Theses/Dissertations - Geosciences

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    Reconstructing the early Paleocene light environments using fossil Platanites from the San Juan Basin, New Mexico, and its implications for stomatal pCO2 reconstructions.
    (2023-08) Geng, Jie, 1997-; Peppe, Daniel J.
    In the last two decades, stomatal proxies have been extensively used to reconstruct the concentrations of paleoatmospheric CO2 (pCO2) based on the well-established negative correlation between stomatal density (SD) and pCO2. However, various light environments within a fossil flora, known to influence many plant traits, including SD, were rarely discussed in most previous pCO2 reconstructions. We collected one well-preserved early Paleocene flora from the San Juan Basin, New Mexico, that is dominated by Platanites. We then analyzed the light environment of this flora based on leaf epidermal cell wall undulations, quantified by undulation index (UI), and the range of leaf carbon isotope. We found a negative correlation between UI and SD, which indicates that leaves under higher light intensities produce higher SD. Importantly, we observed a positive correlation between UI and reconstructed pCO2 using a leaf gas-exchange model, and we recommend future pCO2 reconstructions using stomatal proxies also assess variations caused by canopy light environments.
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    Environmental framework, structural evolution and petroleum potential of the Cambrian Wilberns Formation West-Central Texas
    (1984) Poorman, Stephen Eugene; Beaver, Harold; Baylor University.
    The Wilberns Formation (Upper Cambrian) in the subsurface of west-central Texas is composed predominantly of sandstone units. Subsurface mapping of the Wilberns shows the sandstones to be persistent throughout most of the study area. Analyses of core samples indicate that the upper portion of the Wilberns Formation consists of a lower sandstone facies overlain by a dolomitic interval. The suite of sedimentary structures present in both facies suggests tidal flat deposition and the electric log signatures are similar to those produced by channel and tidal sand bodies. Thus, it is postulated that the Wilberns Formation was deposed on extensive tidal flats in tidal channels. The Fort Chadbourne fault system is a linear zone of deformation trending from Sutton County northward into northeastern Nolan County. This structural zone has uplifted and faulted the Wilberns Formation. The presence of en echelon faults and folds as the predominant structures along the Fort Chadbourne system suggests wrench faulting. The orientation of the faults (N-S) and folds (NE-SW) suggests that the wrench system had left lateral movement produced from compressive forces active during the Ouachita orogeny. Thin section analyses of core samples produced a general paragenetic sequence for the upper sandstone units of the Wilberns Formation consisting of at least four diagenetic stages as follows: (1) Burial and compaction leading to reduction of primary porosity and development of quartz overgrowths. (2) Precipitation of dolomite cement into remaining pore space. (3) Uplift and exposure leading to dissolution of calcareous materials, resulting in development of secondary porosity. (4) Re-burial and precipitation of hematite, pyrite and clays as pore lining and pore fill. Hydrocarbon migration probably occurred during this stage. This sequence of diagenetic events is responsible for the excellent reservoir quality of the sandstone units within the Wilberns Formation; it is found only along the Ford Chadbourne fault system.
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    Hydrogeologic assessment of shallow groundwater flow systems in the Walnut Formation, Central Texas
    (1994) Feckley, David Lee; Baylor University.
    The Walnut Formation is the most clay-rich member of the limestone dominated Lower Cretaceous rocks of central Texas. Due to this clayey nature, the Walnut Formation is a natural target for landfills in this region. Shallow groundwater flow systems present within the Walnut Formation provide baseflow to streams which transect the Walnut outcrop belt in Central Texas. Three sites within the outcrop belt of the Walnut Formation were investigated. The Walnut flow systems are geomorphically and stratigraphically controlled, and recharge zones are influenced by the geomorphic history and subsequent soil formation. Discharge zones are controlled by seasonal influences, stratigraphy, and geomorphic position. Field observations indicated saturated zones in the near surface Walnut Formation may produce hydraulic heads above ground elevation. These saturated zones, within the weathered depth of the Walnut Formation, discharge to main streams by tributary discharge and spring flow. Conceptual models of the flow systems were constructed following field observations, hydrogeolgic testing (slug tests and pumping tests), geochemical analyses, and hydrograph analysis and interpretation. Regionally, the Walnut flow systems relate to the condition of the underlying Paluxy Formation (a minor aquifer). Flow systems of the Walnut Formation are important from a regional perspective due to the potential of non-point source contamination to surface waters by landuse practices on the Walnut outcrop area.
