Browsing by Subject "trace element"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
Item Circulation of North American epicontinental seas during the Carboniferous using stable isotope and trace element analyses of brachiopod shells(2012-07-16) Flake, Ryan ChristopherPrevious studies have identified ???C events in the Carboniferous that imply major shifts in the carbon cycle. However, inherent in this interpretation is the assumption that epicontinental seas are chemically representative of the global ocean. Our study uses stable isotope and trace element analyses of brachiopod shells to examine changes in climate and circulation of the North American epeiric sea. Formations were selected for study to provide shallow marine environments with geographic coverage of North America. These units include the Grove Church and Mattoon Formations (Illinois Basin), Glenshaw Formation (Appalachian Basin), Bird Spring Formation (Bird Spring Basin), and Oread Formation (US midcontinent). In all, 98 brachiopod shells were found to be well preserved based on screening with plane light and cathodoluminescence microscopy of thin-sections, and trace element analyses. Upper Chesterian Grove Church (Illinois Basin) samples have ???C and ???O averages of 1.1% and -3.1% respectively. These low values are interpreted as a local or regional effect caused by terrestrial runoff. Terrestrial influences are also suggested by the depositional environment: nearshore marine. Chesterian samples from the Bird Spring Formation at Arrow Canyon, Nevada average 3.7% and -1.4% for ???C and ???O respectively. The higher ???C and ???O values, compared with samples from the time equivalent Grove Church, likely reflect the freer exchange with the Panthalassa Ocean at this most western edge of North America, and best represent open-ocean conditions. Samples from the Virgilian Ames-Shumway-Plattsmouth cyclothem show a progression of ???C and ???O enrichment moving west from near the Appalachians (1.9% and -3.8%) to the Illinois Basin (3.2% and -2.4%) and finally to the US midcontinent (4.2% and -1.5%). This is interpreted as the transition from nearshore, terrestrial influence with enhanced organic matter oxidation and lower salinity to well-mixed conditions with normal salinities and potential for seafloor ventilation and upwelling. This is supported by published sediment ?Nd(t) values from the Appalachian Basin (?Nd(t) = -9) that increase further westward (?Nd(t) = -6) due to higher influence from the eastern Panthalassa Ocean. Mass balance calculations based on the ???O of the brachiopod shells suggest salinities of 25 and 31 psu for the Appalachian and Illinois Basins, respectively, assuming salinities of 34.5 psu for the US midcontinent. Trace element analyses do not show a systematic east-west trend similar to stable isotopes. In both time slices, spiriferids from the intermediately-located Illinois Basin are enriched in Mg/Ca and Sr/Ca relative to those in other basins. This Mg and Sr enrichment in Illinois Basin brachiopods suggests delivery of Sr-rich fresh waters and restricted circulation in that basin.Item Neogene Low-latitude Seasonal Environmental Variations: Stable Isotopic and Trace Elemental Records in Mollusks from the Florida Platform and the Central American Isthmus(2012-10-19) Tao, KaiThis Ph.D. dissertation integrates stable isotope and trace element geochemistry in modern and fossil gastropod shells to study low-latitude marine paleoenvironments. First, stable isotopes (delta18O and delta13C) and Sr/Ca ratios are used to examine low-latitude temperature and salinity variations recorded in Plio-Pleistocene (3.5-1.6 Ma) fossils from western Florida during periods of high-latitude warming and "global" cooling. The middle Pliocene Pinecrest Beds (Units 7 and 4) and the overlaying Plio-Pleistocene Caloosahatchee Formation generate significantly different delta18O-derived paleotemperatures but identical Sr/Ca ratios. High delta18O values, together with low delta13C values and brackish fauna, indicate that Unit 4 was deposited in a lagoonal environment similar to modern Florida Bay. In contrast, relatively low delta18O and high delta13C values in Unit 7 and Caloosahatchee Formation represent deposition in an open-marine environment. The observed Unit 7 and Caloosahatchee paleotemperatures are inconsistent with middle Pliocene warming event, but consistent with the Plio-Pleistocene cooling trend. To quantify modern upwelling and freshening signals and contrast these signals between the tropical eastern Pacific (TEP) and southwestern Caribbean (SWC), methodologies are developed for reconstructing seasonal upwelling and freshening patterns from modern tropical gastropod shells from Panama using: 1) paired oxygen and carbon isotopic profiles and delta18O-delta13C (delta-delta) correlations, and 2) deviation from baseline delta18O values that represent conditions free of seasonal upwelling or freshening influences. Shell delta18O values normalized to the baseline faithfully record modern conditions of little or no upwelling in SWC and Gulf of Chiriqui, and strong upwelling in the Gulf of Panama, as well as strong freshwater input in most areas. The baseline and delta-delta methods are applied to identify and quantify changes in upwelling and freshening in the Neogene TEP and SWC seawaters associated with the final closure of Central American Isthmus. The records reveal significant upwelling in late Miocene SWC and mid Pliocene TEP waters, strong freshening in SWC waters from 5.7-2.2 Ma, and minimal seasonal upwelling and/or freshening variations in Plio-Pleistocene SWC waters. The reconstructed paleotemperatures agree with the global cooling trend through the late Miocene, but lack evidence for middle Pliocene warming or late Neogene global cooling.