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The Laurentide Ice Sheet was a major driver of global sea-level change during the last deglaciation and may have impacted both atmospheric and oceanic circulation. An understanding of past changes in the ice sheet is important for constraining its interaction with other components of the climate system. Here, we present the geologic context and chronology for ice-sheet fluctuations in eastern Maine, adjacent to the North Atlantic Ocean, thought to be a key player in the termination of the last ice age. Retreat of the Laurentide Ice Sheet through coastal Maine first produced a series of lobate grounding-line moraines, followed by deposition of the prominent Pineo Ridge System, which crosscut the earlier moraine set and which is characterized by extensive ice-contact deltas, closely spaced parallel moraines, and association with eskers. Our new 10 Be surface exposure ages indicate that the Pineo Ridge System, which extends for more than 100 km in eastern Maine and Atlantic Canada, dates to ∼15.3 ka, ∼800 years older than recent estimates. Our data are in accord with inboard minimum-limiting radiocarbon ages of terrestrial materials, which indicate deglaciation as early as 15.3 ka, as well as of marine shells that are as old as 15.0 ka. Both the deglaciation that produced the lobate moraines and the short-lived readvance that led to the Pineo Ridge System occurred during Heinrich Stadial 1. Given that faunal and isotopic evidence indicates that the ocean remained cold during deglaciation of coastal Maine, we infer that ice recession was due to rising summer air temperatures that gave way briefly to cooling to allow minor readvance. Glacial deposits north of the Pineo Ridge System display evidence of ice stagnation and downwasting, suggesting rapid ice retreat following deposition of the delta-moraine complex, coincident with the onset of the Bolling.

The North American Ice Sheet Complex (NAISC; consisting of the Laurentide, Cordilleran and Innuitian ice sheets) was the largest ice mass to repeatedly grow and decay in the Northern Hemisphere during the Quaternary. Understanding its pattern of retreat following the Last Glacial Maximum is critical for studying many facets of the Late Quaternary, including ice sheet behaviour, the evolution of Holocene landscapes, sea level, atmospheric circulation, and the peopling of the Americas. Currently, the most up-to-date and authoritative margin chronology for the entire ice sheet complex is featured in two publications (Geological Survey of Canada Open File 1574 [Dyke et al., 2003]; 'Quaternary Glaciations - Extent and Chronology, Part II' [Dyke, 2004]). These often-cited datasets track ice margin recession in 36 time slices spanning 18 ka to 1 ka (all ages in uncalibrated radiocarbon years) using a combination of geomorphology, stratigraphy and radiocarbon dating. However, by virtue of being over 15 years old, the ice margin chronology requires updating to reflect new work and important revisions. This paper updates the aforementioned 36 ice margin maps to reflect new data from regional studies. We also update the original radiocarbon dataset from the 2003/2004 papers with 1541 new ages to reflect work up to and including 2018. A major revision is made to the 18 ka ice margin, where Banks and Eglinton islands (once considered to be glacial refugia) are now shown to be fully glaciated. Our updated 18 ka ice sheet increased in areal extent from 17.81 to 18.37 million km(2), which is an increase of 3.1% in spatial coverage of the NAISC at that time. Elsewhere, we also summarize, region-by-region, significant changes to the deglaciation sequence. This paper integrates new information provided by regional experts and radiocarbon data into the deglaciation sequence while maintaining consistency with the original ice margin positions of Dyke et al. (2003) and Dyke (2004) where new information is lacking; this is a pragmatic solution to satisfy the needs of a Quaternary research community that requires up-to-date knowledge of the pattern of ice margin recession of what was once the world's largest ice mass. The 36 updated isochrones are available in PDF and shapefile format, together with a spreadsheet of the expanded radiocarbon dataset (n = 5195 ages) and estimates of uncertainty for each interval. (C) 2020 Elsevier Ltd. All rights reserved.