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    Using Sycamore leaves to reconstruct ancient light environments.
    (May 2023) Thorne, Desirae E., 1999-; Peppe, Daniel J.
    Light environments strongly influence the composition and structure of terrestrial ecosystems and climates. Further, light intensity impacts both a plant’s leaf morphologic traits and its chemical composition, making it possible to quantify how these variables change in response to light intensity. Thus, it is possible to use leaf morphology and chemical composition to reconstruct ancient light environments, which can provide critical insights into past environments. Here, we present results focused on the development of a proxy for light availability using leaf size and shape (physiognomy) and chemical composition from modern Sycamore leaves that were grown under varying degrees of light availability in an outdoor light experiment. We found notable differences in leaf physiognomic variables, such as leaf area and perimeter, across light environments; and results from 13C NMR spectroscopy also indicate differences between light environments, with more abundant in lipids and less abundant in lignin found in low light conditions. Physiognomic and geochemical data were used to develop three different multivariate models for predicting daily light integral (DLI) that can applied to the fossil record. Using these models, we analyzed early Paleocene Platanites fossil leaves from the San Juan Basin, New Mexico to reconstructed ancient light conditions to help understand light availability and its impacts on the ecosystem and plant communities of early Paleocene.
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    Structure-reactivity mechanisms of organic matter sorption and photochemical transformation in aqueous environments.
    (May 2023) Leonce, Burke C., 1987-; Hockaday, William C., 1979-
    Black carbon produced from vegetation fires accounts for 15% of soil organic carbon and the dissolved component makes up 10% of organic carbon transported by rivers to the oceans annually. The association of organic carbon with metal oxides and its photochemical transformation are critical factors controlling C turnover in soils and aquatic systems. However, the influence of chemical structure on photodegradation of dissolved organic matter and its sorption to metal oxides remains questionable. The original research in this dissertation is aimed towards better understanding structure-reactivity mechanisms that control organic matter sorption to metal oxides and its photodegradation in aquatic environments. The molar heat of sorption of lignin monomers onto ferrihydrite was 1.17 kJ mol-1 and occurred via an outer-sphere mechanism involving multiple points of attachment. Sorption of lignin monomers was characterized by an exchangeable fraction that was bound to surface sites and within unblocked intraparticle pores and an unexchangeable fraction that was not accessible and/or removable by simple ion exchange. Plant biomass type and its processing via charring were significant drivers of dissolved organic matter (DOM) photolability and its sorption to nano-crystalline boehmite. The photo-bleaching behavior of DOM was captured by a three-component energy-based model. The first component was resistant to photobleaching, whereas the second and third components required a mean energy flux of 42-204 kJ m-2 and 168-1540 kJ m-2, respectively. Model predictions showed that an increase in vegetative fires would decrease photodegradative contributions to DOM cycling in the conifer-dominated east Texas, increase contributions in the hardwood shrub-dominated west, but have no effect in the grass-dominated central regions. The sorption of DOM to nano-crystalline boehmite was related to the preponderance of alkyl attachments to the aromatic backbone. Photolysis of DOM reduced the complexity and total energy of sorption for DOM thereby suggesting that solar-exposed DOM will be less prone to being sorbed, sorbed strongly and hence preserved via sorptive preservation on metal oxide colloids.
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    Quaternary landscape evolution, pedology, and geoarchaeology in central Texas.
    (May 2023) Taormina, Rebecca A., 1991-; Nordt, Lee C.
