David M. Mickelson

Were deforming subglacial beds beneath past ice sheets really widespread

Abstract The concept of widespread, large-strain deformation of subglacial unconsolidated, unfrozen, sediments during Pleistocene glaciations has inconsistencies in the geological record. Numerous properties of tills and related sediments are difficult to reconcile with pervasive strains, as predicted by the deforming bed theory. While accepting glacier-bed deformation as a geological process occurring under certain circumstances, we propose that it was much less widespread than believed by some. Instead, we believe that basal sliding and englacial transport are major ice movement and debris-transport mechanisms.

A numerical investigation of ice-lobe-permafrost interaction around the southern Laurentide ice sheet

Permafrost existed around and under marginal parts of the southern Laurentide ice sheet during the Last Glacial Maximum. The presence of permafrost was important in determining the extent, form and dynamics of ice lobes and the landforms they produced because of influences on resistance to basal motion and subglacial hydrology. We develop a two-dimensional time-dependent model of permafrost and glacier-ice dynamics along a flowline to examine: (i) the extent to which permafrost survives under an advancing ice lobe and how it influences landform development and hydrology, and (ii) the influence of permafrost on ice motion and surface profile. The model is applied to the Green Bay lobe, which terminated near Madison, Wisconsin, during the Last Glacial Maximum. Simulations of ice advance over permafrost indicate that the bed upstream of the ice-sheet margin was frozen for 60-200 km at the glacial maximum. Permafrost remained for centuries to a few thousand years under advancing ice, and penetrated sufficiently deep (tens of meters) into the underlying aquifer that drainage of basal meltwater became inefficient, likely resulting in water storage beneath the glacier. Our results highlight the influence of permafrost on subglacial conditions, even though uncertainties in boundary conditions such as climate exist.

Sedimentology and hydrogeology of two braided stream deposits

Abstract Two approaches were used to quantify the spatial distribution of hydrofacies in braided stream deposits. One approach involved mapping a 50 by 60 by 3.3 m section of a proximal braided stream deposit. In a second study, we generated a 400 by 400 by 2.6 m section of a medial braided stream deposit using a computer model. In both cases we produced three-dimensional images showing connected hydrofacies with high permeabilities that form preferential flow paths. This information was input to a groundwater flow model and flow paths were analyzed by following the transport of imaginary particles. In both systems, particles that were uniformly distributed at the up-gradient end of the model clustered along preferential flow paths during transport, showing that connection among high-permeability facies is a critical factor in hydrogeological investigations involving assessment of contaminant movement and remediation.

Evidence of early Holocene glacial advances in southern South America from cosmogenic surface-exposure dating

Cosmogenic nuclide surface-exposure dating reveals that glaciers in southern South America (46°S) advanced at ca. 8.5 and 6.2 ka, likely as a result of a northward migration of the Southern Westerlies that caused an increase in precipitation and/or a decrease in temperature at this latitude. The older advance precedes the currently accepted initiation of Holocene glacial activity in southern South America by ~3000 yr. Both of these advances are temporally synchronous with Holocene climate oscillations that occurred in Greenland and the rest of the world. If there are causal links between these events, then rapid climate changes appear to be either externally forced (e.g., solar variability) or are rapidly propagated around the globe (e.g., atmospheric processes).

Evidence against pervasively deformed bed material beneath rapidly moving lobes of the southern Laurentide Ice Sheet

Abstract During the Wisconsin Glaciation, the ice lobes of the southern part of the Laurentide Ice Sheet moved rapidly as the result of elevated subglacial water pressure. The rapid movement was probably not accompanied by wide-spread pervasive deformation of unfrozen material under the ice. The till stratigraphy of much of this area is well known, and it is clear that the stratigraphic sequence is generally intact. Therefore, such deformation, if it occurred, would have been restricted to the till sheet that was currently being deposited. However, this seems unlikely as well, because the till of the region commonly contains lenses and clasts of unlithified bedded sand that should have been destroyed by pervasive shearing. If unfrozen deformed-till layers were widely present, they must have been confined to the thin upper parts of till sheets.

Patagonian Glacier Response During the Late Glacial–Holocene Transition

Whether cooling occurred in the Southern Hemisphere during the Younger Dryas (YD) is key to understanding mechanisms of millennial climate change. Although Southern Hemisphere records do not reveal a distinct climate reversal during the late glacial period, many mountain glaciers readvanced. We show that the Puerto Bandera moraine (50°S), which records a readvance of the Southern Patagonian Icefield (SPI), formed at, or shortly after, the end of the YD. The exposure age (10.8 ± 0.5 thousand years ago) is contemporaneous with the highest shoreline of Lago Cardiel (49°S), which records peak precipitation east of the Andes since 13 thousand years ago. Absent similar moraines west of the Andes, these data indicate an SPI response to increased amounts of easterly-sourced precipitation—reflecting changes in the Southern Westerly circulation—rather than regional cooling.

