James B. Benedict

Chronology of cirque glaciation, Colorado front range

Abstract Moraines and rock glaciers in Front Range cirques record at least four, and possibly five, intervals of Holocene glacier expansion. The earliest and most extensive was the Satanta Peak advance, which deposited multiple terminal moraines near present timberline shortly before 9915 ± 165 BP. By 9200 ± 135 BP, timberline had risen to at least its modern elevation; by 8460 ± 140 BP, patterned ground on Satanta Peak moraines had become inactive. Although a minor ice advance may have occurred just prior to 7900 ± 130 BP, there is no evidence that glaciers or perennial snowbanks survived in the Front Range during the “Altithermal” maximum (ca. 6000–7500 BP), or during a subsequent interval of alpine soil formation (ca. 5000–6000 BP). Glaciers were larger during the Triple Lakes advance (3000–5000 BP) than at any other time during Neoglaciation. Minimum ages of 4485 ± 100 BP, 3865 ± 100 BP, and ca. 3150 BP apply to a threefold sequence of Triple Lakes deposits in Arapaho Cirque. After an important interval of soil formation and cavernous weathering, glaciers and rock glaciers of the Audubon advance (950–1850 BP) reoccupied many cirques, and perennial snowbanks blanketed much of the area above present timberline; although the general Audubon snow cover had begun to melt from valley floors by 1505 ± 95 BP, expanded snowbanks lingered on tundra ridge crests until 1050–1150 BP, and glaciers persisted is sheltered cirques until at least 955 ± 95 BP. Following a minor interval of ice retreat, glaciers of the Arapaho Peak advance (100–300 BP) deposited multiple moraines in favorably oriented cirques. Interpretation of Holocene glacial deposits in the Southern and Central Rocky Mountains has been hampered by (1) a heavy reliance upon relative-dating criteria, many of which are influenced by factors other than age; (2) the assumption that glacial advances in high-altitude cirques can be correlated directly with alluvial deposition in far-distant lowlands; and (3) the assumption that glacial advances have necessarily been synchronous throughout the Rocky Mountain region and the world. Although Holocene glacier fluctuations in the Front Range are believed to reflect changes in regional climate, the Front Range chronology does not have particularly close analogs in other parts of North America. Better-dated local sequences are needed before the hypothesis of global synchroneity can be adequately evaluated; until synchroneity has been proven, long-distance correlations and worldwide cycles of recurring glaciation will remain unconvincing.

Frost creep and gelifluction features: A review

Abstract Frost creep and gelifluction are the cold-climate representatives of mass-wasting processes that occur in a broad range of environments. Neither process requires permafrost, and frost creep can be inhibited by its presence at shallow depth. Acting in various combinations, frost creep and gelifluction produce distinctive lobate and terrace-like landforms, which are easy to recognize while fresh and active, but difficult to distinguish from mudflow lobes, earthslides, and similar deposits after they have been modified by other processes. Large frost creep and gelifluction features are currently active in many tundra environments that experience only deep seasonal freezing; thus they are not generally considered to be indicators of permafrost. Most radio-carbon-dated lobes and terraces, however, seem to have originated at times when permafrost was more widespread than it is today. This is true in the Colorado Front Range, where the formation of lobes and terraces appears to have been initiated by rapid melting of ice-enriched permafrost during the warming phases of frost-heave cycles that were centuries or millennia in duration. There is growing evidence that lobes and terraces developed in many parts of the world between about 3000 and 2500 BP; the climatic significance of their formation during this interval is open to several interpretations. Long-term average rates of frontal advance, calculated for deposits in Colorado, Australia, Greenland, Yukon Territory, Alaska, Scotland, and Norway, range from 0.6 to 3.5 mm per calendar year, significantly slower than maximum rates of movement measured on the surfaces of active lobes and terraces in comparable environments; the features are clearly not as effective at transporting debris as was previously supposed. Variations in past rates of downslope soil movement, estimated from close-interval dating of buried humus horizons or plant remains overrun by the advancing fronts of lobes and terraces, provide a sensitive record of climatic change. The dated humus layers are also suitable for detailed pollen analyses and soil chronosequence studies.

LICHEN MORTALITY DUE TO LATE-LYING SNOW: RESULTS OF A TRANSPLANT STUDY

Lines of lichen-covered rocks were installed on the floors of two semipermanent snowpatches above timberline in the Colorado Front Range. All Rhizocarpon subgenus Rhizocarpon thalli died within 5 to 8 yr where the average annual duration of snowcover exceeded 40.3 to 42.7 wk. Thalli of snow-tolerant Rhizocarpon species (especially R. riparium) survived for the duration of the study where snow cover was less persistent, and where meltout occurred by 1 July in occasional years. Lichen mortality is attributed to depletion of thallus carbon reserves in late spring and summer, when meltwater was continuously available at the base of the snowpack, light penetration was negligible, and thalli respired but were unable to photosynthesize. Fast-growing lichen species such as Umbilicaria virginis and Xanthoria elegans were less tolerant of late-lying snow than were slow-growing species such as Lecidea atrobrunnea and R. riparium; species-related differences in susceptibility to snowkill make interspecific diameter ratios unreliable. The lichen-free areas that surround modern Front Range snowbanks are products of the 1950s, when snow accumulation was greater than today. Earlier episodes of massive lichen kill on the floors of Front Range alpine valleys may have resulted from warm (0?C) subnival winter temperatures rather than from very late-lying general snow cover, as was previously believed.

