Arthur S. Dyke

The last glacial maximum

The Melting Is in the Details Global sea level rises and falls as ice sheets and glaciers melt and grow, providing an integrated picture of the changes in ice volume but little information about how much individual ice fields are contributing to those variations. Knowing the regional structure of ice variability during glaciations and deglaciations will clarify the mechanisms of the glacial cycle. Clark et al. (p. 710) compiled and analyzed more than 5000 radiocarbon and cosmogenic surface exposure ages in order to develop a record of maximum regional ice extent around the time of the Last Glacial Maximum. The responses of the Northern and Southern Hemispheres differed significantly, which reveals how the evolution of specific ice sheets affected sea level and provides insight into how insolation controlled the deglaciation. Regional patterns are presented of the timing of ice-sheet and mountain-glacier maxima near the end of the last ice age. We used 5704 14C, 10Be, and 3He ages that span the interval from 10,000 to 50,000 years ago (10 to 50 ka) to constrain the timing of the Last Glacial Maximum (LGM) in terms of global ice-sheet and mountain-glacier extent. Growth of the ice sheets to their maximum positions occurred between 33.0 and 26.5 ka in response to climate forcing from decreases in northern summer insolation, tropical Pacific sea surface temperatures, and atmospheric CO2. Nearly all ice sheets were at their LGM positions from 26.5 ka to 19 to 20 ka, corresponding to minima in these forcings. The onset of Northern Hemisphere deglaciation 19 to 20 ka was induced by an increase in northern summer insolation, providing the source for an abrupt rise in sea level. The onset of deglaciation of the West Antarctic Ice Sheet occurred between 14 and 15 ka, consistent with evidence that this was the primary source for an abrupt rise in sea level ~14.5 ka.

Forcing of the cold event of 8,200 years ago by catastrophic drainage of Laurentide lakes

The sensitivity of oceanic thermohaline circulation to freshwater perturbations is a critical issue for understanding abrupt climate change. Abrupt climate fluctuations that occurred during both Holocene and Late Pleistocene times have been linked to changes in ocean circulation, but their causes remain uncertain. One of the largest such events in the Holocene occurred between 8,400 and 8,000 calendar years ago,, (7,650–7,200 14C years ago), when the temperature dropped by 4–8 °C in central Greenland and 1.5–3 °C at marine, and terrestrial, sites around the northeastern North Atlantic Ocean. The pattern of cooling implies that heat transfer from the ocean to the atmosphere was reduced in the North Atlantic. Here we argue that this cooling event was forced by a massive outflow of fresh water from the Hudson Strait. This conclusion is based on our estimates of the marine 14C reservoir for Hudson Bay which, in combination with other regional data, indicate that the glacial lakes Agassiz and Ojibway, (originally dammed by a remnant of the Laurentide ice sheet) drained catastrophically ∼8,470 calendar years ago; this would have released >1014 m3 of fresh water into the Labrador Sea. This finding supports the hypothesis,, that a sudden increase in freshwater flux from the waning Laurentide ice sheet reduced sea surface salinity and altered ocean circulation, thereby initiating the most abrupt and widespread cold event to have occurred in the past 10,000 years.

The Laurentide and Innuitian ice sheets during the Last Glacial Maximum.