    Central Texas provides a unique environment as a climate ecotone for studying processes of river incision, deposition, and soil formation. This is the first study in central Texas focused on alluvial floodplain and terrace formation of the Brazos and Bosque rivers. Additionally, Buttermilk Creek provides a view of late Quaternary landscape evolution in central Texas along a low-order tributary. The late Quaternary history of central Texas is a complex story of climate and hydrologic changes, sediment supply, and human interaction. From >60 ka near the transition of MIS 5 to 4 to historic times through the end of MIS 1, the stream systems have recorded changes as rivers incised and aggraded.
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    Identification of a thermal maturation high, revised maturity map, and production trends of the Late Devonian Duvernay Formation, Alberta, Canada.
    (May 2023) Yeates, David W., 1990-; Atchley, Stacy C.
    The Late Devonian Duvernay Formation in the Western Canada Sedimentary Basin is a prolific shale reservoir that has been developed since 2010 and produced 250 million barrels of oil equivalent. Regional thermal maturity trends across central and southern Alberta published in the literature are poorly understood and inconsistent with produced fluid types. The focus of this dissertation is to refine thermal maturity trends of the Duvernay Formation. The Duvernay is subdivided into three hydrocarbon production areas that are, from west to east, Kaybob, Willesden-Green, and Innisfail. The maturity proxies used in this study are from solid-state 13C Nuclear Magnetic Resonance (NMR) and Rock Eval’s Tmax along with publicly available Tmax data, produced fluid types, and the structural burial depth trends across the basin. Thermal maturity across these areas increases to the southwest except for Willesden-Green. The southern portion of the Innisfail region reaches oil maturity whereas Kaybob ranges from oil to gas mature. Willesden-Green is markedly more mature than Kaybob as indicated by NMR (P=0.05) and Tmax (P=0.00) proxies even though samples from both areas were collected at similar depths (P=0.24). This thermally anomalous trend is named the Willesden-Green Maturity High (WGMH). The origin of the WGMH is interpreted to be related to an increase in geothermal gradient induced by the convective flow of hydrothermal fluids through a fault and fracture network stemming from the reactivation of the Snowbird Tectonic Zone during the Laramide Orogeny. Successful Duverney hydrocarbon production is related to both thermal catagenesis and optimal reservoir mineralogic composition as estimated from whole core elemental geochemistry. Elemental geochemistry was measured from continuous whole core at a 1-m sample interval using handheld X-ray fluorescence (XRF). Major elements are used as a proxy for mineralogy and redox-sensitive trace element enrichment as an indicator of depositional anoxia. Silica and calcium are used to estimate the relative abundance of quartz and calcite, respectively. High quartz and/or calcite mineral proportions increase rock brittleness and susceptibility to induced fracture stimulation. The eastern portion of the Kaybob region has the most productive oil and gas wells and correspondingly high concentrations of silica and redox-sensitive trace metals.
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    Unstressed groundwater flow in the Brazos River Alluvium Aquifer with implications for temporal ranges in groundwater to surface water interactions.
    (May 2023) McVea, Jairon, 1998-; Yelderman, Joe C.
    Groundwater flow paths and flow rates through alluvium aquifers are often oversimplified. This oversimplification may be due to sediment description limitations or aquifer properties obtained using well hydraulics that assume homogeneity. Groundwater travel times to the Brazos River channel can be longer than anticipated when the aquifer gradient is low and flow paths encounter heterogeneity within the lithologic framework. Conversely, travel times to the river channel can be less than anticipated when gradients are steep and homogeneous coarse sediment dominate the lithology within the aquifer while connecting directly to the river. This thesis investigates unstressed groundwater flow rates and flow paths under natural conditions using detailed lithologic logs to recreate more realistic heterogeneity of the Brazos River Alluvium Aquifer characteristics and a 2D finite element steady state model. These estimations provide important insight for surface to groundwater interactions within alluvium aquifers.
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    Subsurface Stratigraphy of the Strawn and Canyon Groups of west central Texas, Concho and Menard counties
    (1983) Woodard, Jan N.; Grayson, Robert; Parker, Don; Baylor University.
