Laurentide Research Articles

Rise of Great Lakes Surface Water, Sinking of the Upper Midwest of the United States, and Viscous Collapse of the Forebulge of the Former Laurentide Ice Sheet

AbstractGreat Lakes water levels rose 0.7–1.5 m from 2013 to 2019, increasing surface water volume by 285 km3. Solid Earth's elastic response to the increased mass load is nearly known: The Great Lakes floor fell 8–23 mm, and the adjacent land fell 3–14 mm. Correcting GPS measurements for this predicted elastic loading (1) straightens position‐time series, making the evolution of position more nearly a constant velocity and (2) reduces estimates of subsidence rate in Wisconsin, Michigan, and southern Ontario by 0.5–2 mm/yr, improving constraints on postglacial rebound. GPS records Wisconsin and Michigan to have subsided at 1–4 mm/yr. We find this sinking to be produced primarily by viscous collapse of the former Laurentide ice sheet forebulge and secondarily by elastic Great Lakes loading. We infer water on land in the Great Lakes watershed to be total water change observed by GRACE minus Great Lakes surface water smeared by a Gaussian distribution. Water stored on land each year reaches a maximum in March, 6 months before Great Lakes water levels peak in September. The seasonal oscillation of water on land in the Great Lakes basin, 100 km3 (0.20 m water thickness), is twice that in a hydrology model. In the seasons, groundwater in the Great Lakes watershed increases by 60 km3 (0.12 m) each autumn and winter and decreases by roughly an equivalent amount each spring and summer. In the long term, groundwater volume remained constant from 2004 to 2012 but increased by 50 km3 (0.10 m) from 2013 to 2019.

A GIS‐based multi‐proxy analysis of the evolution of subglacial dynamics of the Quebec–Labrador ice dome, northeastern Quebec, Canada

AbstractThe glacial records of the inner‐core regions of the Laurentide Ice Sheet (LIS) document complex yet coherent patterns reflecting ice‐sheet change (e.g. ice‐divide migration), providing unique insights into past glacial conditions. This study develops a conceptual model of subglacial dynamics evolution within a major ice‐dispersal centre of the LIS in northeastern Quebec, Canada using a GIS‐based analysis of the surficial geologic record. Multiple proxies of subglacial conditions (subglacial streamlined landforms, lake density and lake area over thin drift/bedrock) were analysed through grid‐overlay techniques and then classified based on different proxy variables ranging from highly mobile warm‐based to immobile cold‐based conditions. An additional proxy (till blanket) was used to identify areas of thick till deposition, but with few proxies (few lake or landform metrics). Based on local ice‐flow reconstructions, the most ‘relict’ glacial terrain zone (GTZ1) has warm‐based conditions over 66% of its area and is remarkably well preserved, suggesting laterally extensive warm‐based conditions during the oldest identified ice‐flow phase. This relict glacial terrain is partially overprinted by two subsequent ice‐flow phases in spatially restricted zones in the northeast (73% warm‐based), east‐central (41% warm‐based), and northwest (33% warm‐based) of the study area. A zone of more sluggish conditions (only 3% warm‐based) was identified in the highlands at the centre of the study area, characterized by thin till cover, few landforms, yet with large patches of relatively abundant small lakes, indicative of areal scouring. No clear evidence of sustained cold‐based conditions (i.e. high chemical index of alteration values or high 10Be abundances) was found in the study area. These results suggest that warm‐based conditions (active erosion and/or deposition) were uniformly widespread during the earliest ice‐flow phase, later becoming more spatially restricted with broader sluggish ice conditions. These spatially restricted regions of warm‐based subglacial regimes were likely controlled by surrounding and down‐flow ice streaming. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

From source to sink: Glacially eroded, Late Devonian algal “cysts” (Tasmanites) delivered to the Gulf of Mexico during the Last Glacial Maximum