The Late Wisconsinan advance of the Laurentide Ice Sheet started from a Middle Wisconsinan interstadial minimum 27–30 14 C ka BP when the ice margin approximately followed the boundary of the Canadian Shield. Ice extent in the Cordillera and in the High Arctic at that time was probably similar to present. Ice advanced to its Late Wisconsinan (stage 2) limit in the northwest, south, and northeast about 23–24 14 C ka BP and in the southwest and far north about 20–21 14 C ka BP. In comparison to some previous reconstructions of ice extent, our current reconstruction has substantially more Late Wisconsinan ice in the High Arctic, where an Innuitian Ice Sheet is generally acknowledged to have existed, in the Atlantic Provinces, where ice is now thought to have extended to the Continental Shelf edge in most places, and on eastern Baffin Island, where ice probably extended to the fiord mouths rather than to the fiord heads. Around most of the ice margin, the Late Wisconsinan maximum ice extent either exceeded the extent of earlier Wisconsinan advances or it was similar to the Early Wisconsinan advance. Ice marginal recession prior to 14 14 Ck a BP occurred mainly in deep water and along the southern terrestrial fringe. However, Heinrich event 1 probably drew down the entire central ice surface at 14.5 14 C ka BP sufficiently to displace the Labrador Sector outflow centre 900 km eastward from the coast of Hudson Bay. The onset of substantial ice marginal recession occurred about 14 14 C ka BP in the northwest, southwest, and south but not until about 10–11 14 C ka BP in the northeast and in the High Arctic. Thus, the period of maximum ice extent in North America generally encompasses the interval from B24/21 to 14 14 C ka BP, or considerably longer than the duration of the LGM defined as occurring during a period of low global sea level as well as during a time of relative climate stability B18 14 C ka BP. The interval of advance of much of the Laurentide Ice Sheet to its maximum extent (between B27 14 C ka BP and B24 14 C ka BP) coincides with a suggested interval of rapid fall in global sea level to near LGMlevels. r 2001 Elsevier Science Ltd. All rights reserved.

An outline of North American deglaciation with emphasis on central and northern Canada

Publisher Summary This chapter reviews that the Late Wisconsinan North American ice sheet complex consisted of three major ice sheets: (1) the Laurentide Ice Sheet, which was centred on the Canadian Shield, but also expanded across the Interior Plains to the west and south, (2) the Cordilleran Ice Sheet, which inundated the western mountain belt between the northernmost co-terminus United States and Beringia, and (3) the Innuitian Ice Sheet, which covered most of the Canadian Arctic Archipelago north of about 7°N latitude. The ice cover over Newfoundland and the Maritime Provinces of Canada is usually referred to as the Appalachian Ice Complex, because ice flowed out from local centres rather than from the Canadian Shield. All of the peripheral ice sheets were confluent at the Last Glacial Maximum (LGM) with the Laurentide Ice Sheet, and the Greenland Ice Sheet was confluent with the Innuitian Ice Sheet. The nucleus of this complex, the Laurentide, comprised three major sectors, the Labrador Sector, the Keewatin Sector, and the Baffin Sector, named for areas of inception mid probable areas of outflow at LGM and located respectively east, west and north of Hudson Bay. The chapter presents revised maps of North American deglaciation at 500-year and finer resolution. These maps represent an updating of a series prepared nearly two decades ago for the INQUA 1987 Congress.

Forms, response times and variability of relative sea-level curves, glaciated North America

Abstract Relative sea level curves from glaciated North America reveal coherent spatial patterns of response times. In the Laurentide Ice Sheet area, curve half-lives range from 1.2–1.4 ka at the uplift centre to 1.7–2 ka in a ridge of high values inboard of the glacial limit. Half-lives decline from this ridge to less than 1 ka along the margin. In the Innuitian Ice Sheet area, half-lives are about 2 ka at the uplift centre and decline to less than 1 ka at the margin. The central Laurentide response times are about half those of central Fennoscandia. This accords with the theoretical expectation that central response times are inversely proportional to ice sheet radius for ice loads large enough that rebound at the centre is insensitive to lithospheric thickness. The Innuitian central response time indicates that rebound at the centre of this ice sheet, which is much smaller than the Fennoscandian Ice Sheet, remains sensitive to lithospheric thickness. Radial gradients in response times reflect the increasing influence of the lithosphere at sites increasingly closer to the margin. Along this gradient, rebound progresses as though at the centres of smaller and smaller ice sheets. That is, the effective spatial scale of the ice load decreases toward the margin. Near the glacial limit, postglacial isostatic adjustment is complicated by forebulge migration and collapse. This is seen most strongly in the relative sea level record of Atlantic Canada, which has subsided during the Holocene more than 20 m more than the adjacent American seaboard. The relative sea level history of some areas, notably the St. Lawrence Estuary, is complicated by tectonic processes.

