Quaking aspen (Populus tremuloides) has the largest natural distribution of any tree native to North America, ranging from Alaska through the breadth of Canada and south to mid-Mexico. The Laurentide ice sheet occupied most of the current range of P. tremuloides until the late Pleistocene epoch, so this species has undergone a significant, geologically recent range expansion. Surprisingly, range-wide patterns of genetic variation in P. tremuloides have never been described. Using a sample set representing the full longitudinal and latitudinal extent of the species distribution, I have conducted a phylogeographic analysis for P. tremuloides. Preliminary results comparing both nuclear and chloroplast DNA sequences revealed surprisingly low levels of divergence across the range. Because of this remarkably shallow genetic divergence among aspen populations, I used a set of rapidly-evolving molecular markers (microsatellites) to describe patterns of gene flow and diversity and to correlate those patterns with landscape features and histories. I analyzed eight microsatellite loci in 794 individuals from 30 sampling sites. From this multilocus data set, I identified pronounced genetic structuring across the range. Strikingly, sampling sites representing the southwestern portion of the range, the western United States and Mexico, form a distinct cluster. Sites within this southwestern cluster display dramatically reduced within-site genetic diversity but elevated regional genetic diversity, which suggests that populations in the southwestern portion of the range make up a stable edge persisting through multiple climate oscillations. Based on the uniqueness of the southwestern cluster and the climatic differences between the southwest and northern portions of the range, I propose that the southwestern cluster may represent a distinct ecotype. I also identified hotspots of diversity that correspond with potential refugia during the last glacial maximum but additional work is needed to refine these patterns. Further, my findings provide a solid foundation for a range of future studies on adaptive genetic and trait variation in this species.

Earth is characterized by episodes of glaciations and periods of minimal or no ice through geologic time. Using the linear energy balance model (EBM), nonlinear EBM with empirical ice sheet schemes, the general circulation model coupled with an ice sheet model, this study investigates the occurrence and stability of glaciations in geologic time. The simulations since the last glacial maximum (LGM) suggest that the summertime thawline of ice sheets conforms closely to the equatorward edge of the ice sheets and implies the relative stability toward deglaciation. CO2 levels are indispensable in controlling the initiation of ice sheet in the Cretaceous. At low CO2 levels, ice sheets exist in all periods no matter LGM or the last interglacial (LIG) orbital elements; however, at high CO2 levels ice sheets rarely exist. The simulations agree well with recent geological evidence of the hysteresis of glaciations in the Permo-Carboniferous. Gondwanaland reached its glacial maximum when CO2 level was roughly the same or slightly higher than the preindustrial value. With a further increase of CO2, deglaciation dominates and results in an ice free state. Again, if CO2 decreased to the present level, Gondwanaland would be glaciated once more and start a new cycle of glaciation and deglaciation. Simulations from five paleogeography maps in Gondwanaland with a suite of CO2 levels and different orbital elements reveal that paleogeography, CO2 levels and the Milankovitch cycles all contribute to the glaciations of Gondwanaland. This study shows that orbital elements alone are insufficient to account for the evolution of ice sheets. Net radiative forcing caused by greenhouse gases, such as CO2 and solar constant change are the primary drivers to glacial inception or demise. Continental geography, CO2 levels, solar constant change, and the Milankovitch cycles complicate the glacial history of Earth.

