Lake Agassiz Research Articles

A calibrated deglacial drainage chronology for the North American continent: evidence of an Arctic trigger for the Younger Dryas

We present a new deglacial meltwater drainage chronology for the North American ice-sheet complex using a 3D glacial systems model calibrated against a large set of paleo-proxies. Results indicate that North America was responsible for a significant fraction of mwp1-a, with order 1.5 dSv or larger (100 year mean) peak discharges into both the Gulf of Mexico and the Eastern Atlantic and less than 1 dSv into the Arctic Ocean. Our most significant result concerns discharge into the Arctic Ocean. The largest total discharge into the Arctic Ocean (ensemble mean values of 1.0–2.2 dSv) occurs during the onset of the Younger Dryas. The large majority of this discharge is locally sourced with reduction of the Keewatin ice dome being the largest contributor. Given that the only outlet from the Arctic Basin at this time was via Fram Strait into the Greenland–Iceland–Norwegian Seas, we hypothesize that this pulse was the trigger for the re-organization of thermohaline circulation that is thought to have been responsible for the Younger Dryas cold interval. In contradistinction with past inferences and subject to the imperfectly constrained ice-margin chronology, we also find that the Northwest outlet likely dominated much of the post - 13 ka drainage of Lake Agassiz.

An 8500 cal. year multi-proxy climate record from a bog in eastern Newfoundland: contributions of meltwater discharge and solar forcing

Analyses of plant macrofossils, testate amoebae and the degree of peat humification have been combined into a single composite reconstruction of bog surface wetness (BSW) on a coastal plateau bog in eastern Newfoundland. The reconstruction reveals 14 distinctive phases of near-surface water tables commencing at 8270, 7500, 6800, 5700, 5200, 4900, 4400, 4000, 3100, 2500, 2050, 1700, 600 and 200 cal. BP, which may be used to infer changes in the atmospheric water balance of eastern Newfoundland. The first two major phases of pool development follow the final drainage of glacial Lake Agassiz at 8400 cal. BP and the Ungava lakes between ca 7500–6900 cal. BP, respectively. From 7500 cal. BP to the present there appears to be a strong correlation, within dating errors, between reconstructed BSW and the stacked ice rafted debris (IRD) record in the North Atlantic Ocean. Both records may reflect long-term changes in air masses. Comparisons of the BSW reconstruction with records of cosmogenic isotope flux also suggest a persistent link between reduced solar irradiance and increased BSW during the Holocene.

Consistent simulations of multiple proxy responses to an abrupt climate change event

Isotope, aerosol, and methane records document an abrupt cooling event across the Northern Hemisphere at 8.2 kiloyears before present (kyr), while separate geologic lines of evidence document the catastrophic drainage of the glacial Lakes Agassiz and Ojibway into the Hudson Bay at approximately the same time. This melt water pulse may have been the catalyst for a decrease in North Atlantic Deep Water formation and subsequent cooling around the Northern Hemisphere. However, lack of direct evidence for ocean cooling has lead to speculation that this abrupt event was purely local to Greenland and called into question this proposed mechanism. We simulate the response to this melt water pulse using a coupled general circulation model that explicitly tracks water isotopes and with atmosphere-only experiments that calculate changes in atmospheric aerosol deposition (specifically (10)Be and dust) and wetland methane emissions. The simulations produce a short period of significantly diminished North Atlantic Deep Water and are able to quantitatively match paleoclimate observations, including the lack of isotopic signal in the North Atlantic. This direct comparison with multiple proxy records provides compelling evidence that changes in ocean circulation played a major role in this abrupt climate change event.

A highlight of environmental and engineering geology in Fargo, North Dakota, USA

Several unfavorable environmental and engineering geologic conditions exist in Fargo, North Dakota. Dominantly, the behavior of smectitic clays within the proglacial Lake Agassiz sediments of the Sherack and Brenna Formations creates subsoil instability beneath engineered structures in the Fargo area and slope instability within cutbank meanders of the Red River of the North. Unfavorable engineering geologic conditions encountered include: the elastic deformation of clayey glaciolacustrine soils, shrink-swell properties, inadequate bearing capacities, and mass movements. These conditions are responsible for structural failures including the Fargo Grain Elevator in 1955 and the Northern Pacific railroad grade. Bank failures along the Red River are common due to the inherent instability of Brenna Formation smectitic clays which are subject to plastic deformation in the subsurface, with resultant block failure of overlying Sherack Formation. Recent alluvial sediments due to typical fluvial action and the continued seasonal saturation of cutbank meanders within the floodplain also add to soil instability.

