Jeffrey S. Munroe

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.

Development of a Spatial Analysis Method Using Ground-Based Repeat Photography to Detect Changes in the Alpine Treeline Ecotone, Glacier National Park, Montana, U.S.A

ABSTRACT Repeat photography is a powerful tool for detection of landscape change over decadal timescales. Here a novel method is presented that applies spatial analysis software to digital photo-pairs, allowing vegetation change to be categorized and quantified. This method is applied to 12 sites within the alpine treeline ecotone of Glacier National Park, Montana, and is used to examine vegetation changes over timescales ranging from 71 to 93 years. Tree cover at the treeline ecotone increased in 10 out of the 12 photo-pairs (mean increase of 60%). Establishment occurred at all sites, infilling occurred at 11 sites. To demonstrate the utility of this method, patterns of tree establishment at treeline are described and the possible causes of changes within the treeline ecotone are discussed. Local factors undoubtedly affect the magnitude and type of the observed changes, however the ubiquity of the increase in tree cover implies a common forcing mechanism. Mean minimum summer temperatures have increased by 1.5°C over the past century and, coupled with variations in the amount of early spring snow water equivalent, likely account for much of the increase in tree cover at the treeline ecotone. Lastly, shortcomings of this method are presented along with possible solutions and areas for future research.

Soil development along an elevational gradient in the southeastern Uinta Mountains, Utah, USA

Abstract A silt mantle enriched in Ca is important in buffering soils on quartzitic terrane in the southeastern Uinta Mountains. We examined 32 pedons along an elevational gradient on the south slope of the eastern Uintas to determine the amount of the silt and its importance to pedogenesis. The pedons were in the upper montane forest (2700–3000 m), subalpine forest (3000–3400 m) and alpine tundra (3400–3850 m) on stable sites. There were no significant differences in silt cap thickness or in profile quantities of silt (upper 100 cm) by ecoclimatic zone. Similar soils were found in all three ecoclimatic zones, including Eutrocryepts, Dystrocryepts, Haplocryalfs, Cryorthents, and Haplocryolls, in descending order of abundance. Profile quantities of clay, exchangeable cations, and extractable Al and solum thickness are greatest in the alpine zone, probably because the soils there are older than those at lower elevations and soil formation proceeds more rapidly. Base cycling is enhanced in alpine communities dominated by Acomastylis (Geum) rossii as evidenced by large profile quantities and high tissue concentrations of Ca. Profile quantities of total N and organic C are significantly greater (p≤0.05) in the alpine zone, reflecting greater below-ground production and lower rates of organic matter turnover than in soils at lower elevations.

Estimates of Little Ice Age Climate Inferred through Historical Rephotography, Northern Uinta Mountains, U.S.A

Abstract I replicated and analyzed six photographs taken in A.D. 1870 near the subalpine forest-alpine tundra ecotone in the northern Uinta Mountains to quantify changes in the distribution of vegetation. Three dramatic differences were noted. First, the historical photographs document a treeline 60 to 180 m (mean of ∼100 m) lower than at present, with greater depression on west-facing slopes. Given the modern lapse rate for mean July temperature (6.9°C km−1), this difference corresponds to a temperature depression in A.D. 1870 of 0.4 to 1.2°C (mean of 0.7°C). Second, timberline forests in A.D. 1870 were significantly (P < 0.01) less dense, with tree densities approximately half those measured in the modern photographs. Third, the area of floodplain meadows decreased ∼75% from A.D. 1870 to the present. Because the original photographs were taken within a few decades of the end of the Little Ice Age, ca. A.D. 1850, I assumed that differences in vegetation distribution documented in the repeat photographs represent the biotic response to climate warming over the past ∼130 yr. This analysis provides an independent estimate of the magnitude of growing season temperature depression during the Little Ice Age.

Last Glacial Maximum equilibrium-line altitudes and paleoclimate, northern Uinta Mountains, Utah, U.S.A.

