Uranium paleoredox proxies such as enrichment factors and 238U/235U ratios can be used to reconstruct redox conditions in ancient water masses on local and global scales based on the differential behavior of uranium in oxic, suboxic, and anoxic environments. Many studies have focused on black shales due to their high levels of metal enrichment and association with climactic events. However, the influence of local effects on uranium accumulation is uncertain, particularly in ancient epeiric sea environments.
This study consists of two parts: an evaluation of common sequential extraction methods for U analysis (particularly the Tessier-type sequential extraction) and the application of a modified procedure to study the U distribution in samples of the Heebner shale of the Late Pennsylvanian Midcontinent Sea. XRD data were collected to evaluate the effects of the extractions on relevant minerals and to supplement the uranium concentration data for the Heebner samples. Nitrogen isotope excursions that are present in each of the studied outcrops were used for geochemical horizon correlation. The results suggest that sequential extractions can improve the uranium paleoredox methods, but additional modifications may be required to common extraction schemes. These modifications include steps that better isolate organic matter and apatite. The results of a sequential extraction on the Heebner indicate that uranium is largely partitioned between apatite and fractions that are loosely associated with organic matter, although the true associations of U with organic matter are unclear. Apatite condensation surfaces occur predictably and are likely caused by redox cycling in the water column. This may negatively affect uranium redox proxies due to the association of U-enriched apatite with fluctuating rather than persistently anoxic conditions in the water column. The spatial variation in U and apatite accumulation at the three outcrop locations supports the prevailing superestuarine circulation model for black shale deposition in the LPMS, but does not support the hypothesis that an upwelling belt occurred in the Southern area of the Midcontinent shelf.
| Identifer | oai:union.ndltd.org:LSU/oai:etd.lsu.edu:etd-08262015-010240 |
| Date | 22 September 2015 |
| Creators | Lindaman, Maxwell A |
| Contributors | Herrmann, Achim, Wicks, Carol M, Bao, Huiming |
| Publisher | LSU |
| Source Sets | Louisiana State University |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.lsu.edu/docs/available/etd-08262015-010240/ |
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