Gerald Osborn

Holocene glacier fluctuations in the middle Canadian Rocky Mountains

Abstract Holocene glacial advances in the Banff-Jasper-Yoho area of the Canadian Rocky Mountains have been extremely limited in extent. Limiting 14 C dates from two sites within 1 km of contemporary glaciers of fresh terminal moraines indicate that the late Wisconsin Ice Sheet and valley glaciers disappeared prior to 9660 yr B.P. Two subsequent glacial advances are recognized. The earlier Crowfoot Advance is represented by moraines and rock-glacier deposits overlain by Mazama ash (6600 yr B.P.) and is therefore early Holocene or possibly late Wisconsin in age. The late Neoglacial Cavell Advance of the last few centuries is dated by dendrochronology and lichenometry. In addition, there is fragmentary, undated evidence of intermediate-age advance(s), mainly from rock-glacier deposits. All these advances were of limited extent (1–2 km beyond present ice margins) and the Cavell Advance was usually the most extensive. Major exceptions to this pattern occur only where rock glaciers or extensive ice-cored moraines developed during the earlier advance(s?). These deposits were not overrun by glaciers during the Cavell Advance because of their relatively greater downvalley extent and the physical barrier they presented to subsequent glacial advances. Earlier work which postulated more extensive early Holocene advances in the Canadian Rocky Mountains is shown to have inadequate dating control: Many of the features previously attributed to older Holocene events are late Wisconsin in age.

Holocene glacier fluctuations in the Canadian Cordillera (Alberta and British Columbia)

Abstract Although the record of Holocene glacier fluctuations in British Columbia and Alberta is still very fragmentary, much recent progress has been made in their study. Dating of end moraines has been supplemented by studies of lateral moraine stratigraphy and dating of glacially-overridden organic matter upstream of end moraines. Dates from sites close to contemporary glaciers indicate that ice had retreated to about its present limits before 10 ka BP. Prior to deposition of Mazama tephra, a minor pre-Hypsithermal readvance (the ‘Crowfoot Advance’) left deposits in the Rockies and the interior of British Columbia. The earliest known Neoglacial advances occurred 5–6 ka BP in the Coast Mountains and shortly after 4 ka BP in the Rockies. An advance culminating ca. 4-3 ka BP has been reported from Bugaboo Glacier, while evidence of another advance culminating in the period ca. 2.5-1.8 ka BP (the ‘Tiedemann Advance’) is reported from more than one site. Following a recession, the Little Ice Age expansion (the ‘Cavell Advance’) began shortly after 900 BP. This advance culminated in most cases in the 18th or 19th centuries. The Crowfoot, Tiedemann, and Little Ice Age Advances were apparently of roughly the same magnitude. The latter was most extensive in most cases, but at several sites Crowfoot moraines lie slightly farther downvalley than Little Ice Age moraines. During the present century, ice-fronts have generally retreated (particularly rapidly in the period 1920–1950 A.D.), although rates of retreat have decreased in the last three decades and some glaciers have readvanced slightly.

Age of the Crowfoot advance in the Canadian Rocky Mountains: A glacial event coeval with the Younger Dryas oscillation

A suite of sediment core samples was recovered from two lakes, Crowfoot and Bow lakes, that are adjacent to the Crowfoot moraine type locality, to identify and radiocarbon date sediments related to the Crowfoot advance. The Crowfoot moraine system, widely recognized throughout northwestern North America, represents a glacial advance that is post-Wisconsin and pre-Mazama tephra in age. An interval of inorganic sediments bracketed by accelerator mass spectrometry radiocarbon ages of ca. 11,330 and 10,100 [sup 14]C yr B.P. is associated with the Crowfoot moraine. The Crowfoot advance is therefore approximately synchronous with the European Younger Dryas cold event (ca. 11,000-10,000 [sup 14]C yr B.P.). Furthermore, the termination of the Crowfoot advance also appears to have been abrupt. These findings illustrate that the climatic change responsible for the European Younger Dryas event extended beyond the northern Atlantic basin and western Europe. Equilibrium-line altitude (ELA) depressions associated with the Crowfoot advance are similar to those determined for the Little Ice Age advance, whereas Younger Dryas ELA depressions in Europe significantly exceed Little Ice Age ELA depressions. 26 refs., 3 figs., 1 tab.