Abstract The source of reworked Devonian algal “cysts” in last glacial maximum (LGM) sediment in the Gulf of Mexico is traced to their host black shales, which ring the southwestern Great Lakes. The source-to-sink pathway includes intermediate storage in fine-grained LGM glacial lacustrine sediment and till. The “cysts” are pelagic chlorophyllous algae (Tasmanites and Leiosphaeridia), collectively referred to herein as tasmanitids. Radiocarbon dates of syndepositional Gulf of Mexico foraminifera, derived from accelerator mass spectrometry, bracket the Gulf of Mexico sediment age with common tasmanitids from 28.5 ± 0.6–17.8 ± 0.2 cal kyr B.P. Approximately 1400 km north of the Gulf of Mexico, tasmanitids are abundant in Upper Devonian black shales (New Albany, Antrim, and Ohio Shales) that ring the Michigan, Illinois, and Appalachian intracratonic basins. Tasmanitids were eroded from bedrock and incorporated in glacial sediment dating from ca. 28.0–17.6 cal kyr B.P. by the Lake Michigan, and Huron-Erie lobes of the Laurentide Ice Sheet. The physical characteristics of tasmanitids are ideal for long-distance transport as suspended sediment (density: 1.1–1.3 g/cc, size ranging from 63 µm to 300 µm), and these sand-sized tasmanitids traveled with the silt-clay fraction. Thus, the source-to-sink journey of tasmanitids was initiated by subglacial erosion by water or friction, sequestering in till or glaciolacustrine sediment, re-entrainment and suspension in meltwater, and final delivery in meltwater plumes to the Gulf of Mexico. River routes included the Mississippi, Illinois, Ohio, Wabash, Kaskaskia, and many of their tributaries. Reworked Devonian tasmanitids are a previously unrecognized link between their occurrence in deep-water deposits of the Gulf of Mexico and the late Wisconsin glacial history of the Upper Mississippi Valley. We propose that tracking occurrences of tasmanitid concentrations from the source area to sink, along with adjunct proxies such as clay minerals, will facilitate a more refined analysis of the timing and duration of megafloods. This study also demonstrates that isotopically dead carbon, from reworked Devonian tasmanitid “cysts,” can contaminate radiocarbon dating of LGM bulk sediment samples toward older ages.

Isotopic Fingerprints of Ice‐Rafted Debris Offer New Constraints on Middle to Late Quaternary Arctic Circulation and Glacial History

AbstractFor the first time, ice‐rafted debris provenance related to the Quaternary glacial discharge and circulation was constrained using radiogenic Sr, Nd, and Pb isotopes from a sedimentary record (Core BN05) from the central Arctic Ocean. The record is composed of glacial/interglacial intervals estimated to span ~0.5 Ma (Marine Isotope Stages [MIS] 1–14). Nd versus Sr isotope data show two end member groups, further elaborated using Pb isotopes. Glacial data mostly point at North American sources with unradiogenic εNd and high 87Sr/86Sr values, including detrital carbonate (dolomite) layers originating from Paleozoic rocks eroded by the Laurentide Ice Sheet. The opposite end member group including estimated MIS6 and MIS4 samples indicates the Siberian Large Igneous Province eroded by the Eurasian Ice Sheet as the major source, possibly with additional inputs from the East Siberian Ice Sheet. The limited evidence for Siberian Ice Sheet provenance may result from the predominant distribution of this glaciation on the continental shelf providing weak, fine‐grained sediments. Glacial‐time western Arctic circulation inferred from isotopic data is mostly similar to the modern Beaufort Gyre but was more streamlined toward the Fram Strait during some glacial events. Interglacial/major interstadial, presumably sea ice transported sediments, shows a mixture of North American and Siberian sources, consistent with historically observed variations in the Transpolar Drift versus Beaufort Gyre circulation. Isotope pattern in major interglacials (e.g., MIS5e, MIS11, and MIS 13) resembles the recent trend of increased sediment ice transport from the East Siberian/Chukchi Sea, possibly in relation to the ongoing Arctic warming.

Ice retreat in Wilkes Basin of East Antarctica during a warm interglacial.