The objective of this project is to reconstruct a picture of initial Laurentide Ice Sheet retreat at the end of the Last Glacial Maximum (LGM) using geochemical proxies in Gulf of Mexico sediments, and place the reconstruction into global perspective. The project asks two questions. (1) Can a time frame be established for initial retreat of the Laurentide Ice Sheet? (2) If so, how does the timing compare to that of other large ice sheets and mountain glaciers in both hemispheres? Sediment core MD02-2550 from the anoxic Orca Basin offers excellent preservation and a high sediment accumulation rate. Twelve accelerator mass spectrometry 14C dates provide very good age control from 18.36 - 23.88 ka, the transitional period from glacial to deglacial conditions. Paired Mg/Ca and d18O from the planktonic foraminifera Globigerinoides ruber (pink variety) were combined with a matching record from the upper half of the same core from a previous study (Williams et al., 2010), expanding the record to 10.73 - 23.86 ka. Sea surface temperature (SST) derived from Mg/Ca exhibits a mean value of 23.0 ± 0.8°C through the LGM (18.4-23.9 ka), ~3.9°C below the modern summer mean. At 18.4 ka, mean values drop in an anomalous cold snap, exhibiting a mean of 21.7°C that lasted until 17.8 ka. At 17.8 ka, SST begins a recovery warming toward present day conditions. This warming occurs markedly early relative to the onset of the Bølling-Allerød warm period, known best from Greenland ice core records. The d18O of seawater exhibits no sustained shift toward more depleted values that would be consistent with a single major surge of initial meltwater. Instead, d18Osw appears to have been over 1.5 per mil below the modern mean throughout the LGM, persisting through the early deglacial period, and not shifting toward more positive values until well into the Younger Dryas. The corresponding salinity estimates were likewise ~2 psu lower than modern surface waters. Several negative excursions (~1 per mil) during the LGM and deglaciation coincide with millennial-scale retreats of individual lobes along the southern margin of the Laurentide Ice Sheet. These retreats and re-advances have previously been suggested to mirror small short-term excursions in Greenland ice core d18O, that reflects air temperature changes. The consistently depleted d18Osw-ivc values and corresponding salinity estimates through the LGM require a mechanism to create a steady-state lower salinity environment in the northern Gulf of Mexico during the LGM, which would persist as SST changed.

This thesis explores the natural forces controlling variability of the tropospheric oxidants on interannual to glacial-interglacial time scales. The oxidants (primarily OH and ozone) determine the lifetime of many trace gases of human interest, including air pollutants and long-lived greenhouse gases such as methane. The oxidants respond to meteorological conditions, precursor emissions (natural and anthropogenic), and surface and overhead stratospheric boundary conditions, all of which have changed since the Last Glacial Maximum (LGM; ~21ka). This dissertation first examines in mechanistic detail the effect of variability in the lightning source of nitrogen oxides \((NO_x)\) precursors on interannual variability (IAV) of the oxidants in the recent past. An optimized technique is presented to constrain the lightning \(NO_x\) source in the GEOS-Chem global chemical transport model (CTM) to time-varying satellite data from the Lightning Imaging Sensor. This constraint improves the ability of the CTM to reproduce observed IAV in 9-year (1998-2006) hindcasts of tropical ozone and OH. IAV in ozone and OH is more sensitive to lightning than to biomass burning, despite greater IAV in \(NO_x\) from the latter source. The sensitivity of OH to lightning reflects positive chemical feedbacks on ozone production, \(HO_x\) recycling, and loss frequencies. This dissertation next introduces an offline-coupled climate-biosphere-chemistry framework for determining oxidant levels at and since the LGM. Detailed simulations of tropospheric composition are performed by GEOS-Chem driven by meteorological fields from the GISS ModelE general circulation model, land cover from the BIOME4-TG global terrestrial equilibrium vegetation model, and fire emissions from the LMfire model. Time slice simulations are presented for the present day, preindustrial, and two different possible representations of the LGM climate. Sensitivity of the results to uncertainty in lightning and biomass burning emissions is tested. Though well-buffered, all simulations find net reduced oxidative capacities relative to the present day. The most important parameters for controlling tropospheric oxidants over glacial-interglacial periods are changes in overhead ozone, tropospheric \(H_2O\), and lightning. The results are discussed in the context of the ice-core record, particularly for methane. / Engineering and Applied Sciences

The location of fronts has a direct influence on both the physical and biological processes in the Southern Ocean. However, until recently fronts have been poorly resolved by available data and climate models. In this thesis we utilise a combination of high resolution satellite data, model output and ARGO data to improve our basic understanding of fronts. A method is derived whereby fronts are identified as local maxima in sea surface height gradients. In this way fronts are defined locally as jets, rather than continuous-circumpolar water mass boundaries. A new climatology of Southern Ocean fronts is presented. This climatology reveals a new interpretation of the Subtropical Front. The currents associated with the Subtropical Front correspond to the western boundary current extensions from each basin, and we name these the Dynamical Subtropical Front. Previous studies have instead suggested that the Subtropical Front is a continuous feature across the Southern Ocean associated with the super gyre boundary. A comprehensive assessment of the relationship between front locations and wind stress is conducted. Firstly, the response of fronts to a southward shift in the westerly winds is tested using output from a 100 year climate change simulation on a high resolution coupled model. It is shown that there was no change in the location of fronts within the Antarctic Circumpolar Current as a result of a 1.3° southward shift in the westerly winds. Secondly, it is shown that the climatological position of the Subtropical Front is 5-10° north of the zero wind stress curl line, despite many studies assuming that the location of the Subtropical Front is determined by the zero wind stress curl. Finally, we show that the nutrient supply at ocean fronts is primarily due to horizontal advection and not upwelling. Nutrients from coastal regions are entrained into western boundary currents and advected into the Southern Ocean along the Dynamical Subtropical Front. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Submitted.</p>

