2024 Abstracts
2024 Abstracts
POSTER ABSTRACTS
MORNING POSTERS
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Title:
Title: Petrographic Analysis of the Giants Range Batholith in Northeastern Minnesota
Authors: Lauren Pardi, Zsuzsanna Allerton, Christian Teyssier
Abstract: The Giants Range Batholith is a large granitoid characterized by the abundance of quartz, plagioclase, alkali feldspar that stretches 40 km in a northeast-southwest direction and covers about 2500 km 2 in northeastern Minnesota (Allison, 1925). The Giants Range Batholith (~2.7Ga) is in contact with the gabbroic Duluth Complex, the intrusive suite of the well-known Midcontinent Rift System (~1.1 Ga) to the east. The emplacement of the intrusion generated temperature and pressure changes and introduced hydrothermal fluids, resulting in contact metamorphism, hydrothermal alteration, deformation, and reaction textures with varying intensity as a function of distance from the Duluth Complex-Giants Range Batholith contact. Samples collected from the contact westward into the Giants Range Batholith unit allowed for a systematic approach to conduct petrographic analysis with transmitted light optical microscopy. Petrographic observation of hydrothermal alterations is characterized by samples displaying change in the presence of contact metamorphic signatures and reaction textures in the form of recrystallized minerals, myrmekite, sericite/chlorite alterations, etc. The different textures in samples across the Giants Range Batholith inform about the reactions that occurred providing insight into hydrothermal and chemical alterations induced by the emplacement of the Duluth Complex and regional metamorphism.
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Title: Zircon geochronology across the Giants Range Batholith in northeastern Minnesota
Author: Marek Poplawski, Zsuzsanna Allerton, and Christian Teyssier
Abstract: The focus of this study is the hydrothermal effect of the Mesoproterozoic (~1.1 Ga) Duluth Complex (DC)—the westernmost intrusive suite of the Midcontinent Rift System (MRS)— on the Archean (~2.7 Ga) granitic-granodiorite Giants Range Batholith (GRB) footwall in northeastern Minnesota. The DC emplacement resulted in contact metamorphism, sulfide mineralization, deformation, etc., but little is known about the hydrothermal effect and its magnitude recorded in accessory minerals.
The accessory mineral zircon is central to the field of geochronology because of its innate ability to resist weathering processes and high temperatures, retain isotopic signatures, and preserve complex textures that can be dated through the U-Th-Pb chronology system to reveal thermal history (Hoskin, 2014). We are interested in Pb loss as a function of hydrothermal alteration. When plotting ratios of 206Pb/238U and 207Pb/235U a discordia line can be formed; in which the lower intercept is the time of hydrothermal alteration, and the upper intercept signifies crystallization age.
Preliminary data of zircon grains from GRB samples show crystallization age of ~2.7 Ga and Pb loss associated with hydrothermal alteration at ~1.1 Ga. In order to strengthen the data and define the magnitude of the hydrothermal output of DC within host rocks, additional samples were collected systematically—from the DC contact with increased distance westward—to better understand the extent of thermal effects in accessory minerals and to interpret thermal history of ancient terrains.
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Title: Simultaneous Measurement of Phase Transitions and Magnetic Susceptibility of Clay Minerals using a High-Temperature Magnetic Susceptibility Bridge
Authors: Kacie Malone, Rashida Doctor, Joshua Feinberg
Abstract: Magnetic and thermal data have long been used to characterize thermodynamic phase transitions in natural samples. Measurements of magnetic susceptibility as a function of temperature are commonly collected using High Temperature Susceptibility Bridges (HTSBs), while thermal analyses are performed separately, utilizing techniques such as Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC). These measurements are collected on different instruments, necessitating additional time and larger volumes of material for analyses. Here we demonstrate that it is possible to simultaneously collect magnetic and thermodynamic data using a standard HTSB. Because HTSBs heat samples at a relatively constant rate, deviations in the heating rate provide DTA-analogous information, indicating the occurrence of exothermic and endothermic reactions (Doctor & Feinberg, 2022).
Four clay standards with differing magnetic properties serve as a case study to demonstrate the benefits of simultaneous collection of magnetic and thermodynamic information using a HTSB: nontronite, montmorillonite, gibbsite, and kaolinite. Heating curves were created by interpolating and subtracting each sample run with its associated blank measurement. The heating curves in this study were able to accurately replicate literature values for dehydration reactions in nontronite and montmorillonite, as well as dehydroxylation reactions in gibbsite and kaolinite.
These results integrate magnetic and thermal characterization methods and increase data collection efficiency. This combination of techniques allows users to simultaneously identify ferrous and non-ferrous minerals during a single standard geophysical measurement. The ability to identify both clay and iron oxide/hydroxide minerals in soils is helpful to the study of soil health and carbon sequestration. Similarly, this approach can also help differentiate the provenance of glacial tills and delineate alteration and growth of magnetic minerals during thermally based paleomagnetic experiments. Ongoing efforts focus on the thermal and magnetic characterization of a natural soil sample from the Christina River Basin Critical Zone Observatory in southwestern Pennsylvania.
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Title: Not all compost is rad(ish): how compost and soil quality impact radish growth and taste
Authors: Adora Thao, Vang Moua, Bennett Nguyen Prowell, Nana Vang, Sakura Vue, James
Nins, Christof Zweifel, Amanda Patsis, Lindsey Kimmerling, Luis Santiago-Rosario, Lauren Agnew, David Woods, Emilie Snell-Rood, and Cara Santelli
Abstract: Urban agriculture contributes an estimated 15-20% of the global food supply, and here at Urban Roots we grow around 50 crop varieties and 10,000-15,000 pounds of produce each year. We garden on five urban plots in east Saint Paul, the biggest being Rivoli Bluffs. This site is in the flight path of a regional airport and was previously a dump site. We typically add a load of Ramsey County compost to our garden beds annually to restore depleted nutrients each season. This compost was easily accessible, but we noticed that crops grew slowly and not even weeds would grow on the compost pile. We figured something might be wrong with either our soil, the compost we got from Ramsey County, or both!
We investigated our soils to figure out what was affecting our plant growth. Radishes were grown in 5 different soil treatments, 3 soils from the Rivoli site and 2 composts from Ramsey County and Urban Roots. We recorded radish germination rates and dry weight of plant roots and shoots, as well as the overall taste of the vegetables. We measured soil chemistry using ion chromatography, pH meters, and X-ray fluorescence, and sent samples to the University of Minnesota Soil Lab for further analysis.
Radishes grown in the Urban Roots compost did not taste significantly better or worse than those grown in Rivoli soils. However Ramsey compost grown radishes tasted significantly worse than those grown in hilltop and hillside soil from Rivoli Bluff. Compared to the other treatments, Ramsey compost had higher concentrations of salt and nitrate, which has been shown to worsen the taste of vegetables in other studies.
While compost is generally considered beneficial for soil health and plant growth, our results suggest that compost is not always needed and it’s important to consider the source of the compost. We think that the Ramsey County compost is contaminated with trash from street sweeping and community members incorrectly composting, as we’ve found wrappers, plastic dolls, and other materials that aren’t organic or compostable. Trash, road salt, and high nitrate may contribute to this compost producing the least tasty radishes. As an organization that farms and distributes our produce locally, understanding how our soils affect plant growth and taste helps us produce good food and grow healthy communities.
