HR: 1340h
AN: PP33B-1271    [Abstracts]
TI: Signatures of Glacial Erosion and Retreat in the Landscape: Cosmogenic and Numerical Modeling Constraints
AU: * Ward, D J
EM: dylan.ward@colorado.edu
AF: University of Colorado, Dept. of Geological Sciences and Institute of Arctic and Alpine Research, 2200 Colorado Ave, UCB399, Boulder, CO 80309, United States
AU: Anderson, R S
EM: andersrs@colorado.edu
AF: University of Colorado, Dept. of Geological Sciences and Institute of Arctic and Alpine Research, 2200 Colorado Ave, UCB399, Boulder, CO 80309, United States
AU: Briner, J P
EM: jbriner@buffalo.edu
AF: State University of New York at Buffalo, Dept. of Geology, 876 Natural Sciences Complex, Buffalo, NY 14260, United States
AU: Guido, Z S
EM: zackguido@yahoo.com
AB: We use cosmogenic radionuclide (CRN) exposure ages to constrain numerical simulations of deglaciation histories in the Middle Boulder Creek drainage, Colorado Front Range, and the Animas River valley, San Juan Mountains, Colorado. We present 18 new ¹⁰Be exposure ages from glacially polished bedrock sampled in the Middle Boulder Creek valley. All of these ages are younger than the ~19-22 ka terminal moraine age based on ²⁶Al and ³⁶Cl measurements by Schildgen (2000) and Benson et al. (2005). Exposure ages decrease with distance upvalley from the moraine, and the youngest ages in the uppermost valley are uniformly ~13 ka. We include 4 ¹⁰Be ages in a cross section across the mid-valley, which show a pattern of Last Glacial Maximum (LGM) ages (12-14 ka) within the glacial footprint, and older exposure ages (~40 ka) near the trim lines. A similar age trend is seen in the Animas River valley in southwestern Colorado, which was occupied by a lobe of the LGM ice sheet that capped the San Juan mountains. Deglaciation began here ca. 19.4 ka, based on a ¹⁰Be depth profile in a proglacial terrace. A longitudinal transect of exposure ages from glacially polished samples indicates that terminus retreat proceeded at ~15 m/yr until complete deglaciation ca. 12.3 ka. Neither valley has obvious recessional deposits within the LGM glacial footprint. The first-order trend in each valley is a monotonic glacial retreat, but there are other possible retreat scenarios. For instance, we would like to test whether the same trend in ¹⁰Be concentrations could be generated by episodic retreat punctuated by periods of readvance. To investigate these scenarios, we modified the GC2D numerical glacier simulation (see Kessler et al., 2006) to incorporate a CRN accumulation layer. This layer can contain any starting value of CRN concentration. Production over each timestep is scaled to DEM latitude and altitude. Production is taken to be zero in areas covered by more than 10 m of ice. The CRN inventory can also decline due to glacial erosion. We incorporate a selectable erosion rule based on basal sliding or total ice velocity, ice discharge, ice power, or basal shear stress, and calculate the reduction in CRN inventory by the depth stripped in each timestep. We then simulate a glacier responding to equilibrium line altitude (ELA) changes imposed stepwise, gradually, or including short periods of lowering during an overall rise. Each scenario generates a pattern of ages in the CRN layer that can be compared with the map pattern of measured 10Be concentrations. Initial results show that a step-function ELA rise to its present value causes a retreat that is too rapid to explain the range of ages observed in both valleys. A steady ELA rise can replicate the age-distance trend of an individual valley (within error). An episodic retreat with readvances results in a distinctive pattern of discordant ages between tributaries of the same glacial valley. Our CRN dataset includes several samples chosen to discriminate between this pattern and one typical of monotonic retreat.
DE: 0720 Glaciers
DE: 0798 Modeling
DE: 1130 Geomorphological geochronology
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting