<p> Sediment cores from Cascade Lake (68.38&deg;N, 154.60&deg;W) and Shainin Lake (68.34&deg;N, 151.05&deg;W), Arctic Alaska were selected for paleomagnetic analysis to assess 210Pb-14C age control using paleomagnetic secular variation (PSV) and relative paleointensity (RPI) features, and to quantify environmental magnetic variability during the Holocene and late Pleistocene. U-channels were studied through alternating field (AF) demagnetization of the natural remanent magnetization (NRM), and laboratory-induced magnetizations including anhysteretic remanent magnetization (ARM) acquisition, ARM demagnetization, isothermal remanent magnetization (IRM), and hysteresis experiments to determine magnetic mineralogy and grain-size variability. </p><p> Cascade Lake sediment yields a strong, well-defined characteristic remanent magnetization with average maximum angular deviation values of &lt; 2&deg; and average inclinations within 4&deg; of the expected geocentric axial dipole. Correlation of inclination changes with geomagnetic field models, as well as the Burial Lake record ~ 200 km to the west, indicates a variable offset between the Cascade Lake radiometric chronology and the preferred PSV-derived age model (PSV-1), reaching a maximum offset of 1.5&ndash;2.8 kyr during the mid-Holocene. This offset likely results from either a hard-water effect or the incorporation of watershed-stored terrestrial carbon into ¹⁴C samples. The PSV-1 age model extends the Cascade Lake age model to ~ 21 ka. Cascade Lake sediment may be suitable for RPI estimation using the IRM as a normalizer, however three methods of normalization (magnetic susceptibility (kLF), ARM, and IRM) produce similar normalized remanence results. </p><p> Hysteresis experiments and S-ratios for Cascade Lake glacial till and Shainin Lake sediment supports the hypothesis that local bedrock hosts predominantly high-coercivity magnetic material. However, S-ratios from Cascade Lake (~ 21 ka to present) and Shainin Lake (~ 12.6 ka to present) do not appear consistent with Burial Lake S-ratios, and most S-ratio variability is therefore interpreted as a result of site-specific sedimentation processes and background magnetic assemblages. A Younger-Dryas-aged peak in Shainin Lake S-ratios may be revealed by the increased sensitivity of the S-ratio parameter to magnetite at high-coercivity background levels. Cascade Lake S-ratios increase from 10.3 ka to present, potentially indicating Holocene biogenic magnetite production, down-core magnetic dissolution, or eolian input from a fine-grained, low-coercivity magnetic source that is clearly distinct from eolian magnetite at Burial Lake. Anhysteretic susceptibility (k<p style="font-variant: small-caps">ARM</p>)/k<p style="font-variant: small-caps">LF</p> may be a better indicator of this fine-grained magnetite population observed in the north-central Brooks Range, however the origin of this magnetic component remains unclear. This research highlights the potential advantages of supplementing ¹⁴C dating with additional dating methods, and will benefit from ongoing efforts to improve age control (e.g., cryptotephra exploration) and additional magnetic experiments to constrain the source of fine-grained magnetite.</p>

