Theses and Dissertations at Montana State University (MSU)
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Item Soil storage on steep forested and non-forested mountain hillslopes in the Bitterroot Mountains, Montana(Montana State University - Bozeman, College of Letters & Science, 2018) Quinn, Colin Aidan; Chairperson, Graduate Committee: Jean DixonMountain hillslopes are dynamic settings with discontinuous soils affected by a suite of variables including climate, lithology, hydrology, and vegetation. Our study seeks to understand how forest cover influences soil and rock distribution at decadal to century timescales. We focus on a series of post-glacial hillslopes in Lost Horse Creek of the Bitterroot Mountains, Montana. In this system, avalanche paths maintain parallel, topographically similar swaths of forested and non-forested slopes with uniform aspect, lithology, and climate. We combine field observations, fallout radionuclide analysis (210 Pbex & 137 Cs), and remote sensing data to understand both landscape- and fine-scale patterns in soil and rock distribution. Local soil and rock measurements indicate more extensive soil cover (forest = 94.4 + or = 2.6%; non-forest = 88.3 + or = 1.9%) and thicker soils (6cm greater median) in the forested system. We compare landcover-classified rock to topographic metrics from LiDAR data and find a doubling of rock cover (from 40% to 80%) as hillslope angles transition across slopes of ~24-42 ?. Topographic roughness, calculated as the standard deviation of slope, is predictive of only ~60% of total landscape rock cover, but can identify large boulders and coarse-scale outcrops with higher accuracy (79%). These calibrated remote sensing metrics indicate higher rock cover in non-forested regions (34%, compared to 20% in forested areas), though with high uncertainty. Additionally, we measure fallout-radionuclide inventories in soils to explore variations in decadal transport processes and soil residence times. We find distinct 210 Pb and 137 Cs behaviors in forested and non-forested systems, controlled both by unique partitioning of each nuclide within organic and mineral soil horizons, but also due to depth-driven differences in their physical mobility. Average 210 Pb ex inventories in non-forested soils are 33% lower, and half as variable as soils in the forested region (10.45 + or = 0.97 and 15.49 + or = 1.91 kBq/m 2 respectively), while 137 Cs inventories are indistinguishable (4.04 + or = 0.34 and 3.73 + or = 0.42 kBq/m 2). Together, our spatial, field, and isotope analyses suggest forested systems have greater soil storage and longer residence times than non-forested soils, mediated by differences in surface erosion processes within a larger fire disturbance landscape.Item Snowpack driven changes in decadal soil evolution: insights from a 48-year snow manipulation experiment(Montana State University - Bozeman, College of Letters & Science, 2017) Feldhaus, Aaron Michael; Chairperson, Graduate Committee: Jean DixonSoil mantled landscapes are a critical interface that support biological life, weather geologic materials, and develop in response to changes in climate. Climate has long been considered a dynamic control on the evolution of Earth's landscapes. However, we have limited understanding regarding how soils respond to short-term perturbations of key climate variables like precipitation and moisture availability. Furthermore, the timescales over which diverse weathering processes feedback and measurably change soil character are still relatively uncertain, as well as how they respond to swift changes in climate. Here, we explore the role of precipitation in decadal soil evolution by utilizing a 48-year snowpack experiment located in the Greater Yellowstone Ecosystem (GYE) of SW Montana. In this unique field site, we compare soil development across experimental plots with enhanced snowpack, where snow has been doubled (2x) and quadrupled (4x) above ambient conditions for almost five decades. We find that decadal snowpack addition provides multiple pathways for enhancing soil weathering, both physically and chemically. Soils under enhanced snowpack generally contain higher amounts of fine-grained material (clay and silt) and are more acidic (lower soil pH) in nature. Significant (>85%) surface depletions of the fallout radionuclide 210 Pb and reduced surface horizon carbon and nitrogen content, along with reduced above and below ground vegetation biomass provide evidence of increased wind erosion of soils that experience enhanced winter snowpack. Modeling of diffusion-like mixing from 210 Pb profiles also indicates there is increased bioturbation intensity (soil mixing) under enhanced snowpack. We find that snowpack addition, through associated changes in plant communities and vegetation biomass, along with its effects on physical and chemical weathering processes, produces rapid and