Browsing by Subject "Geology"
Now showing 1 - 5 of 5
Results Per Page
Sort Options
Item A 4600-year record of lake level and hydroclimate variability from an eastern Andean lake in Colombia(2016-05) Rudloff, Owen M.; Bird, Broxton Williams; Gilhooly, William, III; Filippelli, Gabriel M.Hydroclimatic variability in the eastern Colombian Andes is examined using a decadally-resolved, multiproxy lake sediment record from Laguna de Ubaque, Colombia. Hydroclimate trends are examined and compared to existing local, regional, and trans-Andean records to enhance existing knowledge of Late Holocene Colombian precipitation and assess potential hydroclimatic forcing mechanisms in tropical South America. Sedimentological analyses, including percent lithics, grain size, C:N and magnetic susceptibility are sensitive to hydroclimate and lake level while charcoal size and concentrations reflect fire variability. Results show that deep lacustrine conditions characterized by laminated deposits were not established until approximately 3500 cal yr B.P., prior to which, terrestrial C:N values and unstructured sediments indicate that drier, marsh-like conditions prevailed. Between 3500 and 2000 cal yr B.P., interrupted only by a 300-year arid interval from 2800 to 2500 cal yr B.P., greatly increased overall clastic deposition indicates a broad precipitation maximum while decreased sand deposition and the preservation of finely laminated sediment indicate deep lake conditions. After 2000 cal yr B.P., decreased clastic deposition suggests reduced precipitation, but the continued accumulation of laminated sediments indicates that conditions were wet enough to fill the basin continuously until the present day. These observations address two of the driving questions of Andean paleoclimate: were the northern and southern Andes in vi phase during the Holocene, or out of phase, and what are the main drivers of Holocene Andean climate? We find that the early part of Ubaque’s record more closely resembles southern Andean precipitation records until 2000 cal yr B.P., at which point it abruptly switches to resemble northern precipitation records. We attribute this to a combination of the southward migration of the intertropical convergence zone (ITCZ), and an increase in eastern Pacific sea surface temperatures (SST). In addition, we find that Colombian hydroclimate records exhibit a bimodal precipitation pattern, which we attribute to their location either on the Andean slopes or in the high interior Andes.Item Exploring Competing Theories of Viscous Emulsion and Fractional Crystallization of the Impact Melt that Formed the Sudbury Igneous Complex(2023-01) Horman, Alexandra Rose; Macris, Catherine A.; Barth, Andrew P.; Gilhooly, William P., III.The Sudbury Igneous Complex (SIC) in Sudbury, Canada is a remnant geologic structure from a meteor impact that occurred ~1.85 Ga. The impact produced ~30,000 km3 of superheated melt which reached >2200 °C. The existing SIC is composed of three compositionally distinct layers, norite, quartz gabbro, and granophyre, which stretch the entire lateral distance of the complex. The presentation of layers in the SIC is unusual for impact melts, and the crystallization path has been debated by scientists. The SIC differs from more common layered mafic complexes because of its intermediate composition, crustal isotopic signature, and large volume of granophyre. This thesis is an investigation of some of the main theories surrounding the SIC and how it crystallized to form such distinct layers. There are two main theories of how the SIC formed its compositionally distinct layers: (1) fractional crystallization and (2) separation by viscous emulsion. The viscous emulsion theory involves isolated droplets of melt separating from the surrounding melt body due to differences in viscosity and density, similar to an emulsion of oil and water. In this study, viscous emulsion theory was investigated experimentally by heating samples of rock from the SIC to the extreme temperatures associated with the Sudbury impact, and then analyzing the cooled experimental products using electron microscopy to determine if there was evidence of textures that would be consistent with expectations for a viscous emulsion. Fractional crystallization was investigated by modeling using the vii software EasyMELTS to evaluate compositions from the SIC to estimate how they would crystallize according to the temperature, pressure, and other properties of the melt. There was no textural evidence of a viscous emulsion found in the experimental products. The models produced compositions similar to what is seen in the SIC but had limited application to fractional crystallization theory.Item Indicators of Euro-American land-use change as geochronologic markers in Midwest floodplain lake sediment archives(2024-02) LaRoche, Kierstin Marie; Bird, Broxton W.; Licht, Kathy J.; Gilhooly, William P., IIIIn association with predicted rising global average temperatures, spring and winter precipitation in the Midwest is projected to increase by up to 30% by the end of this century. Enhanced by the alteration of natural environments, this increase will likely result in more frequent extreme flood events. To best prepare for these circumstances, interest has risen in reconstructing the dynamics between changing climate, altered landscapes, and fluvial systems with age-depth modeling, often using radiocarbon (14C) and 210Pb dating and multi-proxy evidence from floodplain lake sediment archives. Age- depth modeling over the last 300 years can be difficult, however, due to a large plateau in the radiocarbon calibration curve, and 210Pb dating is not reliable for all sediment records. Here, indicators of land-use change, magnetic susceptibility, Rb/Sr, Pb/Zr, and d15N