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This thesis develops and applies new techniques for reconstructing past environments from the speleothem archive. Chapter 3 introduces Ca isotopes as a novel palaeo-aridity proxy. Recent dripwater, farmed calcite and bedrock data show that, in the modern environment, 36% of initial dripwater Ca is removed by prior calcite precipitation at the HS4 dripsite (Heshang Cave, Central China). A speleothem record spanning the 8.2kyr event suggests this value increased to 60% during the event, in response to a decrease in effective rainfall of approximately a third. Modern Ca isotope systematics, and the cycling of other Group II metals, are studied further in Chapter 4. Data from Heshang Cave monitoring samples explore the temporal and spatial controls on these proxy systems. Results highlight the importance of secondary calcite as a potential source of dripwater metals, as well as the significant contribution of particulate calcite to bulk dripwater chemistry. At the HS4 dripsite, over a third of bulk dripwater Ca is present as particulate calcite. This influences bulk dripwater Ca concentrations and isotopes, as well as potentially impacting speleothem growth and chemistry. Chapters 5 introduces CaveCalc, a new numerical model for dripwater and speleothem chemistry and isotopes based on PHREEQC. Key strengths of CaveCalc include its ability to model multiple proxy systems in a single framework, the ability to quantify the extent of open-system dissolution, and the extensible nature of its design. Chapter 6 applies CaveCalc to disentangling the controls on dripwater and speleothem δ¹³C values. Model results are compared with speleothem data. At Heshang Cave, anomalously high δ¹³C is quantitatively explained as the result of CO₂ degassing and prior calcite precipitation, an interpretation made possible by the availability of coupled δ¹³C, a¹⁴C and Ca isotope data. This approach provides a powerful tool to help researchers better interpret dripwater and speleothem δ¹³C data.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1983. / Vita. Includes bibliographical references (p. 268-279).

Thesis (M.S.)--Ohio State University, 1984. / Includes bibliographical references (leaves 134-138). Available online via OhioLINK's ETD Center

A variety of biological and organic reaction mechanisms are studied using powerful tools from experimental and theoretical chemistry. These tools include the precise measurement of kinetic isotope effects (KIEs) and the use of theoretical calculations to predict KIEs as well as determine factors that contribute to reaction acceleration and selectivity. Theoretical analysis of the Swain-Schaad relationship involves the prediction of a large number of isotope effects and establishes the semiclassical boundaries of the relationship. Studies on the mechanism of oxidosqualene cyclase involve the determination of a large number of precise KIEs simultaneously. Transition state models for the Sharpless asymmetric epoxidation have been developed that explain the versatility, high selectivities, and ligand accelerated catalysis of the reaction. Theoretical predictions on the proposed enzymatic mechanism of flavin dependent amine oxidation suggest a hydride transfer mechanism and rules out mechanisms involving covalent intermediates. Finally, a theoretical analysis of Diels-Alder reactions successfully describes the unexpected exo selectivity in some of these reactions.

Unfortunately, chemists can never experimentally unravel a full reaction pathway. Even our ability to define key aspects of mechanisms, such as short-lived intermediates and the even more ephemeral transition states, is quite limited, requiring subtle experiments and subtle interpretations. Arguably the most important knowledge to be gained about the mechanism of a reaction is the structure and geometry of the transition state at the rate-limiting step, as this is where a reaction’s rate and selectivity are generally decided. The Singleton group has developed a methodology for predicting the combinatorial kinetic isotope effects (KIEs) at every atomic position, typically carbon or hydrogen, at natural abundance. A combination of experimental isotope effects and density functional theory (DFT) calculations has greatly aided our ability to predict and understand a reaction’s pathway and transition state geometries. Precise application of this method has allowed for the mechanistic investigation of a myriad of bioorganic, organic, and organometallic reactions. The technique has been applied in the analysis of the catalytic borylation of arenes via C-H bond activation, dynamic effects in the enyne allene cyclization, palladium catalyzed allylic alkylation, the nature of proton transfer in orotate decarboxylase, and the epoxidation of enones with t-butyl hydroperoxide.

A variety of organic reactions provide experimental observations that are not explained by current models of reactivity and selectivity. This dissertation describes a combination of experimental and theoretical studies of such reactions. In the ene reaction of singlet oxygen with tetramethylene, it is found that standard statistical rate theories fail to account for the observed kinetic isotope effects, particularly with regard to their broad temperature independence. Dynamics trajectories are found to account for the observed isotope effects. In the dimerization of cyclopentadiene, novel "dynamic" isotope effects are observed on the 13C distribution in the product, and a method for the prediction of these isotope effects is developed here. In the cycloaddition of diazomethane with dimethylfulvene, it is found that the current model of the mechanism as a [6 + 4] cycloaddition is incorrect, and a new mechanism is proposed. Isotope effects have been measured for the recently reported unusual "on water" quadricyclane cycloadditions, and the implications of these observations toward the mechanism are discussed.

