<p>ABSTRACT Carbon isotope measurements (&#948;13C) across tree rings retain long-term annual and seasonal climate trends that can be used to extend historical records in regions where instrumental observations are limited or unavailable. The &#948;13C value measured from successive tree-rings reflects changes in stomatal conductance, which varies as a stress response to changes in environmental moisture. In relatively dry environments, a decrease in moisture leads to a reduction in stomatal conductance and causes an observable increase in the measured tree-ring &#948;13C composition. Thus, changes in fractionation can be an indicator of varying water-stress associated with changing temperatures. Here, I investigate the use of annually resolved &#948;13C data to identify twentieth century Arctic temperature trends. I present a new annually resolved &#948;13C dataset spanning 50 years (1912-1961) from three Larix cajanderi tree cores collected in far northeastern Siberia. The dataset yields a strong correlation (r = 0.55) with an increase in temperatures associated with the Early Twentieth Century Warming (ETCW) event (1925 to 1946). In order to investigate whether this Arctic-wide temperature anomaly can be identified from other Arctic tree-ring sites, I compiled &#948;13C data from thirteen previously published high-latitude (>62 &#730;N) tree-ring chronologies. The combined dataset, which spanned nearly the entire twentieth century (1900-1998), identified a strong negative relationship (r = -0.53, p < 0.01) between net carbon isotope fractionation and temperature. This Arctic-wide tree-ring dataset showed strong correlation across the ETCW in particular (r = -0.86), as well as across the interval of rapid late twentieth century anthropogenic warming (r = -0.50). Identification of both the natural ETCW and the current anthropogenic Late Twentieth Century Warming (LTCW) event (1966 to 1998) demonstrates the potential for tree-ring &#948;13C records to extend our knowledge of Arctic temperature change beyond the limited historical record.

<p> Lipids preserved in soils and sediments are important proxies in paleoclimate research. However, various growth conditions that affect the organisms synthesizing the lipids can in turn affect the abundance and stable isotopic compositions of the lipids themselves, and, consequently, can introduce significant errors in the paleoclimatic inferences drawn from them. This work examines how two climate proxies based on lipids, namely, glycerol dialkyl glycerol tetraether (GDGT)-based paleotemperature proxies in soils, and paleohydrological proxies based on hydrogen isotopic composition of lipids, respond to variability in environmental and other growth conditions (e.g., carbon source).</p><p> In order to evaluate the role of annual precipitation amount on the distribution of soil GDGTs and on GDGT-based temperature proxies in soils, we studied GDGT distribution in soils collected from two environmental transects in the USA&mdash;a dry, western transect covering six western states and a wet, east coast transect from Maine to Georgia. Our results indicate a significant impact of precipitation amount on soil GDGT distribution, which is related to soil aeration that in turn depends on precipitation amount, and also to soil pH. Our results also indicate that below an annual precipitation of 700-800 mm yr^&ndash;1 the MBT/CBT-temperature proxy based on soil GDGTs is not applicable. Furthermore, due to the distinct GDGT distributions in soils under arid conditions, soil input into lacustrine or marginal marine environments cannot be estimated using BIT index.</p><p> In order to estimate the effects of variability in environmental conditions and utilization of different substrates on D/H_lipid, we studied two heterotrophic organisms&mdash;<i>Haloarcula marismortui</i>, a halophilic archaeon and <i>Tetrahymena thermophila</i>, a ciliated protozoan, in pure cultures. Our results from experiments with <i>H. marismortui</i> indicate that metabolism of different substrates leads to formation of reducing agents (mainly nicotinamide adenine dinucleotide phosphate or NADPH) with distinct D/H signatures, which is reflected in the significant D/H variations in isoprenoidal lipids (ca. 100&permil;). Growth temperature affects growth rate as well as enzyme activities, and salinity of the growth media affects mainly growth rate of <i>H. marismortui</i>, and both cause similar variations in D/H_lipid (ca. 20-30&permil;) that are smaller compared to the substrate-effect. <i>T. thermophila </i>, on the other hand, responds to variations in growth temperature in a different manner. Isoprenoid and fatty acids synthesized by <i> T. thermophila</i> generally become more D-enriched with temperature increase. The isoprenoid ranges from being D-depleted to D-enriched relative to water with temperature increase, but the fatty acids do not display similar patterns. Our results from <i>T. thermophila</i> culture experiments indicate that temperature has a critical control on the D/H ratios of NADPH and possibly also intracellular water, due probably to temperature effects on processes that are related to growth and metabolism of <i>T. thermophila. </i></p>