Late Wisconsin landform distribution and glacier-bed conditions in Wisconsin

Abstract The late Wisconsin Laurentide Ice Sheet advanced across permafrost and reached its maximum extent in Wisconsin between about 18,000 and 15,000 years ago. Deep permafrost persisted in southern Wisconsin until about 14,000 years ago and in northern Wisconsin until about 13,000 years ago. We suggest that during maximum glaciation a zone about 5 km wide in the south and 20 km wide in the north along the margin of the late Wisconsin glacier was frozen to its bed. Meltwater from farther behind the margin, where the bed was at least locally thawed, cut a series of closely spaced tunnel channels through the frozen-bed zone. These channels most likely formed episodically, and they were the source for much of the meltwater-stream sediment deposited in broad outwash plains beyond the ice margin. Frozen-bed conditions near the margin also likely contributed to increased upward shearing of sediment and the accumulation of thicl supraglacial sediment in northern areas. Up ice from the frozen-bed zone the glacier bed was at least locally thawed in a zone about 75 km wide. Extensive drumlin fields formed in the area of the bed that was thawed. By about 13,000 years ago permafrost melted in northern Wisconsin and thawed-bed conditions probably extended to the ice margin throughout Wisconsin and adjacent areas. After about 13,000 years ago in northern Wisconsin the glacier was sliding on its bed and forming drumlins out to the ice margin, and thick supraglacial sediment no longer accumulated.

The southern Laurentide Ice Sheet

Publisher Summary This chapter discusses the advances in the understanding of the southern Laurentide Ice Sheet (LIS) and its deposits since the publication of the Quaternary of the United States in 1965. The late Wisconsin southern LIS consists of thin, gently sloping lobes with low-driving stresses. The southernmost lobes have a wet bed to the margin and surges are probably common. Ice lobes in lowlands may be fed by ice streams. Not until after the late glacial maximum does ice warm to the margin. There are profound differences in the distribution and character of landforms, such as moraines, drumlins, and tunnel channels, and differences are likely to be in subglacial processes, with a soft-deforming bed occurring in places and sliding dominating in others. The extent of early Wisconsin and mid-Wisconsin ice is now thought to have been less extensive than previously interpreted. A wide array of global climate records shows that the LIS responds to climate changes and may also cause changes in climate because of the discharges of melt water and icebergs into the North Atlantic.

Greatlakean Substage: A replacement for Valderan Substage in the Lake Michigan basin

Abstract New evidence from recent field and seismic investigations in the Lake Michigan basin and in the type areas of the Valders, Two Creeks and Two Rivers deposits necessitates revision of late-glacial ice-front positions, rock- and time-stratigraphic nomenclature and climatic interpretations and deglaciation patterns for the period ca. 14,000–7,000 radiocarbon years B.P. The previously reported and long accepted pattern of deglaciation for the Lake Michigan basin started with a regular retreat from the Lake Border Morainic System, with a minor oscillation marked by the Port Huron moraine(s) and then an extensive Twocreekan deglaciation followed by a major (320 km) post-Twocreekan advance (Valders). However, we now record a major retreat between the times of the Lake Border and Port Huron moraines, followed by a gradual retreat from the Port Huron limit and interrupted by a minor standstill (deposition of Manitowoc Till), a retreat (Twocreekan) and a readvance (Two Rivers Till). No Woodfordian or younger readvance was as extensive as had been the preceding one. This sequence argues for a normal, climatically controlled, progressive deglaciation rather than one interrupted by a major post-Twocreekan (formerly Valderan) surge. This revision appears finally to harmonize the geologic evidence and the palynological record for the Great Lakes region. Our investigations show that Valders Till from which the Valderan Substage was named is late-Woodfordian in age. We propose the term “Greatlakean” as a replacement for the now misleading time-stratigraphic term “Valderan”. The type section and the definition of the upper and lower boundaries of the Greatlakean Substage remain the same as those originally proposed for the Valderan Substage but the name is changed.

Latest Pleistocene advance of alpine glaciers in the southwestern Uinta Mountains, Utah, USA: Evidence for the influence of local moisture sources

Cosmogenic surface-exposure 10 Be dating of Last Glacial Maximum (LGM) moraines indicates that glaciers in the southwestern Uinta Mountains remained at their maximum positions until ca. 16.8 0.7 ka, 2 k.y. after glaciers in the neighboring Wind River Range and Colorado Rockies began to retreat. The timing of the local LGM in the south- western Uintas overlaps with both the hydrologic maximum of Lake Bonneville and pre- liminary estimates of the local LGM in the western Wasatch Mountains. This broad syn- chroneity indicates that Lake Bonneville and glaciers in northern Utah were responding to similar climate forcing. Furthermore, equilibrium line altitudes (ELAs) for reconstruct- ed LGM alpine glaciers increase with distance from the Lake Bonneville shoreline, rising from 2600 m to 3200 m over the 120 km length of the glaciated Uintas. This pro- nounced ELA gradient suggests that the magnitude of the latest Pleistocene glacial advance in the western Uintas was due, at least in part, to enhanced precipitation derived from Lake Bonneville; thus, the lake acted as a local amplifier of regional climate forcing. This relationship underscores the sensitivity of alpine glaciers to moisture availability during the latest Pleistocene, and further demonstrates the importance of local moisture sources on glacier mass balance.