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Title: Characterizing iron-rich layers in Bahamian speleothems
Authors: R. Doctor, J. Feinberg, P.M. Chutcharavan, R.L. Edwards, D.A. Richards
Abstract: Reconstructing sea-levels in The Bahamas provides a window into the spatial and temporal variability of the Earth’s past responses to climate change. This project aims to characterize the chemical and physical properties of a thick red layer within Bahamian cave records that coincide with major fluctuations in sea level during Marine Isotope Stage 7 (~225ka). The flowstone of interest has a thick (~1mm) red layer that corresponds to a hiatus in speleothem growth. We characterized this layer using optical, magnetic, and electron microscopy techniques. Based on the mineralogy and morphology of these samples, we hypothesize the iron oxides in this layer formed in a two-part process. Bahamian blue holes have significant stratification of their water chemistry and fluctuations in sea level would change the water chemistry around the flowstone. The main iron-rich material found in the red layer is in the form of framboids, a morphology common to pyrite formed in euxinic and dysoxic conditions. Later oxidation then transformed the pyrite into iron-oxide pseudomorphs such as hematite. Ongoing measurements on these samples include using the electron microprobe to look for remnants of sulfur in the framboids and characterize the layer chemistry with higher resolution.
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Title: Visualization of Agriculturally-Induced Pollution across the Otter Tail Watershed and its Broader Implications for Decline of Manoomin/wild rice
Authors: Madeline Dickenscheidt, Amelia Olsen, Cara Santelli
Abstract: The Otter Tail Watershed of west-central Minnesota lies just at the Minnesota/North Dakota border and extends north, where its headwaters are found within the White Earth Band of Minnesota Chippewa Tribe Reservation. The watershed is rich in bodies of shallow lakes and rivers that are critical for Manoomin (Ojibwe) or wild rice (Zizania palustris) growth; indigenous peoples of White Earth Nation hold rights to rice on both the reservation and ceded territory in the Otter Tail watershed. Land use cover data shows that a majority of the watershed is designated as agricultural use, which heightens concern for wild rice and, in turn, ecosystem health if excess nutrient and chemical pollutant runoff from fertilizers and animal waste were to be introduced into surface and groundwater. Successful growth of wild rice is dependent on a number of factors, including an absence of water level fluctuations and clear, high-quality water. In agriculturally-impacted waters, an overabundance of nutrient anions such as nitrogen and phosphorus in both surface and groundwater can result in eutrophication. This causes algal growth that blocks sunlight and eventually, during the decomposition of organic matter, reduces dissolved oxygen in the water to a level unsuitable for wild rice production. Initial examination of data acquired from the Minnesota Pollution Control Agency (MPCA) and the Minnesota Department of Natural Resources (MNDR) suggests that average levels of both nitrate and phosphorus in surface and groundwater are higher than standards set by the Environmental Protection Agency (EPA) and the MPCA; concentrations are also consistently elevated in groundwater, likely attributed to longer residence times. Large standard deviations across datasets present extremities in concentrations above set limits in certain areas of the watershed, suggesting the existence of impaired bodies of water no longer suitable for Manoomin. Referencing water quality data against land cover data in the watershed points to cultivation and development being the two most immediate causes of water quality degradation. Although fluctuations in water chemistry can occur naturally, the elevated levels of nitrate and phosphorus seen here point to anthropogenic sources due to the extreme levels that were observed. Lack of consistency in sampling patterns diminishes the ability to produce a more definitive conclusion on the evolution of anion concentration over the observed time period (1960-present).
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Title: Deposition of forever chemicals (PFAS) on the Greenland Ice Sheet: an ice core study
Authors: Sydney Waldrop and Joel Barker
Abstract: Per- and polyfluoroalkyl substances (PFAS) were first developed in the United States in the 1930s and their use in consumer and industrial products proliferated in the following decades. These compounds resist degradation for hundreds to thousands of years and thus are colloquially referred to as “forever chemicals”. PFAS bioaccumulate in the environment and organisms and have the potential to pose a long-term environmental and human health risk factor. PFAS are globally distributed in air, water, soil, and animal and human bodies. While specific toxicity indicators are still being sought, it is believed that exposure to PFAS increases the likelihood of high cholesterol, liver damage, birth defects, thyroid disease, and cancers. Greenland is geographically far from point sources of pollution, yet Greenlanders report high levels of PFAS in blood and tissue samples. While bioaccumulation through a marine-based diet has been identified as a possible vector, the potential for direct atmospheric deposition into freshwater sources and food products has not been investigated. The transportation of PFAS to the Greenland Ice Sheet can occur with precipitation and marine aerosol deposition, and a record of this deposition may be preserved in glacier ice and snow. In order to quantify the deposition of PFAS on the Greenland Ice Sheet, two sections from the Dye-2 ice core were analyzed. Section 1 (historical) was deposited between 1723 and 1728, prior to the development of PFAS. Section 2 (modern) was deposited between 1974 and 1978, after the advent of PFAS. Sections 1 and 2 were analyzed for concentrations of 24 PFAS substances using electrospray ionization mass spectrometry. The modern ice core section contained higher concentrations and different PFAS compounds than the historical section, indicating that a) PFAS are being deposited directly onto the Greenland Ice Sheet and b) the ice core drilling technique introduced PFAS compounds into the sample. These results suggest that even remote locations are at risk of PFAS exposure and accumulation and that future snow and ice melt may remobilize PFAS in meltwater, posing potential risks to human and ecosystem health. Further, ice core drilling and storage methods may have to be modified if ice core studies of PFAS are undertaken.
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Title: Digitized Map of the Ruby Gap Complex in Central Australia
Authors: Joshua Schultz, Hannah Tebbens, Christian Teyssier
Abstract: The Ruby Gap Duplex in Central Australia is defined by three regimes of quartzite characterized by varying strain rates and temperatures. Due to the unique structural geology of the region and clear metamorphic signatures present within the units, the Ruby Gap Duplex can be used as a natural laboratory for quartzite deformation studies. To further understand these deformation patterns, a georeferenced map of the duplex was produced using GIS software, drawing from W.J. Dunlap's paper map (1997), and quartzite photomicrograph data. Dunlap’s paper map was scanned and digitized on the free program called Inkscape. The digital version was simplified, focusing on major faults and lithologic units and contacts. This map was transferred to QGIS where quartzite samples were georeferenced. 58 samples were analyzed by using photomicrographs generated by transmitted light petrography and categorized based on deformation regimes. Finally, the samples were georeferenced and color-coded on the digitized map. This data set will allow for a better spatial understanding of deformation patterns associated with the Ruby Gap Duplex.
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Title: Dark Fenton Reactive Oxygen Species Generation at the Sediment Water Interface of Prairie Pothole Wetlands
Authors: Maricia Pacheco, Cole Stenberg, William Arnold, Brandy Toner
Abstract: Globally, small shallow wetlands emit significant amounts of carbon dioxide and methane to the atmosphere. While the role of biotic processes are well studied, the contributions of abiotic reactions to C emissions is poorly constrained. Reactive oxygen species (ROS) are unselective reactants that are capable of oxidizing available carbon. We hypothesize that ROS are produced in the absence of light through dark Fenton reactions at the sediment-water interface (SWI) of Prairie Pothole wetlands. At the SWI, sharp redox gradients, oxic overlying water, and dark conditions are present. These conditions are ideal for the formation of ROS through dark Fenton reactions, which occur when reduced iron and oxygen interact. This study used a combination of field and laboratory based experiments to understand dark Fenton ROS generation. Porewater and sediment was collected from the USGS operated Cottonwood Lakes site located northeast of Jamestown, North Dakota. X-ray absorption near edge structure (XANES) spectroscopy was conducted at the Advanced Photon Source and National Synchrotron Light Source II to identify iron speciation of sediment. We hypothesized that iron sulfide minerals would be the major reductants found in this region. However, our XANES results indicate that most of the iron was found as a silicate mineral with a minor contribution from iron sulfide minerals. These minerals were primarily in Fe(II) valence states. Iron speciation results were used to guide our reactor experiments. ROS generation from synthesized mackinawite and natural samples was measured using a fluorogenic probe (terephthalic acid). Preliminary results suggest that ROS is generated under dark conditions when iron sulfide minerals interact with oxygen. The present study provides information on how abiotic processes impact ROS generation in wetlands.
