The Laurentide Ice Sheet (LIS) was the largest ice sheet during the last glacial period. An accurate representation of its behavior during the last deglaciation is critical to understanding its influence on and response to a changing climate. We use 10Be dating and Bayesian modeling to track the recession of the southwest sector of the Labrador Dome of the LIS along an similar to 500-km-long transect west of Lake Superior during the last deglaciation. This transect reflects terrestrial ice-margin retreat and crosses multiple moraine sets, with the southwestern part of the transect deglaciated by ca. 19 ka and the northeastern part deglaciated by ca. 10 ka. The predominant behavior of the ice margin during this interval is near-constant retreat with retreat rates varying between similar to 59 m/a and 38 m/a. The moraine sets mark standstills and/or readvances that in total constitute only similar to 17% of the retreat interval. The spatial and temporal pattern of ice-margin retreat tracked here differs from existing reconstructions that are based on using isochrons to define ice-margin positions. Acknowledging the uncertainties associated with the modeled ages of ice-margin retreat, we suggest that the overall retreat pattern is consistent with forcing by a gradual increase in Northern Hemisphere, high-latitude summer insolation. The pattern of ice-margin retreat is inconsistent with Greenland ice-core temperature records, and thus these records may not be suitable to drive models of the LIS.

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.

Accurately reconstructing the paleogeography of the Laurentide Ice Sheet (LIS) during Marine Isotope Stage 3 (MIS 3; ca. 57,000 to ca. 29,000 yr B.P.) is critical for understanding glacial growth toward the Last Glacial Maximum (LGM), refining sea-level histories, and studying the Earth system response to rapid climate change events. Here, we present a geochronological data set useful for testing hypotheses of global sea level and refining ice sheet configuration through this interval. Data (n = 735) span the entire MIS 3 interval and consist of 14C determinations (n = 651), cosmogenic exposure ages (n = 52), and optically stimulated luminescence dates (n = 32). On that basis, we hypothesize that the central region of the LIS underwent a dramatic reduction in ice from ca. 52 to 40 ka. Key to this hypothesis are geological records at sites in the Hudson Bay Lowlands (east central Canada) that suggest a marine incursion and development of terrestrial landscapes. We show that these landscapes are consistent with recently published glacial isostatic adjustment predictions that include widespread deglaciation of the eastern (Labrador) sector of the LIS with ice buildup over the western (Keewatin) sector at 42 ka. Ice growth from this minimum toward the LGM is likely to have been rapid. The agreement between this data set and modeling predictions prompts the reassessment of key Late Pleistocene records, including Heinrich events, loess deposition in the continental United States, and sedimentological records from the Gulf of Mexico.

Stratigraphic records from formerly glaciated regions are critical for detailed study of the timing, onset and dynamics of past ice sheets and the palaeoecology of previous ice-free intervals. We examined three stratigraphic sections from an 18-km stretch of the Albany River, Hudson Bay Lowlands, Canada, located at the geographic center for many Late Pleistocene ice sheets. Till characterization and correlation suggest that at least three glacial advances from shifting ice centers within the Labrador sector of the Laurentide Ice Sheet were preserved in these stratigraphic records. Non-glacial units (fluvial, organic-bearing sediments) were constrained via optically stimulated luminescence to two possible periods at ca. 73,000 to 68,000yr BP and ca. 60,000yr BP. Boreal and peatland taxa (Picea, Pinus, Poaceae, Betula, Cyperaceae, Sphagnum) dominated the pollen record at each site, whereas plant macrofossils analyzed at one site confirm the local presence of conifer trees (bark, needles, seed wings), bryophytes (largely Scorpidium spp), herbaceous plants (Caryophyllaceae, Carex, Poaceae), and an aquatic setting (e.g. Potamogeton, ephippia of Daphnia spp). Pollen-derived average summer temperature reconstructions suggested that local temperatures at the Albany sites were between 12 and 15°C, which is similar to present-day estimates for the region (14.2°C). Reconstructed annual precipitation estimates were 580–640mm, which is similar to slightly higher than present-day estimates (564mm). Non-glacial intervals at the Albany sites likely represent abandoned fluvial environments that supported water-logged peatland biota. Results from this research contribute toward ongoing efforts to constrain ice sheet dynamics over North America during the last glacial cycle (e.g. 71,000–14,000yr BP) and provide insight into the complex Late Pleistocene palaeoclimate record at the innermost area of the glaciated region.

