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.

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Ellesmere Island, in the Canadian Arctic, and adjacent sea were covered by ice during the Last Glacial Maximum (LGM, 20-10 ky ago). Postglacial rebound rates, indicative of ice sheet configuration, glacial and deglacial chronologies, and rheologic properties of the underlying mantle, were determined for Makinson Inlet using a new approach based on in-situ accumulation of cosmogenic ³⁶Cl.

Surface and subsurface gravel samples were collected from fourteen paleobeaches at elevations between the sea level and the Holocene marine limit at ca. 105 m a.s.l. Apparent ³⁶Cl ages range from ca. 4 to 13 ky and corrected ³⁶Cl ages (in calendar years) range from 10 ky to recent. Corrected ³⁶Cl ages agree with ¹⁴C ages of organic material from the same paleobeach sequence. Instantaneous uplift rates decrease from the high of 42 m ky^-1 at the beginning of emergence 10 ky ago, to less than 1 m ky^-1 today.

These results show the applicability of the cosmogenic ³⁶Cl exposure dating method in studies of postglacial emergence. The ability to date inorganic surficial materials has two main advantages: (1) the approach may be used on any material, such as rocks and sediments, that has been exposed at the surface due to isostatic rebound; and (2) an arbitrarily large number of samples can be collected at the same location, thereby providing the means of constructing a high-resolution record of exposure and isostatic emergence.

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The accumulation of in-situ ³⁶Cl and ¹⁰Be in soils from the crests of 11 moraines near Bishop Creek, California was used to determine time-integrated erosion rates and exposure ages. Previous studies have provided the exposure history and estimates of erosion for the Bishop Creek moraine sequence allowing for the validation of the ³⁶Cl/¹⁰Be model used in this study. With the two isotopes, simultaneous calculations of exposure ages and erosion rates of the soils have been made. Mean erosion rates of the soils calculated using ¹⁰Be correlate well with those calculated using ³⁶Cl, and range from 19 to 62 mm/ky. Erosion rates from ¹⁰Be have larger uncertainties than those calculated using ³⁶Cl and typically are slightly higher. Meteoric ¹⁰Be used in combination with the two in-situ isotopes yielded an average ¹⁰Be deposition rate (q) of 0.46 ± 0.16 x 10⁶ atoms cm^-2 yr^-1, in agreement with previous estimates.

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ABSTRACT

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A new glacial chronology for equatorial East Africa is developed using in-situ cosmogenic ³⁶Cl measured in 122 boulders from moraines on Mount Kenya and Kilimanjaro. The oldest deposits sampled on Kilimanjaro yield a limiting ³⁶Cl age of >360 calendar kyr (all ³⁶Cl ages are in calendar years, cal. kyr or cal. yr). On Mount Kenya, the oldest moraines give ages of 355-420 kyr (Liki I) and 255-285 kyr (Teleki). Given the uncertainty in our ³⁶Cl ages, the Liki I moraine may correspond to either marine isotope stage 10 or 12, whereas the Teleki moraine correlates with stage 8. There is no evidence for stage 6 on either mountain. The Liki II moraines on Mount Kenya and moraines of the Fourth Glaciation on Kilimanjaro give ages of 28 ± 3 kyr and 20 ± 1 kyr, respectively. They represent the Last Glacial Maximum (LGM) and correlate with stage 2 of the marine isotope record. A series of smaller moraines above the LGM deposits record several readvances that occurred during the late glacial. On Mount Kenya, these deposits date to 14.6 ± 1.2 kyr (Liki IIA), 10.2 ± 0.5 kyr (Liki III), 8.6 ± 0.2 kyr (Liki IIIA) and ~200 yr (Lewis); the corresponding deposits on Kilimanjaro have mean ages of 17.3 ± 2.9 kyr (Fourth Glaciation - Saddle), 15.8 ± 2.5 kyr (Little Glaciation -Saddle), and 13.8 ± 2.3 kyr (Fourth Glaciation - Kibo). These data indicate that the climate of the tropics was extremely variable at the end of the last glacial cycle.

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