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Title: Sugar Beet Production: Investigating the Effects of Cover Crops on Yield, Quality, and Soil Health
Authors: Mehmet Ozturk, Anna Cates, Lindsay Pease
Abstract: Sugar beet is a significant crop in Minnesota, the leading state in U.S. in sugar beet production. However, sugar beet production can cause excessive soil degradation due to the lack of organic matter on the soil surface and the use of heavy machinery, resulting in increased erosion rates. It is substantial to consider implementing sustainable management practices to mitigate these adverse impacts. Integration of cover crops and reduced tillage can prevent or alleviate the current issues and contribute to soil health in this system. Here, we present the findings from the first year of our experiment. The effects of cover crops on soil health, sugar beet quality, and yield were investigated through on-farm and in-station experiments (two farm and two station experiments). After the harvest of wheat or corn, cover crops were planted as both fall and spring cover crops for all experiments, except for one station experiment, where cover crops were interseeded in the early summer. The strip-till method was applied in all experiments and agronomic strategies of farmers were followed for the on-farm experiments. Contrary to some other research, the cover crops did not have a negative impact on the quality or yield of sugar beets in the experiments. However, the combined effects of volunteer wheats and extended termination due times resulted in decreased yield and quality for all plots in one of the experiments. Positive effects of cover crops on soil health were observed in our experiments. In conclusion, the results from the first year demonstrates the potential of cover crops to promote soil health while showing no adverse effects on sugar beet yield and quality, providing valuable insights for sustainable agricultural practices.
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Title: Microbiome sequencing provides insights into microbially-mediated lead transformations in urban gradient soils
Authors: Madeline Taylor, Cara Santelli, Luis Y. Santiago-Rosario, Amanda Patsis, Emilie Snell-Rood
Abstract: Urban ecosystems accumulate lead (Pb) through wastewater, industrial materials, and runoff. Plants store Pb from the soil in their tissues, making urban green spaces potential hotspots for human Pb exposure. Rhizosphere soil microbes transform Pb species and influence Pb availability for plant uptake through processes like chelation and methylation. Understanding the role of soil microbes in Pb cycling could help decrease illnesses from Pb exposure, however, microbial genes related to Pb transformation have not been characterized. We aim to find genes associated with soil Pb transformation through microbiome and metagenomic sequencing. Here, we extracted microbial DNA from the rhizosphere soil of six plant genera and bulk soil at urban and suburban sites in Minneapolis, MN. We used a handheld X-ray fluorescence spectrometer to measure the soil Pb content and found a significant difference between urban and suburban soils. Using targeted bacterial 16S rRNA gene amplification, we are examining microbial abundance, diversity, and taxa to compare data between urban and suburban sites, and between plant genera. We will use the microbiome data to select representative samples for shotgun metagenome sequencing to help identify genes of interest involved in metal transformation and detoxification. We will analyze the metagenomes of microbes living in the high Pb urban site for genes similar to those in known metal detoxification pathways, such as the gene clusters that encode methyltransferases for As and Hg detoxification, as these may indicate genes essential for Pb transformations.
AFTERNOON POSTERS
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Title: Experimental and numerical investigation of dislocation-based transient creep mechanisms in the upper mantle
Authors: Abigail Wilwerding, Shanti Penprase, Andy Wickert
Abstract: Glacial meltwater floods raise river levels downstream, in turn elevating base level for their tributaries, whose lower valleys collect slackwater sediments in lacustrine-like depositional environments. These slackwater deposits—comprising fine sands, silts, and clays transported in floodwaters—record chemical and lithological signatures of their glacial provenance. Here, we decipher the late-stage deglacial history in the upper Mississippi Valley, USA, by analyzing such slackwater sediments. We examine four cores from a terrace 20 m above the Whitewater River, a tributary to the once-proglacial Mississippi River in southeastern Minnesota. The cores contain ~4.88 m of interbedded cm- to mm-thick red, gray, and brown silty clay and sand beds, distinct from the sandy fluvial terrace deposits below. Single-grain Optically Stimulated Luminescence dates indicate the laminated slackwater sediments were deposited at 11.67±1.51 ka (1.16–1.64 m depth) and 11.56±1.44 ka (2.36–2.59 m depth). These dates overlap with the ~11.6–10.6 ka Marquette Advance of the Laurentide Ice Sheet, in which glacial ice reoccupied the Lake Superior basin. This readvance may have driven the dated reoccupations of the southern outlets of both Glacial Lake Agassiz and Glacial Lake Duluth in the modern Lake Superior basin. Deposit colors suggest alternating Agassiz and Superior sources. To test for possible western (Agassiz) and eastern (Superior) meltwater sources, we turn to both classic lithologic descriptions and X-ray Fluorescence (XRF), with the latter providing elemental compositions. Previously studied slackwater deposits from the Superior Lobe were rich in Fe, Cu, Zr, Ti, Mn, Ni, Cr, and Co, whereas Des Moines lobe (Agassiz Basin) deposits were rich in Se, Ca, Ar, Cd, Mo, and Zn. Preliminary analysis of the XRF results confirms that the slackwater deposits are distinct from the locally derived sandy terrace materials in the bottom ∼10cm of the core. Going forward, we will fingerprint these slackwater sediments to source regions throughout the analyzed core. The results from this study will contribute valuable insights into the final meltwater pulses to enter the Mississippi River, including their sources and constraints on paleodischarge, and expand our knowledge of early-Holocene ice-sheet evolution in North America.
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Title: Multidecadal Climate Variability in Large Model Simulations o and Proxy Records of the Past 1000 years
Authors: Alejandro Fernandez, Byron A Steinman, Michael E Mann
Abstract: Internal climate variability is broadly defined as that which is not influenced by external factors to Earth’s climate system, such as volcanic eruptions, orbital configuration, or human impacts. Although the physical mechanisms that cause some modes of internal variability such as ENSO (El Nino – Southern Oscillation) are well understood, observations and climate models suggest that there exist lower-frequency cycles whose underlying causes are still a matter of debate. Such is the case of the Atlantic Multidecadal Variability (AMV), a sea-surface-temperature pattern of variability in the North Atlantic that has been hypothesized as being linked to long-term changes in the Gulf stream and heat transport from the tropical Atlantic. Although evidence exists for semi-oscillatory behavior in the multidecadal frequency band in historic (1850CE – present) simulations and observations, some authors have related this evidence to anthropogenic greenhouse gas emissions and sulfate aerosols. Here, we explore the extent to which Last Millennium simulations (850 – 1850CE) showcase evidence of AMV variability by conducting multivariable, spatiotemporal spectral analysis on the Last Millennium Ensemble (LME) of simulations of the CESM model, as well as other models belonging to the Climate Model Intercomparison Project (CMIP6). In the case of the LME, we take advantage of the ensemble size to estimate the forced signal of variability by computing the ensemble mean, and then estimating each simulation’s internal variability evolution as the temperature field residual. We find that the CESM model shows muted temperature variability in multidecadal timescales, as other models have some evidence of an AMV-like response which may be forced in nature. To complement the model analyses we provide a preliminary compilation and spectral analysis of paleoclimate proxy datasets which span similar timeframes. Our results suggest the absence of a truly oscillatory, internal mode of variability in the multidecadal band as expressed in past millennium simulations, supporting the hypothesis that the observed AMV-like oscillations in models and observations correspond to externally forced variability.
