Browsing by Subject "Stable Isotopes"

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    Progressive deformation, fluid flow and water-rock interaction in the Mexican Fold-Thrust Belt, Central Mexico
    (2010-12) Fitz-Díaz, Elisa
    An integrated study of the Mexican Fold-Thrust Belt (MFTB) in central Mexico provides information on the evolution of the belt and the processes operating to produce structures and fabrics. Key questions addressed concern to the role of lithology in controlling the style of deformation, the role of fluids in facilitating deformation and the timing and sequence of structural development. Overall, the MFTB is thin-skinned and has properties consistent with the critical tapered wedge model of fold-thrust-belt development: there is a decrease in thickness, intensity of deformation and temperature of deformation towards the toe. Critical wedge theory, however, does not explain how deformation accumulates in the rocks within the wedge. In the MFTB, the deformation mechanisms that operate in different segments of the cross-section are strongly controlled by lithology. The dominant rocks in the sedimentary cover are Cretaceous carbonates that vary in facies laterally, with two large carbonate platforms flanked by more thinly bedded basinal carbonates. Kilometer scale thrusts dominate deformation in the platform carbonates (a more brittle behavior), and mesoscopic buckle folds and associated cleavage dominate deformation in the basinal carbonates (a more ductile behavior). Two phases of deformation are recognized, the first, D1 being more intense than the second, D2. The difference in facies distribution is the main reason for the difference between the MFTB and the fold-thrust belt in the southern Canadian Rocky Mountains, which is characterized by large thrust sheets and relatively few buckle folds. Structural observations on the mesoscopic scale allow distinguishing veins (dominantly calcite) of several generations, emplaced early, during and late/after deformation (V1, V2 and V3 respectively). Analysis of δ13C and δ18O in calcite from veins and host-rock shows that the veins confined within thrust slices are isotopically buffered by the host rock and differ in isotopic composition from veins emplaced along major thrusts or cross cutting thrust-slices. The extent of isotopic buffering seems to be controlled by the amount and composition of the fluid interacting with the carbonate sequences rather than by the temperature, which varies between 100 to 300°C along the cross-section. Analysis of δD in water extracted from fluid inclusions trapped in the veins and in clay minerals strongly suggests rock interaction with meteoric fluids in the west (hinterland) and with fluids close to SMOW in the east (foreland) side of the cross-section. The influence of meteoric water was also more important late during D1 than in the early stages of deformation. Stratigraphic constraints indicate that D1 started about 90 Ma on the western side of the MFTB in central Mexico and finished about 65Ma towards the east. Stratigraphic and structural observations in the foreland indicate that D2 took place after 65Ma. K/Ar ages of illite separated from layers of bentonite intensively sheared during folding of D1 lie between 84-77 Ma for one locality and between 64 and 71 in a second locality. Outcrop scale variations in age indicates that illite transformation and the closure of its K/Ar isotopic system are not only controlled by temperature but also by strain around the folds. The data suggest that it took about 5 to 7 Ma to develop individual folds, and this occurred within the time range constrained by stratigraphy. U-Th/He thermochronological ages determined in 21 zircon grains are overall consistent with stratigraphic ages and K/Ar ages.
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    Seasonal and Flood-Induced Variations in Groundwater-Surface Water Exchange Dynamics in a Shallow Aquifer System: Amity Creek, MN
    (2016-01) Jasperson, Jenny
    Excellent brook trout habitat can be found in segments of many streams on the North Shore of Lake Superior. Optimal riverine brook trout habitat includes clear, cold spring-fed water and studies have shown that areas of groundwater upwelling in streams tend to be more important than other site selection variables. Observations of historical brook trout and temperature data in the Amity Creek sub-watershed of Lake Superior South HUC-8 led us to hypothesize that reaches with healthy and stable brook trout communities are likely connected to groundwater storage, exchange, and upwelling. Climate trend models for the Midwest predict future changes in temperature, annual precipitation, and storm event frequency for Northern Minnesota. Streams along the North Shore of Lake Superior are susceptible to increased temperatures and insufficient late summer to early winter flows due to climate change and the unique regional geology. Understanding groundwater-surface water hydrology interactions, watershed connectivity, and related flood-induced geomorphic and hydrologic changes is important because they relate to the overall stability and aquatic health of the stream and the biological communities that inhabit it. The objectives of this study were to identify groundwater storage zones, upwelling zones and seasonal variations; and assess how these relationships change as result of a major flood. A study reach on East Branch Amity Creek is incising through clay-rich glacial tills overlying bedrock and has avulsed multiple times in the past, stranding discrete remnant channels cut into till. A 500-year storm hit Duluth, MN, USA on June 19-20th 2012, producing 8-inches of rain in a 24-hour duration which resulted in flash flooding across the region. Pre-flood and post-flood groundwater and surface water level data were collected through a series of piezometers with pressure transducers and an in-channel stream gage. Stable isotope analyses of Deuterium and Oxygen-18 were conducted on water samples with varying temporal and spatial variability to provide information on watershed and reach scale source hydrology and evaluate flood-induced changes. Results show groundwater upwelling variability within the greater watershed and identify two discrete groundwater storage zones within a smaller study reach. Pre- and post-flood analyses show a correlation between incision in main stem and remnant channels and a lowered water table, following the flood. Isotope analysis indicates a temporary post-flood change in subsurface source water. Because much of the greater North Shore of Lake Superior has the same geology as the study area, the results of this study may provide insight to hydrology studies of other North Shore streams.
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    Stable Isotope Ecology of Gombe National Park: A Modern Analogue for Fossil Hominins
    (2019-09) Nockerts, Rebecca
    Stable isotopes have revolutionized our understanding of the evolution of the human diet. A key early transition in hominin evolution appears to be the increased consumption of C4 grasses and sedges (and/or animals eating C4 plants) and an adaptation towards more open, seasonal environments. The earliest hominins, Ardipithecus ramidus and Australopithecus anamensis, appear to have had diets reliant on C3 foods, interpreted to be similar to the diets of modern chimpanzees. All later hominins after ~3.5 Mya were mixed C3/C4 feeders, though the extent of C4 consumption varied tremendously, both within and between species. Important limitations remain in using modern referential models to interpret this change in dietary ecology. These include the need for an improved understanding of diet to tissue isotope enrichment and studies of appropriate living populations, in comparable environments, with independently known feeding histories. In order to address those limitations, I conducted a comprehensive analysis of the stable isotope ecology of the chimpanzees (Pan troglodytes schweinfurthii) and baboons (Papio anubis) of Gombe National Park, Tanzania. Both chimpanzees and baboons have been invoked as species with ecological adaptations similar to those of fossil hominins. At Gombe these primates share overlapping ranges in a mixed forest/woodland/grassland habitat similar to those inferred for many hominin localities. I combined observational records of their diets and the isotopic composition of their foods to interpret the isotopic signatures of their hair, nails, bones, and teeth. I then compared these results to previously published isotope studies of other populations of Pan, Papio, and fossil hominins and papionins. I found that Gombe plant isotopes varied across plant growth form and plant part, as well as seasonally and across different habitat types. This variation was reflected, to varying degrees, by differences in the isotopic composition of chimpanzees compared to the baboons. Other aspects of the dietary ecology of the Gombe chimpanzees and baboons are reflected by their tissues, including breastfeeding, variable C4 consumption, and differences in meat and insect consumption. These results indicate that even the earliest known hominins consumed substantially more C4 food resources than any known chimpanzee population, and more than many forest-living baboon populations.