    Carbonate rock units of the upper Strawn Group (Desmoinesian) developed on the western flanks of the Llano uplift on a positive topographic platform, the Concho shelf. Late Mississippian and early Pennsylvanian orogenic uplifts along the Texas craton exposed the Ordovician Ellenburger Group carbonate rocks to extensive erosion and provided the paleotopographic unconformity upon which these cyclic limestones and shales were distributed. Orogenic activity related to the advancing Ouachita Fold Belt occurred synchronously with late Strawn deposition. The rising Ouachita orogenic belt initiated a fluvial-deltaic depositional complex that prograded over a slowly subsiding carbonate shelf. In early Canyon time (Missourian) deformation along the Ouachita Fold Belt decreased in intensity and a more stable carbonate platform environment was reestablished on the shelf. Cycles of marine and prodeltaic shales capped by algal limestones characterize the Canyon Group. Paleotopographic variations in the lower Canyon indicate early Missourian tectonic movement in the Ellenburger limestones. In contrast, the wedge-shape geometry of the upper Canyon carbonate units suggest shoreward-building carbonate banks. These banks developed over very porous, water-saturated prodelta shales and gained thickness as accumulating carbonates compressed the unconsolidated distal muds.
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    Paleoenvironmental Variability across the Cretaceous-Tertiary Boundary in the Alberta Foreland Basin, as Interpreted from Fluvial Deposits and Paleosols, Red Deer River Valley, Alberta, Canada
    (2002) Forkner, Robert Murchison; Nordt, Lee C.; Baylor University.
    The integration of sedimentological, paleotectonic and paleopedogenic data across the Cretaceous-Tertiary (KT) boundary of south-central Alberta indicates fluvial aggradation and variability of paleosol morphology in response to foreland orogenesis. The depositional history records an evolution from amalgamated, multi-story, braided sand bodies to accretionary, single-story, overbank-prone meandering deposits. The distribution of paleosols throughout the section is also cyclic. Immature, well-drained paleosols are associated with the braided deposits, whereas mature, poorly-drained paleosols are interbedded with the meandering deposits. Two large-scale aggradational fluvial cycles are observed within the study interval and are interpreted to record variations in sediment supply and tilt of the depositional profile associated with foreland tectonism. Orogenic pulses are reflected in outcrop by amalgamated fluvial deposits interbedded with immature paleosols. Waning orogenesis is characterized by reduced fluvial sedimentation rates and an increase in the number of mature paleosols. Orogenic quiescence is associated with an increase in channel sinuosity, and poorly drained, gleyed, coal-capped paleosols. The KT boundary is located three meters above the tectono-stratigraphic transition from amalgamated, braided fluvial systems with well-drained paleosols to accretionary, meandering fluvial systems with poorly-drained paleosols. A gradual shift towards a more poorly-drained paleosol morphologies and increasingly accretionary fluvial styles suggests that the KT event was not the cause of increasingly cool and wet conditions across the boundary, but that the boundary lies at the inflection point between a well-drained to poorly-drained depositional cycle.
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    Seismic tomography of the former southern margin of Laurentia using teleseismic P and local pg phases.
    (December 2022) Coulibaly, Koundienetia Aicha, 1997-; Pulliam, Jay.
    The compositions, lateral and depth extent, and physical characteristics of geological features in the Texas-Oklahoma region are poorly constrained, just as the events that created them are poorly understood. We used EarthScope’s Transportable Array stations (2008-12) and TexNet stations (2018-present) to produce a 3D model of the region’s crust and upper mantle via seismic traveltime tomography using teleseismic P and local Pg arrivals. The combination of these two arrival types imposes strong constraints on the crust and upper mantle. The resulting tomographic model reveals a fast velocity anomaly in the mantle beneath west Texas, which may be related to subsidence that created the deep Delaware Basin. In east Texas, near the Louisiana border, a fast anomaly extends through the crust in the vicinity of the Sabine Uplift, although it does not coincide with the shallowest expressions of the Uplift. Although the Anadarko Basin appears prominently in the model’s crust, the Southern Oklahoma Aulacogen cannot be identified.
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    Seismic site characterization via joint modeling of horizontal-to-vertical spectral ratios and surface wave dispersion : developing and validating a geophysical tool for deciphering Quaternary stratigraphic architecture of the Monahans dune field, West Texas.