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Title: Discovering the Position of Earth’s Last Geomagnetic Reversal in Sediments from Blackwater Draw, Texas
Authors: Ruby Sandell, Jonathon Stine, Joshua Feinberg
Abstract: The Blackwater Draw Formation (BDF) in Bushland, Texas, composed of alternating layers of loess and soil, is situated in a region known as the Southern High Plains. This region of the southern Midwest lacks significant paleoclimate information due to poor age constraints on the sediments in the BDF. A geochronologic method, referred to as paleomagnetism, is used to better constrain the ages of these sediments by determining the precise locations of ancient magnetic reversals within the stratigraphy. The samples are collected from ~12 m soil core and placed into 7cc non-magnetic plastic cubes, serving as the primary specimens for analysis. The characteristic remanent magnetization (ChRM) is measured using a 2G Cryogenic Superconducting Rock Magnetometer (SRM) that subjects the specimens to step-wise alternating field-demagnetization from 1-100 mT. Preliminary results show consistent zones of normal and reverse polarity that align with known quaternary geomagnetic reversals. The specimens maintained relatively stable magnetic fields under low coercivity conditions (0-10 mT). Locating the position of ancient magnetic reversals within the sediments, thus constraining the age, will relate changes in the sediment record to climate cycles associated with the Early-Middle Pleistocene Transition in North America.
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Title: The role of point defects in the crystal plasticity of olivine
Authors: Hagmar Tinoco Madeira, Lars N. Hansen, Amanda M. Dillman
Abstract: It is well known that the plastic deformation of Earth's lithosphere is largely controlled by the motion of crystal dislocations in olivine. However, the role of the chemical environment in moderating dislocation velocities is still a remaining source of uncertainty. In particular, the chemical environment, such as the oxygen fugacity, influences the concentrations of point defects that may influence dislocation velocity. Overall, dislocation velocities are set by either the rate of dislocation glide or of dislocation climb. It has already been established that dislocation creep in olivine is highly dependent on oxygen fugacity at high temperatures, for which the climb controls the velocity. However, the role of oxygen fugacity has not been investigated at low temperatures, for which glide controls the velocity. Therefore, it is currently unclear if changes in point defect concentrations influence dislocation glide velocities. To address this gap in knowledge, 5 samples consisting of natural single-crystal San Carlos olivine were annealed in a 1-atm gas mixing furnace set to different CO/CO2 ratios (ranging from 50:1 to 1:100) and oxygen fugacities (ranging from 10-11 to 10-2 atm). After annealing, the elastic modulus and hardness of the samples were measured at room temperature, which suppresses dislocation climb, using instrumented nanoindentation with a Berkovich tip. Based on these measurements, the elastic modulus and hardness do not change significantly as a function of oxygen fugacity during annealing, remaining at ~215 GPa and ~13 GPa, respectively. This result demonstrates that, unlike dislocation climb at high temperatures, dislocation glide does not have a dependence on oxygen fugacity and the associated point-defect concentration.
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Title: Dendroclimatology of Northern Minnesota using Thuja occidentalis
Authors: Naomi Schulberg, Daniel Griffin, Kurt Kipfmueller, Andrew Wickert
Abstract: Climate change will likely result in warmer temperatures, higher frequencies of drought, and higher flood risks in northern Minnesota, but sparse long-term climate records exist in the region, limiting our ability to contextualize recent climatic changes. We applied dendrochronological techniques to Thuja occidentalis (northern white cedar), to establish annual to sub-annual long-term records of climate and thus better understand the current context of climate change in northern Minnesota. T. occidentalis presents a promising opportunity to study past climate in northern Minnesota, often living to be over 500 years old, with subfossil wood found to be even older. A pilot study by Kipfmueller showed that T. occidentalis cores from the same stand show synchronized signals which are likely related to environmental factors and can thus be used for climate reconstructions. We created chronologies of earlywood, latewood, and total ring widths from six sites in northern Minnesota, and analyzed their relationship to temperature, precipitation, and drought. Preliminary results show a range of climate responses between sites. The strongest responses identified are a positive relationship with May and June precipitation, and a positive relationship with the Palmer Drought Severity Index from March to August. Additionally, stronger responses to climate were seen at sites in the Boundary Waters Canoe Area Wilderness, as compared to the Cloquet Forestry Center. Future work will involve expanding the current network of chronologies and testing new tree-ring proxy techniques, aiming to identify a stronger climate signal, which can then be used for climate reconstructions.
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Title: Molecular Mechanisms of Algal Growth Enhancement by Phycosphere Bacteria Under Iron Limiting Conditions
Authors: NR Coffey, B Newell, K Manning, RK Stuart, X Mayali, Y Corilo, W Kew, C Dewey, RM Boiteau
Abstract: Iron (Fe) is a critical micronutrient in marine ecosystems, but is difficult to acquire due to its poor solubility. To gain a competitive advantage, some marine microorganisms have developed molecular strategies to bind and uptake Fe from their surroundings. However, most marine algae lack the ability to synthesize these Fe-binding molecules, termed siderophores, and it remains unknown how they access Fe from poorly soluble phases such as dust that represent the major Fe source to the open ocean. Phaeodactylum tricornutum (P. tricornutum) has been a popular diatom to use in nutrient limitation studies as a model organism, as its entire genome is known and its growth is well-characterized. However, most of these studies have been conducted in monoculture, even though P. tricornutum has co-evolved with bacterial partners in environments where low Fe solubility is a dominant ecological control. In this study, we examine how the presence of bacterial partners isolated from P. tricornutum’s phycosphere impact the diatom’s growth under Fe-limited conditions. In experiments using Arizona Test Dust (AZTD, 2-5% Fe2O3 by weight) as the sole Fe source, the presence of one bacterial partner Marinobacter 3-2 significantly improved algal growth under Fe-limited conditions, reaching a final cell density over three times higher than that of the axenic algae. In contrast, Stappia sp. ARW1T significantly inhibited P. tricornutum’s growth, halving the algae’s final cell density after 26 days of growth. Both strains are suspected to produce siderophores, but the identity of those molecules and their potential for use by P. tricornutum remains unknown. Using LC-ESI-MS coupled with novel computational pipelines, we probed the metabolome of axenic P. tricornutum cultures as well as the metabolome of the diatom-bacteria co-cultures to understand the mechanisms behind the observed growth effects. By examining differentially expressed features between high (1 µM) and low (10 nM) Fe conditions in the presence and absence of bacterial partners, we can begin to untangle the molecular basis for algal-bacterial interactions and uncover how bacteria can influence algal growth and resilience.
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Title: Reaction-induced and Background tectonic stresses guide fracture, fluid flow, and serpentinization in the mantle wedge corner
Authors: Jeremiah J McElwee, Ikuko Wada, Kazuki Yoshida, Hiroyuki Shimizu, Atsushi Okamoto
Abstract: During subduction, fluids released by the downgoing slab may react with material in the forearc mantle wedge to form serpentinites. Serpentinization is accompanied by a large volume increase that can stress the surrounding rock and generate fractures, which may act as pathways for more fluids to migrate into the mantle wedge corner, facilitating further serpentinization. However, these reaction-induced fractures are likely dependent on the background tectonic stress state in the mantle wedge corner. We investigate the effect of reaction-induced stresses and background tectonic stresses on fracturing, fluid flow, and serpentinization in the mantle wedge corner of subduction zones using a 2-D brittle-elastic hydraulic-chemical-mechanical discrete element model, and make comparisons to geophysical observations of serpentinization and fluid distributions in subduction zone forearcs.