Proposed Extent of Late Wisconsin Laurentide Ice on Eastern Baffin Island

The apparent outer limit of late Wisconsin Laurentide ice on eastern Baffin Island (roughly correlative with the “classical” Wisconsin of the southern Laurentide margin) is delimited in broad terms on the distal side by the presence of undisturbed glaciomarine deposits for which associated molluscan fauna have 14 C ages beyond the useful radiocarbon age range. This is supported by the distribution of drowned cirques, submerged late Wisconsin glaciomarine deltas, moraines from local ice sources of comparable age, and the absence of main fiord ice moraines. Within this maximum outer limit of late Wisconsin glaciation, only the previously mapped Cockburn Moraine System is associated with marine deposits of finite 14 C age. This system forms a largely continuous and prominent end and lateral moraine overlying till showing extensive tundra polygon development, and it is associated with ice-contact raised marine features dating between 7,500 and 8,500 14 C yr B.P. A map depicting the late Wisconsin ice margin based on these criteria shows that most of the eastern coastal margin of Baffin Island remained ice free throughout the last glacial stade (approximately 8,000 to 25,000? yr B.P.). This interpretation is supported by the occurrence of deposits dated more than 25,000 yr B.P., 30 to 60 km inland from the outer coast, which have not been glacially overridden; the pattern of postglacial isostatic uplift since 8,000 yr B.P.; and the complete absence of features dating between 10,500 and 25,000 yr B.P., despite more than 160 finite dates. Of 66 marine limit dates from the region, 44 percent lie between 7,500 and 8,500 yr B.P., whereas less than 8 percent are between 9,000 and 10,500 yr old. A consideration of the pattern of atmospheric circulation at the last glacial maximum suggests that few cyclonic disturbances penetrated the North American Arctic and that consequent decreased precipitation allowed only minimal glacial expansion.

Meltwater along the Arctic margin of the Laurentide Ice Sheet (8-12 ka) : Stable isotopic evidence and implications for past salinity anomalies

We report a new time series of stable isotopic values for the past 12 ka measured in two species of shallow-water marine bivalves collected from raised marine deposits along the coast of Arctic Canada. Each sample ( n = 60) had been radiocarbon dated previously. The results indicate that between 11.8 and 10 ka the δ 18 O of the shells ranged between 0.25???? and -6????, but values increased after 9 ka as the flux of meltwater to the Arctic Ocean decreased and as the Canadian Arctic channels opened to allow the present surface circulation to become established. Prior to about 9 ka, glacial meltwaters from the northern margin of the Laurentide Ice Sheet must have been entrained in the surface waters of the Arctic Ocean and directed southward to the North Atlantic through Fram Strait. This source of meltwater must be included in late Quaternary models of forcing of the North Atlantic thermohaline circulation. Glacial isostatic uplift of the sills within the Canadian Arctic channels implies that outflow through Fram Strait increased during the Holocene.

Variability in Palaeoeskimo occupation on south-western Victoria Island, Arctic Canada: Causes and consequences

Palaeoeskimo occupation history on western Victoria Island in the Canadian Arctic is inferred on the basis of the abundance of dwelling features according to elevation above sea level. The correlation between elevation above sea level and dwelling age is corroborated with seventy radiocarbon dates. The results suggest that the first occupants arrived in the region approximately 4500 radiocarbon years BP and attained maximum population levels by 4000-3800 BP, which was followed by a sudden decline. Moderate population levels were maintained for the next 600 years, following which, at approximately 3200 BP, there was a further decline. While there were occasional minor population increases following this period, none attained anywhere near the early (4500-3800 BP) levels. While the early and rapid decline of human population from its rapidly established initial maximum level may be attributed to one or more causes, the available evidence suggests that the overhunting of a key resource, musk-ox, cannot be ruled out.

