Central Rocky Research Articles

Climatic and tectonic controls on Late Triassic to Middle Jurassic sedimentation in northeastern Guangdong Province, South China

Stratigraphic analyses document climatic and tectonic controls on the filling of a Late Triassic to early Middle Jurassic (T3–J2) basin that developed on top of a young orogenic belt in southeastern South China. About 2700m of Carnian to Bajocian sedimentary rocks is documented in the Meizhou region, Guangdong Province. The Carnian to Rhaetian sequence is characterized by deltaic facies that are succeeded by Hettangian fluvial, shallow marine and volcaniclastic facies, and by Sinemurian to early Toarcian interdistributary bay and floodplain facies. The late Toarcian to Bajocian sequence comprises proximal alluvial to lacustrine facies that changed upwards to fluvial facies. Fossil assemblages indicate that climatic conditions changed from tropical/subtropical warm humid, to temperate humid, and then to hot arid through the Late Triassic to the Middle Jurassic. Climatically induced changes (e.g., in precipitation, vegetation and erosion) exerted a strong influence on sediment supply, whereas tectonics played a dominant role in stratigraphic evolution, accommodation generation, sedimentation pattern and volcanism. Tectonostratigraphic analysis shows that the T3–J2 basin was initiated on an orogenic belt during late-stage orogeny, and evolved into shallow-marine and volcanic environments and then back to terrestrial facies during the post-orogenic stage. This was followed by regional uplift and the development of a basin-and-range province. The order of these events is similar to that of the central Rocky Mountains, western North America during the Palaeogene. The Mesozoic basin of South China and the Eocene basins of the central Rocky Mountains highlight the importance of subduction-related subsidence above young and broad orogens.

Late Holocene expansion of ponderosa pine (Pinus ponderosa) in the Central Rocky Mountains, USA

AbstractAimPonderosa pine (Pinus ponderosa) experienced one of the most extensive and rapid post‐glacial plant migrations in western North America. We used plant macrofossils from woodrat (Neotoma) middens to reconstruct its spread in the Central Rocky Mountains, identify other vegetation changes coinciding with P. ponderosa expansion at the same sites, and relate P. ponderosa migrational history to both its modern phylogeography and to a parallel expansion by Utah juniper (Juniperus osteosperma).LocationCentral Rocky Mountains, Wyoming and Montana, and Black Hills, Wyoming and South Dakota, USA.MethodsPlant macrofossils were analysed in 90 middens collected at 14 widely separated sites in the northern part of the range of P. ponderosa var. scopulorum. Middens with and without P. ponderosa were 14C dated to pinpoint time of appearance at each site. Sensitivity experiments using a bioclimatic model were used to evaluate potential climatic drivers of late Holocene expansion.ResultsPinus ponderosa colonized the Black Hills region by at least 3850 yr bp (all ages given in calendar years before present). It expanded into the eastern Bighorn Mountains of northern Wyoming by 2630 yr bp, quickly spreading north in the western Bighorns from 1400 to 1000 yr bp. Concurrent with the latter expansion, P. ponderosa spread c. 350 km to the Little Belt and Big Belt Mountains in western Montana, establishing its northern limit and the modern introgression zone between var. scopulorum and var. ponderosa. Expansion in the Central Rockies of P. ponderosa involved two known haplotypes.Main conclusionsPinus ponderosa expanded its range across large parts of northern Wyoming and central Montana during the late Holocene, probably in response to both northward and westward increases in summer temperature and rainfall. The underlying climatic driver may be the same as for the contemporaneous expansion of J. osteosperma, but will remain undetermined without focused development and integration of independent palaeoclimate records in the region.

Provenance and Depositional Ages of Late Paleogene Fluvial Sedimentary Rocks In the Central Rocky Mountains, U.S.A.