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Title: Fate and transport of lead within Twin Cities stormwater catchment ponds using inductively coupled mass spectrometry and synchrotron based X-ray absorption spectroscopy
Authors: Christof Zweifel, Ben Janke, Lea Pollack, Timothy Mitchell, Kaya Koraleski, Jacques Finlay, Emilie Snell-Rood, and Cara Santelli
Abstract: Industrial emissions, legacies of leaded gasoline, and the breakdown of lead paint have created a risk for lead contamination in urban environments. In the Twin Cities Metro Area there are multiple sources of lead emissions from the downtown garbage burner, regional airports, smelters, lead paint, and decades long accumulation from leaded gasoline. Sediments, runoff, and the direct input from industrial sources means stormwater catchment areas have the potential to capture lead and other metals within a unique chemical environment. The chemical transformations that lead undergoes from terrestrial, airborne, and dissolved sources into the sediment captured by stormwater ponds determines the concentration and chemical speciation of lead, ultimately impacting lead bioavailability. Bioavailability determines the impact on pond ecology and potential for remobilization of lead from pond sediments. Concentrations of lead and other heavy metals were determined using mass spectrometry (ICP-MS) for the surface water, bottom water, porewater, and sediment of three well studied ponds within the Minneapolis-St Paul Long Term Ecological Research program (LTER). Lead speciation of Twin Cities soils and pond sediments will be determined in late March 2024 using synchrotron based X-ray absorption spectroscopy (XAS). X-ray fluorescence (XRF) data shows lead concentration in urban soils ranging from 20 to 2,000 (ppm), with the EPA’s action level set to 200 ppm. Sediments at the bottom of the three stormwater ponds had a range from 23 to 300 ppm lead. Sediment lead concentrations in two out of the three ponds exceeded 200 ppm, with lead levels generally higher at greater sediment depths in these two ponds.ICP-MS data shows metals such as chromium, nickel, and copper are concentrated by a factor of 10 within stormwater sediment pore waters as opposed to surface and bottom waters. Porewater lead concentrations are between 0 and 0.002 parts per million (ppm) in the dissolved <0.15 micron phase, which is not significantly different from surface or bottom waters. Metal concentrations and speciation offer insight into the types of sources, processes that affect lead fate and transport, and potential remobilization of metals in urban ponds.
Talk Abstracts
MORNING TALKS
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Title: Investigating Lead Sequestration in the Soil Bacterium Bacillus megaterium
Author: Jamison Ward, Beverly Flood, Cara Santelli, Jake Bailey
Abstract: Lead is a priority pollutant whose environmental occurrence has increased since human industrialization with deleterious consequences for both human and natural ecosystem health. The application of lead-resistant microorganisms for bioremediation has received considerable attention for its advantages over physiochemical remediation strategies. The soil bacterium, Bacillus megaterium, has been previously observed to harbor lead resistance and accumulate lead intracellularly; however, the mechanism for lead storage was not entirely elucidated. One possibility is lead sequestration in microbial polyphosphate (polyP) granules—nanometer-scale inclusions of phosphate polymers used as phosphorus storage and cellular energy reserve. PolyP contains metal atoms as counterions, and lead has been observed to associate with polyP in bacteria grown in lead-rich environments. This project employed a suite of analytical methods to investigate the mechanism of lead resistance in B. megaterium with lead storage in polyP as the working hypothesis. The bacterium was cultured to stationary phase in a lead-amended minimal growth medium and examined using epifluorescence microscopy; cells stained with DAPI and tetracycline were observed to harbor abundant polyP granules. Current efforts include imaging selected specimens via STEM-EDS to further investigate cellular morphology and elemental distribution within polyP granules, thereby elucidating mechanisms of intracellular lead sequestration. Additionally, supernatant samples have been prepared for ICP-MS analysis to evaluate lead concentrations throughout the growth experiment. If soluble lead drawdown is observed, the mechanism of lead accumulation in bacterial polyP granules could prove an effective method to remove lead from contaminated soils or wastewater as B. megaterium is native to soil environments and could be integrated into current wastewater treatment processes. Therefore, novel lead bioremediation techniques derived from this research could be employed to maintain safe drinking water and public greenspaces for Minnesota communities, protecting future generations from the deleterious effects of lead poisoning.
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Title: Evaluating the Redox Evolution of Martian Magmas
Authors: Sanath Aithala, Marc Hirschmann, Maria Buss, Jasper Goldstein
Abstract: The influence of oxygen fugacity (fO2) on igneous processes including crystallization, partial melting, degassing, ascent, and assimilation has long been a subject of interest in planetary geology as its consequences include planetary habitability and petrologic diversity. We seek to better understand the martian interior and its magmatic history by characterizing the processes that produced the rocks in the martian rock record. Variations in terrestrial basalt fO2 are attributed to the influence of plate tectonics recycling surface-materials to the mantle, but martian basalt fO2 variation is even greater despite Mars not having plate tectonics. In addition, some martian magmas undergo extents of oxidation (fO2 increase) throughout crystallization much greater than what is observed in terrestrial magmas. Many workers explain the oxidation histories of martian magmas by invoking shallow oxidative processes such as degassing and crustal assimilation, but do not provide quantitative constraints on their feasibility. We present experimental data that quantitatively constrain the relationship between the oxidation state of multivalent cations - primarily iron - in magmas, to fO2 and we use this relationship to model different scenarios of shallow processes that can explain the oxidative redox history of martian magmas.
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Title: Cranial ontogeny in the Early Cretaceous ceratopsian dinosaur Psittacosaurus lujiatunensis: intraspecific variation by age and taxonomic implications
Authors: Minyoung Son, Peter J. Makovicky, Gregory M. Erickson, Chang-Fu Zhou, and Ya-Lei Yin
Abstract: Before using ontogenetically variable character data and character scorings from immature individuals in maturity assessment and taxonomy of fossils, character state changes across ontogeny should first be evaluated at the species level. Psittacosaurus is one of the best-sampled dinosaur genera in the fossil record and the most speciose genus of dinosaur, with ten species currently accepted as valid. Histological data for Psittacosaurus specimens from China, Mongolia, and Russia have been reported, showing different life history strategies for each analyzed species. However, no study has yet comprehensively examined changes in character states or evaluated characters specific to an ontogenetic stage. Here we examined the post-hatchling variation of Psittacosaurus lujiatunensis based on 30 specimens from the Lujiatun Unit (Barremian, Early Cretaceous) of the lowermost Yixian Formation in Liaoning Province, northeast China. Taphonomic effects were accounted for when identifying and scoring variation. Taphonomic processes inferred from articulated skeletons support the presence of only a single species of Psittacosaurus in the Lujiatun bed, with other proposed Lujiatun species exhibiting different skull morphology due to diagenetic deformation. Based on histological ages, specimens range from less than a year old to fully mature individuals more than ten years old. As previously reported, commonly used body measurements are strongly correlated with age. The efficacy of size-independent character changes for assessing maturity and assigning a specimen to an ontogenetic stage was tested by treating the ontogenetic hierarchy as a form of phylogenetic hierarchy (cladistic ontogeny). We built a character matrix of ontogenetically variable characters of the skull defined as binary or multistate transformation series. The matrix was analyzed with TNT, recovering a strict consensus tree (i.e., ontogram). The consensus ontogram is poorly resolved indicating prevalent intraspecific variation, and is incongruent with the ontogenetic sequence of specimens based on age and size rankings. Although some histologically supported age groups cluster in the ontogram, others are separated by varying patristic distances. Some trait combinations proposed as diagnostic of different Psittacosaurus species are in fact observed across the growth series of P. lujiatunensis, meriting a revision of Psittacosaurus diversity and evolutionary patterns.
