Kurt A. Refsnider

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.

Rock Glaciers in Central Colorado, U.S.A., as Indicators of Holocene Climate Change

ABSTRACT We measured thalli diameters of the lichen Rhizocarpon subgenus Rhizocarpon on 48 individual lobes of 18 rock glaciers and rock glacier complexes in the Elk Mountains and Sawatch Range of central Colorado. Cumulative probability distribution and K-means clustering analyses were used to separate lichen thalli measurements into statistically distinct groups, each interpreted as representing a discrete episode of rock glacier activity driven by an interval of cooler climate. Lichen ages for these episodes were assigned using a growth curve developed for Rhizocarpon geographicum in the nearby Front Range. An early Neoglacial episode, ca. 3080 yr BP, is correlative to other glacial and periglacial activity in the southern Rocky Mountains and surrounding areas and broadly corresponds to an interval of climatic deterioration evident in several other proxies of Holocene climate. The younger two episodes, ca. 2070 and 1150 yr BP, are also coeval with regional (Audubon) glacial and periglacial activity but are thus far not widely recognized in other climate proxies.

Subglacial carbonates constrain basal conditions and oxygen isotopic composition of the Laurentide Ice Sheet over Arctic Canada

Subglacially precipitated carbonate crusts (SPCCs) formed on bedrock and till boulder surfaces adjacent to the Barnes Ice Cap (BIC), central Baffin Island, Arctic Canada, act as unique archives of Laurentide Ice Sheet basal conditions. Uranium-series dating of these features reveals that carbonate precipitation from subglacial meltwater occurred during the Last Glacial Maximum (LGM), requiring warm-based ice in the region at that time. However, the preservation of fragile SPCCs is unlikely beneath erosive warm-based ice, suggesting that the transition to subsequent cold-based conditions took place shortly after the LGM, and glacial erosion in the region occurred dominantly prior to the LGM. The oxygen isotopic composition of the meltwater from which the SPCCs precipitated is indistinguishable from that of the debris-rich BIC basal ice (δ 18 O −24‰ referenced to Vienna standard mean ocean water), but distinct from that of the overlying white Pleistocene ice (δ 18 O ∼−35‰), demonstrating that SPCCs are reliable archives of the isotopic composition of only the basal ice of past ice sheets.

Chronology of the Last Glacial Maximum in the Upper Bear River Basin, Utah

ABSTRACT The headwaters of the Bear River drainage were occupied during the Last Glacial Maximum (LGM) by outlet glaciers of the Western Uinta Ice Field, an extensive ice mass (∼685 km2) that covered the western slope of the Uinta Mountains. A well-preserved sequence of latero-frontal moraines in the drainage indicates that outlet glaciers advanced beyond the mountain front and coalesced on the piedmont. Glacial deposits in the Bear River drainage provide a unique setting where both 10Be cosmogenic surface-exposure dating of moraine boulders and 14C dating of sediment in Bear Lake downstream of the glaciated area set age limits on the timing of glaciation. Limiting 14C ages of glacial flour in Bear Lake (corrected to calendar years using CALIB 5.0) indicate that ice advance began at 32 ka and culminated at about 24 ka. Based on a Bayesian statistical analysis of cosmogenic surface-exposure ages from two areas on the terminal moraine complex, the Bear River glacier began its final retreat at about 18.7 to 18.1 ka, approximately coincident with the start of deglaciation elsewhere in the central Rocky Mountains and many other alpine glacial localities worldwide. Unlike valleys of the southwestern Uinta Mountains, deglaciation of the Bear River drainage began prior to the hydrologic fall of Lake Bonneville from the Provo shoreline at about 16 ka.

Glacial Geology and Equilibrium Line Altitude Reconstructions for the Provo River Drainage, Uinta Mountains, Utah, U.S.A.

ABSTRACT The Provo River drainage in the western end of the Uinta Mountains was glaciated repeatedly during the Pleistocene, and glacial deposits from the Smiths Fork and Blacks Fork glaciations (Pinedale and Bull Lake equivalents, respectively) are well preserved throughout the area. Reconstruction of the Smiths Fork ice extent based on air photo analysis and field mapping reveals that the broad upland surfaces in the Provo River drainage were covered by an ice field from which distributary glaciers emanated. This ice field also covered parts of the Weber and Bear River drainages to the north and the North Fork Duchesne drainage to the east. Equilibrium line altitudes for glaciers in the Provo River drainage were ∼2900 m a.s.l., consistent with previous studies which recognized a dramatic decrease in equilibrium line altitudes toward the western end of the range. End moraine sequences and hypsometric differences between glaciers in the Provo River drainage suggest that ice retreat rates likely differed considerably among the glaciers, reflecting variable dynamic responses to a similar climate forcing during deglaciation.

Variation in glacier length and ice volume of Rabots Glaciar, Sweden, in response to climate change, 1910–2003

Abstract Historical records, photographs, maps and measurements were used to determine changes in the length, geometry and volume of Rabots Glaciär, Sweden, in response to a ∼1°C warming that occurred early in the 20th century. The glacier’s initial rate of retreat from its 1910 maximum was ~2.0 m a–1. After a sharp increase to ∼11.7 m a–1 between 1933 and 1946, the mean retreat rate decreased to ∼5.5 m a-1 between 1946 and 1959. Thereafter the rate of retreat increased to ∼11.0 m a-1 and has remained relatively constant to the present time. Concomitant decreases in ice volume were estimated to be 77.3 × 106m3 between 1910 and 1959, 51.1 × 106m3 between 1959 and 1980, at least 10.4 × 106m3 between 1980 and 1989, and 14.4 × 106m3 between 1989 and 2003. The total volume change over the last 93 years is estimated at ∼153.2 × 106m3 corresponding to 1.6 × 106m3a–1.. The magnitude of the ongoing changes in length and volume suggests that Rabots Glaciär has not yet completed its response to the earlier climatic warming. In contrast, several nearby glaciers, most notably Storglaciären, have completed their adjustments and established new steady-state profiles as a result of having shorter response times.

The projected demise of Barnes Ice Cap: Evidence of an unusually warm 21st century Arctic

As a remnant of the Laurentide Ice Sheet, Barnes Ice Cap owes its existence and present form in part to the climate of the last glacial period. The ice cap has been sustained in the present interglacial climate by its own topography through the mass balance-elevation feedback. A coupled mass balance and ice-flow model, forced by Coupled Model Intercomparison Project Phase 5 climate model output, projects that the current ice cap will likely disappear in the next 300 years. For greenhouse gas Representative Concentration Pathways of +2.6 to +8.5 Wm−2, the projected ice-cap survival times range from 150 to 530 years. Measured concentrations of cosmogenic radionuclides 10Be, 26Al, and 14C at sites exposed near the ice-cap margin suggest the pending disappearance of Barnes Ice Cap is very unusual in the last million years. The data and models together point to an exceptionally warm 21st century Arctic climate.