Abstract The erosion and basin-filling histories of the central Rocky Mountains (Rockies) are critical to the understanding of landscape evolution in the interior of western U.S.A. Ages and sources of fluvial deposits formed during late Paleogene allow the reconstruction of paleogeography within an improved chronostratigraphic framework. Here we use detrital-zircon and single-grain-zircon U-Pb geochronology to constrain the provenance and depositional ages of upper Paleogene fluvial deposits in the central Rockies. A total of 885 concordant detrital-zircon dates from ten fluvial sandstone samples show major age populations of 24–44 Ma, 920–1326 Ma, 1334–1825 Ma, and 1830–3314 Ma, and minor age populations of 45–218 Ma and 227–708 Ma. The youngest population was derived from distal ignimbrite eruptions in western and southwestern North America. The populations older than 45 Ma were derived directly from local Precambrian basement-cored uplifts and recycled from Paleozoic–lower Cenozoic strata. The maximum depositional ages based on the weighted mean U-Pb ages of the youngest clusters of detrital zircons are generally consistent with the available ash radiometric ages for samples of latest Eocene–Oligocene, a time interval with high ignimbrite eruption flux in western and southwestern North America, suggesting that detrital-zircon U-Pb ages can be used to tightly constrain the depositional ages of these strata when pure ash beds are not present and when the strata contain abundant concurrent magmatic zircons. Sandstone samples of latest Eocene age have more Archean zircons (> 2500 Ma) than samples of Oligocene age in the central Rockies, suggesting that the Archean cores of the Laramide ranges were largely exposed until the Oligocene. By combining these results with previously published detrital-zircon provenance results of late Paleogene eolian sandstones in the same region, we suggest that the gradual burial of the Archean cores during the Oligocene resulted from widespread eolian deposition initiated during the latest Eocene–early Oligocene. Generally eastward-flowing fluvial systems and eolian depositional processes supplied unlithified source sediments for each other and promoted sediment recycling during the Oligocene. Furthermore, the abundance of zircons with 37–25 Ma ages in the late Paleogene deposits shows that the ignimbrite eruptions in western and southwestern North America were most active during this time interval.

A World at Risk: Aggregating Development Trends to Forecast Global Habitat Conversion.

A growing and more affluent human population is expected to increase the demand for resources and to accelerate habitat modification, but by how much and where remains unknown. Here we project and aggregate global spatial patterns of expected urban and agricultural expansion, conventional and unconventional oil and gas, coal, solar, wind, biofuels and mining development. Cumulatively, these threats place at risk 20% of the remaining global natural lands (19.68 million km2) and could result in half of the world’s biomes becoming >50% converted while doubling and tripling the extent of land converted in South America and Africa, respectively. Regionally, substantial shifts in land conversion could occur in Southern and Western South America, Central and Eastern Africa, and the Central Rocky Mountains of North America. With only 5% of the Earth’s at-risk natural lands under strict legal protection, estimating and proactively mitigating multi-sector development risk is critical for curtailing the further substantial loss of nature.

Limber pine in the central and southern Rocky Mountains: Stand conditions and interactions with blister rust, mistletoe, and bark beetles

Combined effects of the recent mountain pine beetle (Dendroctonus ponderosae) outbreak and white pine blister rust (caused by Cronartium ribicola) (WPBR) are causing extensive crown dieback and mortality in limber pine (Pinus flexilis) in the central and southern Rocky Mountains. Additional stressors such as climate change and limber pine dwarf mistletoe (Aceuthobium cyanocarpum) may significantly alter stand structure and biodiversity of these ecosystems. The objectives of this study were to: (1) assess site, stand, and tree health conditions in limber pine stands of northern Colorado, Wyoming, and southeastern Montana, (2) quantify impacts of WPBR, bark beetles, and dwarf mistletoe, and (3) determine biotic and abiotic factors that influence the occurrence and incidence of these damage agents.We assessed 22,700 limber pines on 508 plots in limber pine-dominated stands in twenty-five study areas in northern Colorado, Wyoming, and southeastern Montana. Average live limber pine density was 311stemsha−1. Overall, 50% of limber pines were classified as healthy with over 50% of limber pine mortality attributed to bark beetle attacks. White pine blister rust damage was evident on over 60% of declining or dying limber pines. Blister rust was the primary damage agent, occurring on 73% of the plots and 26% of the trees. Bark beetle-caused mortality was found on 75% of plots and 18% of trees. Limber pine dwarf mistletoe was present on 29% of plots and 9% of trees. In study areas that were previously monitored, incidence of WPBR increased over 8–9years by 6%, bark beetle-caused mortality by 17%, while dwarf mistletoe incidence remained the same. We used statistical modeling to determine meteorological, macro and micro site factors, and stand factors that influenced the occurrence and incidence of WPBR, bark beetles, and dwarf mistletoe on limber pines.Declines in health and rise in mortality due to bark beetles and WPBR has left some study areas with low limber pine basal area and stand density. These stands may be at risk for localized extirpation if the minimum viable population thresholds are not maintained and are identified as areas of concern for restoration and conservation efforts. Current condition status and long-term monitoring of limber pine is needed for land manager decision making and facilitation of restoration goals.