    (December 2022) Spears, Brady, 1996-; Pulliam, Jay.; Pulliam, Jay.
    Quaternary deposition for the Monahans dune system is reflected in its stratigraphic architecture through various periods of dune accretion, stabilization, and erosion as well as buried clay, carbonate-rich soils, and playa lake deposits. The complex and discontinuous stratigraphy of Monahans is difficult to capture without extensive borehole investigation. Shallow seismic techniques using Horizontal-to-Vertical Spectral Ratios (HVSR) and surface wave dispersion offer a cost-effective alternative to deciphering Quaternary architecture, but inverse models are subject to ambiguity and nonuniqueness. We jointly model HVSR and surface wave phase velocity dispersion measurements via global optimization to produce best-fit 1D shear wave velocity models for the interpretation of geologic structure and use statistical tools, including posterior probability distributions and parameter correlations, to demonstrate that joint modeling has the potential to impose stronger constraints on model parameters than modeling either seismic dataset alone.
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    Impact induced porosity of terrestrial impact craters : a gravity study of Meteor Crater.
    (August 2022) Mitchell, Christopher D., 1995-; James, Peter Benjamin.
    In this project we gathered and interpreted gravity anomaly data at Meteor Crater, and we used these data to constrain the porosity variations across Meteor Crater that were induced by the crater-forming impact event. We performed a gravity survey at the crater and merged these new data with legacy gravity survey data. From the resulting merged gravity data set, we generated a reduced anomaly map and created a model to estimate the gravitational effects of several stratigraphic units across Meteor Crater. Finally, we used a Bayesian inversion scheme to infer the bulk density and porosity of each stratigraphic unit. The models reflect the creation of impact-induced porosity of ~5-10% around and beneath the crater, supporting previous findings with novel methods and data.
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    Gravitational indications of subduction on Venus.
    (August 2022) Dame, Rudger, 1993-; James, Peter Benjamin.
    While Venus lacks global plate tectonics, there has been evidence of localized subduction on Venus. The goal of this study is to observe and forward model the gravity gradients of a subducted slabs on Earth and Venus. Our modeled results are consistent with subduction in at least three locations on Venus: Artemis Corona, Quetzalpetlatl Corona, and Astkhik Plateau. We also have identified a trend at terrestrial and Venusian subduction zones of a higher geothermal gradient on the overriding plate side of the trench and a lower geothermal gradient on the outboard side. The modeled subducting slabs on Earth and Venus are found to be negatively buoyant. However, the Venus slabs tended to be slightly less negatively buoyant. We hope that these methods will be repeated for smaller corona once VERITAS’s gravity data has been recovered.
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    Application of machine learning and magnetotellurics to aid in subsurface characterization of petroleum and geothermal reservoirs.
    (August 2022) Rau, Elisabeth G., 1994-; Atchley, Stacy C.
    Energy is the foundation of society and with future energy demand expected to increase significantly over the next few decades, solutions contributing to future energy resources are of high interest scientifically, geopolitically, and economically. Data analytics and machine learning provide useful tools to more efficiently and cost-effectively produce petroleum and geothermal resources vital for our energy future. Supervised and unsupervised machine learning can aid in the prediction of sedimentological and reservoir attributes in wells lacking core control to better and more efficiently characterize subsurface petroleum reservoirs. Using tree-based machine learning models, core-observed depositional attributes from the Late Devonian Duvernay Formation in Alberta, Canada may be predicted in wells lacking core control when class proportion and thickness conditions are met. Unsupervised machine learning technique, non-negative matrix factorization with k-means clustering (NMFk), automatically identifies reservoir significance, undetected through the traditional deterministic modelling, within the Duvernay Formation without calibration to core observations. The application of NMFk with petrophysical data may assist in highlighting intervals of interest in advance of core descriptions reducing observer inconsistency and bias and enhancing the quality and relevance of core description for reservoir correlation and mapping. Machine learning methods provide precise, consistent, and objective petrophysical interpretations and reservoir characterization, and increases the consistency and accuracy of resource assessment for petroleum exploration and production. Unsupervised machine learning and magnetotellurics are useful analytical tools to assess prospective geothermal resources in the Tularosa Basin of south-central New Mexico based on heat flow, temperature, porosity, and permeability. The unsupervised machine learning method, NMFk, identifies locations with the highest likelihood of geothermal success, and the passive geophysical method, magnetotellurics can detect subsurface geothermal prospects. The integration of NMFk and MT can provide a 3D assessment of heat flow, temperature, and permeability for geothermal exploration. This research provides innovative methods to aid in the development of efficient and cost-effective approaches for future energy exploration and production.
