Applications of in situ cosmogenic nuclides to problems in Quaternary geology require increasingly accurate and precise knowledge of nuclide production rates. Production rates depend on the terrestrial cosmic-ray intensity, which is a function of the elevation and geomagnetic coordinates of a sample site and the geomagnetic field intensity. The main goal of this dissertation is to improve the accuracy of cosmogenic dating by providing better constraints on the spatial variability of production rates.In this dissertation I develop a new scaling model that incorporates the best available cosmic-ray data into a framework that better describes the effects of elevation and geomagnetic shielding on production rates. This model is based on extensive measurements of energetic nucleon fluxes from neutron monitor surveys and on more limited data from low-energy neutron surveys. A major finding of this work is that neutron monitors yield scaling factors different from unshielded proportional counters. To verify that the difference is real I conducted an airborne survey of low-energy neutron fluxes at Hawaii (19.7° N 155.5° W) to compare with a nearby benchmark neutron monitor survey. Our data confirm that the attenuation length is energy dependent and suggest that the scaling factor for energetic nucleons is 10% higher between sea level and 4000 m than for low-energy neutrons at this location. An altitude profile of cosmogenic 36Cl production from lava flows on Mauna Kea, Hawaii, support the use of neutron flux measurements to scale production rates but these data do not have enough precision to confirm or reject the hypothesis of energy-dependent scaling factors.

This research aims to understand the natural and human influences on erosion in three tributary watersheds to the Mekong River, Yunnan Province, China and to assess the utility of a novel application of isotopic indicators of erosion. It explores how erosion varies through time and space as a function of physical characteristics of the landscape, tectonic forces, and human alteration of the landscape for forestry and agriculture. To accomplish these goals, I use four sediment-associated radionuclides: in situ 10Be, meteoric 10Be, 210Pbex, and 137Cs. These isotopes accumulate in or on sediment grains, and each accumulates to a different depth on the landscape and has a different half-life. Thus, the isotopes can be used to track sediment as it moves across Earth's surface, each providing unique insight into processes occurring over a certain time period (from ~50 to 50,000 years) or eroding to a certain depth on the landscape. The studied watersheds range from 22° to 27° N latitude, and from 200 to 2500 km2 in area. I collected 54 samples of river-borne sediment within the three study watersheds, and measured the concentration of each isotope in every sample. In addition to the measured isotopic concentrations, I utilize on over 20 years of daily sediment yield data at the outlet of each watershed, hillslope steepness, normalized channel steepness (ksn), contemporary land-use data, elevation, and 56 years of mean annual precipitation data (MAP). Long-term erosion rates scale with topographic parameters in two of the three study basins, indicating that topography, or the underlying tectonic forces responsible for topography, control erosion rates over the past 6,000 to 50,000 years. Isotopic data also show that contemporary erosion is higher in cultivated areas than un-cultivated areas, a direct result of agricultural practices. Contemporary sediment yield, however, has not increased notably due to land-use change; however, under-representation of large stochastic events and sediment trapped by agriculture have reduced sediment yield relative to the long-term average in two of the studied watersheds. Overall, the data imply changes in contemporary erosion that are consistent with Chinese policies that promoted deforestation from the 1950's to the late 1980's and conservation from the late 1990's to present. This proves to be a significant finding, as the result of the top-down approach China has taken with conservation policy has been widely called into question in previous studies. While each isotope has the potential to provide unique information regarding erosional processes, in situ 10Be and 210Pbex proved to be the most useful, while meteoric 10Be was the most challenging to utilize. Though interpretation is complex, measuring all four isotopes on the same sediment samples helps to fully realize the potential of in situ 10Be to estimate background erosion by simultaneously allowing for assessment of contemporary and human induced erosion.

Carbon-14, a radioactive nuclide, is used in many industrial applications. Due to its wide range of uses in industry, many workers are at risk of accidental internal exposure to 14C. Being a low energy beta emitter, 14C is not a significant external radiation hazard, but the internal consequences posed by 14C are important, especially because of its long half life of 5730 years. The current biokinetic model recommended by the International Commission on Radiological Protection (ICRP) is a conservative estimate of how radiocarbon is treated by the human body. The ICRP generic radiocarbon model consists of a single compartment representing the entire human body. This compartment has a biological half life of 40 days yielding an effective dose coefficient of 5.8×10-10 Sv Bq-1. This overestimates the dose of all radiocarbon compounds that have been studied. An improved model has been developed that includes and alimentary tract, a urinary bladder, CO2 model, and an "Other" compartment used to model systemic tissues. The model can be adapted to replicate any excretion curve and excretion pattern. In addition, the effective dose coefficient produced by the updated model is near the mean effective dose coefficient of carbon compounds that have been considered in this research. The major areas of improvement are: more anatomically significant, a less conservative dose coefficient, and the ability to manipulate the model for known excretion data. Due to the wide variety of carbon compounds, it is suggested that specific biokinetic models be implemented for known radiocarbon substances. If the source of radiocarbon is dietary, then the physiologically based model proposed by Whillans that splits all ingested radiocarbon compounds into carbohydrates, fats, and proteins should be used.