values, were used to create age-depth control for Shannon Lake, IN, a difficult-to-date 600-year-old oxbow lake of the White River near Indianapolis. Age control for Shannon Lake was completed by correlating the timing in the rise of the same proxies from a previously well-dated lake record from Half Moon Pond, an oxbow lake of the White River near Petersburg, IN. The Shannon and Half Moon records were compared to those of three floodplain lake records of the Ohio River floodplain: Avery Lake, IL, Goose Pond, IN, and Grassy Pond, KY to investigate how these proxies varied in floodplain lake sediment from the lower Ohio River. The land-use indicators provided age-control for Shannon Lake, and the indicators of land-use change for the White River records resembled those of the Ohio River records over the last 600 years, where the onset of Euro-American land-use changes were detected at or around 1750 CE. MS, Rb/Sr, and d15N values displayed the greatest resemblance, while greater variations in Rb/Sr values were observed across the five lake records, potentially due to differences in regional setting or differences in the scale of the White River and Ohio River watersheds.Item A laminated carbonate record of late holocene precipitation/evaporation from Pretty Lake, Lagrange County, Indiana(2016-11-06) Albert, Ashley Lisbeth; Bird, BroxtonLate Holocene hydroclimate variability in the midcontinental United States is not well understood because there is a lack of high-resolution paleoclimate records constraining historical climate patterns for the region. Here, a 2500-year-long multi proxy lake sediment record from a northern Indiana kettle lake is used to examine spatial and temporal scales of drought and pluvial patterns in the Midwest. Oxygen (18Ocal) isotope analysis of authigenic carbonate and the sedimentary lithic abundance (%lithics) are the primary datasets used to evaluate hydroclimate trends with supporting information from total organic matter, total carbonate and magnetic susceptibility. We additionally derive a record of local evaporation by subtracting the isotopic composition of precipitation (18Oprecip) as characterized by the nearby Martin Lake, IN, record, from the Pretty Lake 18Ocal record. The combined Pretty Lake hydroclimate record documents climate variability during the last 2 millennia and shows that the Midwest has experienced a wide range of evaporative regimes during the late Holocene. We notice a consistent relationship between the Pretty and Martin Lake multi-proxy records; where reduced (increased) evaporative periods and higher (lower) lake levels at Pretty Lake mostly align with increased (decreased) Gulf sourced precipitation and stream erosion with longer (shorter) warm seasons at Martin Lake. Early periods of much drier, and weakened warm-season evaporation patterns dominated from 600 BCE to 900 CE. Evidence of a prolonged period of enhanced warm-season pluvial conditions, with less evaporation and higher lake levels, during the Medieval Climate Anomaly (MCA) between 900 to 1350 CE; and a pronounced century of arid conditions throughout the Little Ice Age (LIA) from 1350 to 1700 CE followed by a gradual decrease in evaporation and rising lake levels starting at 1700 CE and continuing to present. These trends track other Midwest regional hydroclimate climate records, but show an anti-phased relationship with records from the High Plains and western United States regions. This supports the idea that a hydrocilmate dipole exists between the Midwest and western United States driven largely by mean state changes in the Pacific North American teleconnection pattern, but with modification by local and in-lake responses to mean climate states.Item A record of spontaneous subduction initiation in the Izu-Bonin-Mariana arc(Nature, 2015-09) Arculus, Richard J.; Ishizuka, Osamu; Bogus, Kara A.; Gurnis, Michael; Hickey-Vargas, Rosemary; Aljahdali, Mohammed H.; Bandini-Maeder, Alexandre N.; Barth, Andrew P.; Brandl, Philipp A.; Drab, Laureen; do Monte Guerra, Rodrigo; Hamada, Morihisa; Jiang, Fuqing; Kanayama, Kyoko; Kender, Sev; Kusano, Yuki; Li, He; Loudin, Lorne C.; Maffione, Marco; Marsaglia, Kathleen M.; McCarthy, Anders; Meffre, Sebastién; Morris, Antony; Neuhaus, Martin; Savov, Ivan P.; Sena, Clara; Tepley III, Frank J.; van der Land, Cees; Yogodzinski, Gene M.; Zhang, Zhaohui; Department of Earth Sciences, School of ScienceThe initiation of tectonic plate subduction into the mantle is poorly understood. If subduction is induced by the push of a distant mid-ocean ridge or subducted slab pull, we expect compression and uplift of the overriding plate. In contrast, spontaneous subduction initiation, driven by subsidence of dense lithosphere along faults adjacent to buoyant lithosphere, would result in extension and magmatism. The rock record of subduction initiation is typically obscured by younger deposits, so evaluating these possibilities has proved elusive. Here we analyse the geochemical characteristics of igneous basement rocks and overlying sediments, sampled from the Amami Sankaku Basin in the northwest Philippine Sea. The uppermost basement rocks are areally widespread and supplied via dykes. They are similar in composition and age—as constrained by the biostratigraphy of the overlying sediments—to the 52–48-million-year-old basalts in the adjacent Izu–Bonin–Mariana fore-arc. The geochemical characteristics of the basement lavas indicate that a component of subducted lithosphere was involved in their genesis, and the lavas were derived from mantle source rocks that were more melt-depleted than those tapped at mid-ocean ridges. We propose that the basement lavas formed during the inception of Izu–Bonin–Mariana subduction in a mode consistent with the spontaneous initiation of subduction.