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    Edge computing with an “Internet of Things” based sensor array : an innovative approach to near real time seismic exploration and monitoring.
    (2020-10-27) Sepulveda, Frank, 1976-; Pulliam, Robert Jay.
    We demonstrate the feasibility of leveraging an Internet of Things (IoT)-based sensor array to orchestrate edge-based (i.e., in a field setting) storage and computing resources capable of characterizing the subsurface, using ambient seismic noise, in near real-time. Our approach enables the continuous assessment of results and the identification of opportunities to modify the sensor array in more optimal configurations depending upon ambient noise field characteristics. Moreover, we can assess the need to leave the array in place for longer or shorter than originally planned with high levels of confidence our survey objectives have been met. Over the course of our four deployments (i.e., Texas – May 2017, Nevada – June 2017, Texas – July 2018, and Nevada – May 2019) we developed an edge-based framework that utilized commercially available communication infrastructure, digitizers, embedded systems, and an established distributed database (i.e., DataStax Enterprise; DSE) to store and process sensor array data, in a field setting. This framework allowed us to overcome real-world performance limiters (e.g., bandwidth, power, etc.) commonly encountered while carrying out remote seismic exploration or monitoring. Moreover, it provides an alternative solution to centralized (i.e., cloud-based) data storage and processing strategies that often have more demanding network capacity and reliability requirements. The use of DSE (powered by Apache Cassandra), as our edge-based distributed database, is central to the scalability and reliability of our framework. To the best of our knowledge, no one has attempted to use DSE or Cassandra, on an embedded system, as a seismic sensor array’s edge-based datastore. Our use of DSE is beneficial in the following three ways: 1) it supports the highly scalable write-heavy workloads common to sensor arrays, 2) it allows for the use of the same fault tolerant distributed database across a variety of commercially available hardware (e.g., embedded systems, servers, etc.), and 3) it seamlessly maintains and replicates data along a user defined continuum of locations (i.e., “edge to cloud”). We believe geoscientists can use our edge-based solution to improve existing and develop novel methods to characterize the subsurface.
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    Groundwater/surface water interactions : gravel pit lakes in the Brazos River Alluvium Aquifer.
    (2022-05-02) Dawson, Claudia, 1998-; Yelderman, Joe C.
    Urban expansion and the subsequent need for construction materials are increasing aggregate mining activities in many areas around the world. In central Texas much of the aggregate has come from alluvial sediments in the Brazos River valley which also comprise the Brazos River Alluvium Aquifer. Aggregate mining is exempt from many reclamation requirements and the result is a series of post-mining gravel pit lakes in the Brazos River Alluvium Aquifer exposing the aquifer to surface conditions. This study examines two gravel pit lakes in the Brazos River Alluvium Aquifer to determine if the lakes act as sinks, recharge sites, or flow-through groundwater-fed lakes. Data based on water elevation, specific conductivity, isotopic values, and a two-dimensional finite element model suggest that both lakes are flow-through systems throughout the year.
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    Paleogene climate reconstruction using paleosol mineral assemblages, San Juan Basin, New Mexico.
    (2022-05-09) Price, Nicole Ashley, 1996-; Dworkin, Stephen I.