Grass Phytolith Analysis

The vegetative history of eastern North Da kota is broadly known from pollen and multi-proxy investigations of lake basins from across the re gion (Clark et al. 2001; Dean and Schwalb 2000; Wright et al. 2004). These records suggest that parkland or savanna rapidly developed following glacial retreat and subsequent drainage of glacial Lake Agassiz to the south (10,800 to 9400 14C year B.P.) with true grasslands (or prairie) emerging by 9000 14C year B.P. Changes in grassland com position of the region is poorly documented in the fossil record throughout the Holocene. While stable carbon isotopes offer some insight (Clark et al. 2001), this proxy is limited in that it cannot dis tinguish between changes in forest cover from C3 grasslands. Fossil phytolith assemblages are used to infer changes in Holocene grassland vegetation and cli mate at the Rustad site. The evidence presented comes from two stratigraphic columns allowing evaluation of local variability in phytolith assem blages and validation of results. The soils and sedi ments sampled in this study span much of the last 9,000 years. Several major environmental changes are hypothesized to have occurred during this time and should be detectable with opal phytolith analy sis. First, the early Holocene (12,000 to 8000 years B.P.) was a period of major climatic warming. Grassland response to this warming should show up in the phytolith record as a shift from a C3 grass dominated cover to a mixed grass prairie including both tall (mesic adapted) and short (xe ric adapted) C4 grasses along with diverse C3 grass taxa. Second, sometime during the mid-Holocene (8000 to 5000 years B.P.) many, but not all, re gional paleoenvironmental records suggest one or more periods of higher temperatures and greater aridity, broadly labeled the Altithermal. This mid Holocene aridity should be detectable in the phytolith record as a relative increase in phytolith morphotypes from the xeric-adapted, C4 short grasses such as blue grama, side oats grama, and buffalo grass. The goal of this chapter is to report the find ings of phytolith analysis and offer some prelimi nary interpretations. The analysis focuses on the grass phytolith portion of the assemblages, al though there is great potential for additional in formation from the arboreal phytolith component and from other proxy data, including carbon iso topes, terrestrial snails, and vertebrate fauna. METHODS

The dynamic balance between organic acids and circumneutral groundwater in a large boreal peat basin

Large raised bogs and patterned fens cover 56% of the landscape in the Glacial Lake Agassiz region of northern Minnesota (USA). Organic acids supply most of the acidity in the surface water of the bogs, but groundwater upwelling from the underlying glacial deposits neutralizes these organic acids within deep peat. Substantial concentrations of organic acids also occur in the surface waters of fens mixed with variable amounts of inorganic solutes contributed by groundwater discharge. We used a triprotic analog model to determine the extent to which organic acids in fen and bog waters behave as strong or weak acids. The modeling approach optimized charge balance by calibrating estimates of mole site density in the DOC (dissolved organic carbon) of surface and pore waters with estimates of triprotic acid dissociation constants. Before the calibration process, all of the bog waters and 76% of the fen waters had more than +20% imbalance in charge balance. After calibration, more than 75% of all waters were electrochemically balanced within 20%. In the best calibration, the mole site denisty of bog DOC was estimated as ∼0.05 mmol/mmol C., approximately six times smaller than that estimated for fen DOC or the DOC in the fen deeper fen peats that underlie all bog landforms. The three modeled de-protonation constants were; pK a1=∼3.0, pK a2=∼4.5 and pK a3=∼7.0 for the bog DOC, and; pK a1=∼5.2, pK a2=∼ 6.5 and pK a3=∼7.0 for the fen DOC. Bog DOC, behaves as a strong acid despite its small mole site density. The DOC in bog runoff can therefore theoretically acidify the surface waters in adjacent fens wherever these waters do not receive sufficient buffering alkalinity from active groundwater seepage.

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 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). 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.

Geophysicists

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). 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.