Glaciation in the Great Basin of the Western United States

Abstract Forty individually named ranges, plateaus, and massifs draining wholly or partly into the Great Basin of the western United States show definite evidence of Pleistocene glaciation. The most obvious deposits are a family of moraines designated, among other names, “Tioga”, “Angel Lake”, and “Pinedale”. Such moraines generally can be traced from range to range away from described type moraines. These deposits have been numerically assigned to Late Wisconsinan glaciation in the Wasatch Range, White Mountains, Boulder Mountain, and Sierra Nevada on the basis of radiocarbon and surface-exposure ages, and have been assigned to Late Wisconsinan time in several other ranges on the basis of relative-age studies. The type Angel Lake moraine, and most other equivalent moraines across the Great Basin, are thick, hummocky, lobate piles of till rather than looping ridges. The thicknesses of the moraines (often 60+m) can be explained by heavy debris loads, and/or glacial advance, retreat, and readvance to the same positions a number of times, which is consistent with recent evidence that multiple Late Wisconsinan advances, possibly related to Heinrich and Dansgaard-Oeschger events, occurred in the Sierra Nevada. Pre-Angel Lake deposits occur in many Great Basin ranges, but it is currently difficult or perhaps impossible to determine if these deposits are equivalent to each other and what their relationship is to pre-Tioga deposits in the Sierra Nevada. Numerical ages are rare and relative-age studies suggest that pre-Angel Lake deposits may be products of more than one glaciation. Mapped pre-Angel Lake glaciers were longer than their Angel Lake counterparts, but the length differences do not translate into large differences in ELA depression. There is evidence of two minor latest Pleistocene or early Holocene advances in some ranges, judging from the presence of overlying Mazama tephra and/or weathering comparisons to local Angel Lake moraines. In the latter part of the Holocene, ELAs were sufficiently high that only the highest, wettest ranges developed Neoglacial glaciers. There does not appear to be a consistent pattern of latest Pleistocene/Holocene glacial fluctuations along an east–west transect through the Cordillera, or even through the Great Basin.

Potential glacial evidence for the younger dryas event in the Cordillera of North and South America

There is debate as to whether or not the Younger Dryas (YD) climatic event affected areas outside of the North Atlantic region. The potential of alpine glacial deposits to record the YD is of interest because alpine glaciers would be likely to leave a morainal record even during minor, short-lived advances caused by a period of deteriorating climate. In the Cordilleran ranges of North and South America there are several candidate moraines, many bracketed between radiocarbon ages of ca. 12 and 10 ka, which may be YD in age. The best examples are in the Canadian and American Rockies, the Ecuadorian, Peruvian and Bolivian Andes, and around the Patagonian Ice Cap. In most cases temporal brackets are not narrow enough and some radiocarbon ages may not be accurate enough to allow correlation at the present time. There are two cases of Lateglacial readvance whose ages are interpreted to match or fall within the YD interval: an advance of the Crowfoot Glacier in Canada is interpreted to have occurred within the period ca. 11.3 and 10 14C ka BP, and an advance of the Reschreiter Glacier in Ecuador is interpreted to have culminated in the period ca. 11 to 10.6 14C ka BP. The latter interpretation depends on an assumption that lake-level variations were caused by glacier advances and retreats.

Lateral-moraine stratigraphy and neoglacial history of Bugaboo Glacier, British Columbia

Abstract Most studies of alpine glacial history have concentrated on terminal moraines, but such moraines provide dates only for maximum extents of glaciers. Some lateral moraines have the potential for providing more complete glacial histories, because deposition by distal-flank accretion may result in sequences of stacked tills. Interpretation is aided by (1) the approximation that any continuous surface within the moraine parallel to the moraines distal flank is isochronous, (2) inset gulley-fill deposits, which provide information on ice-margin history, and (3) incorporated organic material which can be radiocarbon dated. Lateral moraine stratigraphy of Bugaboo Glacier in southeastern British Columbia suggests that the glacier was fairly close to its maximum Neoglacial extent about 2500 14C yr B.P. A slow general expansion punctuated by at least one minor retreat continued from that time and culminated probably in the last century.

Latest pleistocene and early Holocene fluctuations of glaciers in the Canadian and northern American Rockies

In the Canadian and northern American Rockies small-scale fluctuations of glaciers during and after late Wisconsinan deglaciation have been identified using stratigraphy of superimposed glacial deposits, geomorphology, and records of glacial advance in lake and peat sediments. Several authors have proposed that valley-glacier readvances of relatively modest scale followed the late Wisconsinan maximum. Evidence for some of these is difficult to evaluate from the literature and no deposits recording such readvances have been numerically dated. Several minimum-limiting radiocarbon dates for deglaciation suggest ice had retreated to within a few tens of km of present limits by ca. 12 ka BP, and to very near present limits by ca. 11 ka BP. One or more minor glacial advances, or stillstands during recession, evidently occurred in valley heads prior to 11 ka BP. Part of the evidence consists of moraines a few km downvalley from subsequently emplaced Little Ice Age and Crowfoot moraines. The Piper Lake moraine in the Mission Mountains of Montana is overlain by Glacier Peak G tephra, and thus has a minimum age of 11,200 14C years BP. The Crowfoot Advance, a minor advance of about the same magnitude as Little Ice Age advances, occurred between ca. 11,300 and 10,000 14C years BP and is thought to be related to the Younger Dryas climatic reversal. Crowfoot moraines have been found from Jasper National Park on the north to the Mission Mountains on the south. Crowfoot ELAs were an average of 40 m lower than modern ELAs around the Canada-U.S. border. There have been past proposals of significant early Holocene glacial advances in the Rockies, i.e. after Crowfoot time but before the LIA, but recent evidence indicates that glaciers then were less extensive than at present.