The Neoproterozoic succession of the central Rocky Mountains, Canada

Abstract The Neoproterozoic succession exposed in the central Canadian Rocky Mountains comprises a thick (2 to 5 km) sequence of marine clastic rocks deposited after major subsidence associated with continental scale rifting. A major transverse south to north change in lithofacies separates the succession into southern more distal and northern more proximal sequences. The southern sequence (Miette Group) comprises the sandstone-conglomerate dominated McKale Formation (new) overlain by the argillite and siltstone dominated East Twin Formation (new). Locally, black pyritic slate and limestone of the Cushing Creek Formation (new) occurs at the base. The Cushing Creek Formation was deposited in a deep euxinic basin with only minor sandstone turbidite deposition. The lower part of the McKale Formation was deposited in basin-floor turbidite complexes, whereas the upper part, with its thick interlayered argillite intervals and uncommon slump structures, was deposited in lower slope environments. A thick varicoloured argillite unit separates the lower and upper McKale Formation in northeastern exposures. This unit, mappable into the lower part of the Framstead Formation and correlative with the Old Fort Point Formation, records an abrupt reduction in the supply of clastic material associated with the termination of glaciation. The East Twin Formation represents a transition from slope sedimentation at the base to shallow-marine sedimentation at the top. It records the reduction of the supply of coarse clastic material into the basin and the filling of the Neoproterozoic basin. The northern sequence (Misinchinka Group) comprises in succession: diamictite of the Vreeland Formation (new); argillite with local carbonate olistoliths, medial sandstone to conglomerate and, in the east, basal interbedded sandstone and argillite of the Framstead Formation (new); carbonate of the Chowika Formation; and argillite and siltstone of the Cut Thumb Formation (new). Locally interbedded siltstone, argillite and sandstone of the Paksumo Formation (new) occur at the base of the exposed succession. The Paksumo, Vreeland Formation and basal part of the Framstead Formation record glacially influenced slope sedimentation. The Paksumo Formation and basal part of the Framstead Formation are primarily turbidite deposits, whereas diamictites of the Vreeland Formation are primarily resedimented mass flow and glaciogenic ‘rain-out’ deposits. The lower part of the Framstead Formation records an abrupt reduction in the supply of clastic material associated with a post-glacial eustatic sea level rise. Large olistoliths of shallow-water carbonate in both the lower and upper parts of the Framstead Formation indicate deposition in a slope environment and instability of the adjacent carbonate platform. Sandstone to conglomerate turbidite-filled channels in the middle part of the Framstead Formation record a relative sea level drop and a temporary breakdown of the adjacent carbonate platform. The return to fine clastic slope sedimentation in the upper part of the Framstead Formation records a relative sea level rise. The overlying Chowika carbonate platform records a dramatic reduction in clastic sediment supply and a relative sea level drop. The abruptly overlying Cut Thumb Formation represents a relative sea level rise and a change to deeper water siliciclastic shelf sedimentation that is not reflected in the south. Tectonism near the end of Neoproterozoic Windermere sedimentation resulted in the inversion of part of the initial deep basin-floor to form an outboard high on the west and in the reactivation of the transverse facies-change zone, as a north-side-down rather than south-side-down feature.

Characterization of 13 microsatellite markers for Calochortus gunnisonii (Liliaceae) from Illumina MiSeq sequencing.

Premise of the study:Microsatellite primers were designed for Calochortus gunnisonii (Liliaceae), a montane lily species of the central and southern Rocky Mountains, using next-generation DNA sequencing. The markers will be used to investigate population structure, genetic diversity, and demographic history.Methods and Results:Thirteen polymorphic microsatellite loci were isolated from C. gunnisonii using Illumina MiSeq next-generation DNA sequencing and bioinformatic screening. The mean number of alleles per locus ranged from 4.15 to 5.92 (avg. = 4.97). Observed and expected heterozygosity ranged from 0.077 to 0.871 and 0.213 to 0.782, respectively. The primers were also tested for cross-species amplification value with C. flexuosus, C. nuttallii, C. kennedyi var. kennedyi, and C. subalpinus.Conclusions:These primers will be useful for genetic and evolutionary studies across C. gunnisonii’s range within the southern and central Rocky Mountains. Furthermore, these markers have proven valuable for cross-species amplifications within Calochortus.