<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>The Peruvian/Bolivian Altiplano is an important hydrologic system for paleoclimate reconstruction because it is unique in its ability to record climate variability associated with the near-continental scale South American summer monsoon (SASM), which is responsible for much of the precipitation over the Amazon basin and the southern subtropics. Over long timescales moisture on the Altiplano fluctuates in intensity due to changes in precessional insolation forcing as well as teleconnections to decadal-to-millennial scale abrupt temperature shifts in the Northern hemisphere Atlantic. These long-term changes in moisture transport to the Altiplano have been observed in multiple paleoclimate records, including drill core records and paleo-lake level records, as apparent advances and retreats of large lakes in the terminal basin occupied by the Salar de Uyuni and the Salar de Coipasa. </p><p>Presented here are the results from three studies that utilize different methods to create a refined reconstruction of paleohydrology, as well as paleoclimate, on the Altiplano. A major goal of this research is a more detailed understanding of millennial scale climate variability as it relates to insolation changes and abrupt warming and cooling in the north Atlantic. The first study discusses the creation of a paleohydrologic profile to reconstruct north-south hydrological history using previously reported lake core sediment records the northern and southern basins of the Altiplano, and a new 14 m core from the Salar de Coipasa representing the last ~45 ka. The second study uses a terrestrial hydrology model to simulate lake level changes through time given changes in precipitation and temperature. The third study uses strontium isotopic measurements of carbonates and halites in a 220-m core from the Salar de Uyuni to determine how source waters to the southern basin have changed through time. </p><p>The paleohydrologic profile in the first study is constructed using records from three major basins within the Altiplano: Lake Titicaca in the north, and Salar de Coipasa and Salar de Uyuni in the south. The new continuous sediment core from Salar de Coipasa indicates a lake that has fluctuated between deep and shallow phases for the last 45 ka. Lacking sufficient calcium carbonate, we instead take advantage of the general correlation between d18O and d13C in closed basin lakes to approximate water balance using d13C from organic carbon. This reconstruction is validated with diatom paleoecological records. The isotope measurements and diatom records indicate that from 45-36 ka Coipasa was moderately deep, consistent with paleoshoreline evidence of paleolake Minchin (46-36 ka). From 36-26 ka a shallow lake <10 m deep occupied the Coipasa basin. During the LGM (26-21 ka) the lake varied from moderate to shallow and during the Holocene (< 10 ka) the lake evolved from a shallow lake to a salt flat. </p><p>The hydrologic model in the second study was run through many scenarios including increases in precipitation, decreases in temperature, and combinations of the two. During the LGM southern Altiplano lakes fluctuated between 3,660 - 3,700 masl. Model results suggest that during this period basin wide precipitation increased up to 250 mm/yr compared to modern values dependent on a temperature decrease of 5 °C relative to modern values. To create a lake at elevation 3,760 masl consistent with the highest paleolake phase (Tauca, ~16 ka) the model requires an increase of 350 mm/yr compared to modern values dependent on a 5 °C decrease in temperature (relative to modern values). An increase in temperature alone of 2 °C above modern values causes Lake Titicaca water level to decrease ~30 m, creating a closed basin lake. Results indicate that Lake Titicaca outflow is necessary to sustain large lakes in the southern basin, providing ~40-60% of total input via the Rio Desaguadero. </p><p>Analysis of a 220 m core from the Salar de Uyuni suggests periods of alternating wet and dry phases (indicated by alternating mud and salt units respectively) at the salar. Evident in the record is a transition at ~60 ka from sediments consistent with dry conditions ("playa lakes") to sediments consistent with deep lakes ("great lakes"). It has been shown that rivers and lakes in the Bolivian and Peruvian Altiplano display a range of Sr isotopic ratios that can be connected to the lithologies of specific drainage basins. Measurements of Sr ratios of the alternating halites and carbonate sediments are used to determine when paleolakes in the Salar were supplied by flow from the northern and central basins of the Altiplano, and when they were more a product of increased precipitation in the Uyuni basin. The results from Sr isotope analysis suggest that prior to ~60 ka the primary source of Sr to the Uyuni was local runoff and direct precipitation. Following the state change from the "play lakes" phase to the "great lakes" phase Sr isotope measurements suggest a significant influence from more radiogenic waters originating in the central and northern Altiplano basins. The reason for this state change is attributed to a combination of a general increase in precipitation following the onset of the MIS-4 (~70 ka) glacial period and downcutting of the Laka Jahuira hydrologic divide, which connects Lago Poopó in the central basin to the Salar de Coipasa. </p><p>This approach of reconstructing hydrology using the combination of multiple paleolake records, hydrological modeling, and isotopic tracers allows for a better understanding of how precipitation and temperature changes affect the advance and retreat of large lakes on the Altiplano, and ultimately a more accurate understanding of how decadal-to-millennial forcings influence the climate of the subtropical Andes.</p> / Dissertation

<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>

<p> The forest-tundra-short wave feedback is the dominant short wave (SW) vegetation feedback at mid-to-high northern latitudes and is an important feedback in Earth&rsquo;s climate system, especially due to its potential role in modulating glacial cycles. Little research has been done on how the strength of this feedback might vary with the background climate state. It is hypothesized that the feedback has generally strengthened over the last four million years. The feedback mechanism is hypothesized to be weaker under warm Northern Hemispheric conditions when tundra is primarily confined to the high Arctic than under cooler conditions in which the forest-tundra boundary lies generally south across the interiors of the large continental land masses. To test the hypothesis of the weakened/strengthened feedback, an Earth System Model of Intermediate Complexity is used that consists of a newly-developed simple dynamic terrestrial vegetation model coupled to a general circulation atmospheric model and a slab ocean. The response to the same orbital forcing ("cold orbit", favorable to Northern Hemispheric glacial inception) is analyzed for two eras: the PRISM mid-Pliocene Warm Period and the pre-industrial Holocene. </p><p> The change in top-of-atmosphere short wave net radiation (TOASW_net) that is attributable to including interactivity of vegetation in the systemic response to orbital forcing is decomposed into the product of three terms: the short wave vegetation feedback, an effective orbital forcing term, and the amplification of this effective orbital forcing by the climate system when vegetation cover is held fixed. Further analysis is carried out to determine why these terms differ between each era (mid-Pliocene and pre-industrial). </p><p> The results show that the change in TOASW_net that is attributable to including interactivity of vegetation in the systemic response to orbital forcing is about four times as strong in the pre-industrial as in the mid-Pliocene. The mid-to-high latitude SW vegetation feedback is about twice as strong for the pre-industrial as for the mid-Pliocene. This SW vegetation feedback is stronger in the pre-industrial mostly because its climate system is more sensitive in boreal spring to climate-induced changes in vegetation for various reasons, many of which boil down to geography. Surface albedo change is the principle mechanism by which the forest-tundra-short wave feedback operates, but it is discovered that there is also a component of this feedback that operates through the interactions between atmospheric reflectivity and vegetation. The results suggest that the forest-tundra-short wave feedback in glacial inception strengthens as the baseline climate cools from early Pliocene levels, with important implications for the start of major Northern Hemispheric glaciation and the increasing amplitude of glacial-interglacial oscillations over the last few million years.</p><p>