Overbank deposition along the concave side of the Red River meanders, Manitoba, and its geomorphic significance

AbstractSlow earth sliding is pervasive along the concave side of Red River meanders that impinge on Lake Agassiz glaciolacustrine deposits. These failures form elongated, low‐angled (c. 6 to 10°) landslide zones along the valleysides. Silty overbank deposits that accumulated during the 1999 spring freshet extend continuously along the landslide zones over hundreds of metres and aggraded the lower slopes over a distance 50 to 80 m from the channel margin. The aggradation is not obviously related to meander curvature or location within a meander. Along seven slope profiles surveyed in 1999 near Letellier, Manitoba, the deposits locally are up to 21 cm thick and generally thin with increasing distance from, and height above, the river. Local deposit thickness relates to distance from the channel, duration of inundation of the landslide surface, mesotopography, and variations in vegetation cover. Immediately adjacent to the river, accumulated overbank deposits are up to 4 m thick. The 1999 overbank deposits also were present along the moderately sloped (c. 23 to 27°) concave banks eroding into the floodplain, but the deposits are thinner (locally up to c. 7 cm thick) and cover a narrower area (10 to 30 m wide) than the deposits within the landslide zones. Concave overbank deposition is part of a sediment reworking process that consists of overbank aggradation on the landslide zones, subsequent gradual downslope displacement from earth sliding, and eventually reworking by the river at the toe of the landslide. The presence of the deposits dampens the outward migration of the meanders and contributes to a low rate of contemporary lateral channel migration. Concave overbank sedimentation occurs along most Red River meanders between at least Emerson and St. Adolphe, Manitoba. © Her Majesty the Queen in right of Canada.

Flow path oscillations in transient ground-water simulations of large peatland systems

Transient numerical simulations of the Glacial Lake Agassiz Peatland near the Red Lakes in Northern Minnesota were constructed to evaluate observed reversals in vertical ground-water flow. Seasonal weather changes were introduced to a ground-water flow model by varying evapotranspiration and recharge over time. Vertical hydraulic reversals, driven by changes in recharge and evapotranspiration were produced in the simulated peat layer. These simulations indicate that the high specific storage associated with the peat is an important control on hydraulic reversals. Seasonally driven vertical flow is on the order of centimeters in the deep peat, suggesting that seasonal vertical advective fluxes are not significant and that ground-water flow into the deep peat likely occurs on decadal or longer time scales. Particles tracked within the ground-water flow model oscillate over time, suggesting that seasonal flow reversals will enhance vertical mixing in the peat column. The amplitude of flow path oscillations increased with increasing peat storativity, with amplitudes of about 5 cm occurring when peat specific storativity was set to about 0.05 m −1.

Modeling the history of Lake of the Woods since 11,000 cal yr B.P. using GIS

Because of differential isostatic rebound, many lakes in Canada have continued to change their extent and depth since retreat of the Laurentide Ice Sheet. Using GIS techniques, the changing configuration and bathymetry of Lake of the Woods in Ontario, Manitoba, and Minnesota were reconstructed for 12 points in time, beginning at 11,000 cal yr B.P. (∼9.6 14C ka B.P.), and were also projected 500 years into the future, based on the assumption that Lake of the Woods continued to have a positive hydrological budget throughout the Holocene. This modeling was done by first compiling a bathymetric database and merging that with subaerial data from the Shuttle Radar Topography Mission (SRTM). This DEM file was then adjusted by: (1) isobase data derived from Lake Agassiz beaches prior to 9000 cal yr B.P. (∼8.1 14C ka B.P.) and (2) modeled isostatic rebound trend analysis after 9000 cal yr B.P. Just after the end of the Lake Agassiz phase of Lake of the Woods, only the northernmost part of the basin contained water. Differential rebound has resulted in increasing water depth. In the first 3000 years of independence from Lake Agassiz, the lake transgressed >50 km to the south, expanding its area from 858 to 2857 km2, and more than doubling in volume. Continued differential rebound after 6000 cal yr B.P. (∼5.2 14C ka B.P.) has further expanded the lake, although today it is deepening by only a few cm per century at the southern end. In addition, climate change in the Holocene probably played a role in lake level fluctuations. Based on our calculation of a modern hydrological budget for Lake of the Woods, reducing runoff and precipitation by 65% and increasing evaporation from the lake by 40% would end overflow and cause the level of the lake to fall below the outlets at Kenora. Because this climate change is comparable to that recorded during the mid-Holocene warming across the region, it is likely that the area covered by the lake at this time would have been less than that determined from differential isostatic rebound alone.