The objective of this dissertation is to apply radiogenic isotopes extracted from marine sediments to investigate aspects of global climate change over the past 25 kyr, especially ocean and atmospheric circulation, continental aridity, and hydrology. By focusing on the geochemical records from marine sediments and authigenic precipitates preserved in these sediments, I aim to better understand climate forcing and feedback mechanisms, which are critical to models of climate change. Firstly, I have investigated the dynamics of the Intertropical Convergence Zone (ITCZ) over the past 25 kyr in the eastern equatorial Pacific by fingerprinting dust provenance using radiogenic isotopes (Nd, Sr, Pb) and trace elements (Fe, Si, Ba) in the detrital fraction of marine sediments along a transect across the equator at 110ºW. Results from this study suggest no glacial-Holocene difference in the mean position of the ITCZ, but a more northerly, possibly stronger, deglacial ITCZ. Secondly, I have applied Nd isotope ratios from authigenic precipitates extracted from marine sediments and those from fish debris to trace past intermediate water circulation changes on glacial-interglacial and millennial timescales. The authigenic Nd isotope record from the Florida Straits suggests a reduced circulation of Antarctic Intermediate Water (AAIW) into the tropical North Atlantic during the Younger Dryas (YD) and Heinrich 1 (H1) events, associated with a significant reduction in AMOC. However, in the Southern Caribbean, apparent deviations in the Nd isotopic compositions between the acid-reductive leachate and the fish debris suggest that the leachate method is not reliable at this location and that it needs to be tested in more detail in various oceanic settings. In the Southern Caribbean, the fish debris Nd isotope results suggest a two-step recovery of the upper North Atlantic Deep Water during the last deglaciation. Comparing our new fish debris Nd isotope data to authigenic Nd isotope data for sediments from the Florida Straits and the Demarara Rise, we propose that glacial and deglacial AAIW does not penetrate beyond the lower depth limit of modern AAIW in the tropical North Atlantic. Both studies suggest a tight connection between Atlantic intermediate water circulation variability and high-latitude North Atlantic climate change.