<p> This thesis explores the concepts and techniques of observation-model comparisons of the natural variability of the near-surface ocean on three different time scales. The emphasis on natural variability includes removing the all-time trend and seasonality of the data. All analyses used model outputs of the Community Climate System Model version 3.5 (CCSM3.5). </p><p> The first work, <u>An Ensemble Observing System Simulation Experiment of Global Ocean Heat Content Variability,</u> introduces the use of ensemble of model time series to study how a set of observations and how they are processed can capture the statistics of the system being observed. The technique is applied to global ocean heat content (OHC) down to 700m as observed and processed by the In-Situ Analysis System 2013. This study found that before the implementation of the global Argo program (1990-2005), the observed variability is too significantly biased by the low spatial resolution of the observations to return any meaningful estimates of global OHC variability with a median correlation score of 60% and a signal to noise ratio (SNR) of 1.9. The Argo era (2005-2013) is found to do a much better job at estimating global OHC variability to a median correlation score to 95% and an SNR of 14.7. However, this is only true for annual running means and longer; sub-annual variability is still unreliably resolved. </p><p> The second work, <u>Probability Angular Momenta of Multidecadal Oscillations of the North Atlantic,</u> explores concepts in non-equilibrium statistical mechanics, specifically probability angular momentum, as new tools in observation-model comparisons. The indices analyzed include an index related to the Atlantic Multidecadal Oscillation (AMO) and indices related to other oscillations thought to influence the observed variability in the AMO; the atmospheric North Atlantic Oscillation (NAO), the subsurface-ocean Atlantic Meridional Overturning Circulation (AMOC), and outflow from the Labrador Sea (LSO). The PAM analysis was found to detect cycles of the same magnitude and sign as traditional analyses for the simulated indices; for example, the -NAO leads +AMOC by 2 years, +AMO leads -NAO by 10-20 years, and PAMV leads +AMOC by 2-20 years, although the PAM results typically had too low of confidence to support any conclusions from the observed data. The PAM technique also returned a novel insight; a staistically-significant oscillation in the simulated LSO and AMO on the order of 400-1000 years. Since the model output has a time span of only 720 years, this indicates that the PAM technique may be able to detect modes of oscillation with periods on the order of or longer than the time span of the data analyzed, something that cannot be done to any statistical significance via traditional correlation and spectral techniques. </p><p> The final work, <u>PhaseMap: Comparison of Late Pleistocene Surface Temperature Proxies to an Accelerated CCSM3 Simulation,</u> compares the simulated ocean surface in a CCSM3 model run forced using the last 300,000 years of climate forcings to 50 paleotemperature proxies from deep ocean cores around the world. The accelerated model, which was accelerated 100x to simulate 300,000 years of climate in 3,000 model years, was found to agree poorly with the core proxies. While the core proxies correlate strongly with greenhouse gas, ice volume, and sea level forcings, the model results primarily follow the local insolation. It is unclear from this analysis whether this disagreement results from the model being too sensitive to insolation forcing, not sensitive enough to other forcings, or from the fact that the model's subsurface ocean doesn't respond quickly enough to the accelerated forcings. </p><p> These three different fields of ocean study are also inter-compared to explore their individual strengths and weaknesses, and where the techniques of one field may be useful in another. The modern subsurface ocean observations are plagued with uncertainties, and applying the observing system properties to a model was shown to help interpret the uncertainties associated with the spatio-temporal variability in the number and frequency of observations as well as the methodology used to create global maps from these observations. Paleoceanographers often have to work with proxy data that are unevenly sampled in time, and techniques commonly used to mitigate this issue (e.g. Lomb-Scargle method of periodogram estimation) can be used in modern studies where observational data is not available for short periods of time. </p><p> These works explore and propose techniques and concepts regarding surface and near-surface ocean variability on different temporal scales. They also highlight the importance of establishing connections across disciplines working on these different temporal scales. Together, these results improve our understanding of the role of the ocean on the climate system we depend on, and how different disciplines in ocean science can work together to improve our understanding even further.</p><p>