High Arctic Holocene temperature record from the Agassiz ice cap and Greenland ice sheet evolution

Significance Reconstructions of past environmental changes are important for placing recent climate change in context and testing climate models. Periods of past climates warmer than today provide insight on how components of the climate system might respond in the future. Here, we report on an Arctic climate record from the Agassiz ice cap. Our results show that early Holocene air temperatures exceed present values by a few degrees Celsius, and that industrial era rates of temperature change are unprecedented over the Holocene period (∼12,000 y). We also demonstrate that the enhanced warming leads to a large response of the Greenland ice sheet; providing information on the ice sheets sensitivity to elevated temperatures and thus helping to better estimate its future evolution. We present a revised and extended high Arctic air temperature reconstruction from a single proxy that spans the past ∼12,000 y (up to 2009 CE). Our reconstruction from the Agassiz ice cap (Ellesmere Island, Canada) indicates an earlier and warmer Holocene thermal maximum with early Holocene temperatures that are 4–5 °C warmer compared with a previous reconstruction, and regularly exceed contemporary values for a period of ∼3,000 y. Our results show that air temperatures in this region are now at their warmest in the past 6,800–7,800 y, and that the recent rate of temperature change is unprecedented over the entire Holocene. The warmer early Holocene inferred from the Agassiz ice core leads to an estimated ∼1 km of ice thinning in northwest Greenland during the early Holocene using the Camp Century ice core. Ice modeling results show that this large thinning is consistent with our air temperature reconstruction. The modeling results also demonstrate the broader significance of the enhanced warming, with a retreat of the northern ice margin behind its present position in the mid Holocene and a ∼25% increase in total Greenland ice sheet mass loss (∼1.4 m sea-level equivalent) during the last deglaciation, both of which have implications for interpreting geodetic measurements of land uplift and gravity changes in northern Greenland.

A Glacial Isostatic Adjustment Model for the Central and Northern Laurentide Ice Sheet based on Relative Sea-level and GPS Measurements

The thickness and equivalent global sea level contribution of an improved model of the central and northern Laurentide Ice Sheet is constrained by 24 relative sea level histories and 18 present-day GPS-measured vertical land motion rates. The final model, termed Laur16, is derived from the ICE-5G model by holding the timing history constant and iteratively adjusting the thickness history, in four regions of northern Canada. In the final model, the last glacial maximum (LGM) thickness of the Laurentide Ice Sheet west of Hudson Bay was ~3.4-3.6 km. Conversely, east of Hudson Bay, peak ice thicknesses reached ~4 km. The ice model thicknesses inferred for these two regions represent, respectively, a ~30 per cent decrease and an average ~20-25 per cent increase to the load thickness relative to the ICE-5G reconstruction, which is generally consistent with other recent studies that have focussed on Laurentide Ice Sheet history. The final model also features peak ice thicknesses of 1.2-1.3 km in the Baffin Island region, a modest reduction relative to ICE-5G and unchanged thicknesses for a region in the central Canadian Arctic Archipelago west of Baffin Island. Vertical land motion predictions of the final model fit observed crustal uplift rates well, after an adjustment is made for the elastic crustal response to present-day ice mass changes of regional ice cover. The new Laur16 model provides more than a factor of two improvement of the fit to the RSL data (?2 measure of misfit) and a factor of nine improvement to the fit of the GPS data (mean squared error measure of fit), compared to the ICE-5G starting model. Laur16 also fits the regional RSL data better by a factor of two and gives a slightly better fit to GPS uplift rates than the recent ICE-6G model. The volume history of the Laur16 reconstruction corresponds to an up to 8 m reduction in global sea level equivalent compared to ICE-5G at LGM.