Effects of temperature and precipitation on snowpack variability in the Central Rocky Mountains as a function of elevation

AbstractWe employ a regression‐based methodology to study the impact of temperature and precipitation on snowpack variability as a function of elevation in the Central Rocky Mountains. Because of the broad horizontal coverage and thermal heterogeneity of the measurement sites employed, we introduce an elevation correction based on the sites' climatological temperature. For the elevation range investigated (1295–2256 m), and assuming an average atmospheric lapse rate of −6.5°C/km, we find a mostly linear relationship between effective elevation and correlation of temperature or precipitation with snow water equivalent and snowpack duration. We estimate a threshold elevation, 1560 ± 120 m, below (above) which temperature (precipitation) is the main driver of the snowpack. This threshold elevation is robust under a range of assumed atmospheric lapse rates. Locations below this elevation are likely to be affected by projected rising temperatures, with important effects on ecosystems and economic activities dependent on snow.

MODELING STREAMFLOW AND WATER QUALITY SENSITIVITY TO CLIMATE CHANGE AND URBAN DEVELOPMENT IN 20 U.S. WATERSHEDS.

Watershed modeling in 20 large, United States (U.S.) watersheds addresses gaps in our knowledge of streamflow, nutrient (nitrogen and phosphorus), and sediment loading sensitivity to mid-21st Century climate change and urban/residential development scenarios. Use of a consistent methodology facilitates regional scale comparisons across the study watersheds. Simulations use the Soil and Water Assessment Tool. Climate change scenarios are from the North American Regional Climate Change Assessment Program dynamically downscaled climate model output. Urban and residential development scenarios are from U.S. Environmental Protection Agency’s Integrated Climate and Land Use Scenarios project. Simulations provide a plausible set of streamflow and water quality responses to mid-21st Century climate change across the U.S. Simulated changes show a general pattern of decreasing streamflow volume in the central Rockies and Southwest, and increases on the East Coast and Northern Plains. Changes in pollutant loads follow a similar pattern but with increased variability. Ensemble mean results suggest that by the mid-21st Century, statistically significant changes in streamflow and total suspended solids loads (relative to baseline conditions) are possible in roughly 30-40% of study watersheds. These proportions increase to around 60% for total phosphorus and total nitrogen loads. Projected urban/residential development, and watershed responses to development, are small at the large spatial scale of modeling in this study.

If Long-Term Resistance to a Spruce Beetle Epidemic is Futile, Can Silvicultural Treatments Increase Resilience in Spruce-Fir Forests in the Central Rocky Mountains?

Within the Central Rocky Mountains, spruce beetle populations have the potential to rapidly transition from endemic to epidemic levels in the spruce-fir (Engelmann spruce and subalpine fir) forest type. Conventional management has focused on creating resistance to spruce beetle outbreaks by manipulating the overstory density and composition. Three silvicultural treatments, single tree selection, group selection, and shelterwood with reserves, were established in a spruce-fir forest in northern Utah with the goals of increasing both resistance and resilience to outbreaks. Resistance and resilience metrics were explicitly defined. Pre-harvest and two post-harvest measurements were used to assess how the different silvicultural treatments influenced the metrics. The shelterwood with reserves was the only treatment to meet both the resistance and resilience criteria. This treatment, while not traditionally used, created a stand structure and composition that will be most resilient to climate induced increases in spruce beetle caused tree mortality. However, there will be a trade-off in composition and structure, especially Engelmann spruce, after a spruce beetle epidemic because the created structure is more uniform with fewer groups and gaps than commonly observed in spruce-fir forests. With changing climatic conditions, proactive forest management, such as the shelterwood with reserves in the spruce-fir forest type, is the best method for increasing short-term resistance and long-term resilience to spruce beetle outbreaks.