The magnitude of global sediment flux from streams to the oceans over the last 5 Ma is poorly quantified, yet important for predicting future fluxes and deciphering the relative control of tectonic uplift, climate change, vegetation, and related feedback mechanisms on landscape evolution. Despite numerous proxy studies on global sediment delivery to the oceans, it remains uncertain whether bulk sedimentation increased, decreased, or remained approximately constant across one of the most significant global climate changes: the Plio-Pleistocene transition. New developments and strategies in the application of cosmic-ray-produced isotopes, in part developed by this thesis, provide records of pre-historic denudation of confined fluvial catchments in Texas and Yukon. Non-glaciated, tectonically passive regions were targeted in contrast to other studies on modern sedimentation rates in order to isolate the climate influence from glacial and tectonic controls. The results suggest that average catchment temperature, and surficial processes and other factors such as vegetation cover associated with temperature, are the primary controls on the variation in landscape denudation in regions lacking tectonics and direct glacial cover. Specifically, warmer temperatures yield higher denudation rates, both at the scale of glacial-interglacial climate change and over the Plio-Pleistocene transition. The implication is that stream sediment flux to the ocean from tropical and temperate regions was higher during the Pliocene than in the Quaternary. However, this may have been balanced by an increase in sediment flux from regions covered by warm-based glaciers during glacial periods, or by increased temporary continental storage during interglacial periods.

Thesis (Ph.D.) - University of Glasgow, 2008. / Ph.D. thesis submitted to the Department of Geographical and Earth Sciences, Faculty of Physical Sciences, University of Glasgow, 2008. Includes bibliographical references. Print version also available.

Dissertation (PhD)--University of Arizona, Tucson, Arizona, 2005.

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<p>  </p> <p>Reconstructing past glacier and ice-sheet extents is important to better understand how glacial systems have responded to past climate changes in hope of constraining predictions of their responses to ongoing anthropogenic climate warming. As such, the most recent period of climatic variations, from the Last Glacial Maximum (LGM, ca. 21 ka) through today, is of great interest as a prominent example of how ice has reacted to past climatic warming events. Surface exposure dating utilizing cosmogenic nuclides can directly constrain when past ice deglaciated in current and former glacial landscapes. Numerous studies have utilized long-lived cosmogenic radionuclides (i.e., 10Be, 26Al) in polar regions to reconstruct glacial systems. However, due to prevalent non-erosive cold-based ice, prior nuclides from pre-LGM can be preserved. </p> <p>The research described in this dissertation applies <em>in situ </em>cosmogenic 14C (<em>in situ</em> 14C), an emerging geochronometer, to polar glacial landscapes in Sweden and Antarctica to constrain the timing and rate of glacial ice retreat. <em>In situ </em>14C more closely reflects the post-LGM deglacial signal in polar regions because it is less likely to preserve prior nuclides (inheritance) under minimally erosive ice. Our cosmogenic 10Be–26Al–14C concentrations near the Riukojietna ice cap, the last remaining ice cap in Sweden, combined with a sedimentary record from a proximal proglacial lake, indicate the ice cap likely survived during a warm period in the Holocene, but was less extensive than today. The <em>in situ</em> 14C exposure data from nunataks in western Dronning Maud Land (DML), East Antarctica indicate significant coastal thickening (up to 850 m) not predicted by models to date. In addition, this work dates the timing of post-LGM ice surface lowering in two drainage basins in western DML. These results demonstrate the significant contribution of <em>in situ</em> 14C in polar regions.</p> <p>In addition to applications of <em>in situ</em> 14C in polar regions, this work also describes the development of a compositionally dependent <em>in situ</em> 14C production rate calculator. The ability to extract <em>in situ</em> 14C from samples which quartz cannot be separated (either quartz-poor or fine-grained) would allow new avenues of research. The computational framework will be a useful tool in efforts to broaden the utility of <em>in situ</em> 14C to quartz-poor and fine-grained rock types. </p>

Soil erosion undermines agricultural productivity, limiting the lifespan of civilizations. For agriculture to be sustainable, soil erosion rates must be low enough to maintain fertile soil, as was present in many agricultural landscapes prior to the initiation of farming. However, there have been few measurements of long-term pre-agricultural erosion rates in major agricultural landscapes. We quantified geological erosion rates in the Midwestern U.S., one of the world’s most productive agricultural areas. We sampled soil profiles from 14 native prairies and measured concentrations of the cosmogenic nuclide 10Be and chemically immobile elements to calculate physical erosion rates. We used the erosion rates and measurements of topographic curvature to estimate a pre- agricultural topographic diffusion coefficient. We find pre-agricultural erosion rates of 0.0001–0.1 mm yr-1 and a site-averaged diffusion coefficient of 0.005 m2 yr-1. The pre- agricultural erosion rates and diffusion coefficient we measured are both orders of magnitude lower than anthropogenic values previously measured in adjacent agricultural fields. The pre-agricultural erosion rates are one to four orders of magnitude lower than the 1 mm yr-1 soil loss tolerance value assigned to these locations by the U.S. Department of Agriculture. Hence, as currently defined, tolerable soil loss will lead to unsustainable erosion of Midwestern soils. However, quantifying natural erosion rates via cosmogenic nuclides provides a means for more robustly defining rates of tolerable soil loss and developing management guidelines that promote soil sustainability.