Pedogenic gradients for iron and aluminum accumulation and phosphorus depletion in arctic and alpine soils as a function of time and climate

Pedogenically significant chemical extracts of iron, aluminum, and phosphorus are related mainly to time in chronosequences from Baffin Island (Canadian Arctic), the alpine Sierra Nevada and Wind River Range (western United States), the alpine Khumbu Glacier area (Himalaya, Nepal), and the alpine Southern Alps (New Zealand). Based on the accumulation index for Fe and Al, and the depletion index for P in each region, the following ranking of chemical pedogenic development is obtained: Southern Alps > Khumbu Glacier area ≅ Wind River Range > Sierra Nevada > coastal Baffin Island > inland Baffin Island. The ranking mainly follows regional climate, with the greatest accumulation and depletion to the warmest and wettest environment, and the least accumulation and depletion in the coldest and driest environment.

Experiments on Lichen Growth II. Effects of a Seasonal Snow Cover

Seasonal growth of the foliose lichens Xanthoparmelia lineola and X. subdecipiens was monitored for 26 mo at two transplant localities in a montane meadow on the East Slope of the Colorado Front Ra...

Experiments on LICHEN growth. 1. Seasonal patterns and environmental controls

Radial growth of mature Xanthoparmelia lineola and X. subdecipiens thalli (n = 10 thalli, 170 lobes) was measured bimonthly for 2 yr at a transplant locality on the East Slope of the Colorado Front Range (altitude 2,565 m). Growth occurred during every measurement period, but was most rapid in May, June, July, and August. Ninety-three percent of seasonal growth-rate variation during the first year of the study, and 92% during the second year, are explained by differences in the duration of daytime, snow-free thallus moisture (moisture available for photosynthesis); the strength of the relationship suggests that a positive net carbon assimilation rate is translated almost immediately into lobe elongation. No negative correlation was found between radial growth and thallus moisture recorded at night or beneath snow. In the cool, continental environment of the study area, duration of photosynthetically significant thallus moisture is influenced more strongly by air temperature, day length, and snow cover than by total precipitation. At higher altitudes and latitudes, where lichenometry is used for dating, these factors will have even greater importance.

USE OF SILENE ACAULIS FOR DATING: THE RELATIONSHIP OF CUSHION DIAMETER TO AGE

Growth-rate curves for Silene acaulis, a pioneer tundra cushion-plant species, were constructed for the Front Range of north-central Colorado by (1) measuring the diameters of 27 plants on moraines at the front of Arapaho Glacier in 1985 and 1987, and (2) measuring the largest cushions on seven historically dated substrates 9 to 82 yr old. Both curves are sigmoidal. Growth begins slowly, accelerates to a maximum rate of 2 to 3 cm yr-1, and then decreases. A lifespan of a century or less makes Silene acaulis unsuitable for dating Little Ice Age moraines in Front Range cirques. The species, however, can provide minimum age estimates for landforms and erosional features younger than 75 to 100 yr.

A Review of Lichenometric Dating and its Applications to Archaeology

Lichenometry—a method developed by geologists for dating Holocene moraines and other landforms—has many potential applications in archaeology. Maximum-diameter lichenometry can suggest ages for features that were initially lichen-free, such as the moai of Easter Island, and rock surfaces exposed by toolstone quarrying. Size-frequency analysis can provide dates for structures built of lichen-covered rocks, such as game-drive walls and blinds, meat caches, and tent rings. Both methods require local calibration curves, best constructed by measuring lichens on substrata of known exposure age. Most lichenometric studies have involved yellow members of the crustose genus Rhizocarpon, which grow slowly and can live for as long as 10,000 years. Lichenometry has been particularly successful on siliceous rock types in arctic, subarctic, and alpine-tundra environments. The effects of wildfire and of competition from foliose lichens make the technique less well suited for forested terrain. Few data are available for tropical or desert environments or for calcareous substrata. The reliability of a lichenometric date will depend on the quality of the calibration curve, the size of the sample, the nature and postoccupational history of the substratum, and the ability of the archaeologist to recognize potential disturbance factors. An ecological perspective is essential. Known archaeological applications and problems are discussed.

Techniques in Lichenometry: Identifying the Yellow Rhizocarpons

Species of Rhizocarpon subgenus Rhizocarpon are distinguished from each other based on differences in external thallus morphology, internal anatomy of fruiting bodies, and chemistry of intracellular and extracellular lichen substances. Differences in geographical distribution and ecology aid in identifying certain species. Using the procedures described in this paper, and keys such as that of J. Poelt (1988, this volume), it should be possible for a lichenometrist with no previous taxonomic experience to identify rhizocarpons to the section level with little difficulty, and to the species and subspecies levels under many circumstances. In regions where the subgenus has been inadequately studied (e.g., the Rocky Mountains of western North America), specimens having intermediate characteristics may prove difficult to identify using existing keys.

Footprints in the Snow: High-Altitude Cultural Ecology of the Colorado Front Range, U.S.A.

Prehistoric human occupation of the Front Range summit region was influenced by a unique combination of environmental factors: (1) a location at the headwaters of four major river systems in the se...