Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado Professional Paper 1782 U.S. Department of the Interior U.S. Geological Survey Cover photo: Site of former Lake Emma (foreground) and Emery Peak (4,057 meters) to the south, northern San Juan Mountains, Colo. Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado By Paul E. Carrara Professional Paper 1782 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt, Director U.S. Geological Survey, Reston, Virginia: 2011 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1–888–ASK–USGS. 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Suggested citation: Carrara, P.E., 2011, Deglaciation and postglacial treeline fluctuation in the northern San Juan Mountains, Colorado: U.S. Geological Survey Professional Paper 1782, 48 p. iii Contents Abstract ..........................................................................................................................................................1 Introduction.....................................................................................................................................................2 Background and Purpose ....................................................................................................................2 Geography and Geology .....................................................................................................................3 Vegetation Zones in the San Juan Mountains .................................................................................3 The Timberline-Treeline Ecotone and Controlling Factors .............................................................3 Climate ....................................................................................................................................................5 Acknowledgments ................................................................................................................................5 Glacial Geology of the San Juan Mountains .............................................................................................7 Extent of Pinedale Glaciers ................................................................................................................7 Timing of Deglaciation..........................................................................................................................8 Radiocarbon Ages ......................................................................................................................8 Cosmogenic Ages ........................................................................................................................8 Cirque Moraines and Present-Day Snowfields .............................................................................10 Postglacial Forestation and Holocene Treeline in the Northern San Juan Mountains ...................17 Introduction .........................................................................................................................................17 Telluride Water Well (2,665 m asl) ...................................................................................................19 Molas Lake (3,205 m asl) ....................................................................................................................19 Little Molas Lake (3,370 m asl) ..........................................................................................................21 Black Mountain Lake (3,413 m asl) ..................................................................................................21 Placer Gulch Bog (3,600 m asl) .........................................................................................................21 California Gulch Bog (3,615 m asl) ...................................................................................................22 Bog Near Hurricane Basin (3,660 m asl) .........................................................................................22 Eureka Gulch Bog (3,665 m asl) ........................................................................................................27 Lake Emma site (3,740 m asl).............................................................................................................29 Radiocarbon-Dated Conifer Fragments ................................................................................29 Pollen Analysis ...........................................................................................................................33 Fossil Insects .............................................................................................................................34 Picayne Gulch Bog (3,750 m asl) ......................................................................................................37 Discussion .....................................................................................................................................................37 Holocene Treeline and Climatic Fluctuations in the San Juan Mountains ..............................37 Early Holocene Intensification of the Summer Monsoon? ..........................................................39 Conclusions ..................................................................................................................................................40 References Cited..........................................................................................................................................41 Appendix A Evidence of Higher Treelines During the Holocene in Western North America, as Indicated by Radiocarbon Ages or Cross-Dated Conifer Remains..............................................................................................................................45 iv Figures 1. Map showing northern San Juan Mountains and selected sites discussed in this study ....................................................................................................................................2 2. Map showing study sites near the headwater region of Animas River ..............................4 3. Diagram showing trajectories of major air masses that cross the San Juan Mountains ....................................................................................................................6 4. Photograph of site of the former Lake Emma in 1979 .............................................................7 5. Map showing limits of Wisconsin (Pinedale) glaciation in the western San Juan Mountains ...................................................................................................................9 6. Photograph of Molas Lake area ...............................................................................................10 7. Diagram of composite stratigraphic section at Lake Emma site ........................................11 8. Diagram of stratigraphic sections and radiocarbon ages from bogs in northern San Juan Mountains ..................................................................................................12 9–12. Photographs of: 9. Highland Mary Lakes area ..............................................................................................15 10. Yankee Boy moraine in Yankee Boy Basin ....................................................................16 11. Grenadier moraine, northern base of Grenadier Range .............................................16 12. Snowfield at northern base of Trinity Peaks in Grenadier Range .............................18 13. Stratigraphic cross section and radiocarbon ages from Telluride water well ................20 14. Photograph of Placer Gulch bog exposure near Evening Star Mine .................................23 15. Photograph of California Gulch, looking upvalley .................................................................25 16. Map of Hurricane Basin area ...................................................................................................26 17. Photograph of bog near Hurricane Basin...............................................................................27 18. Diagram showing changing apparent elevation of the Hurricane Basin core on the basis of spruce/pine ratios ..................................................................................28 19–21. Photographs of: 19. Eureka Gulch bog study site ............................................................................................29 20. Lake Emma and adjacent Sunnyside Mine around 1900 AD .....................................30 21. Cross section of spruce fragment from Lake Emma site ............................................31 22–25. Diagrams of: 22. Percentages of various types of pollen from Lake Emma .........................................35 23. Concentrations of various