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Title: A comprehensive database of dissolved riverine rare earth elements (REEs): Seeking the controlling factors from the global perspective
Authors: Weiming Ding, Yves Plancherel, Andrew Wickert, Kate Davies, Chandranath Basak, Xin-Yuan Zheng
Abstract: Dissolved rare earth elements (REEs) in rivers carry useful information on continental weathering, aqueous geochemical processes, and anthropogenic contamination. They also contribute significantly to cycles of REEs and their isotopes in the global ocean. Aquatic geochemistry (such as pH and river-water chemical compositions) has been proposed to influence dissolved riverine REEs patterns and concentrations. Nonetheless, the influence of lithology on the dissolved riverine REEs pool has been overlooked since early times, even though riverine REEs stem from dissolution of parental rocks. In addition, a recent study speculated that truly dissolved riverine REEs patterns resemble seawater REEs patterns based on local observations, highlighting the potential origin of oceanic REEs evolution from the riverine REEs end-member. However, the relationship between riverine and seawater REEs characteristics remain unexplored on a global scale. A compilation of riverine REE data and detailed analysis of the controls on REE concentrations and patterns in global rivers is, therefore, valuable.
To better understand the behavior and distribution of riverine REEs, we compiled dissolved riverine REEs concentration data. This new compilation includes 1981 dissolved and 114 truly dissolved REEs datapoints sampled from ~160 rivers and their tributaries in 28 countries. Using this compilation, we provided discharge-weighted annual gross riverine REEs fluxes, and updated net riverine REEs fluxes to the ocean by compiling the riverine REEs removal percentages during estuarine mixing processes, providing more robust constraints on the REEs input to the ocean. In addition, direct comparisons between truly dissolved riverine REEs and seawater REEs pools were conducted. Seawater-like REEs patterns can be observed in finer-filtered truly dissolved riverine REEs fractions. Nonetheless, further chemical fractionations of truly dissolved riverine REEs are still needed to decrease the REEs concentrations and Ce anomalies. Furthermore, Geographic Information System analyses was applied to extract lithological information in the watershed for each REEs sampling point based on their coordinates according to global lithological database. Our data confirm the significant influence of watershed lithologies on the riverine pH, REEs concentrations, patterns and Eu anomalies, which underscores the importance of REEs characteristics in the geological terrains on shaping the riverine REEs.
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Title: Unraveling viscous remanent magnetization of cave formations
Authors: Emma Kostecki, Plinio Jaqueto, & Josh Feinberg
Abstract: The efficacy of using viscous remanent magnetization (VRM) as a tool for dating geological materials has been debated since no general method has been established. Successful applications involve Hadrian’s Wall, Icelandic glacial floods, and movements of boulders due to tsunamis. The technique relies on the presence of the fine scale grain sizes of magnetic minerals that are able to acquire new magnetization when a new orientation is established. One of the materials that has not been explored are speleothems. Speleothems are secondary carbonate formations in caves, such as stalagmites and stalactites, and are natural recorders of the Earth’s dynamic past. During formation, trace concentrations of magnetic minerals are incorporated into their matrix and record the Earth’s magnetic field at the time of formation. Results from all over the globe show the ubiquitous presence of a secondary magnetic component. Although they have not been studied, these secondary magnetizations offer the possibility to be used as a dating tool for past tectonic events, past human occupation in caves, and paleofloods.
In this study, we explored the magnetic mineralogy and the acquisition of viscous remanent magnetization of a stalagmite sample. Our goal was to establish a robust analytical time-dependent expression for the acquisition of VRM. We applied two protocols to unravel the viscous component. We demagnetized at small temperature steps to define the temperature range where the viscous component is defined, and calculated the grain size distribution that is required for this acquisition. Next, we simulated the acquisition of VRM in a controlled laboratory environment, by changing the initial states of the specimens. We were able to define the time required to gain a new vector magnetization. By fitting a curve to this acquisition, the mathematical model can be used to date orientation changes in the sample.
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Title: Historical snow-drought-streamflow relationships in a sensitive aquatic ecosystem: Understanding the consequences of snow drought for wild rice
Authors: Paige R. Voss, G. H. Crystal Ng, Daniel J. Larkin, William (Joe) Graveen, Shaoqing Liu
Abstract: Wild rice, or Manoomin/Psiη (Ojibwe/Dakota), is an aquatic grass native to the Upper Midwest that grows annually in slow moving lakes and rivers. Wild rice holds great cultural and spiritual significance as a sacred relative for Ojibwe, Dakota, and other Indigenous peoples. However, wild rice has declined significantly in response to the environmental stressors triggered by Euro-American settler-colonialism, including changes in water levels and flow rates. As part of a university-tribal collaboration, our work focuses on a culturally significant wild rice river on the Lac du Flambeau Band of Lake Superior Chippewa Indians Reservation in Northern Wisconsin, which has experienced a decline in wild rice in some reaches. Ongoing research shows that wild rice abundance is positively correlated with greater snow accumulation, among other factors. We hypothesize that the hydrologic perturbations caused by low snowfall exacerbate the preexisting stressors affecting wild rice. Since snowmelt magnitude and timing plays a large role in discharge dynamics in this region, differences in snow accumulation have potential consequences for wild rice during its crucial germination, submerged, and floating-leaf life stages. Comparing the historic spring discharge of years characterized by high snowfall to those experiencing snow droughts, we see consistently lower discharge rates during the germination and submerged stage following snow drought winters, even when the winter precipitation was comparable to that of high snow years. Furthermore, discharge levels remain higher late into the summer following winters with high snow accumulation. This potential link between snowfall, water levels, and wild rice raises concern because current climate projections show considerable decreases in snowfall, snow depth, and snow season length for the Upper Midwest by 2100.
AFTERNOON TALKS
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Title: Terrestrial, deep, subsurface microorganisms fuel metabolism with linked organic and inorganic sulfur cycles
Authors: Amanda Patsis, Cara Santelli, Cody Sheik
Abstract: Organosulfur compounds (OrgS) have been increasingly recognized as a central aspect of biogeochemical S cycling in marine and freshwater surface systems, providing substrates for growth and a source of inorganic sulfur to drive chemoautotrophy. The importance of OrgS compounds in the terrestrial deep subsurface, however, remains unexplored. Here, we employ shotgun metagenomic sequencing of deep biosphere communities from Soudan Mine to probe the genetic potential for OrgS cycling and to elucidate the role these compounds play in supporting deep terrestrial life. Our findings demonstrate that a taxonomically diverse microbial community has the capacity to support an active OrgS cycle, with dimethyl sulfide and taurine playing a central role. Organic and inorganic sulfur cycling processes are inextricably linked in this system, with the assimilation and mineralization of OrgS compounds impacting the availability of inorganic S substrates. Specifically, OrgS degradation provides a key connection between the organic and inorganic S cycle through the generation of sulfite or sulfide, which can then be used in redox reactions to generate ATP. Our results are further contextualized through a comparison to other deep biosphere communities sequenced through the Census of Deep Life, which supports the importance of a linked organic and inorganic S cycle and highlights the role of OrgS as a source and sink of inorganic S compounds across the terrestrial deep subsurface. Continued exploration into the scope and scale of microbially driven OrgS cycling will refine our understanding of deep subsurface biogeochemical sulfur cycling and how it impacts larger Earth systems.