measurable changes in the weathered state of soils. Our results indicate that short-term, decadal perturbations in snowpack significantly alter weathering mechanisms in this landscape, which measurably overprint thousands of years of soil development. These findings provide novel insight into the fundamental role of climate on short-term soil evolution and have significant implications for how mountainous or snowpack-dominated systems may respond to perturbations in climate.Item Morphologic and climatic controls on soil evolution in the Bitterroot and Sapphire Mountains, Montana(Montana State University - Bozeman, College of Letters & Science, 2017) Benjaram, Sarah Sarojini; Chairperson, Graduate Committee: Jean DixonTo 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 (RA t). 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.Item Meteoric 10Be in lake sediment cores as a measure of climatic and erosional change(Montana State University - Bozeman, College of Letters & Science, 2017) Jensen, Rachel Elizabeth; Chairperson, Graduate Committee: Jean Dixon; Jean L. Dixon was a co-author of the article, 'A quantative framework for understanding past environmental change using 10 BE in lake sediments' submitted to the journal 'Earth surface processes and landforms' which is contained within this thesis.; Jean L. Dixon was a co-author of the article, 'Meteoric 10 BE in Blacktail Pond, Yellowstone, as a recorder of paleoenvironmental change' submitted to the journal 'GeoSciences' which is contained within this thesis.Developing tools that trace Earth-surface processes is necessary to quantify the complex controls on geomorphological, geochemical, and climate records. This thesis explores the potential of one such tool, meteoric 10 Be. The delivery of meteoric 10 Be to the surface varies with precipitation and its adsorption to sediment has proven useful in studies of erosion. These characteristics indicate that meteoric 10 Be in lake sediments varies under changing climate and changing sediment influx, making it a potential recorder of past climate and landscape processes. To examine the controls on meteoric 10 Be concentrations in lake sediments, we develop a model that predicts concentrations of 10 Be in lake sediments as a function of atmospheric flux, sedimentation rate, and terrigenous input. The model was applied to two published datasets of 10 Be profiles in lake sediments from different settings to assess the sensitivity of individual controls on 10 Be concentration. Results show that while a variety of environmental conditions influence 10 Be in lake sediments, these can be quantified with surprisingly simple parameters. Assessment of the relative importance of model parameters requires the comparison of 10 Be concentrations in well-dated lake cores to independent paleoenvironmental proxies. We further validate this model and explore the application of meteoric 10 Be in lake archives, by exploring a new system, Blacktail Pond in Northern Yellowstone, for which a wealth of paleoenvironmental data exists. We present new meteoric 10 Be data in the core, and compare to model predictions based on sedimentation rates of both autochthous and allocthonous sediments and changes in the flux of meteoric 10 Be with precipitation. Surprisingly, patterns of measured 10 Be concentrations in Blacktail Pond sediments show little relationship to predicted concentrations, despite being of similar magnitude. Based on this analysis, we suggest that small lake systems may be most problematic for 10 Be analyses as they are most sensitive to changing 10 Be concentrations relative to changes in model parameters. This work provides a new quantitative framework to assess the control of sedimentation rates, inputs of allocthonous sediments, and hydroclimate in determining the 10 Be concentrations measured in lake sediments, and highlights the potential and limitations of meteoric 10 Be in quantifying past environmental changes.Item Assessment of streambank erosion along the North Fork Flathead River, northwestern Montana(Montana State University - Bozeman, College of Letters & Science, 1988) Ruth, John HelmsItem A productivity index model for Montana soils(Montana State University - Bozeman, College of Agriculture, 1989) Sandor, Stephen PaulItem The effect of unit weight and rainfall intensity on the erosion of unprotected slopes(Montana State University - Bozeman, College of Engineering, 1968) Rowlison, Dale LeroyItem Development and testing of a modified ground sediment trap(Montana State University - Bozeman, College of Agriculture, 1985) Bush, Leslie CarolItem Non-point source pollution control using dryland vegetative filter strips(Montana State University - Bozeman, College of Agriculture, 1999) Fasching, Richard AllanItem A hydromechanical erosion model for surface-mined areas(Montana State University - Bozeman, College of Agriculture, 1983) Cai, Guangrong