Morphologic and Climatic Controls on Soil Evolution
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MORPHOLOGIC AND CLIMATIC CONTROLS ON SOIL EVOLUTION IN THE BITTERROOT AND SAPPHIRE MOUNTAINS, MONTANA by Sarah Sarojini Benjaram A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Earth Sciences MONTANA STATE UNIVERSITY Bozeman, Montana April 2017 ©COPYRIGHT by Sarah Sarojini Benjaram 2017 All Rights Reserved ii DEDICATION I would like to thank my advisor, Dr. Jean Dixon, for her guidance in carrying out this project and writing this thesis. Thank you for your good example and your patience. Thanks to my committee members, Dr. Tony Hartshorn and Dr. Dave Mogk, for their input on my project. Tony, thanks especially for visiting me in the field during the summer. I am grateful to George Furniss for his hospitality in letting me stay at his beautiful place in the Bitterroots. I couldn’t have managed such a long field trip without taking breaks at the little house. Thanks to Dr. Andrew Wilcox for his insight, advice and support. Thanks to Sam Shelafo, Darrell Schwartz, and Aaron Feldhaus for helping me out in the field and Karsten Lorentz for help in the lab. iii TABLE OF CONTENTS 1. INTRODUCTION .....................................................................................................1 Intent of this MS Thesis .......................................................................................1 Background ..........................................................................................................1 Thesis Format and Outline ...................................................................................7 References Cited ..................................................................................................9 2. MORPHOLOGIC AND CLIMATIC CONTROLS ON SOIL EVOLUTION IN THE BITTERROOT AND SAPPHIRE MOUNTAINS ....................................16 Introduction ........................................................................................................16 Legacies of Past Climate on Landscape Morphology ....................................16 Landscape Morphology and Soil Cover ........................................................19 Implications of Bedrock Exposure for Chemical Weathering .......................22 Study Area .........................................................................................................25 Tectonic History .............................................................................................26 Climate History ..............................................................................................29 Topography and Geomorphology of the Bitterroot and Sapphire Mtns ........30 Approach and Methods ......................................................................................32 Topographic Analysis ....................................................................................35 Field Methods and Sampling .........................................................................37 Analysis of Land Cover Distribution .............................................................39 Analysis of Chemical Weathering Extent ......................................................42 Results ................................................................................................................46 Topographic Variability Across Study Catchments.......................................46 Soil Cover and Thickness ..............................................................................52 Chemical Weathering Intensity ......................................................................52 Discussion ..........................................................................................................56 Assessing the Glacial Imprint on the Landscape ...........................................56 Interpretation of Soil Cover Distribution and Geochemistry .........................59 The Influence of Modern Climate on Soil Evolution ....................................63 Effect of Lithology on Catchment Characteristics .........................................64 Conclusions ........................................................................................................66 References Cited ................................................................................................70 3. SUMMARY ........................................................................................................83 iv TABLE OF CONTENTS – CONTINUED APPENDICES…………………………………………………………………………...85 APPENDIX A: Calibration of the Rock Exposure Index ......................................86 APPENDIX B: Sample Field Data ........................................................................90 APPENDIX C: Sample GIS-Derived Data ............................................................93 APPENDIX D: Elemental Composition of Field Samples from XRF ..................96 APPENDIX E: Calculated CDFs and Mass Transfer Coefficients .......................99 APPENDIX F: Notes on the Utilization of Cosmogenic Nuclides......................101 APPENDIX G: Literature Review .......................................................................108 APPENDIX H: Detailed Sampling Methods .......................................................137 APPENDIX I: Distribution of Land Cover Type with Slope Gradient ...............140 v LIST OF TABLES Table Page 2.1. Mean elevation and slope of 11 study catchments .....................................49 2.2. Hypsometric integrals of 11 study catchments ...........................................49 2.3. CDFs and taus of surface samples in Lost Horse and Rye .........................55 2.4. Variation in slope gradient by lithology in the Sapphires ..........................55 2.5. Variation in slope gradient by lithology in the Sapphires ..........................66 vi LIST OF FIGURES Figure Page 1.1. Illustration of contrasting landscapes ...........................................................4 1.2. Concept sketch ..............................................................................................8 2.1. Idealized slope-elevation distributions of fluvial and glacial landscapes ...19 2.2. Location map ..............................................................................................26 2.3. Geologic maps ............................................................................................34 2.4. String grid used for estimating soil cover ...................................................39 2.5. Estimation of soil and rock cover from remotely sensed data ....................42 2.6. Comparison of Ti and Zr for use as immobile element ..............................45 2.7. Elevation and slope histograms ..................................................................47 2.8. Slope versus elevation of 11 catchments ....................................................48 2.9. Hypsometric integral versus catchment area ..............................................48 2.10. Basin slopes binned by elevation ..............................................................51 2.11. Soil thickness, CDF, and tau histograms ..................................................53 vii ABSTRACT To what extent is chemical weathering governed by a landscape’s topography? Two neighboring mountain ranges in the northern Rockies of western Montana, USA, provide an ideal natural laboratory in which to investigate the relationship between soil chemical weathering, persistence of soil cover, and topography. We also examine the connection between the topography and climate history. The mountain ranges we explore are the previously glaciated Bitterroot Mountains, which consist of steep, rock-dominated hillslopes, and the neighboring unglaciated Sapphire Mountains which display convex, soil-mantled hillslopes. Soil thickness measurements, soil and rock geochemistry, and digital terrain analysis reveal that soils in the rock-dominated Bitterroot Mountains are less thoroughly weathered than those in the Sapphire Mountains. These differences are even greater when we adjust weathering for rock fragments and consider surface weathering intensity at a landscape scale using our newly developed metric, the rock- adjusted chemical depletion fraction (RACDF) and rock-adjusted mass transfer coefficient (RAτ). The Bitterroots overall are 30% less weathered than the Sapphires despite higher mean annual precipitation in the former, with an average RACDF of 0.38 in the postglacial Bitterroots catchment and 0.61 in the nonglacial Sapphire catchment, suggesting that 38% of rock mass is lost in the conversion to soil in the Bitterroots, whereas 61% of rock mass is lost in the nonglaciated Sapphires. Though we find little evidence for modern climate influence on weathering, data suggest that precipitation may influence slope thresholds for soil cover. Forested soils persist on slopes that are 5° higher in the Bitterroots than the Sapphires (25° and 20° respectively), based on land cover data. Because the previously glaciated Bitterroots are less weathered despite being wetter, we conclude that the glacial history of this landscape exerts more influence on soil chemical weathering than does modern climate. However, while previous studies have correlated weathering intensity with topographic parameters such as slope gradient, we find little topographic indication of specific controls on weathering in these complex systems. 1 CHAPTER 1 INTRODUCTION Intent of this MS Thesis Significant advancement has been made in our understanding of the controls on soil development and chemical