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Title: Soil Carbon and Nitrogen Stocks Across Hillslopes Underlain by Continuous Permafrost in the Northern Arctic Foothills, Alaska, USA
Authors: I.A. Ainuddin, N.A Jelinski, R. Matamala, C.L. Ping, J.D. Jastrow
Abstract: Constraining the variability of soil organic carbon (SOC) and total nitrogen (TN) stocks across hillslopes in permafrost-mantled terrain of the low Arctic remains a significant challenge for improving uncertainties in global estimates of permafrost SOC stocks. Despite studies focusing on SOC and TN stocks across regional climate gradients in the northern circumpolar region, the lack of quantitative SOC and TN stock data across hillslope toposequences introduces large uncertainties in SOC estimates at regional and global scales. We investigated SOC and TN stocks across hillslopes at two locations in the Arctic Foothills of Alaska, USA (Happy Valley and Sagwon Hills). Average SOC and TN stocks for the 0-1 m depth interval were high (51.5 7.6 kg C m-2 and 2.75 0.34 kg N m-2) and linearly related (R2 = 0.74, p < 0.0001) but unlike soils of other permafrost and non-permafrost landscapes, variability was greatest within rather than between hillslope positions. Furthermore, SOC and TN stocks in the top 1m did not exhibit strong patterns by hillslope position nor were they closely associated with major geomorphic parameters (i.e. slope and curvature) that are typically predictors of SOC and TN stocks across most landscapes. The unique small-scale variability in ground ice content, cryoturbation, patterned ground, and organic layer thickness on these broad, Low Arctic sites contributes to the relatively homogeneous distribution of SOC and TN stocks across hillslope positions. This work underscores the need for and importance of local toposequence studies to underpin broader regional-scale predictive efforts.
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Title: Solutes as tracers of streamflow components in an agricultural headwater catchment in Southern Brazil
Authors: Alice Dambroz, Paul Capel, Jean Minella, Fábio Mallmann, João Henrique Silva, Felipe Bernardi, Larissa Werle, Fabio Schneider and Gustavo Merten
Abstract: Streamflow depends on precipitation and the hydrological characteristics of the watershed. Surface runoff (fast flow) is one component of streamflow which happens during rainfall events, temporarily increasing discharge. Another component is baseflow (slow flow), which slowly discharges water to a stream in-between events. Understanding the components of streamflow is important for water resources management, especially for maintaining adequate discharge over the year to avoid impairments to the environment. Solute concentrations during events may be used as end-members to infer on the sources of fast and slow flows. No-till agriculture, topographic gradients, a geological transition, and different soil classes characterize Guarda Mor catchment in southern Brazil. Rainfall and streamflow are monitored in this catchment, at a high temporal resolution. Manual water samples are collected during events to quantify calcium (Ca) and magnesium (Mg) concentrations. Eight rainfall events that occurred between April 2022 and September 2023 were analyzed. The baseflow index (BFI) was calculated for each event by using the dissolved concentrations in an end-member mixing analysis (EMMA) and by a hydrograph separation modeling tool. For the EMMA analysis, the maximum Ca and Mg concentrations in each event were considered as the slow flow end-members, while for fast flow, concentrations in rainfall were used. Eckhardt’s digital filter was used for hydrograph separation of individual events, and median BFI were between 0.84 and 0.99. Median BFI in Ca-EMMA ranged from 0.66 to 0.98, and between 0.60 and 0.92 for Mg-EMMA. Eckhardt and Mg-EMMA’s BFI have positive correlations to 24 and 48h antecedent precipitation, and negative correlations to 120 and 168 h antecedent precipitation, discharge, and rainfall’s kinetic energy. Differently, Ca-EMMA’s BFI has a positive correlation to 24 and 48 h antecedent precipitation, discharge, and rainfall’s kinetic energy. Comparing the shifts in fast flow can provide information on stream recharge over the year and aid the planning of management practices that improve water infiltration.
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Title: Seasonal variation of CO and CO2 concentrations and fluxes above an urban footprint in Greater New York
Authors: Trey A. Maddaleno, Dylan B. Millet, Michael P. Vermeuel, Katelyn L. Rediger, Jeff Peischl, Emily B. Franklin, Delphine K. Farmer
Abstract: Carbon monoxide (CO) and carbon dioxide (CO2) play key roles in the urban atmosphere as pollutants, combustion products, and endpoints of volatile organic compound (VOC) oxidation. In complex urban environments, the two species can also be used as tracers to quantify the importance of VOC emissions from combustion & traffic relative to non-combustion sources including volatile chemical products. However, this potential is underexploited as few studies have combined direct flux measurements of CO, CO2, and VOCs in urban environments. During both a summer and wintertime sampling period, the ‘Fluxes of Reactive Organic Gases in New York’ (FROG-NY) project used the eddy covariance method from two heights (32.5 and 60 m) on a cellular tower in Long Island, NY to directly quantify trace gas fluxes over an urban environment. Fast, simultaneous measurements of CO2, CO, and N2O were collected alongside comprehensive VOC measurements from H3O+ and I- chemical ionization time-of-flight mass spectrometers. Here, we present a first look at the observed CO and CO2 fluxes and concentrations along with their connection to traffic sources.
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Title: Experimental and numerical investigation of dislocation-based transient creep mechanisms in the upper mantle
Authors: Diede Hein, Lars N Hansen
Abstract: Transient creep of olivine in the upper mantle plays an important role in large-scale Earth processes such as glacial isostatic adjustment and postseismic creep, as well as (exo-)planetary tidal heating and orbital dynamics. Yet, an experimentally confirmed microphysical understanding of transient creep across all timescales relevant to Earth processes remains elusive.
The time-dependent dissipation of strain energy during transient creep manifests as attenuation, Q-1, in the frequency domain. Therefore, constitutive equations of proposed transient creep mechanisms should be able to predict the attenuation in polycrystalline olivine subjected to forced oscillations. Here we present numerical investigation of the nonlinear constitutive equations of two contending mechanisms in the frequency domain and comparisons thereof to the mechanical results of a set of high-stress, forced-oscillation experiments on polycrystalline olivine performed in a deformation-DIA coupled with synchrotron analysis techniques. Key microstructural variables needed to inform these comparisons, such as grain size, viscous anisotropy, and dislocation density, were obtained from electron backscatter diffraction and dislocation decoration.
Numerical analysis of the constitutive equations yields high Q-1 values, up to ~5, which is similar to the experimental observations. We find that the experimental observations are consistent with predictions from the backstress model for the grain sizes and dislocation densities of our samples. When extrapolated to lower stress amplitudes, the backstress mechanism produces approximately linear behavior and behaves as a Burgers model in frequency space, suggesting that dislocation interactions may contribute to seismic wave attenuation as well.