Detailed studies of coastal cliff exposures through two end moraines form the basis for a model of ice-marginal sedimentation in large ice-contact glacier-fed lakes. The input to the ice-marginal environment directly from the glacier included subglacial till and subaquatic flow tills. The subaquatic flow till (thinly bedded diamicton) was deposited in an apron (up to 1 km wide) along the ice margin. An upward gradient of pore-water pressure immediately beyond the ice margin, causing heaving and dilation of the sediments, initiated debris flows of glacially derived debris (subaquatic flow tills). Most of the stratified sediments in the ice-marginal zone entered the lake by way of a large proglacial stream. Sedimentation was dominated by quasi-or near-continuous density underflows that resulted in the deposition of a sequence of thick rhythmites. The glacier in the Lake Erie basin most likely behaved like an ice stream, with its movement controlled predominantly by a deforming bed of glacial debris, separating the glacier sole from underlying predeposited sediments. The deforming bed is preserved as a massive diamicton layer, interpreted here as subglacially deposited till.

We examine pollen, macrofossils and sedimentological proxies from the Ridge Site, an 18‐m sequence of glacial and non‐glacial sediments exposed along the bank of the Ridge River in the southern Hudson Bay Lowlands (HBL), Canada. As the HBL is located in the previously glaciated region of North America, palaeorecords from this region have important implications for understanding ice‐sheet palaeogeography and climate for the late Pleistocene. Two diamicton units were interpreted as subglacial till deposited by a glacier flowing toward the south‐southwest (lower diamicton) and west‐southwest (upper diamicton), respectively. Confined between these tills is a 6‐m non‐glacial unit, constrained to Marine Isotope Stage 3 (MIS 3; c. 57 000 to c. 29 000 a BP) by three radiocarbon dates. Quantitative analyses of the pollen record (dominated by Sphagnum, Cyperaceae, Pinus, Picea, Salix, Alnus and Betula) suggest that average summer temperature (June, July, August) was 14.6±1.51 °C, which is similar to that of the present day at the site. Total annual precipitation was 527±170 mm as compared to 705 mm present‐day. The macrofossil record confirmed the local presence of Betula, Salix and conifers. Our results, in combination with other records from the periphery of the Laurentide Ice Sheet, suggest that vast boreal forest‐type vegetation, along with a drier interstadial climate, existed in the region during MIS 3. We also compare pollen‐derived palaeoclimate reconstructions from the Ridge Site with reconstructions from a previously published site along the Nottaway River, HBL, which was dated to MIS 5a–d (c. 109 000 to c. 82 000 a BP). This comparison suggests that, with additional data, it may be possible to differentiate MIS 3 and MIS 5 deposits in the HBL on the basis of relative continentality, with MIS 3 characterized by lower total annual precipitation, and MIS 5 by values similar or greater than present‐day.

The Nipigon channels, located to the west and northwest of Lake Nipigon, Ontario, are thought to have enabled the eastward drainage of meltwater from glacial Lake Agassiz during the last deglaciation. Here we present the first direct ages of flood deposits in two of these channels using 10Be surface exposure dating. Five 10Be ages of a coarse-grained deposit near the Roaring River in the Kaiashk channel complex indicate deglaciation and cessation of water flow by ∼11 070 ± 430 years. To test for inherited nuclides in boulder samples, we also measured the 10Be concentrations of the undersides of two boulders at the Roaring River site. Five 10Be ages of boulders atop a large bedform near Mundell Lake in the Pillar channel complex indicate deglaciation and cessation of water flow by ∼10 770 ± 240 years. Two 10Be ages of nearby bedrock are slightly younger (10 340 ± 260 and 9860 ± 270 years). The 10Be ages from the two sites are statistically indistinguishable and indicate that Laurentide Ice Sheet recession occurred rapidly in the region. We used clast diameters and channel dimensions at the Mundell Lake site to estimate paleodischarge and evaluate the possibility that meltwater drainage influenced climate conditions. We estimate a large maximum discharge of 119 000–159 000 m3·s−1 at the site. However, the timing of meltwater discharge at both Roaring River and Mundell Lake is not contemporaneous with abrupt climate events.