Nineteen former valley glaciers were reconstructed for their Last Glacial Maximum (LGM) extents in the northern Uinta Mountains, Utah, U.S.A. Mean equilibrium-line altitudes (ELAs) calculated by four methods (accumulation-area ratio, toe-headwall altitude ratio, lateral moraines and cirque floors) range from 3050 to 3300 m a.s.l. Modern mean summer temperatures (T s ) at the ELAs range from 8.7° to 11.2°C, while modern winter precipitation (P) ranges from 354 to 590 mm snow water equivalent (SWE). Based on the difference in elevation of mean ELAs across the range, LGM P values must have ranged from 940 to 3040 mm SWE, assuming the modern summer lapse rate was the same during the LGM. A T s depression of 5.5°C is required for these precipitation values to plot in the range of modern ELA values. The reconstructed increase in P at the western end of the range is 10 times the modern increase, reflecting the influence of pluvial Lake Bonneville. Assuming ELA depression (ΔELA) resulted from this P increase and a uniform 5.5°C T s decrease, the regional LGM ΔELA was approximately 900 m.

Organic Carbon Pools and Genesis of Alpine Soils with Permafrost: A Review

Abstract Soils with mountain permafrost occupy 3.5 million km2 worldwide, with 70% in central Asia. High-mountain environments have “warm” permafrost, with surface permafrost temperatures of -0.5 to -2 °C and deep active layers (2 to 8 m). From a global database of 41 sites and 312 pedons, alpine soils with permafrost are strongly acid (pH = 5.0 to 5.5), have intermediate cation-exchange capacities (20 to 25 cmolc/kg) and base saturation (44% to 85%), and commonly have an isotic mineral class. Soil organic carbon is concentrated in the upper 30 to 40 cm, with profile density averaging 15.2 ± 1.3 kg m-2 (range = <1.0 to 88.3 kg m-2), which is comparable to temperate grasslands (13 kg m-2) but substantially less than moist arctic tundra (32 kg m-2). Mountain soils with permafrost contain 66.3 Pg of soil organic carbon (SOC), which constitutes 4.5% of the global pool. In contrast, the SOC pool in the Arctic is 496 Pg (33% of the global pool). Alpine soils with deep active layers contrast strongly with high-latitude soils in areas of continuous permafrost. Permafrost in the upper 2 m induces cryoturbation in the profile, acts as a barrier to water movement, and generates cooler temperatures resulting in greater SOC levels. High-elevation and high-latitude soils are experiencing warming of air temperature and permafrost and a thickening of the active layer.

Soil Development in Low-Arctic Tundra of the Northern Brooks Range, Alaska, U.S.A.

Thirty-six Gelisols were investigated in the vicinity of Galbraith Lake and Toolik Lake (68°30′N, 149°30′W) in northern Alaska. The soils formed on surfaces previously interpreted as ranging from ~110 ka BP to late Holocene in age. Sixteen of the profiles (44%) contained evidence of cryoturbation and were classified as Turbels, while the others were classified as Orthels. While most of the Orthels are developed in coarse-textured outwash, all of the Turbels are restricted to finegrained till and colluvium with ice-rich permafrost. There were no significant differences in profile quantities of weathering products, such as H+ ion, clay, silt, silt-plus-clay, and loss on ignition, between Orthels and Turbels. However, there were significant or nearly significant differences in profile quantities of H+ ion, silt, and silt-plus-clay as a function of age, with the lowest values in Holocene soils, intermediate values for the two younger drifts, and greater values for the oldest surface. A key finding of this study is that despite pervasive cryoturbation, soil properties can be used to determine relative age in the Low Arctic. Our results confirm previous age estimates that Itkillik II surfaces are of late Wisconsin age, with Itkillik I surfaces being somewhat older.

