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The relative quantities of 16O and 18O (δ18O) in water molecules within ice cores has proven critical for reconstructing temperatures going back hundreds of thousands of years. However, the δ18O record can be skewed through several sources, such as isotopic diffusion, meltwater percolation, and exchanges between atmospheric water vapor and the surface snow. All of these can reduce the accuracy of δ18O-based temperature reconstructions. Two climatological events are explored to determine whether they may have been post-depositionally modified by one or more of these mechanisms. Within three ice cores extracted in 2011 from Mt. Ortles in the Italian Alps, the δ18O record from the 2003 European Heatwave might be preserved in a shallow annual layer tentatively attributed to 2002. Additionally, the δ18O minimum in winter 1946/1947 exceeds the expected level given that winter 1946/1947 was not extremely cold. A chronology based on counting annual pollen layers was used to explore if a high-resolution chronology would suggest shifting the anomalous δ18O records in-phase with air temperature data. The glaciological/meteorological conditions were modeled using EISModel to determine air temperature and snow mass balance conditions when these δ18O records were formed. The locations of ice lenses in the ice core stratigraphy were identified to explore the possible relationship between meltwater refreezing and isotopic offsets in 2003 and the 1940s. Results showed the decoupling between δ18O and local air temperature in 2003 and the 1940s persisted, and modelling suggests that all the snow that accumulated over ~9 months melted during the 2003 European Heatwave. These observations imply that the anomalous δ18O records may stem from post-depositional effects. Due to the lack of ice lenses concurrent with the anomalous δ18O records and because EISModel-derived melt rates during the 2003 European Heatwave and literature-derived melt rates in the 1940s all exceeded the low melt rate scenarios suitable for meltwater-induced isotopic fractionation, meltwater percolation is not a likely explanation of the 2003 and 1940s δ18O anomalies. EISModel suggests the anomalous δ18O values around 2003 may be explained by isotopic exchange between the surface snow and atmospheric water vapor or fractionation from sublimation/deposition within the snowpack. The extent to which these processes may modify δ18O in high altitude/low latitude environments is unknown, indicating further investigation is needed.

Rapid changes in global and local climate and weather, noticeable over our lifetimes, have spurred heated public and political debate over the cause of these climatic changes, whether or not we can reverse, stop or slow down this process, and how to create adequate models that can predict Earth's changing climate. To create the most accurate models of future climate, many researchers look to past greenhouse intervals for insight; intervals of time identified by high modeled atmospheric CO2, a lack of glacial deposits, and geologic evidence of globally high sea-levels--all of which suggest little to no glacial ice and a relatively monotonous climate. However, recent stratigraphic and geochemical evidence from the Cretaceous 'supergreenhouse' interval document compelling evidence to support dynamic greenhouse climates. The goals of the studies hereafter described are to document and describe orbital-scale (104-105 yr) climate fluctuations recorded by δ18O as a proxy for glacio-eustasy and sea surface temperature and µNd as a proxy for continental weathering and wet/warm or dry/cool climate during two global greenhouse intervals: the Late Silurian and Late Devonian. The first study (Chapter 1) explores whether glacio-eustasy was the driver for orbital-scale shallowing-upward cycles developed in Late Silurian and early Late Devonian tropical and subtropical greenhouse climates. Two intra-cycle δ18O conodont apatite trends were observed: asymmetric trends demonstrating progressive δ18O increases coincident with facies shallowing, or symmetric trends demonstrating initially decreasing, then increasing δ18O values. These isotopic trends and intra-cycle magnitudes support the hypothesis that these cycles developed in response to glacio-eustasy during glacial stages, implying that Late Silurian and Late Devonian greenhouse climates were more dynamic than traditionally assumed. Relationships between orbital-scale continental weathering flux and glacial-interglacial marine cycles was first assessed comparing intra-cycle µNd and δ18O values from Middle Pennsylvanian icehouse cycles (Chapter 2). Observed conodont δ18O trends support previous interpretations that sampled cycles were generated by glacio-eustasy (30-50 m magnitudes) combined with