Establishing the atmospheric expression of abrupt climate change during the last glacial termination is key to understanding driving mechanisms. In this paper, we present a new 14C chronology of glacier behavior during late-glacial time from the Scottish Highlands, located close to the overturning region of the North Atlantic Ocean. Our results indicate that the last pulse of glaciation culminated between ~12.8 and ~12.6 ka, during the earliest part of the Younger Dryas stadial and as much as a millennium earlier than several recent estimates. Comparison of our results with existing minimum-limiting 14C data also suggests that the subsequent deglaciation of Scotland was rapid and occurred during full stadial conditions in the North Atlantic. We attribute this pattern of ice recession to enhanced summertime melting, despite severely cool winters, and propose that relatively warm summers are a fundamental characteristic of North Atlantic stadials. his work was supported by NSF grant EAR‐9118375 and National Geographic/WAITT Foundation grant 450‐16. A.E. Putnam acknowledges support from the Comer Family Foundation, the Lenfest Foundation, a Lamont‐Doherty Earth Observatory postdoctoral fellowship, and NSF grant EAR‐1554990. The data reported and discussed in this paper are listed in the references, tables, and supporting information. peer-reviewed

Knowledge of the West Antarctic Ice Sheet (WAIS) response to past sea level and climate forcing is necessary to predict its response to warmer temperatures in the future. The timing and extent of past interior WAIS elevation changes provides insight to WAIS behavior and constraints for ice sheet models. Constraints prior to the Last Glacial Maximum (LGM) however, are rare. Surface exposure ages of glacial erratics near the WAIS divide at Mt. Waesche in Marie Byrd Land, and at the Ohio Range in the Transantarctic Mountains, range from ∼10 ka to >500 ka without a dependence on elevation. The probability distribution functions (PDF) of the exposure ages at both locations, are remarkably similar. During the last glaciation, maximum interior ice elevations as recorded by moraines and erratics were reached between 10 ka and 12 ka. However, most exposure ages are older than the LGM and cluster around ∼40 ka and ∼80 ka. The peak in the exposure age distributions at ∼40 ka includes ages of alpine moraine boulders at Mercer Ridge in the Ohio Range. Comparison of the PDF of exposures ages from the Ohio Range and Mt. Waesche with the temperature record from the Fuji Dome ice core indicates that the youngest peak in the exposure age distributions corresponds to the abrupt warming during the Last Glacial termination. A prominent peak in the Ohio Range PDF corresponds to the penultimate termination (stage 5e). During the intervening glacial period, there is not a consistent relationship between the peaks in the PDF at each location and temperature. A combined ice sheet/ice shelf model with forcing scaled to marine δ 18 O predicts that interior WAIS elevations near the ice divide have varied ∼300 m over the Last Glacial cycle. Peaks in the PDF correspond to model highstands over the last 200 ka. In the simulated elevation history, maximum ice elevations at Ohio Range (+100 m) and Mt. Waesche (+60 m) occur at ∼10 ka, in agreement with observations from these sites. During collapse of the marine portion of the WAIS, ice elevations at Ohio Range and Mt. Waesche are drawn down at least 200 m below the present ice elevation. The good correspondence between the model results and observations at both the Ohio Range and Mt. Waesche supports the conclusion that interior WAIS highstands do not occur during glacial maximums. Rather, the highstands are controlled primarily by increased accumulation during temperature maximums that occur early in the interglacials. Interior down-draw events follow highstands, resulting from the arrival of a wave of thinning triggered by retreat of the WAIS grounding line coupled with decreasing accumulation rates.