    Certain preserved minerals are stable within specific climate conditions, thus their presence can indicate the type of climate that was present during their deposition. The climate after the Cretaceous/Paleogene extinction event is unique based upon preceding circumstances and thus requires analysis. Through the analysis of outcrops within the Nacimiento Formation of the San Juan Basin, New Mexico, the paleoclimate of this time period can be reconstructed. This is done through X-ray diffractometry, where climatically significant minerals can be identified and quantified to lead to a conclusion. It had been discovered that kaolinite, one such mineral climate indicator, was present throughout all of the observed sections, indicating the climate was never completely dry. However, some sections contained more kaolinite than others, indicating varied climate across regions, but note that the climate was never dry due to the lack of calcite present. Thus, the climate was wet during the Nacimiento deposition.
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    Early Paleocene plant community and paleoclimate reconstruction of the Nacimiento Formation from the San Juan Basin, New Mexico.
    (2022-05-03) Gygi, Danielle J., 1997-; Peppe, Daniel J.
    Following the major extinction event at the Cretaceous-Paleogene (K-Pg) boundary, the Earth experienced a trend of long-term warming, punctuated by several short-term ‘hyperthermal’ events with associated carbon isotopic excursions in the early Paleogene. The early Paleocene climate record and the plant community response to the K-Pg extinctions and during and after hyperthermal events has been well studied in the northern Great Plains of North America but relatively little is known about the floral and climatic record at lower latitudes. The lack of data from terrestrial southern North American basins and from the middle Paleocene across North America limits our understanding of how Paleocene plant communities evolved and responded to climate change in the past and make it impossible to fully characterize regional patterns. This in turn inhibits our ability to understand how modern plant communities will respond to the unprecedented climate change the Earth is currently experiencing. The San Juan Basin in northwestern New Mexico and southern Colorado represents an ideal stratigraphic record for expanding our knowledge on plant community recovery and response to climate change, because unlike other basins in North America, the San Juan Basin preserves a remarkable, nearly continuous, record of both fossil floras and mammalian turnover that spans the early and middle Paleocene. Additionally, the early Paleocene floral record collected from the Nacimiento Formation in the San Juan Basin has been well characterized and documents a flora that is considerably more diverse than the contemporaneous floras in North America.
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    Plant cuticle as a proxy for paleoecology and paleoatmospheric composition : modern calibration and application to the early Paleocene of the San Juan Basin, New Mexico.
    (2022-02-18) Milligan, Joe N., 1992-; Peppe, Daniel J.
    Human alteration of land cover and combustion of fossil fuels has elevated atmospheric CO2, causing the Earth to warm. The geological record provides an archive of how organisms and ecosystems respond to changes in climate and atmospheric CO2 allowing us to assess the implications of a warmer planet. This dissertation focuses on using fossil plant cuticle to reconstruct ancient ecosystems and atmospheric conditions. In particular, I develop, test, and apply leaf cuticle proxies to (1) reconstruct canopy structure and (2) atmospheric CO2 of the early Paleocene from the San Juan Basin (SJB) New Mexico. I reconstructed canopy structure using single species and multi-species approaches. I used light experiments to quantify the response of cell size and undulation and carbon isotopes of modern Platanus occidentalis. I then developed and applied a proxy based on cell wall undulation for daily light integral (DLI) to fossil Platanites leaves from early Paleocene floras in the SJB. The distribution of high DLI values from fossil leaves indicate that either (1) most of the canopy mass is within the upper portion of the crown or (2) leaves exposed to more sunlight are preferentially preserved. I then used a novel proxy to reconstruct leaf area index (LAI) based on the cell wall undulation and aspect ratio of multiple species using dispersed plant cuticle from seven localities in the SJB. I find that reconstructed LAI is comparable to modern broadleaf forests and indicates a stable canopy in the early Paleocene. Changes in LAI were only weakly associated with changes in climate, instead, likely linked to changes in the SJB fluvial system evolution. I then evaluate early Paleocene CO2 by applying a leaf gas-exchange model to Platanites leaves of four early Paleocene localities from the SJB. I first calibrate the model using P. occidentalis and P. acerifolia, in which the model accurately predicts present-day CO2. Applying the model to the early Paleocene, I find CO2 fluctuates between ~660 to 1140 ppm between 65.66 and 64.59 million years ago. These elevated levels of atmospheric CO2 during the early Paleocene are aligned with the elevated temperature during this period.