The impact of air mass circulation dynamics on Late Holocene paleoclimate in northwestern North America

Paleoclimate records from northern British Columbia, southwestern Yukon, and adjacent Alaska suggest that Late Holocene climate may have been influenced by specific air mass circulation dynamics. The Aleutian lowpressure index (ALPI) is a measure of sea level pressure fluctuations in the Pacific Northwest associated with the Aleutian low (AL) pressure system. In this study, we show that the AL has a strong influence on historical climate change in the study area and explore the relationships between ALPI polarity and changes in late Holocene paleoclimate records. Analyses of weather station data in the study area indicate positive correlations (r > 0.63) between mean wintertime (December– March) temperature and ALPI values; total wintertime snowfall accumulation and total precipitation show moderate and weak negative correlations, respectively. A Late Holocene increase in exotic western hemlock (Tsuga heterophylla) pollen has been observed in regional paleoclimate studies. A sustained positive ALPI phase during the late Holocene is considered as a causative mechanism. Under such conditions, warm maritime air masses would more frequently penetrate inland, potentially resulting in eastward pollen transfer, enhanced growing conditions at coastal sites, and an increase eastwards in the range limit of these species. This study indicates that apparent conflicts in the timing and magnitude of Late Holocene climate change may be the result of a strong regional climate-forcing mechanism that exhibits both temporal and geographical variation. r 2002 Elsevier Science Ltd and INQUA. All rights reserved.

Evaluation of proposed early-Holocene advances of alpine glaciers in the North Cascade Range, Washington State, USA: constraints provided by palaeoenvironmental reconstructions

Many palaeoenvironmental reconstructions from across western North America indicate that the early to mid-Holocene was warmer and drier than present. The wide distribution of these records suggests that relatively mild and arid conditions were regionally ubiquitous during the early Holocene. In contrast, two recently proposed advances of alpine glaciers in the Cascade Range of Washington State, corresponding to equilibrium-line altitude depressions of 350 to 450 m, are placed in the early Holocene. The juxtaposition of the proposed major expansion of early-Holocene alpine glaciers with the constraint provided by a number of palaeoenvironmental reconstructions from directly adjacent areas results in palaeoclimatic gradients that are untenable. The apparent diachronous behaviour of adjacent palaeoenvironmental records may be an artifact of dating errors.

A multi-proxy holocene record of environmental change fromthe sediments of Skinny Lake, Iskut region, northern British Columbia,Canada

A stratigraphic record from a lake in the Central Plateau Regionof northern British Columbia reveals changes in environment and inferredclimate during the Holocene. Upon deglaciation (ca. 11500 BP), Skinny Lakebecame an embayment of an ice-dammed lake. High clastic sedimentationrates, an unstable landscape, and cool, possibly wet conditions likelypersisted until the early Holocene (ca. 9000 BP). From ca. 9000–8300 BPdeclining lake levels coupled with warm and dry conditions resulted in theformation of a prominent marl bed. A colonizing shrub and herb assemblagepersisted from 9000 BP until about 8300 BP when it was replaced by a spruce(Picea) and subalpine fir (Abieslasiocarpa) forest under slightly cooler and moister conditions. Themiddle Holocene was warmer-than-present, however, decreasingtemperature and increasing precipitation trends characterize the period fromca. 6000 BP–3000 BP. The transition to modern climate at 3000 BP isevident primarily in the lithostratigraphic record and corresponds with theinitiation of the Tiedemann glacial advance (ca. 3300 BP) in thesouth-coastal mountains of British Columbia. A significant change infossil pollen occurs at ca. 2400 BP and is characterised by an increase in pinepollen accompanied by decreases in alder (Alnus), spruceand fir. This also coincides with an increase in west-sourced exoticwestern hemlock (Tsuga heterophylla) and cedar type(Cupressaceae) pollen possibly transported by regional changes in air masscirculation patterns associated with Aleutian Low dynamics. This studydemonstrates that both lithostratigraphic and biotic proxies are helpful inreconstructing the timing and nature of climate change and that each may havevarying sensitivities to a particular type of change.