Efforts to improve sea level forecasting on a warming planet have focused on determining the temperature, sea level and extent of polar ice sheets during Earth's past interglacial warm periods1-3. About 400,000 years ago, during the interglacial period known as Marine Isotopic Stage 11 (MIS11), the global temperature was 1 to 2 degrees Celsius greater2 and sea level was 6 to 13 metres higher1,3. Sea level estimates in excess of about 10 metres, however, have been discounted because these require a contribution from the East Antarctic Ice Sheet3, which has been argued to have remained stable for millions of years before and includes MIS114,5. Here we show how the evolution of 234U enrichment within the subglacial waters of East Antarctica recorded the ice sheet's response to MIS11 warming. Within the Wilkes Basin, subglacial chemical precipitates of opal and calcite record accumulation of 234U (the product of rock-water contact within an isolated subglacial reservoir) up to 20 times higher than that found in marine waters. The timescales of 234U enrichment place the inception of this reservoir at MIS11. Informed by the 234U cycling observed in the Laurentide Ice Sheet, where 234U accumulated during periods of ice stability6 and was flushed to global oceans in response to deglaciation7, we interpret our East Antarctic dataset to represent ice loss within the Wilkes Basin at MIS11. The 234U accumulation within the Wilkes Basin is also observed in the McMurdo Dry Valleys brines8-10, indicating11 that the brine originated beneath the adjacent East Antarctic Ice Sheet. The marine origin of brine salts10 and bacteria12 implies that MIS11 ice loss was coupled with marine flooding. Collectively, these data indicate that during one of the warmest Pleistocene interglacials, the ice sheet margin at the Wilkes Basin retreated to near the precipitate location, about 700 kilometres inland from the current position of the ice margin, which-assuming current ice volumes-would have contributed about 3 to 4 metres13 to global sea levels.

Glacier expansion on Baffin Island during early Holocene cold reversals

The North Atlantic was a key locus for circulation-driven abrupt climate change in the past and could play a similar role in the future. Abrupt cold reversals, including the 8.2 ka event, punctuated the otherwise warm early Holocene in the North Atlantic region and serve as useful paleo examples of rapid climate change. In this work, we assess the cryospheric response to early Holocene climate history on Baffin Island, Arctic Canada, using cosmogenic radionuclide dating of moraines. We present 39 new 10Be ages from four sets of multi-crested early Holocene moraines deposited by cirque glaciers and ice cap outlet glaciers, as well as erratic boulders along adjacent fiords to constrain the timing of regional deglaciation. The age of one moraine is additionally constrained by in situ14C measurements, which confirm 10Be inheritance in some samples. All four moraines were deposited between ∼9.2 and 8.0 ka, and their average ages coincide with abrupt coolings at 9.3 and 8.2 ka that are recorded in Greenland ice cores. Freshwater delivery to the North Atlantic that reduced the flux of warm Atlantic water into Baffin Bay may explain brief intervals of glacier advance, although moraine formation cannot be definitively tied to centennial-scale cold reversals. We thus explore other possible contributing factors, including ice dynamics related to retreat of Laurentide Ice Sheet outlet glaciers. Using a numerical glacier model, we show that the debuttressing effect of trunk valley deglaciation may have contributed to these moraine-building events. These new age constraints and process insights highlight the complex behavior of the cryosphere during regional deglaciation and suggest that multiple abrupt cold reversals—as well as deglacial ice dynamics—likely played a role in early Holocene moraine formation on Baffin Island.

Surface and subsurface Labrador Shelf water mass conditions during the last 6000 years

Abstract. The Labrador Sea is important for the modern global thermohaline circulation system through the formation of intermediate Labrador Sea Water (LSW) that has been hypothesized to stabilize the modern mode of North Atlantic deep-water circulation. The rate of LSW formation is controlled by the amount of winter heat loss to the atmosphere, the expanse of freshwater in the convection region and the inflow of saline waters from the Atlantic. The Labrador Sea, today, receives freshwater through the East and West Greenland currents (EGC, WGC) and the Labrador Current (LC). Several studies have suggested the WGC to be the main supplier of freshwater to the Labrador Sea, but the role of the southward flowing LC in Labrador Sea convection is still debated. At the same time, many paleoceanographic reconstructions from the Labrador Shelf focussed on late deglacial to early Holocene meltwater run-off from the Laurentide Ice Sheet (LIS), whereas little information exists about LC variability since the final melting of the LIS about 7000 years ago. In order to enable better assessment of the role of the LC in deep-water formation and its importance for Holocene climate variability in Atlantic Canada, this study presents high-resolution middle to late Holocene records of sea surface and bottom water temperatures, freshening, and sea ice cover on the Labrador Shelf during the last 6000 years. Our records reveal that the LC underwent three major oceanographic phases from the mid- to late Holocene. From 6.2 to 5.6 ka, the LC experienced a cold episode that was followed by warmer conditions between 5.6 and 2.1 ka, possibly associated with the late Holocene thermal maximum. While surface waters on the Labrador Shelf cooled gradually after 3 ka in response to the neoglaciation, Labrador Shelf subsurface or bottom waters show a shift to warmer temperatures after 2.1 ka. Although such an inverse stratification by cooling of surface and warming of subsurface waters on the Labrador Shelf would suggest a diminished convection during the last 2 millennia compared to the mid-Holocene, it remains difficult to assess whether hydrographic conditions in the LC have had a significant impact on Labrador Sea deep-water formation.

