Application of Hydrogen and Oxygen Isotope Ratios of Water to the Study of Evaporation, Palaeoclimate, and Ancient Marine Conditions
Triple-oxygen and hydrogen isotopes of water are critical to aid our understanding of the hydrological cycle on the modern and ancient Earth. As such, this thesis presents advancements in techniques and in the understanding of the isotopic behaviour of water that undergoes evaporation in different conditions. An application of these findings is made to palaeoclimate data, and an expansion of the use of water isotopes from hydrated minerals is applied to a novel mineral system. Of fundamental importance to the hydrological cycle is the process of evaporation. Yet, under- standing how water isotopes behave as they experience evaporation remains poorly quantified. One limitation is the ability to confirm theoretical models or palaoeclimatic data sets with accurate, precise, and controllable experiments. To overcome this, I developed a widely applicable experimental method that permits the measurement of the triple-oxygen and hydrogen isotope evolution of an evaporating fluid and its coincident vapour that is both highly accurate and with a precision and sampling rate previously unobtainable. Additionally, the methodology permits a wide range of variables to be controlled or manipulated, allowing future scholars to thoroughly explore the effect of differing climatic conditions on the full suite of water isotopes as they undergo evaporation. One such variable explored is the morphology of an evaporating basin. Whilst currently neglected in studies that examine lacustrine palaeoclimate records, I present experimental data for both idealised and real basin morphologies (generated by 3D printing) that highlights a significant effect on the rate of isotopic change as a function of basin morphology, despite constant environmental conditions. Experimental data suggest that unless the surface area to volume (SA:V) ratio of a lake is considered, palaeoclimatic data from two lakes with distinct morphologies but undergoing evaporation under identical climates could be misinterpreted. Similarly, the results from the real basin morphology experiments suggest that rapid changes in SA:V ratio could result in inflections in the isotopic record that are solely a response to morphology as opposed to changes in climate. Understanding from the evaporation experiments is then applied to gypsum hydration water (GHW) data obtained from Lake Petén Itzá, Guatemala. The lake is in the northern neo-tropics and provides a complete record of climate variability over the last 43 ka. Using the GHW isotopic data in combination with a Monte Carlo and combined hydrological-climate re-analysis models, I determine relative differences between the cooler, drier stadial periods that are recorded in the sediments. In addition, multiple proxies indicate that intra-stadial variability is recorded at Lake Petén Itzá and this mirrors variation observed in polar oxygen isotope records. This suggests a close climate teleconnection between the poles and the tropics during these periods. Finally, I explore the use of oxygen and hydrogen isotopes in sedimentary talc. This is achieved by experimentally determining the fractionation factors between amorphous Mg-silicate and the water from which it precipitates. This is followed by calculating the fractionation factors as these precipitates undergo synthetic metamorphism to true talc. Isotopic measurements from Neoproterozoic samples are measured and the isotopic composition of the ancient precipitating fluid is estimated using the uncertainty of these parameters and Monte Carlo modelling. This thesis provides a strong experimental backbone for future examination of evaporating water under a variety of palaeoclimatic conditions and contexts. It demonstrates how insights from evaporation experiments can be applied to interpreting palaeoclimate data and how hydrated minerals of various types, and from a range of geological settings and times, can be used to provide robust information about the climate and fluids from which they formed.
Palaeoclimate, Water isotopes, Triple-oxygen, Gypsum, Talc
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