Well-dated paleorecords from periods prior to the Last Glacial Maximum (LGM) are important for validating models of ice sheet build-up and growth. However, owing to glacial erosion, most Late Pleistocene records lie outside of the previously glaciated region, which limits their ability to inform about the dynamics of paleo-ice sheets. Here, we evaluate new and previously published chronology data from the Missinaibi Formation, a Pleistocene-aged deposit in the Hudson Bay Lowlands (HBL), Canada, located near the geographic center of the Laurentide Ice Sheet (LIS). Available radiocarbon (AMS = 44, conventional = 36), amino acid (n = 13), uranium-thorium (U-Th, n = 14), thermoluminescence (TI, n = 15) and optically stimulated luminescence (OSL, n = 5) data suggest that an ice-free HBL may have been possible during parts of Marine Isotope Stage 7 (MIS 7; ca. 243,000 to ca. 190,000 yr BP), MIS 5 (ca. 130,000 to ca. 71,000 yr BP) and MIS 3 (ca. 29,000 to ca. 57,000). While MIS 7 and MIS 5 are well documented interglacial periods, the development of peat, forest bed and fluvial deposits dating to MIS 3 (n = 20 radiocarbon dates; 4 TL dates, 3 OSL dates), suggests that the LIS retreated and remained beyond, or somewhere within, the boundaries of the HBL during this interstadial. Ice sheet models approximate the margin of the LIS to Southern Ontario during this time, which is 700 km south of the HBL. Therefore, if correct, our data help constrain a significantly different configuration and dynamicity for the LIS than previously modelled. We can find no chronological basis to discount the MIS 3 age assignments. However, since most data originate from radiocarbon dates lying close to the reliable limit of this geochronometer, future work on dating the Missinaibi Formation using other geochronological methods (e.g. U-Th, OSL) is necessary in order to confirm the age estimates and strengthen the boundaries of the LIS during this period. (C) 2016 Elsevier Ltd. All rights reserved.

During deglaciation of the Great Lakes region, events occurred that may have resulted in the burial of archaeological sites. Such events include oscillations of the ice margin that could have resulted in the direct burial of archaeological sites by till, or blocking of meltwater drainage passageways at lower elevations, resulting in the flooding of former land surfaces in front of the glacier and possibly the deposition of lake sediments on abandoned archaeological sites. Changes in the routing of glacier meltwater as the ice margin receded-combined with glacial isostatic adjustment and climatic change-created situations where former land surfaces became inundated and, with sediment deposition, possible burial of archaeological sites.Examples of these various types of events are described, including the Arkona-Whittlesey (ice advance and blockage of outlets at lower elevations), Kirkfield-Main Algonquin and the Nipissing Great Lakes transgressions (glacial isostatic adjustment), and the glacier re-advance to the Marks and Dog Lake moraines (Marquette advance), an example where there is a possibility that a habitable preexisting landscape was overridden and covered with till, and areas immediately in front of the ice were rapidly flooded by ponded meltwater. Climatic change and the glacial meltwater bypassing of the Great Lakes may have combined to form closed-basin lakes within the basins of the Great Lakes, and concurrent and subsequent glacial isostatic adjustment resulted in the eventual flooding of once exposed forest beds and potential archaeological sites on the floors of the present-day Great Lakes.