A quasi-annual record of time-transgressive esker formation: implications for ice-sheet reconstruction and subglacial hydrology

Abstract. We identify and map chains of esker beads (series of aligned mounds) up to 15 m high and on average ∼ 65 m wide in central Nunavut, Canada, from the high-resolution (2 m) ArcticDEM. Based on the close 1 : 1 association with regularly spaced, sharp-crested ridges interpreted as De Geer moraines, we interpret the esker beads to be quasi-annual ice-marginal deposits formed time-transgressively at the mouth of subglacial conduits during deglaciation. Esker beads therefore preserve a high-resolution record of ice-margin retreat and subglacial hydrology. The well-organised beaded esker network implies that subglacial channelised drainage was relatively fixed in space and through time. Downstream esker bead spacing constrains the typical pace of deglaciation in central Nunavut between 8.1 and 6.8 cal kyr BP to 165–370 m yr−1, although with short periods of more rapid retreat (> 400 m yr−1). Under our time-transgressive interpretation, the lateral spacing of the observed eskers provides a true measure of subglacial conduit spacing for testing mathematical models of subglacial hydrology. Esker beads also record the volume of sediment deposited from conduits in each melt season, thus providing a minimum bound on annual sediment fluxes, which is in the range of 103–104 m3 yr−1 in each 6–10 km wide subglacial conduit catchment. We suggest that the prevalence of esker beads across this predominantly marine-terminating sector of the Laurentide Ice Sheet is a result of sediment fluxes that were unable to backfill conduits at a rate faster than ice-margin retreat. Conversely, we hypothesise that esker ridges form when sediment backfilling of the subglacial conduit outpaced retreat, resulting in headward esker growth close to but behind the margin. The implication, in accordance with recent modelling results, is that eskers in general record a composite signature of ice-marginal drainage rather than a temporal snapshot of ice-sheet-wide subglacial drainage.

Lake Agassiz drainage bracketed Holocene Hudson Bay Ice Saddle collapse

The duration and intensity of the early Holocene freshening events has been explained by subglacial outburst(s) from Lake Agassiz, the collapse of a Laurentide Ice Sheet ‘saddle’ overlying Hudson Bay, and/or a combination of the two events. Our field evidence provides new radiocarbon ages and geomorphologic observations to assess the deglacial history of this important region, allowing for revision of the sequence of events. We show that the collapse of the Hudson Bay Ice Saddle in southwestern Hudson Bay occurred between 8.57 ± 0.28 ka BP and 8.11 ± 0.19 ka BP. This event was preceded by at least one subglacial-drainage event through numerous newly-mapped subglacial channels onshore of southwestern Hudson Bay. Lake Agassiz may have experienced multiple subglacial drainage events prior to the final HBIS collapse, accounting for the timing discrepancies in freshwater cooling observed in the North Atlantic. Importantly, this new work links the chronology of events on the southwest (land-based) side of the HBIS to the northeast (marine-based) side of the ice sheet. Additionally, this work provides a potential analog for the behaviour of other ice sheets whose beds lie well below sea level, such as the West Antarctic Ice Sheet, during periods of warming climate.

