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Keith Brugger

Keith A. Brugger, "The non-synchronous response of Rabots Glaciär and Storglaciären, northern Sweden, to recent climate change:a comparative study," in Annals of Glaciology, Volume 46, pp 275-282, 2007

Abstract: Rabots Glaciär and Storglaciären, two small valley glaciers in the Swedish Arctic, have not behaved synchronously in response to recent climate change. Both glaciers advanced late in the 19th century and then began to retreat in response to a 1°C warming that occurred around 1910. By the mid-1980s the terminus and volume of Storglaciären had essentially stabilized, so it may have completed its response to the earlier warming. In contrast, ongoing thinning and retreat of Rabots Glaciär are substantial and suggest its response time is considerably longer. A time-dependent numerical model was used to investigate each glacier's response to perturbations in mass balance. This modeling suggests that, for small perturbations, volume timescales for Storglaciären and Rabots Glaciär are 125and 215 years, respectively. Another measure of response time (i.e. length response time) yields somewhat lower values for each glacier; however, what is significant is that by either measure and accounting for uncertainties, the response time for Rabots Glaciär is consistently about 1.5 times longer than that for Storglaciären. This implies that their non-synchronous behavior is likely due to differences in response times. The latter ultimately result from markedly different long! itudinal geometries (particularly near the termini), velocity profiles and specific net balance gradients.

Kurt A. Refsnider ('04) and Keith A. Brugger, " Rock Glaciers in Central Colorado, U.S.A., as Indicators of Holocene Climate Change," in Arctic, Antarctic, and Alpine Research, Volume 39, pp 127-136, 2007

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. Link: Click Here

Keith A. Brugger, " Cosmogenic 10Be and 36Cl ages from Late Pleistocene terminal moraine complexes in the Taylor River drainage basin, central Colorado, USA," in Quaternary Science Reviews, Volume 26, pp 494-499, 2007

Abstract: Cosmogenic surface-exposure ages from boulders on a terminal moraine complex establish the timing of the local last glacial maximum (LGM) in the Taylor River drainage basin, central Colorado. Five zero-erosion 10Be ages have a mean of 19.5±1.8 ka while that for three 36Cl ages is 20.±72.3 ka. Corrections for modest rates ( 1mmka 1) of boulder surface erosion result in individual and mean ages that are generally within 2% of their zero-erosion values. Both the means and the range in ages of individual boulders are consistent with those reported for late Pleistocene moraines elsewhere in the southern and middle Rocky Mountains, and thus suggest local LGM glacier activity was regionally synchronous. Two anomalously young (?) zero-erosion 10Be ages (mean 14.4±0.8 ka) from a second terminal moraine are tentatively attributed to the boulders having been melted out during a late phase of ice stagnation.
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Keith Brugger, and Kurt Resnider '04, Matthew Whitehill '99(UMM alumni), "Variation in Glacier Length and Ice Volume of Rabots Glacier, Sweden, in reponse to climate change, 1910-2003," In Annals of Glaciology, Volume 42, Number 1, August 2005,   pp 180-188: International Glaciological Society.

Abstract:   Historical records, photographs, maps, and measurements were used to determine changes in the length, geometry, and volume of Rabots Glaciär 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 x 106 m3 between 1910 and 1959, 51.1 x 106 m3 between 1959 and 1980, at least 10.4 x 106 m3 between 1980 and 1989, and 14.4 x 106 m3 between 1989 and 2003.  The total volume change over the last ninety-three years is estimated at ~153.2 x 106 m3 corresponding to 1.6 x 106 m3 a-1.  The magnitude of the ongoing changes in length and volume suggest 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.

Keith Brugger, "Late Pleistocene climate inferred from the reconstruction of the Taylor River glacier complex, southern Sawatch Range, Colorado."   In Geomorphology, Volume 75, Issues 3-4, May 2006,   pp 318-329: Elsevier.

Abstract:   Ice surface topography of a late Pleistocene glacier complex, herein named the Taylor River Glacier Complex (TRGC), was reconstructed on the basis of detailed mapping of glacial landforms combined with analyses of aerial photos and topographic maps.  During the last glacial maximum (LGM) the TRGC covered an area of 215 km2 and consisted of five valley or outlet glaciers that were nourished by accumulation in cirques basins and/or upland ice fields. 

Equilibrium-line altitudes (ELAs) for the glaciers of the TRGC were estimated using the accumulation-area ratio method, assuming that ratio to be 0.65 ± 0.05.  ELAs thus derived ranged from about 3275 to 3400 m, with a mean of 3340 ± 60 m. A degree-day model (DDM) was used to infer the climatic significance of the LGM ELA.  With no appreciable differences in precipitation with respect to modern climate, the ELA implies that mean summer temperatures during the LGM were ~7.6 °C cooler than today.  The DDM was also used to determine the temperatures required to maintain steady-state mass balances for each of the reconstructed glaciers.  The required reductions in summer temperature vary little about a mean of 7.1 °C.  The sensitivity of these results to slight (±25%) changes assumed for LGM precipitation are less than ±0.5 °C.  Even under an LGM climate in which precipitation is assumed to be substantially different  (±50%) than the present, mean summer temperatures must be on the order of 7.0 to 8.5 °C lower to depress equilibrium lines to LGM altitudes.  The greater sensitivity of the ELA to changes in temperature suggests that glaciation in the region was driven more by decreases in summer temperature rather than increases in precipitation. Link: Click Here