Alternative routing of Lake Agassiz overflow during the Younger Dryas: new dates, paleotopography, and a re-evaluation

Overflow from glacial Lake Agassiz has been implicated in affecting late-glacial ocean circulation and climate. The timing of the diversion of Lake Agassiz overflow away from its southern routing (to the Gulf of Mexico) correlates closely with the Younger Dryas ca 11–10 14C ka. New models of paleotopography in the eastern outlet region of Lake Agassiz, adjusted for differential isostatic rebound, show that overflow to the Superior basin would have occurred when the lake formed its Herman, Norcross, and Tintah beaches, just after ∼10.9 14C ka, if the Laurentide Ice Sheet (LIS) had retreated from the Thunder Bay, Ontario, region by that time. However, the absence of coarse flood deposits normally associated with large overflows, plus the morphology of possible overflow channels in the Thunder Bay region, suggest that Agassiz overflow may not have been routed into the Great Lakes during the Younger Dryas, as previously believed. In addition, non-carbonate radiocarbon dates in the Thunder Bay area are all younger than 10 14C ka, suggesting that this area may not have been deglaciated until after the Younger Dryas, thus preventing Lake Agassiz overflow through this area. An alternative route of overflow from Lake Agassiz during the Younger Dryas could have been through the northwestern (Clearwater) outlet to the Arctic Ocean, although all research to date shows that this region was covered by ice until ∼10 14C ka. On the other hand, it is possible that older and coarser overflow sediment may now lie buried beneath finer deposits associated with the 10 14C ka Marquette readvance of the LIS, which ponded waters in valleys between Lake Agassiz and Lake Superior, or that eastward overflow in this area was not catastrophic.

A high-resolution, absolute-dated Holocene and deglacial Asian monsoon record from Dongge Cave, China

We present a continuous record of the Asian monsoon over the last 16 ka from δ 18O measurements of stalagmite calcite. Over 900 oxygen isotopic measurements providing information on shifts in monsoon precipitation are combined with a chronology from 45 precise 230Th dates. δ 18O and therefore Asian monsoon intensity generally follows changes in insolation, although changes in δ 18O are generally accommodated in abrupt shifts in contrast to smoothly varying insolation, indicating that threshold effects may be important. δ 18O decreased dramatically (∼3‰) at the start of the Holocene (∼11.5 ka) and remained low for ∼6 ka. Four positive δ 18O events centered at 11225±97 yr BP (1.05‰), 10880±117 yr BP (1.15‰), 9165±75 yr BP (1.4‰), and a double event centered at 8260±64 yr BP (1.1‰) and 8080±74 yr BP (1.0‰) punctuated this period of high monsoon intensity. All four events correlate within error with climate changes in Greenland ice cores. Thus, the relationship between the Asian monsoon and the North Atlantic observed during the glacial period appears to continue into the early Holocene. In addition, three of the four events correlate within error with outburst events from Lake Agassiz. The decline of monsoon intensity in the mid-late Holocene is characterized by an abrupt positive shift in δ 18O which occurs at 3550±59 yr BP (1.1‰ in ∼100 yr). In addition, the Holocene is punctuated by numerous centennial- and multi-decadal-scale events (amplitudes 0.5 to 1‰) up to half the amplitude of the glacial interstadial events seen in the last glacial period. Thus, Holocene centennial- and multi-decadal-scale monsoon variability is significant, although not as large as glacial millennial-scale variability. The monsoon shows a strong connection with northern South American hydrological changes related by changes in ITCZ position. Spectral analysis of the δ 18O record shows significant peaks at solar periodicities of 208 yr and 86 yr suggesting variation is influenced by solar forcing. However, there are numerous other significant peaks including peaks at El Niño frequencies (observed for high-resolution portions of the record between 8110 and 8250 yr) which suggest that changes in oceanic and atmospheric circulation patterns in addition to those forced by solar changes are important in controlling Holocene monsoon climate. In addition, for this high-resolution portion, we observe a distinctive biennial oscillation of the Asian monsoon, which has been associated with the Tropospheric Biennial Oscillation (TBO).