Moraines and late-glacial stratigraphy in central Lake Superior

AbstractSeismic-reflection surveys of the Isle Royale sub-basin, central Lake Superior, reveal two large end moraines and associated glacial sediments deposited during the last cycle of the Laurentide Ice Sheet in the basin. The Isle Royale moraines directly overlie bedrock and are cored with dense, acoustically massive till intercalated down-ice with acoustically stratified outwash. Till and outwash are overlain by glacial varves, a lower red unit and an upper gray unit.The maximum extent of late Younger Dryas-age readvance into the western Lake Superior basin is uncertain, but it was probably controlled by both ice dynamics and climate. Our data indicate that during retreat from the maximum, the ice paused just long enough to construct the outer of the two moraines, >100 m high, and then retreated to the inner moraine, during which time most of the lower glacial-lacustrine sequence (red varves) was deposited. Retreat from the inner moraine coincided with a marked flux of icebergs at the calving margin and a change to gray varves. Rapid retreat may be related to both an influx of meltwater from Glacial Lake Agassiz about 10,500 cal yr BP and retreat of the calving margin down an adverse slope into the Isle Royale sub-basin.

Regional and global changes during Heinrich Event 1 affecting macrobenthic habitat: Ichnological evidence of sea-bottom conditions at the Galicia Interior Basin

Abstract Heinrich events (HEs) are climatic changes, occurring during the Pleistocene, related to massive discharge of freshwater from the Laurentide Ice Sheet, through the Hudson Strait, recognized in the sedimentary record through distinctive layers of ice-rafted detritus (IRD), the so-called Heinrich layers (HLs). Environmental changes during HEs influence significantly in marine biota (i.e. modifications in composition and abundance of assemblages), with ecological and evolutionary consequences. This point has been only partially addressed in particular groups (e.g. microorganisms), whereas interactions with other groups remain understudied. Here, we analyse ichnological features of the Heinrich layer 1, associated to Heinrich Event 1 (HE1), from several gravity cores at the Galicia Interior Basin (NW Iberian Peninsula) to test the influence of this HE1 on environmental parameters, such as bottom and pore-water oxygenation, as well as benthic food availability, conforming the macrobenthic habitat. Freshwater input during the first phase of the HE1 caused unfavorable conditions (probably highly dysoxic to anoxic) for tracemakers, as revealed by the absence of both discrete traces and a well-developed mottled background. However, the tracemaker community was reestablished shortly after deposition of the ice-rafted detritus layer (Heinrich layer) of HE1, as reflected by the significant increase in diversity and abundance of traces (Planolites, Thalassinoides, Thalassinoides-like, Chondrites, and Zoophycos), revealing a major shift to oxic bottom and pore waters and likely benthic food. This global/general pattern, though, is affected by the regional setting and by the associated predominant sedimentation processes, leading to a variable incidence of paleoenvironmental changes associated with HE1. The ecological impact on macrobenthic biota, in term of changes in diversity and abundance of tracemakers, by HE1 soon attenuates, resulting in a negligible evolutionary impact.

Comment on: Deglaciation of the Greenland and Laurentide ice sheets interrupted by glacier advance during abrupt coolings

We present new 10Be surface exposure ages from boulders on bedrock to directly date northwest Laurentide ice-sheet deglaciation through a wide swath of the western Canadian Shield that had no previous reliable temporal constraints on ice-margin retreat. Uplift-corrected boulder 10Be surface exposure ages are 13.9 ± 0.2 ka (n = 6) at a site on the western edge of the Slave Craton and 12.4 ± 0.2 ka (n = 5, 1 outlier) at a second site ∼110 km up-ice to the east. These direct 10Be ages for ice-margin retreat are ∼2.4 kyr and ∼1.6 kyr older, respectively, than the canonical deglacial chronology for the northwest Laurentide ice sheet that is based on minimum-limiting 14C dates. We infer an ice-margin retreat rate of 60–100 m yr−1 between the two sites over an interval spanning the transition from the Allerød warm period into the Younger Dryas cold period. This is significantly slower than the rapid >800 m yr−1 retreat rate for the northwest Laurentide ice sheet inferred from earlier deglacial chronologies, which has been hypothesized as a potential source of meltwater forcing for the Younger Dryas cold period. These first direct ages on spatio-temporal patterns of deglaciation in this data-poor region suggest that additional refinement of the deglacial chronology is needed to test hypotheses on the relation between ice-sheet retreat, associated meltwater discharge, abrupt climate change, and rapid sea-level rise.

