Browsing by Subject "Paleoenvironment"
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Item Quantification of magnetic components in sediments with applications in paleoenvironmental studies.(2011-12) Lascu, IoanThe present dissertation is a collection of papers investigating the magnetic properties of sediments. The main aim of the work presented here is to study the magnetic characteristics of sedimentary deposits by using a methodology that efficiently quantifies the contributions of various ferrimagnetic components in sediments, and to exemplify how this model can be used to make inferences about past climatic and environmental variability. Magnetic minerals in sediments have long been used as indicators of variability in the factors controlling sediment deposition, and sediment-magnetic properties can be interpreted in terms of the processes controlling the fluxes of various magnetic components. Ferrimagnetic minerals, such as magnetite, are strong magnetically, and tend to dominate the signal from bulk measurements. Two sedimentary ferrimagnetic components that play a major role in shaping the magnetic record with time: a detrital component and a biogenic component. The detrital component of magnetic assemblages probably accounts for the greater proportion of the magnetic signal in many records, and therefore has been the focus of most environmental magnetism studies. The processes that control detrital records are mostly tied to local hydrology, climate, and vegetation cover. However, there is strong evidence that many magnetic assemblages are dominated by autochthonous magnetic particles, which in most cases are produced as a result of direct biologic control. Knowing the contribution of each of these components to the total mass of ferrimagnetic material becomes important when making inferences about past climatic or environmental conditions. The theoretical mixing model devised here using the characteristics of detrital and biogenic end members was tested on lake sediments from Minnesota. The analysis incorporates both spatial and temporal effects on magnetic record. We have investigated the history of sediment flux to Deming Lake, Minnesota, for the past 1000 years. Our results reveal several episodes of reduced precipitation, during which less sediment is mobilized from the catchment by overland flow and runoff. The most prominent episode occurred at the end of the Little Ice Age, indicating that this time period was not only cold but might have been drier than previously thought. The spatial control on sediment-magnetic properties was established via a survey of the magnetic properties of surface sediments from several Minnesota lakes. The magnetic properties are controlled by the competing fluxes of detrital and biogenic particles, according to location in the basin, while the position of the oxic-anoxic interface controls whether biogenic magnetite is formed in the sediment or in the water column, with implications in the preservation of intact versus collapsed bacterial chains. The thesis concludes with an incursion into the magnetic properties of chemical sediments from caves, or speleothems. The magnetic recordings preserved in calcite speleothems hold enormous potential for paleomagnetic and paleoenvironmental reconstructions. Speleothems lock in magnetization instantly, are not affected by post-depositional effects, and can be dated with high precision. The natural remanence in speleothems is carried mainly by magnetite, and the main remanence acquisition mechanism is depositional, through physical alignment of detrital magnetic grains parallel to the Earth's magnetic field. Future speleothem magnetism studies should benefit from increasingly sensitive magnetometers, operating at high spatial resolution, that are able to resolve short-term geomagnetic variability, and characterize events such as geomagnetic excursions at an unprecedented scale.Item Stable Isotope Paleoenvironmental Reconstructions of the Late Pleistocene and Early Miocene Sites on Rusinga and Mfangano Islands, Lake Victoria, Kenya(2016-12) Garrett, NicolePaleoenvironmental reconstructions are a key component when trying to understand a species evolutionary history as environmental pressures play a critical role in the evolution of species. This dissertation, on the Late Pleistocene and early Miocene deposits of Rusinga and Mfangano Islands, Lake Victoria, Kenya, uses carbon and oxygen isotope analyses of paleosol carbonates, bulk sedimentary organic matter, isolated biomarkers from organic matter, and mammalian tooth enamel to reconstruct the various habitats available to the mammalian communities. Present within the Late Pleistocene deposits on Rusinga and Mfangano Island are Middle Stone Age tools, indicating the presence of at least one population of early Homo sapiens. The analysis included within (Chapter 1) of the Late Pleistocene habitats indicates stream- or spring-side woodlands within a larger C4 grassland with climates drier than present today. The early Miocene deposits on Rusinga include a well-preserved primate community with remains of the primitive hominoid Ekembo as well as specimens from Dendropithecus and the lesser-known catarrhines Limnopithecus and Nyanzapithecus. The paleoenvironmental reconstructions for the early Miocene deposits (Chapters 2-3) provide an important evolutionary context for the evolution and diversification of catarrhines, including the earliest members of our own ape-human lineage. The analysis of the Early Miocene habitats indicates a temporally and spatially dynamic mixture of C3 habitats was available to the faunal community, which included close-canopy habitats experiencing CO2 recycling as well as environments with plants undergoing light and or water stress (i.e., unshaded areas). The data from the early Miocene also indicates there were distinct habitats present at Rusinga, with the younger habitats exhibiting either an increase in mean annual temperature, an increase in aridity (evaporation), a decrease in mean annual precipitation or some combination of the three climatic factors. Overall, this analysis indicates the early Miocene primates from Rusinga were able to cope and even thrive in a dynamic and varied landscape by inhabiting both closed and open habitats, including woodlands, bush/shrublands or woody grasslands. For the primitive hominoids, this level of habitat flexibility suggests it may have been an important primitive characteristic for apes.