Effects of glacier meltwater on the algal sedimentary record of an alpine lake in the central US Rocky Mountains throughout the late Holocene

The effects of alpine glaciers on the hydrology, physical features, and biogeochemistry of lakes have been investigated over contemporary time scales. However, the influence of these factors on algal communities over longer time scales remains unclear, yet is critical to paleolimnological interpretation of environmental change in alpine regions. We examined sedimentary algal pigments and fossil diatom assemblages in two proximal lakes with equivalent local climates, one glacier-fed and one snow-fed, in the central Rocky Mountains (USA) to determine how glacier meltwater has altered algal records over the last 3,000 years. Differences between the records of the two lakes intensified during the Medieval Climate Anomaly and the Little Ice Age, with the glacier-fed lake exhibiting an overall increase in fossil algal pigment concentrations and greater diatom assemblage turnover. Starting 1,000 years ago, the glacier-fed lake in this study showed evidence of nitrogen enrichment from glacier meltwater, as indicated by increasing relative abundances of Asterionella formosa and, to a lesser extent, Fragilaria crotonensis. Since the Little Ice Age, diatom species richness declined in the glacier-fed lake, and further decreased following the 1950s, while assemblage turnover increased. These results demonstrate that glaciers can strongly alter the algal sedimentary record and should be considered when interpreting high-elevation lake records.

Middle to late Cenozoic cooling and high topography in the central Rocky Mountains: Constraints from clumped isotope geochemistry

The timing and mechanism of the formation of the high elevations of the central Rocky Mountains (Rockies) in the western interior of North America remain controversial. We examine the middle and late Cenozoic paleorelief between the Great Plains and the central Rockies using clumped isotope geothermometry of carbonate cements of fluvial and eolian sandstones. Our petrographic observations suggest that the studied carbonate cements were formed primarily during eodiagenesis in near-surface conditions and retain primary paleoclimate and paleoelevation signals. Carbonate clumped isotope temperatures decrease ∼10°C, and calculated water δO18 values decrease ∼4‰ from ∼35Ma to ∼5Ma in both the central Rockies and the western Great Plains, following middle and late Cenozoic global cooling trend. A persistent longitudinal gradient in temperature and δO18 value existed between the central Rockies and the Great Plains during the Oligocene and Miocene, with temperatures 4–8 °C higher, and calculated water δO18 values 3–6‰ higher in the Great Plains. These observations suggest the mean elevation of the central Rockies was ∼1km higher than the western Great Plains during the middle and late Cenozoic, similar to the present-day regional relief. When placed in the context of other paleoaltimetry studies and geological observations, our findings support the hypothesis that the high mean topography of the Rockies was developed during the late Eocene, possibly related to isostatic adjustment, or dynamic uplift caused by foundering of lower mantle lithosphere or the Farallon slab, or both.

Estimates of large magnitude Late Cambrian earthquakes from seismogenic soft‐sediment deformation structures: Central Rocky Mountains

AbstractA variety of unusual early post‐depositional deformation structures exist in grainstone and flat‐pebble conglomerate beds of Upper Cambrian strata, western Colorado, including slide scarps, thrusted beds, irregular blocks and internally deformed beds. Thrusted beds up to tens of centimetres thick record thrust movement of a part of a bed onto itself along a moderate to steeply inclined (15° to 40°) ramp, locally producing hanging wall lenses with fault‐bend geometries. Thrust plane orientations are widely distributed, and in some cases nearly oppositely oriented in close proximity, indicating that they did not form as failures acted upon by gravity forces. Irregular bedded to internally deformed blocks are isolated on generally flat upper bedding surfaces. These features represent parts of beds that detached, moved up onto and some distances across, the laterally adjacent undisturbed bed surfaces. Deformation of thin intervals of mud on the ocean floor by moving blocks rules out the possibility of storm‐induced deformation, because the mud was not eroded by high shear stresses that would accompany the extremely large forces required to produce and move the blocks. Finally, internally deformed beds are characterized by large blocks, fitted fabrics of highly irregular fragments and contorted lamination, which represent heterogeneous deformation, such as brecciation and liquefaction. The deformation structures were produced by earthquakes linked to the reactivation of Mesoproterozoic, crustal‐scale shear zones in the central Rockies during the Late Cambrian. Analysis of the deformation structures indicates very large body forces and calculated earthquake‐generated ground motion velocities of ca 1·6 m sec−1. These correspond to moment magnitudes of ca 7·0 or more and a Mercalli Intensity of X+. These are the only known magnitude estimates of Phanerozoic (other than Quaternary) large‐intensity earthquakes for the Rocky Mountain region, and they are as large as, or larger than, previous estimates of Proterozoic earthquakes along these major shear zones of the central Rockies.

Influence of ecological and geological features on rangewide patterns of genetic structure in a widespread passerine.