Knowledge of the timing and extent of past West Antarctic Ice Sheet (WAIS) elevation changes provides insight to the WAIS response to sea level and climate forcing, as well as constraints for ice sheet models. Trimlines and erratics indicate that maximum ice elevation at the Ohio Range, located near the WAIS divide, was 125 m above the present ice surface and was reached at ~ 10 ka. However, 3 He and 10 Be exposure ages of most granitic erratics and supra-glacial boulders in adjacent blue ice areas are older than the last highstand and provide constraints on earlier interior WAIS elevation changes. Although diffusive loss of 3 He occurs in quartz, an excellent correlation between 3 He and 10 Be concentrations in samples up to 400 ka is observed in the Ohio Range, demonstrating the utility of 3 He measurements in Antarctic quartz. The correlation is used to calculate “equivalent 10 Be exposure ages” from the larger 3 He data set. The exposure ages occur in distinct clusters 10–20 ka, 70–80 ka, 130–140 ka and 180–190 ka that we relate to pulses in the emergence of englacial debris in localized blue ice ablation areas. A combined ice sheet/ice shelf model predicts that WAIS elevations near the Ohio Range have varied only ~ 125 m over the last 200 kyr with highstands at ~ 10 ka and ~ 130 ka, in good agreement with observations of trimlines and peaks in the exposure age distribution at the Ohio Range. In the model, interior WAIS elevations are controlled by the interaction of accumulation rates and ice dynamics related to ice sheet/ocean interactions at the grounding line. WAIS highstands occur early in interglacial periods during intervals of warmer temperatures and high accumulation rates, prior to the arrival of a wave of thinning. We hypothesize that the correspondence of rising regional WAIS elevation with debris pulses relates to increased ice (debris) flux and ablation in the local blue ice areas resulting from the warmer temperatures. The occurrence of clasts with exposure ages up to 200 ka on the WAIS surface indicates elevation changes near the Ohio Range were limited in extent during stage 5e. The geologic observations, combined with model results, place constraints on WAIS geometry during stage 5e and limit the WAIS contribution to the higher sea levels observed during the last interglacial to ~ 3 m.

To further evaluate the potential use of Mg/Ca and Sr/Ca ratios as a paleothermometer in the shell carbonate of the blue mussel Mytilus edulis, we grew juvenile mussels (∼15 mm shell height

Remains identified as those of a woolly mammoth (Mammuthus primigenius) dated at 12,200 ± 55 14C yr B.P. were recovered while excavating in a complex sequence of glaciomarine sediments in Scarborough, Maine, USA. The mammoth was found in the top meter of a fossiliferous unit of mud and sand laminites. These sediments were deposited during a marine regressive phase following the transgression that accompanied northward retreat of the margin of the Laurentide ice sheet. A Portlandia arctica valve from the underlying transgressive unit provides a minimum age of 14,820 ± 105 14C yr B.P. for local deglaciation. The mammoth, an adult female, died in midwinter with no evidence of human involvement. Tusk growth rates and oxygen-isotope variation over the last few years of life record low seasonality. The mammoth was transported to the site as a partial carcass by the late-glacial proto-Saco River. It sank in a near-shore setting, was subjected to additional disarticulation and scattering of elements, and was finally buried in sediments reworked by the shallowing sea.

Hill of granite on east side of Carver’s Harbor—It affords fine opportunity for studying glacial erosion and disruption of the hills and general surface —Original deposits of Maine, rocks principally clay, which in time was consolidated into slates, and finally passed into granite and other rocks —Boulders used as rasping, polishing and scratching tools—Terrace-like steps at a quarry west of Carver’s Harbor—A huge slab removed 200 or 300 ft, and placed upon its edge—Nature personified—Thoughts on seeing men at work upon a boulder quarried ages before man appeared upon the globe—Moraines of doubtful existence in the State of Maine—Some hills bare of boulders; others covered with them—The motion of the glacier slow.

A lateral moraine band on Mount Waesche, a volcanic nunatak in Marie Byrd Land, provides estimates of past ice sheet surface elevations in West Antarctica. Helium-3 and chlorine-36 surface exposure ages indicate that the proximal part of the moraine, up to 45 meters above the present ice surface, was deposited about 10,000 years ago, substantially later than the maximum ice extent in the Ross Embayment. The upper distal part of the moraine may record multiple earlier ice sheet high stands. A nonequilibrium ice sheet model predicts a delay of several thousand years in maximum ice levels at Mount Waesche relative to the maximum ice extent in the Ross Sea. The glacial geologic evidence, coupled with the ice sheet model, indicates that the contribution of the Ross Sea sector of the West Antarctic Ice Sheet to Holocene sea level rise was only about 3 meters. These results eliminate West Antarctic ice as the principle source of the large meltwater pulse during the early Holocene.