Late Pleistocene Palaeoenvironments of the Southern Lake Agassiz Basin, USA

AbstractMacroscopic plant remains, pollen, insect and mollusc fossils recovered from a cut bank on the Red River in North Dakota, USA, provide evidence that an extensive wetland occupied the southern basin of Lake Agassiz from 10 230 to 9900 14C yr BP. Marsh‐dwelling plants and invertebrates had colonised the surface of a prograding delta during the low‐water Moorhead Phase of Lake Agassiz. A species of Salix (willow) was abundant along distributary channels, and stands of Populus tremuloides (aspen), Ulmus sp. (elm), Betula sp. (birch), and Picea sp. (spruce) grew on the better‐drained sand bars and beach ridges. Most of the species of plants, insects, and molluscs represented as fossils are within their existing geographic ranges. Based on a few species with more northerly distributions, mean summer temperature may have been about 1–2°C lower than the present day. No change in species composition occurred in the transition from the Younger Dryas to Preboreal. At the time that the wetland existed, Lake Agassiz was draining either eastward to the North Atlantic Ocean or northwestward to the Arctic Ocean. The wetland was drowned during the Emerson Phase transgression that resulted in meltwater draining southward to the Gulf of Mexico after 9900 14C yr BP. Copyright © 2005 John Wiley & Sons, Ltd.

The Allerød–Younger Dryas–Holocene sequence in the west-central Champlain Sea, eastern Ontario: a record of glacial, oceanographic, and climatic changes

The aragonite mineralogy and geochemistry of the mollusc faunas preserved at Navan and Bearbrook, Ontario, serve as proxies of original seawater chemistry. The composite section spanning 12,980–10,980 cal yr BP includes the Younger Dryas (YD) paleoclimatic oscillation. Oxygen isotopes demonstrate the onset of cooling with the YD event, in addition to the lowering of marine values by the influx of isotopically light glacial meltwater from Lake Agassiz. Impact of cooling and dilution is reduced or eliminated with the start of the Holocene, when water temperatures and salinities for Champlain Sea (CS) seawater were 8–16 °C and 27–34 ppt, respectively. Overall, oxygen isotope values deceased to −3.5% during the YD mainly due to freshening by glacial meltwater. Carbon isotopes confirm the rise in atmospheric CO 2 concentration at the YD–Holocene transition. Marine strontium isotope values for the Allerød–YD–earliest Holocene range from 0.709151 (16,210 cal yr BP) to 0.709145 (12,980 cal yr BP) and 0.709142 (10,950 cal yr BP). The oceanographic changes recorded for the CS are in agreement with the evolutionary phases of Lake Agassiz and deglaciation dynamics of the Laurentide Ice Sheet. The volume and direction of meltwater discharge from Lake Agassiz alternated between the Gulf of Mexico during the Allerød, via the Great Lakes through the CS to the North Atlantic during the YD, and back to the Gulf of Mexico during the early Holocene, but with diminished impact.

Strandline analysis in the southern basin of glacial Lake Agassiz, Minnesota and North and South Dakota, USA

Glacial Lake Agassiz, a large Pleistocene lake, left behind strandlines at multiple elevations within its basin. Prior researchers have used these strandlines to identify four relatively stable water level stages, but tracing them over long distances is often problematic. In this study, the elevation of the Tintah strandline is returned to its original designation, and a newly interpreted stable lake level at 1020–1030 ft (311–314 m) is referred to as the Upham level, based on a concentration of ridges and a spit at that elevation. Cores from strandline and lagoon complexes from the Milnor to Tintah beaches reveal a decreasing thickness of strandline sediment associated with shorter aggregate-length strandlines. This relationship is most likely controlled by sediment starvation, greater water level fluctuations and shallower water. Beaches near the head of the southern outlet spillway indicate that shoreline occupation at the Lower Campbell level was either very short-lived or strandline development was hampered by shallow water. The modern-day sill elevation between the southern outlet spillway and the lake basin is found to be 3 km south of White Rock, Minnesota, at an elevation of 974 ft (297 m). All but the Campbell strandlines at the southern outlet are interpreted as the result of a regression driven by incision of the southern outlet spillway.