Geological and ecological features restrict dispersal and gene flow, leading to isolated populations. Dispersal barriers can be obvious physical structures in the landscape; however microgeographic differences can also lead to genetic isolation. Our study examined dispersal barriers at both macro- and micro-geographical scales in the black-capped chickadee, a resident North American songbird. Although birds have high dispersal potential, evidence suggests dispersal is restricted by barriers. The chickadee's range encompasses a number of physiological features which may impede movement and lead to divergence. Analyses of 913 individuals from 34 sampling sites across the entire range using 11 microsatellite loci revealed as many as 13 genetic clusters. Populations in the east were largely panmictic whereas populations in the western portion of the range showed significant genetic structure, which often coincided with large mountain ranges, such as the Cascade and Rocky Mountains, as well as areas of unsuitable habitat. Unlike populations in the central and southern Rockies, populations on either side of the northern Rockies were not genetically distinct. Furthermore, Northeast Oregon represents a forested island within the Great Basin; genetically isolated from all other populations. Substructuring at the microgeographical scale was also evident within the Fraser Plateau of central British Columbia, and in the southeast Rockies where no obvious physical barriers are present, suggesting additional factors may be impeding dispersal and gene flow. Dispersal barriers are therefore not restricted to large physical structures, although mountain ranges and large water bodies do play a large role in structuring populations in this study.

High-elevation paleoenvironmental change during MIS 6–4 in the central Rockies of Colorado as determined from pollen analysis

Paleoecological studies from Rocky Mountain (USA) high elevations encompassing the previous interglacial (MIS 5e) are rare. The ~10-m composite profile from the Ziegler Reservoir fossil site (2705m asl) of central Colorado allows us to determine paleoenvironments from Marine Oxygen Isotope Stages (MIS) 6– 4 using pollen zones that are approximately equivalent to marine oxygen isotope stages. During Pollen Zone (PZ) 6 time, pollen assemblages dominated by Artemisia (sagebrush) suggest that alpine tundra or steppe occurred nearby. The transition to PZ 5e was characterized by a rapid increase in tree pollen, initially Picea (spruce) and Pinus (pine) but also Quercus (oak) and Pseudotsuga menziesii (Douglas-fir). Non-arboreal pollen (NAP) types increased during PZ 5d, while Abies (fir) and Juniperus (juniper) increased during PZ 5c. Pollen evidence suggests that temperatures during PZ 5b were as cold as during PZ 6, with the site again surrounded by alpine tundra. Picea dominated during PZ 5a before the onset of cooler conditions during PZ 4. The MIS 6–MIS 5e transition here was similar to the MIS 2–MIS 1 transition at other Rocky Mountain sites. However, the Ziegler Reservoir pollen record contains evidence suggesting unexpected climatic trends at this site, including a warmer-than-expected MIS 5d and cooler-than-expected MIS 5b.

Middle Cenozoic uplift and concomitant drying in the central Rocky Mountains and adjacent Great Plains

When and how the central Rocky Mountains (Rockies) of western North America gained modern topography remain controversial questions. We reconstruct the middle and late Cenozoic topography along a transect that extends from the Great Plains of western Nebraska across the central Rockies in Wyoming, based on reconstructed surface-water δ D values from volcanic glass δ D values. Our data show gradual increases of surface-water δ D values in both the central Rockies and adjacent Great Plains during middle and late Cenozoic time, and the establishment of a similar-to-present surface-water δ D gradient between the central Rockies and western Great Plains before earliest Oligocene time. These observations suggest that the region underwent differential uplift to form relief similar to that of today before earliest Oligocene time. This uplift has caused regional drying and the gradual increase of surface-water δ D values over the past 35 m.y. When placed in the context of other paleoaltimetry studies, our work suggests that along our transect, the central Rockies and adjacent Great Plains underwent uplift during the late Eocene, and have not undergone any large-magnitude (>∼500 m) uplift since that time.

Not in warfare alone: Conical timber lodges in the Central Rocky Mountains and Northwestern Plains

Conical timber lodges are a well known but little investigated archaeological feature of the Central Rocky Mountains and Northwestern Plains. These perishable wood structures are commonly regarded as war lodges or short-term shelters built by Plains Indian groups during the Protohistoric and Historic periods. Archaeological, ethnohistorical, and ethnographic data show that this interpretation is too narrow. Conical timber lodges served a diversity of purposes as temporary shelters, special activity sites, and domiciles.