The exposures at the St. Marys Cement Inc. quarry at St. Marys, Ontario, have been studied by geologists since the mid-1950s. This paper summarizes previous and new information collected in 2009, 2010, and 2012 and discusses the record of sediments resting on the bedrock surface that have been exposed during quarry operations. The exposed sediments illustrate the dynamic behavior of the Laurentide Ice Sheet in this area during the Late Wisconsin. Evidence for subglacial meltwater activity, shifting ice-flow directions during till deposition, a local or regional erosion event, possible subaerial exposure, and ice-marginal sedimentation is observed in the sequence of tills and stratified sediments exposed in the quarries. It also highlights how rapid lateral facies changes, complex contact relationships, and steep erosional contacts can prove to be challenging for correlation and extrapolation of subsurface units into three-dimensional stratigraphic models. Fourteen units were identified of which the lower six were deposited during the Nissouri age (Catfish Creek Drift). This included five layers of till deposited by a glacier that alternated from flowing out of the Lake Huron basin to that of a regional flow to the south-southwest. An angular unconformity cuts into these sediments and separates them from a finer-grained sediment sequence (primarily of Port Bruce age) consisting of glaciofluvial gravel, two fine-textured till layers, and rhythmically bedded glaciolacustrine sediments, all overlain by the uppermost till, the Rannoch Till, and Mitchell Moraine ice-marginal fan sediments.

Till sampling near the Lac des Iles Pd mine north of Thunder Bay, Ontario, Canada has led to the identification of heavy mineral dispersal trains in till as unique as the local Pd deposits. The till at Lac des Iles was deposited by ice actively flowing in a 240 degrees direction. It is commonly thin and ideal for exploration sampling.Several dispersal trains of chrome-andradite garnet and chromite occur within the area surrounding the Mine Block Intrusion (MBI). Cr-andradite garnet is normally a hybrid, assimilation-type indicator mineral; however, here the Cr-andradite is of a low-temperature, cryptocrystalline form, suggesting that hydrothermal alteration, possibly associated with the Lac des Iles late-stage mineralizing event, may have played a significant role in its formation. A major Cr-andradite + chromite dispersal train occurs over the MBI. It is a narrow, ribbon-shaped, composite train that trends 240 degrees, is c. 600 m in width and exceeds 5 km in length. Its farthest up-ice source appears to be located to the NW of the margin of the MBI, in rocks of the Northern Lac des Iles Intrusion (NLDI-I). The distal part of the dispersal train SW of the MBI may contain additional indicator mineral grains from a septa of ultramafic rocks associated with the North Lac des Iles intrusion that lies west of the MBI. The accompanying chromite dispersal trains are shorter and appear to be derived from similar source areas as well as an area in the vicinity of the Baker zone.The indicator mineral contribution of the main Roby (ore) zone is difficult to assess due to a sampling gap but appears to be minimal, probably because this zone occurs in the interior of the intrusion and is hosted mainly by Cr(2)O(3)-poor gabbro. The only PGM are rare grains of Pt and Pd arsenides, which are uncommon in the mineralized zones but are stable in near-surface till.

The Surficial Geochemistry Case Studies Project in the Lake Nipigon region involved detailed Quaternary mapping and multimedia geochemical sampling within five case study areas. Two of these areas, Lac des Iles and Tib Lake, contain known platinum group element (PGE) mineralization. The other three case study areas feature drainage catchments with lakes that contain anomalous levels of PGEs in bottom sediment. Surficial media sampled included till, soils, stream sediment, lake sediment, peat, surface water, and groundwater. Over Archean terrain, such as at Lac des Iles and Tib Lake, there is excellent geochemical contrast between the PGE prospective rock type (mafic to ultramafic intrusive) and the surrounding rock type (e.g., granitoid rocks). This geochemical contrast is mirrored in the geochemistry of most surficial media sampled during this project. Over the Nipigon Embayment, the geochemical contrast between mafic and ultramafic rocks (e.g., “Seagull”-type intrusions) and the surrounding diabase sill rocks is inherently weaker due to the relatively high background levels for copper, palladium, and gold in the Nipigon diabase sills. However, the results of stream-water geochemistry over the Seagull and Disraeli ultramafic intrusions highlight their unique geochemical footprint on the surficial landscape, in particular with respect to Cr, Mg, and Ni, compared with the surrounding Nipigon diabase. In addition, lake sediments underlain by the Seagull, Disraeli, and Hele intrusions have distinctly elevated Cr concentrations and Gd/Yb ratios compared with the surrounding areas underlain by Nipigon diabase sills or Sibley Group rocks. Therefore, exploration value can be maximized over the Nipigon Embayment by exploiting these geochemical contrasts in surficial media to discriminate between ultramafic rocks and the surrounding Nipigon diabase sills. The results of this study highlight the importance of chromium concentrations in surficial media as a diagnostic feature for the presence of ultramafic rocks, regardless of their age or location. In general, a cost-effective exploration strategy for PGE mineralization includes targeting the associated metals (Cr, Ni) within drift deposits (C-horizon till) and drainage media (stream sediment, lake sediment) to vector to prospective mafic–ultramafic intrusive rocks, prior to detailed (property scale) follow-up, involving the determination of base metals and PGEs within soil, till, stream sediment, and peat samples. Relative to the metals copper, nickel, and chromium (ppm levels), the PGEs have significantly lower initial concentrations (ppb levels), are less mobile in the surficial environment, have significantly shorter glacial dispersion trains, and are less reliably determined at the laboratory.