Reply to Carlson (2020) comment on “Deglaciation of the Greenland and Laurentide ice sheets interrupted by glacier advance during abrupt coolings”

This Review quantitatively and qualitatively describes 10 representative Baffin Island fjords chosen to explore the nature of a glaciation in delivering sediment to a fjord, with focus on deglacial (within 13–6.7 kyr BP) and postglacial conditions (within 8.2–0 kyr BP). Fjords are compared using observed sediment thicknesses (49–175 m, averaged) and volumes (0.7–14.6 km3), and modeled sediment yields: (1) deglacial 140–1060 t/m2/yr; (2) paraglacial 45–350 t/m2/yr; (3) neoglacial 30–150 t/m2/yr; (4) Little Ice Age 70–440 t/m2/yr; and (5) Anthropocene 60–290 t/m2/yr. Modeled sediment loads, when converted to deposit volumes, compare favorably to observations from geophysical surveys and terrestrial mapping, or separately to sediment accumulation rates (SAR) determined from marine sediment cores. Fjord-averaged SAR during deglaciation range from 4.6 to 25 mm/yr, and 0.6–3.1 mm/yr for the postglacial interval. Fjords directly impacted by the Laurentide Ice Sheet (central and northern Baffin Island) are 2–3 times larger in dimensions than southern Baffin Island fjords, with ∼4.5x deglacial and ∼5x postglacial larger deposits, including well-defined ice-proximal outwash deposits. To the south, Penny Ice Cap associated fjords with no withholding sills, lose 30–40% of their incoming fjord sediment to Broughton Trough that dissects the continental shelf. Sunneshine is the only sampled fjord that was neither directly nor indirectly impacted by the Wisconsin continental ice sheet; Sunneshine responded instead to local maritime conditions and an alpine glaciation.

Last glacial maximum ecology and climate from terrestrial gastropod assemblages in Peoria loess, western Kentucky

ABSTRACTThe Rocks loess section, in unglaciated western Kentucky, provides a high‐resolution environmental record during the last glacial maximum onset. The Peoria Silt (9 m thick) contains 26 terrestrial gastropod species, with up to 15 species within a single 5 cm interval. Thirteen radiocarbon ages, using shells or charcoal, range between 30 and 24.5 cal ka; younger loess has been leached or eroded. Stratigraphic shifts in gastropod assemblages imply significant cooling, particularly ~27 cal ka, as solar insolation was decreasing and the southern Laurentide Ice Sheet rapidly advancing. Midwestern to southern species (e.g. Anguispira kochi, Gastrocopta pentodon, Hawaii miniscula, Helicodiscus parallelus, Vallonia perspectiva) occur only in the lowermost Peoria Silt (~30–27 cal ka). In contrast, cold‐tolerant species (Columella alticola, Vertigo modesta, Vallonia gracilicosta) occur only in full glacial Peoria Silt (27–24.5 cal ka). Inferred mean July temperatures, from mutual climatic range methods, range from ~23 °C at 30 cal ka, cooling to ~18 °C by 26 cal ka; about 3–8 °C cooler than today (~26 °C). Superimposed on this cooling trend are multi‐centennial variations in detrital carbonate, fossil shell concentrations, palaeotemperature estimates, and oxygen isotope values (Vertigo, Discus, Helicodiscus). The finer‐scale variations imply relatively synchronous fluctuations in glacial sediment supply, loess sedimentation, and climate.

Minimum limiting deglacial ages for the out-of-phase Saginaw Lobe of the Laurentide Ice Sheet using optically stimulated luminescence (OSL) and radiocarbon methods

AbstractTwenty-four new optically stimulated luminescence (OSL) and radiocarbon ages from sediment cores in nine lakes associated with the Shipshewana and Sturgis moraines in northern Indiana and southern Michigan estimate when recession of the Saginaw Lobe of the Laurentide Ice Sheet was underway in the southern Great Lakes region, USA. Average OSL ages of 23.4 ± 2.2 ka for the Shipshewana Moraine and 19.7 ± 2.2 ka for the Sturgis Moraine are considered minimum limiting deglacial ages for these recessional moraines. The much younger radiocarbon ages are consistent with other regional radiocarbon ages from lakes, and record climate amelioration around ~16.5 cal ka BP. Early recession of the interlobate Saginaw Lobe was well underway by 23.4 ± 2.2 ka, when the adjacent Lake Michigan and Huron-Erie lobes were a few hundred kilometers farther south and near their maximum southerly limits. The results provide the first time constraints when sediment from the Lake Michigan and Huron-Erie lobes began filling the accommodation space left by the Saginaw Lobe. The difference between the oldest radiocarbon and OSL age is 7400 yr for the Shipshewana Moraine and 3400 yr for the Sturgis Moraine.