[1] Interior ice elevations of the West Antarctic Ice Sheet (WAIS) during the last glaciation, which can serve as benchmarks for ice-sheet models, are largely unconstrained. Here we report past ice elevation data from the Ohio Range, located near the WAIS divide and the onset region of the Mercer Ice Stream. Cosmogenic exposure ages of glacial erratics that record a WAIS highstand 125 m above the present surface date to 11.5 ka. The deglacial chronology prohibits an interior WAIS contribution to meltwater pulse 1A. Our observational data of ice elevation changes compare well with predictions of a thermomechanical icesheet model that incorporates very low basal shear stress downstream of the present day grounding line. We conclude that ice streams in the Ross Sea Embayment had thin, lowslope profiles during the last glaciation and interior WAIS ice elevations during this period were several hundred meters lower than previous reconstructions. Citation: Ackert, R. P., Jr., S. Mukhopadhyay, B. R. Parizek, and H. W. Borns (2007), Ice elevation near the West Antarctic Ice Sheet divide during the Last Glaciation, Geophys. Res. Lett., 34, L21506, doi:10.1029/ 2007GL031412.

The glacial geology of Maine records the northward recession of the Late Wisconsinan Laurentide Ice Sheet, followed by development of a residual ice cap in the Maine-Quebec border region due to marine transgression of the St. Lawrence Lowland in Canada. The pattern of deglaciation across southern Maine has been reconstructed from numerous end moraines, deltas and submarine fans deposited during marine transgression of the coastal lowland. Inland from the marine limit, a less-detailed sequence of deglaciation is recorded by striation patterns, meltwater channels, scattered moraines and waterlain deposits that constrain the trend of the ice margin. There is no evidence that the northern Maine ice cap extended as far south-west as the Boundary Mountains and New Hampshire border. Newly-obtained radiocarbon ages from marine and terrestrial ice-proximal environments have improved the chronology of glacial recession in Maine. Many of these ages were obtained by coring Late-glacial sediments beneath ponds and lakes. Data from this study show that the state was deglaciated between about 14.5 and 10.0 ka BP (14C years). The coastal moraine belt in southern Maine was deposited by oscillatory ice-margin retreat during the cold pre-Bolling time. Rapid ice recession to northern Maine then occurred between 13 and 11 ka BP, during the warmer Bolling/Allerod chronozones. Radiocarbon-dated pond sediments in western and northern Maine show lithological evidence of Younger Dryas climatic cooling and persistence of the northern ice cap into Younger Dryas time. A large discrepancy still exists between radiocarbon ages of deglaciation in coastal south-western Maine and the timing of ice retreat indicated by New England varve records in areas to the west. Part of this problem may stem from the uncertainty of reservoir corrections applied to the radiocarbon ages of marine organics.

Ice-wedge casts in northern Nova Scotia and the relation of the casts to the outwash that contains them indicate that the ice wedges formed in a permafrost environment after the accumulation of the outwash. This permafrost environment is tentatively correlated with pollen zone L-3 of the Gillis Lake deposit, Cape Breton Island, Nova Scotia, and with the Valders time of the midcontinental sequence.

The Five Islands Formation is a late Pleistocene outwash deposit. It forms a discontinuous raised terrace between the Cobequid Hills and the north shore of the Minas Basin, Nova Scotia. Three lithosomes may be distinguished. At Advocate Harbor, sea bluffs and numerous sand pits expose well-stratified sandy gravels and openwork gravels of the glaciolittoral lithosome. These were deposited in a wave-agitated environment similar to tie modern Advocate Bay. Here the surface of the terrace is molded into a lagoon enclosed by spits and backed by a wave-cut bluff, closely resembling the modern Advocate Harbor. Between Cape Spencer and Five Islands, river valleys are rilled with a glaciodeltaic lithosome. Bottomset clay deposits and sand and gravel foreset and topset deposits contain ice-contact features and, rarely, casts and molds of Portlandia glacialis. The glaciodeltaic lithosome is disconformably overlain by a sheet of kettled glaciofluvial gravels exhibiting a relict braided pattern on its surface. The mar...

Deposits of Kaskawulsh Glacier express major climatic changes of the last 10,000 years. A great retreat, miles up valley from present terminus, occurred 12,000 to 9,000 yr ago, leaving drift on valley walls and alluvial fans. Winds added up to 8 feet of loess on these deposits and buried 9700 year-old vegetation near the present terminus. During Slims nonglacial interval weathering developed a red-yellow paleosol in this loess. In lower parts of a bog, just above the Neoglacial terminal moraine, dominant grass pollen and moss spores suggest a wetter climate than today. Just above the 3300-yr-old date in this bog, peat with sedge, spruce, and artemesia pollen show a change to a drier climate. The climatic change and reactivated outwash signal the Neoglacial advance 2600 to 3000 yr ago. Kaskawulsh reached its terminal position approximately 300 yr ago, and was there as late as 145 yr ago.