To better understand how the evolution of Cenozoic deep-water circulation related to changes in global climate and ocean basin configuration, we generated Nd isotope records from Ocean Drilling Program sites in the southeastern Atlantic to track deep water mass composition through time. We used fossil fish debris from ODP Sites 1262-1264 (Leg 208), spanning present-day water depths of 2500-4750 m, to reconstruct the isotopic signature of deep waters over the past ~53 Ma. The data indicate an initial transition from relatively non-radiogenic values (??Nd=~-10) at 53 Ma to more radiogenic values (~-8.5) at ~32 Ma. From ~32 Ma to 3.85 Ma, the Nd signal becomes more nonradiogenic, ~-12.3 at the top of the record. Comparison of our data with Nd isotopic records derived from a North Atlantic Fe-Mn crust show similar non-radiogenic values (~-10.5) in the 53??32 Ma interval and a trend toward more non-radiogenic values beginning at ~20 Ma. The data likely reflect an overall shift from a Southern Ocean deep water source to the ultimate incursion of deep waters from the North Atlantic. The non-radiogenic values at the base of the record reflect a Southern Ocean source of deep water. The shift toward more radiogenic values indicates an increased contribution of Pacific waters to the Southern Ocean source as the tectonic gateways changed after ~35-33 Ma. The subsequent trend toward more non-radiogenic Nd isotope values is approximately concurrent with the increase of benthic foraminiferal ??18O values, based on comparison with a compilation of global data. Thus, changes in oceanic gateway configuration in addition to overall cooling and the build-up of continental ice on Antarctica may have altered the Nd isotope character of Southern Ocean deep waters during the early Oligocene.

Three stalagmites, JSD 01, JSD 02, and L12, were collected from Libo county in the southern Guizhou provence. JSD 01 and JSD 02 are 14cm long stalagmites from Golden Lion Cave and L12 is a 7.1cm long stalagmite from Dragon Spring Cave. Subsamples were drilled along the central axis for JSD 01V while subsamples of JSD 01H were obtained along the horizontal axis. Subsamples were analyzed for carbon and oxygen stable isotopes. The age determination for stalagmites were done by U-Th dating. The JSD 01 dates range from 93,700 to 95,200 yr B.P.; the JSD 02 dates are from 87,900 to 88,200 yr B.P.. The L12 dates are from 750 to 9,560 yr B.P.. The stalagmite record from Golden Lion Cave is correspondent to the marine isotope stage 5 (MIS 5) in age. The £_18O values of stalagmite JSD 01 record in MIS 5c are ranging from -6.58‰ to -5.17‰ ; £_13C values are from -9.19‰ to -6.84‰. Stalagmite JSD 01H records are ranging from -8.11‰ to -5.7‰ for £_18O and from -9.67‰ to -2.46‰ for £_13C. The £_18O values for stalagmite JSD 02 record in MIS 5c are from -8.15‰ to -7.12 ‰ and are from -9.09‰ to -5.19‰ for £_13C. The results indicate a warm and wet climate prevailing during MIS 5c and MIS 5b in Guizhou. An intensification of summer mosoon would increase in precipitation and vegetation above the cave. Result from JSD record shows data obtained from the horizontal axis may be younger in age than vertical axis in terms of sampling direction. The stalagmite L12 contains two sections: (1) Dates range from 700 to 2,400 yr B.P. between top and 35mm: the £_18O values are ranging from -8.23‰ to -7.07‰ while £_13C values are ranging from -11.61‰ to -6.98 ‰; (2) The £_18O values are ranging from -10.52‰ to -7.54‰ and £_13C values are from -10.75‰ to -6.98‰ between 35mm and 71mm. Results suggest a warm and wet climate prevailing during 9,000-9,600, with lighter £_18O values reflecting a strong summer monsoon. A decline of summer monsoon occurred around 9,000 yr B.P. as shown by a sharp increase in the £_18O. This sudden climatic change might lead to a decrease in vegetation and soil loss above the cave, resulting in the cease growth of the stalagmite. Around 2,400 yr B.P. the growth of stalagmite was resumed. The lighter £_13C values and heavier £_18O values represent a cooler and drier climate in late Holocene. After ~1,000 yr BP, a deforestation event occurred in the area probably which was induced by human activity.

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