James Jones

Micah J. Jessup, James V. Jones III, Karl E. Karlstrom, Michael L. Williams, James N. Connelly, and Matthew T. Heizler, "Three Proterozoic Orogenic Episodes and an Intervening Exhumation Event in the Black Canyon of the Gunnison Region, Colorado." In Journal of Geology , Vol   114, p. 555-576

ABSTRACT:   U/Pb zircon/titanite geochronology, in situ monazite geochronology, and 40Ar/39Ar thermochronology provide an unusually complete data set for reconstructing the tectonic history of Proterozoic rocks exposed in the Black Canyon, Gunnison, Colorado. These new geochronologic data record three protracted orogenic episodes and an exhumation event between orogenic pulses: (1) Yavapai orogeny (1741-1689 Ma), (2) exhumation marked by an angular unconformity beneath post-Yavapai, pre-Mazatzal quartzites, (3) Mazatzal orogeny (postquartzite deposition), and (4) 1434-1403 Ma intracratonic tectonism. Supracrustal rocks of the Black Canyon succession were deposited or crystallized at or prior to 1741 _ 4 Ma and were intruded by the 1713 _ 2 Ma Pitts Meadow granodiorite. Paleoproterozoic hightemperature metamorphism ( 1 650 _ C) associated with the Yavapai orogeny occurred between 1741 and 1690 Ma.   Deposition of interorogenic quartzites took place after ~ 1700 Ma. The Vernal Mesa monzogranite was emplaced at 1434 _ 2 Ma followed by metamorphism ( ~ 600 __ 50 _ C and ~ 3 _ 1 kbar) at <= 1403 _ 23 Ma. 40Ar/39Ar thermochronology records Mesoproterozoic middle crustal temperatures of 350 _ -500 _ C, with the highest temperatures occurring near the Vernal Mesa monzogranite and the NE-striking Black Canyon shear zone. The area cooled through 350 _ C by ~ 1385 Ma but variably cooled through 300 _ C from 1370 to 1100 Ma, suggesting long-term residence of rocks above the 250 _ C isotherm at ~ 10 km crustal depths. When these results are combined with geologic data to construct generalized pressure/temperature/time/deformation paths ( PTtD ), a new template for the evolution of Proterozoic rocks of southwestern Colorado and the southwestern United states emerges. Link: Click Here


James V. Jones III and James N. Connelly    (In press) "Proterozoic tectonic history of the Sangre de Cristo Mountains, Colorado: Rocky Mountain Geology." I n   Rocky Mountain Geology, Vo                                 l 41, p. 79-116

Abstract: Field studies and U-Pb geochronology in the Sangre de Cristo Mountains, southern Colorado, provide new constraints on the Proterozoic tectonic evolution of southern Laurentia. Protoliths for basement gneisses and amphibolites were formed in an arc environment and underwent early penetrative deformation and metamorphism (D 1 and M 1 ) during formation of the Yavapai province. D 1 deformation produced penetrative, subvertical, northwest-striking fabrics (S 1 ) in rocks exposed throughout the range and is interpreted to have occurred during long-lived arc formation and accretion across northwest-striking tectonic boundaries. The ages of D 1 and M 1 are constrained by a suite of 1750-1730-Ma calc-alkaline intrusions in the southern part of the range and might have occurred as late as ca. 1710 Ma in the northern part of the range. The northeast-striking tectonic grain that was developed regionally during the culmination of the Yavapai orogeny is not recognized locally. Post-orogenic granitoid plutons were emplaced at 1695±2 Ma and 1682±3 Ma, broadly coeval with deposition of locally derived quartzite at the surface. Magmatism and sedimentation during this time are interpreted to represent contemporaneous responses to crustal extension during the ca. 60-m.y. inter-orogenic period between the Yavapai and Mazatzal orogenic events. D 2 deformation is interpreted to represent the Mazatzal orogeny locally, and involved northwest-directed shortening and dextral shear localized along subvertical, northeast-trending high-strain zones. D 2 was accompanied by amphibolite-facies metamorphism (M 2 ) at 1637±6 Ma, and the quartzite is inferred to have been deformed during this time. Mesoproterozoic deformation (D 3 ) produced a northeast-striking, subvertical tectonic foliation and localized shear zones between 1420 and 1412 Ma. D 3 deformation was bracketed by the emplacement of two newly dated granitic intrusions at 1434±2 Ma and 1407±6 Ma. The map-scale geometry of these intrusions and coeval deformational fabrics suggest that ca. 1.4-Ga granites were emplaced into a broadly compressional stress field during subhorizontal northwest-southeast shortening. These new data and observations indicate that ca. 1.4-Ga granites are not anorogenic, consistent with tectonic models suggesting that widespread magmatism was broadly synchronous with intracontinental orogenesis at ca. 1.4 Ga.
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