A total of 123 bulk and serial enamel samples were obtained from modern goats, horses and yaks from southern Tibet for C and O isotope analysis. The δ13C and δ18O values of tooth enamel were compared with the δ13C values of local vegetation and the δ18O values of local waters to examine the relationship between the isotopic composition of modern herbivores and their environment. The δ13C values of enamel samples from horses range from -11.2‰ to -13.9‰ with an average δ13C value of -12.7 ± 1.0‰ (n=13). The δ13C values of yak tooth enamel range from -7.3‰ to -14.2‰, averaging -10.1±1.4‰ (n=84). The goat teeth have δ13C values ranging from -7.8‰ to -12.1‰, with a mean of -10.2 ± 1.2‰ (n=26). These enamel δ13C values indicate that these modern herbivores were feeding predominantly on C3 plants, consistent with the current dominance of C3 vegetation in the region. Some of the samples have δ13C values between -7.3 and -9‰. Although these higher δ13C values could suggest consumption of some C4 plants by the animals, the lack of significant seasonal δ13C variations within individual teeth indicates that these higher enamel δ13C values are more likely due to consumption of C3 plants experiencing water stress and/or some CAM plants. This suggests that the "cut-off" δ13C value for a pure C3 diet can be as high as -8‰ in water-stressed environments. The δ13C variations within and between species primarily reflect the variations in the δ13C values of the C3 plant foodstuffs consumed by the animals. The δ13C values of tooth enamel do not show a trend with increasing elevation. Oxygen isotopic compositions of tooth enamel varied widely within and between species. In contrast to the small intra-tooth δ13C variations within individual teeth, serial enamel samples display large intra-tooth δ18O variations, reflecting seasonal variations in the oxygen isotopic composition of meteoric water. The mean δ18O values of tooth enamel from goats showed a correlation with water δ18O values, suggesting that the δ18O of tooth enamel can be used as a proxy for the δ18O of meteoric water. Unfortunately, the oxygen isotopic compositions of water and tooth enamel do not show a clear trend with increasing elevation in the study area, suggesting that quantitative reconstruction of paleoelevation in this region using reconstructed δ18O values of paleo-meteoric water from fossil tooth enamel or other O-bearing minerals may not be warranted. For a given elevation/environment, horses have the lowest enamel δ18O values while goats display the highest enamel δ18O values among the three species studied. The large inter-species δ18O variations are due to differences in physiology and diet/drinking behavior of the animals. This confirms the importance of species-specific studies when interpreting δ18O data of fossil mammalian teeth in a stratigraphic sequence as a record of paleoclimate changes. / A Thesis submitted to the Department of Geological Sciences in partial fulfillment of the requirements for the degree of Master of Science. / Spring Semester, 2007. / December 15, 2006. / Tibet, Oxygen, Herbivore, Enamel, Carbon, Isotopes, Altitude, Paleoelevation, Paleoclimate / Includes bibliographical references. / Yang Wang, Professor Directing Thesis; A. Leroy Odom, Committee Member; William Parker, Committee Member.

Includes bibliographical references. / The Sverdrupfjella Group in western Dronning Maud Land forms part of a 1200 Ma to 900 Ma orogenic belt which experienced a thermal overprint at around 500 Ma. Although the degree of tectonic reworking during this later event remains uncertain, evidence for late fluid alteration is widespread. In this study, the high-grade metamorphic rocks which make up the Sverdrupfjella Group were sampled in the central Kirwanveggen area. The stable isotope and whole-rock composition of these rocks have been determined, in order to test whether zones of intense fluid-rock interaction can be used as evidence for Pan African overprinting of Grenvillian orogenesis in western Dronning Maud Land.

Nitrogen is the only plant nutrient taken up as both a cation (NH4 +) and anion (NO3-). Nitrate is considered the "safe" form of N and NH/ is generally thought to be toxic, especially at high levels. High NH/ /NO3- ratios are thought to be toxic because they result in a rhizosphere pH low enough to damage root membranes, induced cation deficiencies, and build-up of NH3 caused by delayed NH/ assimilation. These factors can be minimized in hydroponic culture. The objective of these studies was to quantify the effects of high NH4 + IN 0 3- ratios on nitrification and growth of wheat in hydroponics. Two cultivars of wheat (Triticum aestivum L.) were grown to maturity with either 15% or 80% of the N supplied as NH4+. The effect of using CL- versus so/- as counter ions to NH4 + was also examined. Yield was not significantly affected by NH4 + ratio or counter ion. Seed protein was increased from 15 to 19% with high NH4 +. Harvest index was reduced from 52 to 48% with 80% NH4 +, but was unaffected by counter ion. Rates of nitrification in hydroponic culture are not well quantified and could result in significant conversion of NH4+ to NO3- before plant uptake. An isotopic dilution study was conducted to quantify rates of nitrification in hydroponic culture. A 2 x 2 x 2 factorial design was used to examine the effect of pH (5.8 or 7.0), inoculation with nitrifying bacteria, and the presence of plants. This study was done with wheat grown in vigorously-aerated, 2-L bottles. Each bottle contained 10 g of diatomaceous earth to provide surface area for microbial growth. Nitrate began to accumulate in 5 din unplanted, inoculated bottles at pH 7.0; in 20 d at pH 5.8 with inoculation; but did not begin to accumulate in non-inoculated bottles (pH 5.8 or 7.0) until day 30. Nitrate never accumulated in any of the planted bottles, most likely because plants consumed the No 3- that was produced. Calculations from the isotopic dilution measurements indicated that the rate of nitrification averaged 58 μmol No 3- L-1 d-1 in the planted bottles, and averaged 270 μmol NO3- L- 1 d-1 in unplanted bottles. Nitrification was likely reduced in the planted bottles because the reduced concentration of NH4 + limited nitrification. To provide rapid, inexpensive measurement of nutrient concentration in hydroponic solution, five colorimetric tests were evaluated. Tests for NO3- and PO4 were accurate and reliable, but the tests for SO4^2-, Si 0 2, and Fe need additional refinement.

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Global 𝛿^18O indicates that the late Pliocene/early Pleistocene (LP/EP) was characterized by quasi-cyclic variations of continental ice volume with a period at ∼ 41- kyrs. However, it is well-known that high-latitude summer insolation intensity, the mechanism conventionally believed to control ice sheet growth/decay, is paced by cycles near 21-kyrs. This mismatch constitutes the "41-kyr World Problem." Isolating Northern Hemisphere ice volume change during the LP/EP is a promising approach for testing recent 41-kyr World hypotheses. A previous study of planktic 𝛿^18O at ODP Site 625, located in the northern Gulf of Mexico, documented negative isotopic excursions attributed to episodic discharge of ^16O-enriched meltwater to the study site via the Mississippi River. However, the lack of a benthic 𝛿^18O stratigraphy left the Site 625 time-scale highly uncertain. Here, we present a Site 625 benthic 𝛿^18O stratigraphy for the LP/EP, which improves the Site 625 time-scale through correlation to the global 𝛿^18O signal of the LRO4 Stack. The new Site 625 chronology suggests the previous age-model may have artificially enhanced the 1/_41-kyr frequency, while reducing natural temporal variability. Upon comparing the timing of twelve planktic 𝛿^18O anomalies at Site 625 to cycles of obliquity and June 21, 65°N insolation, no clear relationship is observed. However, comparison of Site 625 planktic 𝛿^18O to benthic records clearly indicates that northern Gulf of Mexico surface waters were at times severely impacted by locally and/or regionally manifested influences beyond the global norm, thus demonstrating the need for further inquiry of planktic 𝛿^18O anomalies.

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