<p> The age of the Ruby Ranch Member (RRM) of the Cedar Mountain Formation in East-Central Utah was recently constrained using carbon isotope chemostratigraphy to span known excursions associated with the late Aptian. The RRM is characterized by calcrete horizons that are thought to occur across the C10 carbon isotope excursion. Along with carbonate stable isotope analyses and the region&rsquo;s paleo-position in a depositional basin on the leeward rain shadow of the Sevier Orogenic belt, this interval is hypothesized to coincide with an aridification event. Our research objective is to quantify the extent of this aridity using clumped isotope paleothermometry (<i>n</i> = 7) and paleoprecipitation proxies (n = 51) for samples collected across the C10 chemostratigraphic interval. Two weathering indices, CIA-K and CALMAG, were applied to data obtained using X-ray fluorescence spectrometry. Using these proxies, we determined mean annual precipitation across the RRM at its type section. Precipitation values (<i> n</i> = 27) obtained through CIA-K for identified paleosol horizons ranged between 795 and 1275 mm/year, and through CALMAG ranged between 735 and 1042 mm/year. Precipitation values decreased through the C10 interval which may indicate increased aridity. Clumped isotopes provided ?47 values ranging from 0.647 to 0.693&permil;. Paleotemperature measurements (n = 4) from accepted carbonate samples were between 27.9 and 46.3 &deg;C. Isotopic compositions of water calculated from carbonates ranged between -4.4&permil; and -1.9&permil; VSMOW. Precipitation values and temperatures were not lowest during the C10 interval. Temperatures peaked at the end of the C10 interval and decreased afterward, indicating a potential for cooler, more arid conditions. These results suggest that carbon cycle changes during the mid-Cretaceous may have influenced paleoclimate conditions experienced in terrestrial settings.</p><p>

Recent successful drilling operations at Lake El'gygytgyn, NE Russia have recovered sediment cores back to 3.6Ma, representing the longest time-continuous sediment record of past climate change in the terrestrial Arctic. Analysis of the hydrogen isotopic ratio (δD) of specific organic biomarkers allows reconstruction of past hydrological conditions, thereby providing a powerful tool for reconstructing past Arctic climate changes. Compound specific isotopic analysis of sedimentary lipids from this remote basin provides new insights into the climate evolution of the Arctic, capturing the mechanisms and dynamics of the last two glacial-interglacial transitions, potentially enhancing the accuracy of modeled future climate change projections and presenting an opportunity to estimate past polar amplification of climate change. The results of this research document the first continuous, high fidelity continental record of reconstructed δD in precipitation from terrestrial plant leaf waxes in the High Arctic spanning the last 120 ka. The hydrogen isotopic composition of lipid biomarkers were determined from previously obtained Lake El'gygytgyn sediment cores and compared with other multi-proxy evidence of past climate change within the lake basin. The modern isotope hydrology and controls on the δD lipid signal were first established within the El'gygytgyn Basin from modern precipitation, stream and lake waters, ice cover as well as modern vegetation, water column and lake bottom surface sediments in order to provide a modern context to properly constrain and interpret paleoclimatic proxy data. Reconstructed δD records of paleoprecipitation and temperature at Lake El'gygytgyn lead other northern hemisphere climate records (e.g. North Greenland Ice Core Project, NGRIP δ18O records) and are in phase with other continental and Antarctic climate records, suggesting early high northern latitude continental warming prior to established glacial-interglacial transitions. The data set generated here leads to multiple avenues of future work and provides critical insights into Arctic paleoclimate and paleohydrology, contributing to our understanding of high latitude environmental change over geological timescales. Collectively, the results of this dissertation research will provide a context for paleoclimate reconstructions and future organic geochemical and stable isotope analysis. Future application of compound-specific H isotope analyses to long drill cores (recovered in 2009; ∼315m of sediment) will potentially provide a quantitative high-resolution record of paleoclimatic and paleoenvironmental changes spanning the last 3.6 Ma.