Surface exposure dating of the Pierre Sublobe of the James Lobe, Laurentide Ice Sheet

AbstractThe Laurentide Ice Sheet of the last glacial period terminated in several lobes along its southern margin. The timing of maximum extent may have varied among the terminal lobes owing to internal ice sheet dynamics and spatially variable external controls. Some terminal ice lobes, such as the westernmost James Lobe, remain poorly dated. To determine the timing of maximum ice extent in this key location, we have mapped glacial deposits left by the Pierre Sublobe in South Dakota and applied 10Be surface exposure age dating on boulders on moraine ridges associated with three distinct late Quaternary glacial drifts. The oldest and most extensive “Tazewell” drift produced variable 10Be surface exposure ages spanning 20–7 ka; the large range is likely attributable to moraine degradation and subsequent boulder exhumation. The oldest ages of about 20 ka are probably limiting minimum ages for the Tazewell moraine surfaces. By contrast, exposure ages of the youngest “Mankato” drift of the easternmost Pierre Sublobe tightly cluster at about 16 ka. This age for the Pierre Sublobe is consistent with the nearby Des Moines Lobe, suggesting both acted together.

Extratropical cyclones over the North Atlantic and western Europe during the Last Glacial Maximum and implications for proxy interpretation

Abstract. Extratropical cyclones are a dominant feature of the midlatitudes, as their passage is associated with strong winds, precipitation and temperature changes. The statistics and characteristics of extratropical cyclones over the North Atlantic region exhibit some fundamental differences between pre-industrial (PI) and Last Glacial Maximum (LGM) climate conditions. Here, the statistics are analysed based on results of a tracking algorithm applied to global PI and LGM climate simulations. During the LGM, both the number and the intensity of detected cyclones were higher compared to PI. In particular, increased cyclone track activity is detected close to the Laurentide ice sheet and over central Europe. To determine changes in cyclone characteristics, the top 30 extreme storm events for PI and LGM have been simulated with a regional climate model and high resolution (12.5 km grid spacing) over the eastern North Atlantic and western Europe. Results show that LGM extreme cyclones were characterised by weaker precipitation, enhanced frontal temperature gradients and stronger wind speeds than PI analogues. These results are in line with the view of a colder and drier Europe, characterised by little vegetation and affected by frequent dust storms, leading to reallocation and build-up of thick loess deposits in Europe.

Chronology of advance and recession dynamics of the southern Green Bay Lobe of the Laurentide Ice Sheet, south-central Wisconsin, USA

AbstractWe used a combination of accelerator mass spectrometry (AMS) radiocarbon dating, optically stimulated luminescence (OSL) age estimates, and stratigraphic data from cores collected along the southern margin of the Green Bay Lobe (GBL) of the Laurentide Ice Sheet to provide new information on the timing and dynamics of the end of advance of the GBL and the dynamics of the ice sheet while very near its maximum position. Coring at multiple sites along the margin of the GBL indicate that ice had reached a stable position near its maximum extent by 24.7 ka; that ice advanced several kilometers to the Marine Isotope Stage 2 maximum position sometime shortly after 21.2 ka; and that ice remained at or beyond that position through the time interval represented by an OSL age estimate of 19.2 ± 3.2 ka. The timeline developed from these chronological data is internally consistent with, and further refines, AMS radiocarbon ages and OSL age estimates previously published for the southern margin of the GBL. It also provides new chronological control on the expansion of the GBL from its late Marine Isotope Stage (MIS) 3 extent to its MIS 2 maximum.