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Title: Conserving Soil During Perennial Crop Termination
Authors: Jake Kundert; Manbir Rakkar; Jessica Gutknecht; Jacob Jungers
Abstract: Mechanical termination of crops can negatively affect soil biological, chemical, and structural characteristics. Perennial crops do not require annual termination and can improve these same soil characteristics, which has catalyzed interest in the development of new perennial crops. Advanced lines of the perennial grass intermediate wheatgrass (Thinopyrum intermedium [Host] Barkworth and Dewey; IWG) have been bred for increased seed size and marketed as Kernza® perennial grain, but little is known about how this new crop can be terminated for subsequent annual crop production in rotations that enhance agricultural productivity and environmental sustainability. Five methods of terminating IWG were tested in Rosemount, Minnesota. Treatments included mechanical tillage using a chisel plow (CHI), undercutter (UND), and disc (DSC), along with no-till treatments of glyphosate (GLY) and a repeated-mowing control (CTRL). Treatment effects on IWG mortality, soil carbon dioxide (CO2) emissions, bulk density, aggregate stability, soil carbon stocks and soybean yield were measured. Daily CO2 fluxes differed by treatment (p < 0.05) on only one of 19 sample dates, and cumulative soil CO2 emissions over the course of the growing season did not differ across treatments. Bulk density decreased relative to baseline in all treatments except CTRL. Aggregate stability remained unchanged in all treatments except CTRL, which increased from the baseline. Soil carbon stocks did not change in any treatment. Soybean yield was highest in GLY but was not significantly different from CHI or UND. Soil structure, soil carbon stocks and soil CO2 emissions were unaffected by tillage and no-till IWG termination treatments. However, tillage followed by pre-planting harrowing proved ineffective at terminating IWG and required subsequent summer herbicide applications. Therefore, additional tillage events may be required to fully terminate IWG when herbicide use is prohibited.
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Title: Fluvial Mobility of Hadrosauroid Dinosaur Skeletons: Experimentally Testing Transport Potential in Extinct Reptiles
Authors: Michael Chiappone, Michele Guala, Raymond Rogers, Peter Makovicky
Abstract: The fossil record is our primary window into understanding extinct ecosystems, and understanding the taphonomic processes that shape it is integral to interpreting the record of extinct life. Bones preserved in fluvial environments make up a major part of the terrestrial vertebrate fossil record, and unsteady state flows (flooding, crevasse splays, debris flows) are often invoked as drivers of mobility and burial of skeletal elements. Actualistic experiments have explored the fluvial taphonomy of mammal skeletons, but other terrestrial vertebrates, especially extinct clades, have only been sparingly studied. Because of this, researchers have raised concerns that the morphologies of non-mammalian bones would result in different hydrodynamic properties and mobility patterns. Being known from extensive remains across the globe, hadrosauroid dinosaurs make an ideal group for this kind of taphonomic study. Here we investigate the mobility of postcranial bone models of the hadrosauroid, Eolambia caroljonesa, and skull elements of Edmontosaurus regalis, using 3d printed bones weighted to feasible densities in a large-scale flume setup. We find that in unsteady state flow conditions at two discharge levels, our bone models sort into statistically significant transport groups based on movement distance. These groups are broadly similar to Voorhies Groups, although the modes of mobility and groups certain elements sort into is variable due to differing morphologies from the sheep skeletal elements we tested under the same conditions. While distance-based groups were evident among hadrosauroid bones, such groupings were less clear among the more continuously distributed sheep bones. Voorhies Groups were originally constructed from qualitative observations on the order and mode of bone movement rather than transport distance or competent velocity. In the quantitative context of our study, it was not possible to match the sheep skeleton to the originally established Voorhies Groups in a statistically significant and non-arbitrary manner due to their somewhat continuous distribution of transport distances. We also find that both shape and mass were significantly correlated with transport distance by both hadrosaur postcrania and sheep bones, which is suggestive that travel distance in absence of other obstacles relies on both of these characteristics.
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Title: Isotopic evidence reveals limited mobility in the short-legged rhinoceros Teleoceras major at Ashfall Fossil Beds, USA
Authors: Clark T. Ward, Brooke E. Crowley, Ross Secord
Abstract: We use carbon (C), oxygen (O) and strontium (Sr) isotopes to evaluate multiple types of mobility in Teleoceras major from Ashfall Fossil Beds, Nebraska. Ashfall is a mid-Miocene (ca. 12 Ma) site that preserves hundreds of T. major skeletons in a volcanic ash-filled watering hole. This Laggerstätte provides a temporal “snapshot” of T. major population dynamics and allows us to test three possible types of mobility: (1) sex-specific (male) ontogenetic dispersal; (2) seasonal migration; and (3) retreat to a refugium in response to the catastrophic volcanic event. To test for dispersal, we bulk sampled lower m2’s and m¬3’s from five males and eight females; m2’s formed after weaning but before cow-calf separation, while m3’s formed between cow-calf separation and sexual maturity. To test for seasonal migration, we serially sampled m2’s and m3’s from two males and two females. Lastly, we compared isotope data for T. major with other ungulates from Ashfall to detect possible perimortem mobility and niche partitioning. We found only one significant isotopic shift between bulk-sampled m2’s and m3’s for either sex: C isotopes for female m2’s were ca. 0.5‰ higher than female m3’s. This is too small of a difference to worry about confidently assign a single driver. There were also no significant isotopic differences between sexes for either m2’s or m3’s. Oxygen isotopes oscillated by ca. 2‰ for all serially sampled teeth, which likely reflects seasonal climate fluctuations. In contrast, the range in C isotopes was <1‰, and the range in Sr isotopes was within analytical uncertainty (±0.00003). Finally, O and Sr isotopes distinguish T. major from co-occurring ungulates, especially horses. Combined, these results suggest that the sampled T. major individuals were local to Ashfall throughout their lives. They did not arrive from elsewhere (due to ontogenetic dispersal or catastrophe), nor did they seasonally migrate. To avoid inbreeding and intraspecific competition, individuals may have socially (rather than spatially) dispersed among co-occurring groups. Small but significant isotopic differences among ungulates indicate niche partitioning; Teleoceras major at Ashfall likely inhabited wet habitats. Modern rhinoceros species rely on mud wallows for thermoregulation and social interactions, and T. major may have been restricted to wet habitats for similar biological or social reasons.
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Title: Assessing Dinoflagellate Cysts and Geochemical Signals in Surface Sediments of the Northern Bering Sea
Author: Vincy Y Winifred, Vera Pospelova, Evangeline Fachon, Kenneth Neil Mertens and Donald M Anderson
Abstract: Dinoflagellate cysts as well as geochemical parameters were analyzed from 26 surface sediment samples in the northern Bering Sea. Known ocean environmental gradients such as sea-surface temperature and salinity, duration of the sea-ice cover and primary productivity, along with the measured sedimentary geochemical proxies were used to examine the influence of upper water masses on the cyst assemblages. Total concentrations of the organic-walled dinoflagellate cysts vary spatially and range from 726 to 11,673 cysts g-1, with an average of 4,535 cysts g-1. The highest cyst concentrations were observed in the area south of St. Lawerence Island that is influenced by the relatively warm, nutrient-rich Alaska Coastal Current and terrestrial runoff. Well-preserved cysts were recovered in all the samples, with a total of 35 taxa. The cysts of autotrophic Operculodinium centrocarpum sensu Wall and Dale (1966), and heterotrophic Islandinium minutum were the most abundant. Other common dinoflagellate cysts taxa were Spiniferites spp., Spiniferites elongatus, Alexandrium spp., mostly A. catenella and Brigantedinium spp.
Measured sedimentary biogenic silica abundances (% Opal) have consistently higher values (10.7-19.7%) in the area southwest of St. Lawerence Island and lower values in Norton Sound and the Bering Strait. The sedimentary Total Organic Carbon (TOC) measurements range from 0.2 to 1.5%, averaging at 0.6%. The highest TOC values were found in the same area where high biogenic silica values were observed. The spatial distribution patterns of dinoflagellate cysts and the geochemical parameters are complex in the region and influenced by a combination of environmental drivers.