This thesis aims to combine long high-resolution sediment data with information on sedimentation processes in and around Saanich Inlet, to better understand the seasonal sedimentation mechanisms in Saanich Inlet from the late Pleistocene to the early Holocene. These understandings are used to establish sediment provenance in Saanich Inlet with seasonal to sub-seasonal resolution, and to help understand paleoclimatic variations between 10700 and 2150 14C calibrated (cal) a BP (= cal yr BP). In Saanich Inlet, elemental variability within laminated sediments is largely controlled by mineralogical changes in the sediment caused by changes in sediment provenance and primary productivity. In particular, on Vancouver Island, gold and platinum group elements (PGE), occurring in soils as a result of weathering of metamorphic and igneous rocks during the process of pedogenesis, are only observed within a restricted area of naturally occurring minerals, which concentrate those elements in soil. Therefore, preferential deposition of such minor elements can be strongly related to a change in sediment provenance. Chalcophile elements, such as Cu, have successfully proven to be reliable indicators of heterogenic-origin and/or bottom water redox conditions. Furthermore, sediment provenance along the western slope basin near Bamberton are distinguished and identified by a strong expression of enriched excess CaCO3 in both organic (high Ca/Ti and Sr/Ca) and inorganic (high Ca/Ti and low Sr/Ca) forms. This dissertation is divided into five related and complementary parts. The first chapter introduces Saanich Inlet and provides an overview of climatic and oceanographic patterns. The second chapter explains the methodology used in this work, and the third and fourth chapters, which are intended for publication, examine paleoclimatic and oceanographic variability during selected intervals of the Holocene in sub-seasonal (third chapter) and multi-annual to decadal (fourth chapter) scales. The last chapter summarizes the conclusions. Because of this format, some repetition of the introductory material exists in the main chapters.

<p> My research focuses on reconstructing past climate in southern Arizona using cave deposits called speleothems. However, this necessitates a broader perspective than simply a geochemical time series, and therefore, I also investigate modern cave systems using a combination of modeling and observational datasets. Finally, cave deposits are fundamentally non-renewable resources, and sampling for past climate reconstruction can be destructive, unlike other cave uses. My last investigation is focused on developing possible best practice recommendations for paleoclimate scientists and other cave stakeholders moving forward. </p><p> We developed two new stalagmite records of past climate variability in southern Arizona over the past 7000 years. Past climate reconstruction from two caves (Cave of the Bells and Fort Huachuca Cave) highlights insolation control of southern Arizona hydroclimate from 7000-2000 years before present. Additionally, comparison between two stalagmites with different seasonal sensitivities uncovers a few eras of multi-decade long droughts in southern Arizona, which align with other regional reconstructions of past climates and elucidate forcings on Southwest paleoclimate as emergent from both external (insolation) and internal climate variability in the Pacific and Atlantic Ocean basins. Although the oxygen isotopic signal of cave calcite in speleothems is complex, agreement with these other records indicates that the speleothem records from these caves primarily record a climate signal. </p><p> Modeling and monitoring of modern caves both helps us interpret paleoclimate records and enhances our understanding of cave systems in their own right. Modeling of Cave of the Bells dripwaters demonstrates the effect of storage and mixing on the dripwater oxygen isotope signal; non-climate processes can imprint on dripwater variability on multidecadal timescales. Monitoring shows that on very small spatial scales, every cave is