Eric M. Leonard

CLIMATIC CHANGE IN THE COLORADO ROCKY MOUNTAINS: ESTIMATES BASED ON MODERN CLIMATE AT LATE PLEISTOCENE EQUILIBRIUM LINES

A comparison of modern climates at late Pleistocene glacier equilibrium lines in the Colorado Rocky Mountains with the range of climates which occur at the equilibrium lines of modern glaciers worldwide allows an evaluation of the combinations of temperature and precipitation change which would have been necessary to sustain the late Pleistocene glaciers. Modern climatic conditions at late Pleistocene equilibrium lines at 12 sites in 7 ranges are approximated using instrumental and snow survey data. The comparison provides paired values of temperature and precipitation changes which would maintain the glaciers at their late Pleistocene maximum positions. If no change occurred in total precipitation or in seasonal distribution of precipitation between the late Pleistocene and the present, an approximately 8.5?C summer temperature depression would have been necessary to sustain the late Pleistocene glaciers. A 10 to 13?C late Pleistocene temperature depression, which has been suggested on the basis of other lines of evidence, would have been accompanied by a reduction of at least 44%o in fall-through-spring precipitation compared to present-day values.

Geomorphic and tectonic forcing of late Cenozoic warping of the Colorado piedmont

Late Cenozoic warping of the Colorado piedmont involved interplay of tectonic forcing, river erosion, and isostatic response to erosion. Modeled erosional isostasy closely replicates the observed pattern of deformation, but accounts for only about half its magnitude. The remainder reflects tectonic rock uplift that increases southward across the piedmont, likely reflecting proximity to the northward-propagating Rio Grande Rift. This differential uplift triggered differential erosion, concentrated on southern piedmont river systems, particularly the Arkansas River, which led in turn to differential isostatic rock uplift focused on the Arkansas drainage. Covariation of tectonic uplift, erosion, and isostatic compensation across the piedmont reflects a positive feedback between uplift-induced erosion and erosion-induced isostasy, which has progressed to the point that isostatic uplift is approximately equal to the initial tectonic forcing.

Aminostratigraphy of Peruvian and Chilean Quaternary marine terraces

Amino acid enantiomeric ratio (D/L) data for over 200 molluscs from Pleistocene marine terraces on the coast of Peru and Chile are reported. They are used to evaluate the utility of this dating method for terraces in this region. Analyzed samples represent both local relative age sequences and regionally-correlative deposits identified over a broad latitude range (14-310S). Aminozones are defined on the basis of data for Protothaca, supplemented with results for Mulinia and other genera. Electron spin resonance (ESR) data serve to calibrate two aminozones from Peru (ca. 15°S) as being correlative with Oxygen Isotope Stages 5 and 7 of the marine isotope record. The latitudinal trends of enantiomeric ratios for Stage 5 samples, plotted against current mean annual temperature (CMAT), are similar to those observed for Protothaca from the North American Pacific coast. The sea level record preserved in the Peru-Chile terraces is complex and usually not clearly resolved geomorphically. Typical uplift rates of 0.1-0.2 m/ka are inferred from terrace age and elevation data, with higher rates being observed where the Nazca Ridge is being subducted beneath the coast at about 15°S. Evidence of terrace reoccupation by sea levels with substantially different ages is found at several sites of apparent low uplift.

Low uplift rates and terrace reoccupation inferred from mollusk aminostratigraphy, Coquimbo Bay area, Chile

Abstract Mollusk aminostratigraphy of Quaternary marine terrace sediments at Coquimbo Bay, Chile, combined with recently available electron spin resonance (ESR) ages, necessitates revision of the northern Chilean relative sea-level and terrace chronology. Protothaca and Mulinia d -alloisoleucine/ l -isoleucine values define four aminozones which are consistent with available ESR ages. Terrace reoccupation during successive high sea-level stands is inferred on the basis of litho- and aminostratigraphically defined unconformities in terrace sediments. ESR data and a nonlinear kinetic racemization model give approximate numerical ages for the aminozones and thus yield estimates of net uplift rates. These rates, averaged over intervals of one to several hundred thousand years, have ranged from less than 0.1 m/1000 yr to no more than 0.2 m/1000 yr. Such slow uplift is the cause of terrace reoccupation, as the amount of uplift between successive glacioeustatic high sea-level stands is frequently not sufficient to isolate an earlier-formed abrasion platform from rising sea level during a subsequent high stand.

Assessing climatic and nonclimatic forcing of Pinedale glaciation and deglaciation in the western United States

New 10 Be surface exposure ages from adjacent valleys in the upper Arkansas River basin, Colorado (United States), indicate that Pinedale maxima culminated asynchronously at 22.4 ± 1.4, 19.2 ± 0.2, 17.8 ± 0.6, and 15.8 ± 0.4 ka, but that deglaciation initiated synchronously between ca. 16 and 15 ka. These data are combined with published glacial chronologies across the western United States, and indicate that although the ages of Pinedale terminal moraines vary within individual ranges as well as regionally, most western United States glaciers remained near their Pinedale termini until ca. 16 ka, at which time widespread deglaciation commenced. We hypothesize that the near-synchronous demise of glaciers across the western U.S. between ca. 15 and ca. 13 ka was driven by the first major Northern Hemisphere warming following the Last Glacial Maximum, but that some differences in Pinedale culmination ages can be explained by nonclimatic factors intrinsic to individual valleys. These results suggest the need for caution in focusing exclusively on climate forcings to explain apparent asynchrony in Pinedale maxima.