A regiäo de Monte Verde (Camanducaia, MG) está sìtuada a 1500 m de altitude, na porção sul da serra da Mantiqueira. Alvéolos do relevo serrano, como o entroncamento dos córregos do cadete e da Minhoca com o rio Jaguari, junto à sua área urbana, propìciaram o acúmulo de sucessões métricas de sedimentos argilo-arenosos em condiçöes favoráveis à formação de material turfoso e à preservação de palinomorfos. Nesta área, o presente estudo reconstitui, a partir da integração dos registros palinológico, sedimentológico (granulometria e minerais pesados) e geoquímico/isotópico (C e N) de três testemunhos rasos (até 230 cm), a evoluçäo e os possíveis controles sedimentares e paleoclimáticos do cenário paleoflorístico no Quaternário tardio, com especial atençäo para a Floresta de Araucária. O contexto geral registrado nos testemunhos é de planície de inundação fluvial, com cobertura arbórea próxima, e influência varìável de fluxos de encosta. Na parte montante do vale do cadete, obtiveram-se idades compreendidas entre 38695 - 40522 anos cal A.P., em 220 cm de profundidade, e 2060 - 1880 anos cal 4.P., em 5 cm. Na parte inferior da coluna, até cerca de 16000 anos A.P., há indícios de aumento de distalidade para cima. Entre cerca de 16000 e 3500 anos A.P., evidencia-se redução da influência da matéria orgânica de plantas terrestres, em detrimento de algas, possivelmente em momento de abeÍura relativa da cobertura florestal. Os últimos 3500 anos seriam de restabelecimento da cobeÍura florestal, possivelmente já nos moldes da existente hoje, com redução gradual do aporte de areia. Mais a jusante do mesmo vale, as idades ficaram compreendidas entre 26764-26023 anos cal 4.P., em 210 cm de profundidade, e 2350 - 2150 anos cal A.P., em 10 cm, A parte inferior do intervalo, mais antiga que 20830-20370 anos cal A.P., é dominada por influência de áreas fontes locais, graníticas. Em 20000 anos A.P., fontes distais, metamórficas, passam a atuar, com aumento da influência de algas. Nos últimos 5000 a 6000 anos A.P., tem-se o readensamento da Floresta de Araucária, com manutenção de brejos, sob condiçöes climáticas frias e úmidas. Os resultados sugerem que as mudanças climáticas nos últimos 40 mil anos em Monte Verde não foram acentuadas a ponto de exercer grande impacto florístico e que houve predomínio de Araucaria e elementos associados a essa floresta durante todo intervalo estudado, sob condições climáticas frias e úmidas. / he Monte Verde region (Camanducaia, MG) is located in the southern portion of the Mantiqueira Range, with 1500 m of elevation. Relief alveoli, as the junction of cadete and Minhoca creeks with Jaguari River, sited close to the urban area, propitiated the accumulation of metric sucessions of sandy-mud sediments, under cond itions favorable to the formation of peat material and preservation of paìynomorphs. ln this area, the present study integrates palynological, sedimentological (grain size and heavy minerals) and geochemical/isotopic (C and N) records of three shallow cores (up to 230 cm deep) to reconstruct the evolution and possible sedtmentary and paleoclimatic controls of the Laie Quaternary paleofloristic scenario, with special emphasìs in the Araucaria Forest. The general sedimentary context recorded in the cores is a river floodplain close to tree cover areas, and with variable influence of slope flows. On the upstream Cadete valley, were obtained ages of 38695-40522 cal years BP, at the deep of 220 cm, and 2060-1880 cal years BP, at 5 cm. From the bottom of the column until about 16000 BP, there is evidence of increased distality upwards The time interval between about 16000 and 3500 yr BP shows a reduction in the influence of organic matter derived from terrestrial plants, to the detriment of algae, possibly in a moment of relative opening of the forest cover. The last 3500 years would be characterized by the restoration of forest cover, possibly similar to the existing today, with gradual reduction of the sand supply ln the downstream of the same valley, the ages vary from 26,764 fo 26,023 cal years BP, at 210 cm deep, to 2350 to 2150 cal years BP, at 10 cm. The lower part of the sedimentary column, older than 20830-20370 AP\' is dominated by the influence of local granitìc sources areas. ln 20000 years BP, distal metamorphic sources begin to act paralel with a increased influence of algae. over the last 5000-6000 years, a increase of Araucaria Forest is recorded, with maintenance of weflands under cold and wet climatic conditions. This results suggest that climate changes during the last 40 k years in Monte Verde region were not sufficient to exert great floristic impact. Araucaria and associated forest elements predominated during the whole studied interval, under cold and wet weather conditions.

Few studies utilise modern species distribution data and modeling to make predictions for examining potential fossil localities. Instead, species distribution modeling is often used for palaeoenvironmental interpretations. Using palaeoclimate data to model potential past distributions for a species provides a prediction showing areas where its fossil remains may be found. In this study, the current, Last Glacial Maximum, and Last Interglacial potential distributions of the arid steppe-obligate saiga antelope (Artiodactyla: Bovidae, Saiga tatarica) were modeled using the species distribution model Maxent. Few fossil records exist, but available fossil locality records were used to validate both palaeo models, resulting in speculative predictions about where the saiga may have lived. Known fossil localities of saiga from the Last Glacial Maximum time period were located within predicted moderately suitable environments, while four of seven Last Interglacial fossil localities were located within predicted moderately suitable environments, suggesting that models can accurately identify areas where fossils for the saiga can be found. Specifically, these models suggest saiga fossils may be located in northwestern and northeastern China, the western and central regions of the Middle East, and southern Alaska. The predicted areas in northeastern China are of particular interest because saiga fossils have not been identified in this region, but some palaeontologists theorize that northeast China may have been suitable for saiga in the past. The models lend credence to this argument.

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