Detailed facies mapping along Lake Erie and Lake Ontario Bluffs, plus other studies illustrate that sedimentological studies, especially those with geomorphic or landform control, have had three main effects on the Wisconsinan stratigraphy of Ontario: (1) improved understanding of depositional processes and environments of several major rock stratigraphic units, without altering the stratigraphic framework, (2) aided correlation of drift sequences, and (3) questioned previous interpretations and stratigraphic correlations of drift sequences. Thus sedimentological analysis can not be separated from stratigraphy because the interpretation of depositional environnments of many mapped strata relies on their geometry and the inclusion of regional data. The geomorphic control provided by sedimentological study of surface landforms is also important because assessment of older buried sediments such as those at the Scarborough Bluffs has been hampered by the failure to determine landform control. The Late Wisconsinan stratigraphy of Southern Ontario generally remains unchanged, except for questions on the role of climate versus ice margin dynamics. The pre-Late Wisconsinan stratigraphy is scarce and not well defined, yet sedimentary studies support the presence of glacial ice in the Ontario Lake basin for all of the Middle Wisconsinan and possibly earlier, including the formation of the Scarborough delta. Large channel cut and fill sequences in the Toronto area (Pottery Road Formation), initially interpreted as resulting from subaerial erosion, were probably formed by subaqueous or subglacial meltwater erosion. If so, the pre-Late Wisconsinan stratigraphy in southern Ontario changes because the Pottery Road Formation may not be an Early Wisconsinan correlative of the St. Pierre beds. The channel example illustrates that stratigraphie correlation without sedimentological investigations may be misleading.

Many previously published studies of the behaviour of Pt and Pd in till and soils have been done in areas of complex stratigraphy or very thin overburden cover, making the interpretation of soil results difficult because of the many variables associated with these settings. At the Lac des Iles mine site in northwestern Ontario, there are excellent exposures of the overburden in a series of exploration trenches. Glacial dispersal trains can be observed in till (C horizon) geochemistry (e.g., Ni, Cr, Cu, and Co). Regional geochemical dispersal trains of elements, such as Ni, Cr, Mg, and Co associated with the North Lac des Iles intrusion, can be detected for about 4 km beyond the western margin of the Mine Block intrusion. Entire dispersal trains range from 5 to 7 km in length and about 1 to 2 km in width. The dispersal of North Lac des Iles intrusion rock fragments tends to mask the response of the Mine Block intrusion. Dispersal trains of Pt and Pd are not well defined and tend to be very short, <1 km in length, due to the initial low concentrations of these elements in C-horizon till samples from the Lac Des Iles area. An exception to this is the Pd dispersal train originating from the high-grade zone that is up to 3 km long. Pd, Pt, Ni, and Cu appear to be moving both within and out of the soil system downslope into surface and shallow groundwater. It is suggested that these elements, to varying degrees, are moving in solution. Airborne contamination from mine operations of the humus has adversely affected the ability to determine the effectiveness of humus sampling for mineral exploration at Lac des Iles. The airborne contamination likely influences the geochemical results from surface water, shallow groundwater, and near-surface organic bog samples, particularly for the elements Pd and Pt.