HR: 13:30h
AN: H43D-01    [Abstracts]
TI: Temporal Variations in ²³⁴U/²³⁸U Activity Ratios in Four Mississippi River Tributaries
AU: * Grzymko, T J
EM: tgrzymk@tulane.edu
AF: Tulane University Department of Earth and Environmental Sciences, Office 110 Dinwiddie Hall, New Orleans, LA 70118 United States
AU: Marcantonio, F
EM: fmarcan@tulane.edu
AF: Tulane University Department of Earth and Environmental Sciences, Office 110 Dinwiddie Hall, New Orleans, LA 70118 United States
AB: In 2004 we sampled the four tributaries that are the major contributors to the Mississippi River in terms of water discharge, i.e., the Arkansas, Missouri, Upper Mississippi, and Ohio rivers. Each river was sampled four times over the course of the year at variable levels of discharge in an attempt to constrain the causes of the temporal variations of ²³⁴U/²³⁸U activity ratios in the lower Mississippi River at New Orleans. The tributary uranium data support the idea that lower river uranium isotope and elemental systematics are controlled by a simple mass balance of the source tributary discharges. Furthermore, the uranium isotope ratios of the individual tributaries show coherent patterns of variability. Specifically, the data obtained from the four sampling trips yielded similar patterns of temporal variation in the ²³⁴U/²³⁸U activity ratios of all of the rivers, although the absolute values of these ratios were distinctly different from one river to the next. The pattern was such that the highest ²³⁴U/²³⁸U activity ratios were observed during the highest flow associated with the spring freshet while the lowest ratios occurred during the summer. For example, in the Missouri River, the ²³⁴U/²³⁸U activity ratios varied from 1.51 (February 12) to 1.37 (April 14) to 1.34 (July 16) to 1.37 (November 12), while in the Ohio River the same ratios varied from 1.36 (February 12) to 1.29 (April 14) to 1.21 (July 16) to 1.23 (November 12). The apparent seasonal pattern of these ratios in each tributary has led to several ideas as to the causes of the observed trends. The first, and most obvious, is that in each individual drainage basin there are various source tributaries that contribute to the uranium isotope systematics of the main stem of the tributary of interest. It follows that the variations in the uranium activity ratios may be caused by spatial variations in the source rock chemistry of the drainage basin. Other more complex scenarios can also be envisioned and will be discussed. For example, we explore the possibility that the highest ratios associated with the spring freshet are a consequence of snow melt and the flushing of ²³⁴U from fresh surfaces created via physical weathering associated with the winter freeze-thaw cycles.
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 1806 Chemistry of fresh water
DE: 1871 Surface water quality
SC: Hydrology [H]
MN: 2005 Joint Assembly