The impact of recent climate change on permafrost distribution was evaluated by repeating the 1964 survey of Roger Brown along the Alaska Highway from Whitehorse, YT to Fort St. John, BC in August 2007 and 2008. Results demonstrate that: (1) significant degradation of permafrost has occurred over the past four decades, especially in the southernmost part of the route where 67% of the permafrost sites in 1964 no longer exhibit perennially frozen conditions; (2) the mapped southern limit of discontinuous permafrost appears to have shifted roughly 75 km northward; (3) most of the permafrost still present in the study area is in peat or under thick organic mats, which probably relates to a large thermal offset or to the latent heat requirements of thawing permafrost; and (4) that where permafrost has persisted, it is very thin, discontinuous, at temperatures just below 0&deg;C, and its location may relate in part to the existence of atmospheric temperature inversions in the region. Changes in permafrost are attributed to significant climatic warming, primarily in winter, at rates of 0.4&deg;C to 0.5&deg;C per decade from 1965-2008. The results augment the very limited number of field studies of long-term change to permafrost in Canada, and are relevant to northern residents who must adapt to changing permafrost conditions.

No description available.

The recent, rapid decline in Arctic summer sea ice extent has prompted questions as to the rates and magnitude of previous sea ice decline and the affect of this physical change on ice-related ecosystems. However, satellite data of sea ice only extends back to 1978, and mapped observations of sea ice prior to the 1970s are sparse at best. Inventories of boreal ecosystems are likewise hampered by a paucity of investigations spanning more than the past few decades. Paleoclimate records of sea ice and related primary productivity are thus integral to understanding how sea ice responds to a changing climate. Here I examine modern sedimentation, decadal-scale climate change in the recent past, and centennial- to millennial-scale changes of the past 400 ka using both qualitative and quantitative diatom data in concert with sedimentology and organic geochemistry. Diatom taxonomy and corresponding ecological affinities are compiled in this study and updated for the Bering Sea region and then used as recorders of past climate changes. In recent decades, the Pacific Decadal Oscillation and the strength of the Aleutian Low are reflected by subtle changes in sediment diatom assemblages at the Bering Sea shelf-slope break. Farther back in time, the super-interglacial, marine isotope stage (MIS) 11 (428 to 390 ka), began in Beringia with extreme productivity due to flooding of the Bering Land Bridge. A moisture-driven advance of Beringian glaciers occurred while eustatic sea level was high, and insolation and seasonality both decreased at the global peak of MIS 11. Atlantic/Pacific teleconnections during MIS 11 include a reversal in Bering Strait throughflow at 410 ka and a relationship between North Atlantic Deep Water Formation and Bering Sea productivity. Finally, concentrations of the biomarker-based sea ice proxy, IP25, are compared to sea ice concentration across the Bering and Chukchi seas. Changes in the concentration of IP25 in the sediments may be driven by the length of time that the epontic diatom bloom lasts. When combined with a sediment-based proxy for sea surface temperatures, IP 25 can be used to reconstruct spring ice concentration.

<p> Long-term records of cyclic droughts are valuable for understanding future changes in hydrologic patterns and constraining climate models. A 3,300-year long record of &delta;¹⁸O and &delta;¹³C values from endogenic carbonate from Foy Lake, Montana is used to infer such droughts during the early Holocene. From 10.8 to 9.6 kyr BP, &delta;¹⁸O and &delta;¹³C values are low, indicating a period dominated by a cooler, less evaporative climate. Both records exhibit strong cyclicity in the ~200 yr range, which is inferred to be a solar cycle. A dramatic shift towards a warmer, drier climate occurs after 9.6 kyr BP in a step pattern, dramatic in the &delta;¹³C record. Warming occurs after the transition. Stochastic cyclicity dominates with weaker, but statistically significant, periodicities ranging from 40-70 yrs. These are believed to be an expression of the Atlantic Multidecadal Oscillation. This larger synoptic climate signal may be important to the overall Pacific Northwest climate. </p>