Early-Holocene simulations using different forcings and resolutions in AWI-ESM

The earliest part of the Holocene, from 11.5k to 7k (k = 1000 years before present), is a critical transition period between the relatively cold last deglaciation and the warm middle Holocene. It is marked by more pronounced seasonality and reduced greenhouse gases (GHGs) than the present state, as well as by the presence of the Laurentide Ice Sheet (LIS) and glacial meltwater perturbation. This paper performs experiments under pre-industrial and different early-Holocene regimes with AWI-ESM (Alfred Wegener Institute–Earth System Model), a state-of-the-art climate model with unstructured mesh and varying resolutions, to examine the sensitivity of the simulated Atlantic meridional overturning circulation (AMOC) to early-Holocene insolation, GHGs, topography (including properties of the ice sheet), and glacial meltwater perturbation. In the experiments with early-Holocene Earth orbital parameters and GHGs applied, the AWI-ESM simulation shows a JJA (June–July–August) warming and DJF (December–January–February) cooling over the mid and high latitudes compared with pre-industrial conditions, with amplification over the continents. The presence of the LIS leads to an additional regional cooling over the North America. We also simulate the meltwater event around 8.2k. Big discrepancies are found in the oceanic responses to different locations and magnitudes of freshwater discharge. Our experiments, which compare the effects of freshwater release evenly across the Labrador Sea to a more precise injection along the western boundary of the North Atlantic (the coastal region of LIS), show significant differences in the ocean circulation response, as the former produces a major decline of the AMOC and the latter yields no obvious effect on the strength of the thermohaline circulation. Furthermore, proglacial drainage of Lakes Agassiz and Ojibway leads to a fast spin-down of the AMOC, followed, however, by a gradual recovery. Most hosing experiments lead to a warming over the Nordic Sea and Barents Sea of varying magnitudes, because of an enhanced inflow from lower latitudes and a northward displacement of the North Atlantic deep convection. These processes exist in both of our high- and low-resolution experiments, but with some local discrepancies such as (1) the hosing-induced subpolar warming is much less pronounced in the high-resolution simulations; (2) LIS coastal melting in the high-resolution model leads to a slight decrease in the AMOC; and (3) the convection formation site in the low- and high-resolution experiments differs, in the former mainly over northeastern North Atlantic Ocean, but in the latter over a very shallow subpolar region along the northern edge of the North Atlantic Ocean. In conclusion, we find that our simulations capture spatially heterogeneous responses of the early-Holocene climate.

Preliminary Map Showing Quaternary Geology of the Mooresville East 7.5-Minute Quadrangle, Indiana

The Mooresville East 7.5-minute quadrangle is situated near the maximum limit of Wisconsin Episode glacial deposits in northeastern Morgan County. Till and outwash of Wisconsin Episode age and Holocene alluvium are the dominant surficial materials in the quadrangle. Wisconsin Episode eolian sand, loess, and colluvium are also present, with eolian sand and colluvium being adjacent to the West Fork White River valley and loess blanketing Wisconsin Episode till and outwash. Holocene (post-glacial) alluvium is found in the West Fork White River and White Lick Creek valleys and tributaries. Mississippian Borden Group bedrock controls many aspects of the surficial geology of the southeastern sector of the quadrangle. For much of this area, bedrock controls the topography and is less than 20 ft below the ground surface. It is mantled on the summits and shoulders of interfluves mainly by Wisconsin till and minor amounts of sand and gravel (outwash) and eolian sand. The depth to bedrock (Borden Group) is greater along the trend of the modern West Fork White River valley, with depths greater than 100 ft. This bedrock paleovalley is filled with mainly Wisconsin Episode outwash (sand and gravel), and the paleovalley was a major path for meltwater in central Indiana during the Wisconsin Episode, from around 27,000 to 19,000 years ago. The Wisconsin Episode maximum limit lies just outside the quadrangle boundary to the south and east. The Laurentide Ice Sheet reached its maximum limit 24,000 years ago, based on radiocarbon dating of organics within till at the terminal moraine in the adjacent Cope quadrangle. Evidence for a readvance of the ice sheet is found in the Mooresville East quadrangle, which occurred 21,700 years ago; the southern limit of the readvance runs through the central part of the quadrangle, just south of State Highway 144.