River Warren boulders, Minnesota, USA: catastrophic paleoflow indicators in the southern spillway of glacial Lake Agassiz

Boulders resting on meltwater‐sculpted and striated‐granite bedrock near the head of the southern outlet spillway of glacial Lake Agassiz are used to generate paleodischarge calculations. The rounded nature of many boulders suggests fluvial transport and a corestone origin. The distribution of boulders in clusters and linear trains records the interaction of clasts during transport and deposition. The geomorphology of the spillway with streamlined erosional remnant hills is characteristic of other large flood spillways. Using the Manning equation and a variety of empirical equations to determine paleovelocity, preferred discharges between 0.364 and 0.102 Sv are calculated. These discharges agree well with flood discharges using modeling methodologies, and most likely represent ephemeral and catastrophic flood events linked to either episodic incision at the outlet or the result of rapid inputs of meltwater to Lake Agassiz.

River Warren boulders, Minnesota, USA: catastrophic paleoflow indicators in the southern spillway of glacial Lake Agassiz

River Warren boulders, Minnesota, USA: catastrophic paleoflow indicators in the southern spillway of glacial Lake Agassiz

Paleolimnology of Lake Manitoba: the Lithostratigraphic Evidence

Lake Manitoba, the largest lake in the prairie region of North America, is one of the most intensively studied lacustrine basins in western Canada. New AMS14C dating, together with mineralogical, geochemical, and lithostratigraphic analyses of the 14-m-thick, offshore sediment sequence, document a complex Holocene history in which water levels and limnological conditions were controlled by the interplay of changing climate, variable river and groundwater inflow, and differential isostatic rebound. Varves, ice-rafted debris, and clast-rich laminated sediment record deposition in the lake when the basin was part of proglacial Lake Agassiz. As Agassiz retreated northward, Lake Manitoba became isolated by about 850014C yrs BP, and for the next 800 years was characterized by mainly shallow water to dry conditions. Deeper and more stable water conditions returned to the Lake Manitoba basin by 770014C yrs BP probably due to the damming effect of differential isostatic rebound and decreasing aridity. For the next 3000 years, relatively stable lake levels and water compositions were maintained, reflecting a delicate balance between differential isostatic rebound, climate, inflow of the Assiniboine River, and groundwater contribution. At ~450014C yrs BP the Assiniboine River was rerouted to its present easterly path, by-passing Lake Manitoba, and resulting in loss of a significant component of the lake’s hydrologic budget. Water levels dropped and the offshore sediments were once again subaerially exposed. By 370014C yrs BP, a cooler and wetter climate, together with continued southward transgression of water, compensated for the loss of fluvial input, resulting in reflooding of the basin. By about 2000 years ago the lake had evolved from a shallow, saline, and alkaline pool to its present depth and extent.

Simulation of the cold climate event 8200 years ago by meltwater outburst from Lake Agassiz

The cold climate anomaly about 8200 years ago is investigated with CLIMBER‐2, a coupled atmosphere‐ocean‐biosphere model of intermediate complexity. This climate model simulates a cooling of about 3.6 K over the North Atlantic induced by a meltwater pulse from Lake Agassiz routed through the Hudson strait. The meltwater pulse is assumed to have a volume of 1.6 × 1014 m3 and a period of discharge of 2 years on the basis of glaciological modeling of the decay of the Laurentide Ice Sheet (LIS). We present a possible mechanism which can explain the centennial duration of the 8.2 ka cold event. The mechanism is related to the existence of an additional equilibrium climate state with reduced North Atlantic Deep Water (NADW) formation and a southward shift of the NADW formation area. Hints at the additional climate state were obtained from the largely varying duration of the pulse‐induced cold episode in response to overlaid random freshwater fluctuations in Monte Carlo simulations. The model equilibrium state was attained by releasing a weak multicentury freshwater flux through the St. Lawrence pathway completed by the meltwater pulse. The existence of such a climate mode appears essential for reproducing climate anomalies in close agreement with paleoclimatic reconstructions of the 8.2 ka event. The results furthermore suggest that the temporal evolution of the cold event was partly a matter of chance.