Divergence with gene flow within the recent chipmunk radiation (Tamias).

Increasing data have supported the importance of divergence with gene flow (DGF) in the generation of biological diversity. In such cases, lineage divergence occurs on a shorter timescale than does the completion of reproductive isolation. Although it is critical to explore the mechanisms driving divergence and preventing homogenization by hybridization, it is equally important to document cases of DGF in nature. Here we synthesize data that have accumulated over the last dozen or so years on DGF in the chipmunk (Tamias) radiation with new data that quantify very high rates of mitochondrial DNA (mtDNA) introgression among para- and sympatric species in the T. quadrivittatus group in the central and southern Rocky Mountains. These new data (188 cytochrome b sequences) bring the total number of sequences up to 1871; roughly 16% (298) of the chipmunks we have sequenced exhibit introgressed mtDNA. This includes ongoing introgression between subspecies and between both closely related and distantly related taxa. In addition, we have identified several taxa that are apparently fixed for ancient introgressions and in which there is no evidence of ongoing introgression. A recurrent observation is that these introgressions occur between ecologically and morphologically diverged, sometimes non-sister taxa that engage in well-documented niche partitioning. Thus, the chipmunk radiation in western North America represents an excellent mammalian example of speciation in the face of recurrent gene flow among lineages and where biogeography, habitat differentiation and mating systems suggest important roles for both ecological and sexual selection.

A computational approach to Quaternary lake-level reconstruction applied in the central Rocky Mountains, Wyoming, USA

Sediment-based reconstructions of late-Quaternary lake levels provide direct evidence of hydrologic responses to climate change, but many studies only provide approximate lake-elevation curves. Here, we demonstrate a new method for producing quantitative time series of lake elevation based on the facies and elevations of multiple cores collected from a lake's margin. The approach determines the facies represented in each core using diagnostic data, such as sand content, and then compares the results across cores to determine the elevation of the littoral zone over time. By applying the approach computationally, decisions are made systematically and iteratively using different facies classification schemes to evaluate the associated uncertainty. After evaluating our assumptions using ground-penetrating radar (GPR), we quantify past lake-elevation changes, precipitation minus evapotranspiration (ΔP−ET), and uncertainty in both at Lake of the Woods and Little Windy Hill Pond, Wyoming. The well-correlated (r=0.802±0.002) reconstructions indicate that water levels at both lakes fell at >11,300, 8000–5500, and 4700–1600calyr BP when ΔP−ET decreased to −50 to −250mm/yr. Differences between the reconstructions are typically small (10±24mm/yr since 7000calyr BP), and the similarity indicates that our reconstruction method can produce statistically comparable paleohydrologic datasets across networks of sites.

A north-south moisture dipole at multi-century scales in the Central and Southern Rocky Mountains, U.S.A., during the late Holocene

Lake-level changes since ca. 3.6 kilo-annum (ka) at Emerald Lake in the Upper Arkansas River Basin of west-central Colorado coincide in time with changes of the opposite direction at Lake of the Woods in northwestern Wyoming. The contrast provides evidence of a multi-centennial moisture dipole across the Southern and Central Rocky Mountains' region similar to one associated with the effects of the El Nino Southern Oscillation (ENSO) on annual to decadal time scales today. Cores and ground penetrating radar (GPR) profiles from Emerald Lake show that deep-water muds accumulated as extensively across the lake basin as today at ca. 3.6–3.0 and 1.4–0.8 ka, and nearly as extensively at 2.5–1.9 ka. The extensive muds indicate episodes of high water at Emerald Lake and date to times when Lake of the Woods was low. Nearshore sand layers at Emerald Lake indicate that water levels fell during the intervening centuries, including when bristlecone pine chronologies have documented repeated multi-decadal droughts in the Upper Arkansas River Basin. Water levels were also low, based on the absence of nearshore mud accumulation, before ca. 3.6 ka, and dramatically lower (>2 m in the currently 4.5-m deep lake) before a sharp rise in water levels by ca. 5.7 ka. A basin-wide change in sediment accumulation patterns, consistent with an expansion and deepening of the lake at ca. 5.7 ka, correlates with regional cooling and similar evidence of increased effective moisture at Lake of the Woods and other sites throughout central North America. The step increase in moisture availability may relate to a global-scale reorganization of climatic patterns, which developed as the mid- and high-latitudes cooled in response to a decline in summer insolation.