake floor begins, secondary gypsum grows within already-deposited sediment, and clay pelletization in the capillary fringe of surface sediment leads to basin deflation and the accumulation of adjacent eolian lunettes. Continued water level decline leads to pedogenesis. Subsequent lake level rises in all of these models may initiate sedimentation in any part of the hydrological cycle, and a complete cycle in the sedimentary record appears to be uncommon.

Modern sedimentation and hydrology in Lake Tyrrell, Victoria

Abstract Lake Tyrrell is a large ephemeral salt lake, the level of which is controlled by climate and groundwater. Up to a metre of water fills the basin during the wetter and cooler winter season, but evaporates during the summer, precipitating up to 10 cm of halite. Each year essentially the same pool of ions is redissolved by this annual freshening. The small percentage of gypsum precipated (< 2%) in the surface salt crust reflects the low calcium content of the brine which, in turn, is a function of the negligible net discharge of calcium from the groundwater system. The small influx of fine‐grained clastic sediment to the lake floor comes from surface runoff, wind, and reworking of older sediment from the shoreline. The Lake Tyrrell basin lies in a setting in which three different groundwater types, identified by distinct salinities, interact with surface waters. A refluxing cycle that goes from discharging groundwater at the basin margin, to surface evaporation on the lake floor, to recharge through...

Pre-Quaternary microfossils—A guide to errors in radiocarbon dating

The presence of pre-Quaternary microfossils and anomalously old radiocarbon dates frorh fine-grained organic material in Lake Manitoba sediment suggests that old noncarbonate carbon is contaminating these fine-grained deposits. The relative abundance of the pre-Quaternary microfossils in these muds is used as a guide to correcting the anomalous 14C dates.