Reexamination of the Mawson "Tillite" (Jurassic) of Antarctica indicates that the deposit is of volcanic origin and consists largely of both primary and reworked volcanic materials. No evidence diagnostic of a glacial origin was found.

A multidisciplinary study of crustal movements in coastal Maine has shown that parts of the state9s coastal zone are subsiding. This subsidence is associated with seismic activity that has persisted locally since at least the 1800s. Bedrock structural studies have not produced evidence of major Holocene faulting, but measured horizontal rock strains may be related to Mesozoic graben development in the adjacent Gulf of Maine. The crustal down warping is most apparent in eastern Maine, where releveling data indicate a modern subsidence rate of as much as 9 mm/yr. The anomalously rapid sea-level rise resulting from this subsidence has caused erosion and submergence of archeological sites and historical man-made structures. However, the altitudes of glaciomarine deltas and the distribution of dated archeological sites show that the present subsidence rates have not persisted throughout Holocene time.

The highlands of west-central Maine, including the Longfellow and Boundary Mountains, were overridden at least twice, probably three times, by a continental ice sheet during Wisconsinan time. These episodes are indicated by at least five widely separated exposures displaying two-drift sequences composed of thick lodgment tills separated by glaciolacustrine and fluvial sediment. Radiocarbon dates on an intertill pollen- and wood-bearing deposit at New Sharon, Maine, record a major nonglacial interval that ended more than 52,000 B.P. The last ice sheet, actively building end moraines along the Maine coast 13,500 yr ago, had thinned, separated, stagnated, and dissipated over the Longfellow and Boundary Mountains at least by 12,500 B.P. Contemporaneous stagnation, throughout and southeast of the mountains, is evidenced by the distribution and volume of ice-contact stratified drift. Coupled with this is the lack of evidence for receding active ice margins associated with either the Laurentide Ice Sheet or a late-glacial locally centered ice cap suggested by some workers. In addition, the highest cirques in the study area, floored at approximately 930 m, reveal no evidence of reactivation during and subsequent to the dissipation of the last ice sheet.

Parts of coastal and central Maine underwent marine submergence between approximately 13,500 and 12,500 yrs ago. An ice-contact marine delta near Cherryfield, Maine, was formed during this time. Ten radiocarbon dates of basal peat samples from a kettle hole on the delta ranged from 5,310 ± 100 yrs B.P. at a depth of 2.21 m to 9,170 ± 160 yrs B.P. at a depth of 5.28 m. Peat from depths of 5.55 m yielded dates within the statistical error of the oldest date. The radiocarbon dates indicate a time lag of approximately 2,000 yrs between the formation of the delta and the earliest deposition of organic matter in the kettle hole.

The late-glacial stratigraphy of the northern part of the Kennebec River valley of Maine indicates the following sequence of events. (1) As the last extensive ice sheet dissipated in Maine, the sea inundated parts of the deglaciated and isostatically depressed region and deposited the Presumpscot Formation. (2) During emergence, outwash of the Embden Formation was gradationally deposited on the Presumpscot Formation. (3) As emergence progressed, the Embden and Presumpscot Formations were deeply incised by the early Kennebec River. (4) Outwash (?) of the North Anson Formation was later deposited within this incision. The geologic sequence of events, radiocarbon dates in the area, and correlation of events of the Maine, Quebec, New Brunswick, and Nova Scotia region suggest that the last extensive ice sheet to cover the region dissipated more rapidly in eastern Maine and New Brunswick than in the highlands of New Hampshire and western Maine. The data also suggest that ice persisted in the highlands during the time of the Cary-Port Huron Interstade, perhaps dissipating during the time of the Port Huron- Valders Interstade of the mid-continental sequence. These highlands probably later became a center for ice accumulation and radial outflow. This ice cap probably grew synchronously with the main Valders ice sheet to the north, with cirque glaciers on Mount Katahdin, Maine, and with the ice cap in southwestern Nova Scotia.