different, and even sites within the same cave respond uniquely to surface climate. Most notably, calcite oxygen isotopic composition, used to reconstruct past climate, shows seasonal variability unrelated to dripwater and surface rainfall oxygen isotope variability. Substantial oxygen isotope disequilibrium is identified at numerous caves sites in southern Arizona, and this understanding aligns with a growing number of cave studies that demonstrate the long-held assumption of isotopic equilibrium in cave systems may not always be valid or that the way in which we define isotopic equilibrium insufficiently captures the variety of processes controlling the oxygen isotopic composition of speleothems. Overall, however, monitoring can identify stalagmites that are more sensitive to surface climate and less sensitive to these in-cave processes by identifying sites with dripwater variability responses to surface rainfall variability and sites that precipitate close to oxygen isotopic equilibrium. </p><p> Finally, a major missing component in speleothem research is the fact that speleothems take thousands and sometimes hundreds of thousands of years to form. They are non-renewable resources on human timescales, and habitat for myriad microbes that have yet to be identified. Removal of speleothems for paleoclimate research is one of the only destructive uses of these deposits. With that in mind, I also analyze current methods of collecting speleothems and develop a framework based on two surveys of scientists and stakeholders to assist scientists and managers when evaluating potential methods of incorporating cave conservation into the speleothem sampling process. </p><p> Thus, I approach caves from a variety of angles and timescales, from the past through the present to the future, illuminating caves as complex scientific and social systems.</p>

<p> Petrography and phase equilibria involving the minerals trona (Na₂CO₃&bull;NaHCO₃&bull;2H₂O), nahcolite (NaHCO₃), and shortite (Na₂CO₃&bull;2CaCO₃) from the Eocene Green River Formation provide information on the paleoenvironments that controlled their formation during deposition and diagenesis. Shortite and trona are exclusive to the Wilkins Peak Member (WPM) of the Bridger Basin (BB), WY, whereas nahcolite is the primary Na-carbonate mineral in the contemporaneous Parachute Creek Member of the Piceance Creek Basin (PCB), CO. Trona from the BB and nahcolite from the PCB are stratigraphically associated with oil shale, suggesting deposition in perennial, density stratified saline lakes. Preserved primary textures of trona and nahcolite show that they formed at the air-water interface as microcrystalline chemical muds, which supports the hypothesis that precipitation occurred in contact with the early Eocene atmosphere. New experiments (temperature vs. <i>p</i>CO₂) in the NaHCO₃ -Na₂CO₃-CO₂-H₂O system show that nahcolite forms at a minimal <i>p</i>CO₂ concentration of ~ 680 ppm at 19.5 &deg;C, 1 atm, which is lower than the <i>p</i>CO₂ determined by Eugster (1966) (1330 ppm and 1125 ppm with NaCl added). These new results anchor the minimum <i> p</i>CO₂ of the early Eocene atmosphere at ~ 680 ppm. </p><p> Shortite formed diagenetically during burial in the BB as displacive crystals, fracture fills, and pseudomorphous replacements of a precursor Na-Ca-carbonate in carbonate mudstone and oil shale. Experimental results on the thermal stability of shortite in the Na₂CO₃-CaCO₃-H₂O system show that it forms at temperatures > 55 &deg;C, 1 atm, and 1.1m Na₂CO3 via the reaction: Na₂CO₃&bull;CaCO₃&bull;2H₂O_(pirssonite) + CaCO_3(calcite) = Na₂CO₃&bull;2CaCO_3(shortite) + 2H₂O. The large area over which shortite occurs in the WPM indicates saline pore fluids existed in the buried lacustrine sediments and that, at times, giant Na-CO₃-rich saline alkaline lakes existed in the BB during WPM time. The thermal stability of shortite, coupled with vitrinite reflectance data and inferred regional geothermal gradients, establish that the WPM was buried to depths of ~ 1,500 m and experienced post WPM erosion of ~ 800 m.</p>