Master of Science (MS)
Geology and Geophysics
Recent studies demonstrate that important information on sulfur source, oxidation pathway, transport pattern, and reaction kinetics of the oxidation reactions of reduced sulfur gases in the atmosphere may be uncovered by measuring multiple sulfur and oxygen isotope compositions of the oxidation product — secondary atmospheric sulfate (SAS). SAS from Earth’s distant past, however, is rarely preserved in the rock record because of its high solubility and small quantity. Caliche, a pedogenic carbonate formed in arid to semi-arid continental settings and common in the geologic record, could serve as a repository for ancient SAS. Two major groups of caliche samples: 1) modern caliches actively developing on diverse parent materials in the southern High Plains (Texas and New Mexico) and 2) Eocene to Miocene fossil caliches developed on volcaniclastic sediments in the northern High Plains (northwestern Nebraska) of North America were studied in addition to some fossil soil carbonate nodules and gypcretes for comparison. The concentration of carbonate-associated sulfate (CAS) in a typical caliche sample ranges from 10 to 1900 ppm. The Δ17O anomaly in CAS ranges from +0.12 to +2.32‰ with most values higher than the background value of +0.05‰ for seawater sulfate, indicating the presence of an SAS component derived from the oxidation of sulfur gases by O3 or H2O2. Highly positive Δ17O anomalies (up to +2.32‰) in two of the fossil caliche samples have not been encountered in any modern aerosol and rainwater sulfates collected from the mid-latitude sites and may instead record a transient sulfur-oxidation event associated with volcanic eruptions in the past.
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Howell, Katie Jane, "Caliche as a geologic repository for atmospheric sulfate" (2006). LSU Master's Theses. 1388.