Spatial Patterns of Glacial Erosion at a Valley Scale Derived From Terrestrial Cosmogenic ¹⁰Be and ²⁶Al Concentrations in Rock

Abstract

The fundamentally geographic issue of the amounts and spatial patterns of erosion necessary to produce classic glacial landforms such as U-shaped valleys has been debated by scientists for over a century. Terrestrial cosmogenic nuclide (TCN) measurements in glacially abraded bedrock were used to determine patterns of glacial erosion and to quantify the amount of rock removed during the last glaciation along valley-side transects in Sinks Canyon, Wind River Range, Wyoming, and the South Yuba River, Sierra Nevada, California. Surface exposure ages from bedrock and erratic samples obtained during this study indicate last deglaciation between 13–18 ka in the South Yuba River and 15–17 ka in Sinks Canyon. These ages are in agreement with previously published glacial chronologies. In both areas, samples from valley cross sections revealed a pattern of erosion during the last glaciation that decreased toward the lateral limit of ice extent, as predicted by numerical models, while transects further upstream recorded >1.4 meters of bedrock removal throughout. The effects of varying interglacial erosion and surface exposure histories on modeled glacial erosion depths were tested, validating the methodology used. The results demonstrate that the TCN technique, applied at the valley scale, provides useful insight into the spatial pattern of glacial erosion. Extensive sampling in areas with limited erosional loss may provide detailed records of erosion patterns with which to test predictions generated by models of ice dynamics and erosion processes.

Key Words:

• cosmogenic nuclides
• glacial erosion
• Sierra Nevada
• Wind River Range.

Acknowledgments

We are grateful to D. Granger for the use of his chemistry laboratory, the University of Missouri Branson Field Camp, and the late Laurence B. James for base camp accommodation. We thank P. Bierman, A. Heimsath, and an anonymous reviewer for constructive reviews. This research was funded by NSF grant SBR-9631437.

Notes

^a w=weathered bedrock, s=striated bedrock, p=polished bedrock, e=erratic boulder.

^b Geographic (in decimal degrees).

^c Total Al concentration in quartz determined by flame atomic absorption spectrometry and assigned 5% uncertainty.

^d Data are normalized to NIST SRM 4325 assuming ¹⁰Be/⁹Be=3.05 × 10⁻¹¹. Carrier ¹⁰Be/⁹Be=1 × 10⁻¹⁵.

^e Scaled to geomagnetic latitude (see text for details) and assuming no erosion. Altitude /latitude scaling factors based on those of Stone (2000).