Latest Pleistocene history of pluvial Lake Franklin, northeastern Nevada, USA

At its maximum extent during the last glacial cycle, Lake Franklin covered 1100 km 2 of the Ruby Valley of northeastern Nevada, making it one of the largest pluvial lakes between Lakes Bonneville and Lahontan. Mapping of shorelines, surveying of topographic profiles, and radiocarbon dating of gastropod shells were employed to reconstruct the latest Pleistocene history of the lake. During the first half of the Last Glacial Maximum (LGM), Lake Franklin covered ∼42% of its maximum area. This extent increased to ∼60% during the second half of the LGM. Some radiocarbon ages suggest that the lake briefly rose to near its highstand between 20 and 18 ka, but the best constrained rise occurred ca. 17 ka, when the lake rapidly transgressed to its highstand elevation of 1850 m. This rise was synchronous with highstands of nearby pluvial lakes, implicating a regional shift in the balance between precipitation and potential evaporation. The lake dropped to 1843 m, before rising once more to 1850 m ca. 16.0 ka. After falling and stabilizing at 1843 m again ca. 15.4 ka, the lake rapidly regressed to 70%) by 14.8 ka. This regression was synchronous with the fall of Lake Bonneville from the Provo shoreline and the regression of Lake Lahontan from the Sehoo shoreline. Radiocarbon ages and stratigraphic evidence document a final transgression in the latest Pleistocene that reached 1820 m (34% of maximum area) ca. 13.0 ka, synchronous with the Recess Peak Glaciation in the Sierra Nevada, and overlapping with the start of the Younger Dryas and minor transgressions of Lakes Bonneville, Lahontan, and Owens. The correspondence of this Lake Franklin history with other climate archives from this region underscores the value of pluvial lake deposits as sources of paleoclimate information and indicates synchronous forcing of climate changes during the last glacial-interglacial transition.

Development of a Midge-Based Summer Surface Water Temperature Inference Model for the Great Basin of the Western United States

ABSTRACT Surface sediment recovered from 51 lakes in the Uinta Mountains of northeast Utah was analyzed for subfossil chironomid remains, and incorporated in a midge-based inference model for summer surface water temperature (SSWT). The lakes in the calibration set spanned elevation, depth, and summer surface water temperature ranges of 900 m, 12.7 m, and 11.3°C, respectively. Redundancy analysis (RDA) identified four variables, SSWT, depth, specific conductivity, and Al concentration, that could account for a statistically significant amount of variance in the chironomid distribution, with SSWT accounting for the largest amount of variance. The Uinta Mountain calibration set was merged with a previously developed calibration set from the Sierra Nevada, California, in order to develop a midge-based inference model for SSWT applicable to subfossil chironomid stratigraphies from the Great Basin. A variety of statistical approaches, such as weighted averaging (WA), weighted averaging-partial least squares (WA-PLS), and partial least squares (PLS) were used to assess model performance. The best inference model for SSWT, based on a 3-component WA-PLS approach, had robust performance statistics (r2jack = 0.66, RMSEP = 1.4°C). The newly expanded inference model will enable more accurate estimates of late Pleistocene and Holocene thermal regimes and help address many outstanding questions relating to long-term and recent climate change in this region.

Properties of Alpine Soils Associated with Well-Developed Sorted Polygons in the Uinta Mountains, Utah, U.S.A

ABSTRACT Twelve profiles of alpine soils associated with well-developed, but currently inactive, sorted polygons were investigated in the Uinta Mountains of northeastern Utah. The summit upland in the Uintas does not appear to have been glaciated, and these soils are considerably older than those located in lower elevation glacial valleys. Profiles of soils from the polygon centers reveal a dark, organic-rich surface horizon developed in loamy loess overlying a series of redder, sandy B horizons locally exhibiting strongly developed platey structure and pockets of coarser sediment. Irregular and broken horizons at depth reflect extensive cryoturbation during episodes when the sorted polygons are active. Overall morphology and profile quantities of weathering products in these soils are similar to those previously reported for alpine tundra soils in the Uintas; however, the cryoturbated horizons are limited to soils in the patterned ground. A developmental model for these soils emphasizes the combined role of pedogenic processes operating during interglaciations (accumulation of organic matter and loess, translocation of silt and clay, chemical weathering) and periods of cryoturbation (distortion of horizons, redistribution of weathering products within the solum). Outstanding questions include the timing and relative duration of cryoturbation episodes, the timing of loess deposition, and the overall age of the soils.