Evidence of glacial activity during MIS 4 in the Rocky Mountains, Colorado, USA

ABSTRACT

The Ziegler Reservoir fossil site near Snowmass Village, Colorado, provides a rare opportunity to examine environmental conditions in the Rocky Mountains during marine isotope stage (MIS) 4 (71–57 ka). Although recognized as a global-scale cold event, MIS 4 is typically absent from Rocky Mountain glacial chronologies because the geologic evidence was covered or destroyed during the subsequent, and more extensive, MIS 2 (Pinedale; 29–14 ka) glaciation. Ziegler Reservoir lies beyond the Pinedale glacial extent, which allowed for the preservation of a long-lived sequence of eolian sediments deposited in a lacustrine environment that spans from late MIS 6 (ca. 140 ka) through early MIS 3 (ca. 55 ka). Sediments dating to MIS 4 exhibit a significant increase in clay-sized particles, suggesting that the source areas, most likely nearby glacio-fluvial deposits, were enriched with fine-grained material at that time. We hypothesize that the elevated clay content was the result of rock flour production by nearby valley glaciers that were active in the Rocky Mountains during MIS 4. The results of our study illustrate how recognizing indirect evidence of glacial activity can result in a more complete record of past climate conditions than what could be achieved by the study of moraines alone.

KEYWORDS:

• Glacial chronology
• marine isotope stage 4
• Rocky Mountains
• Ziegler Reservoir fossil site

Acknowledgments

The authors recognize the significant contributions of Ken Pierce to our understanding of the glacial history within the Rocky Mountains; he will be missed. We thank Kathleen Springer, Janet Slate, and two anonymous reviewers for providing constructive reviews of this article. We also thank Jeremy Havens for assistance with the figures.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

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Additional information

Funding

This project was funded in part by the U.S. Geological Survey Climate and Land Use Change Research and Development Program. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.