HR: 1340h
AN: H43D-0397    [Abstracts]
TI: Temporal Variations in $^{234}$U/$^{238}$U Activity Ratios in the Lower Mississippi River due to Changes in Source Tributary Discharges
AU: * Grzymko, T J
EM: tgrzymk@tulane.edu
AF: Tulane University, Department of Earth and Environmental Sciences, New Orleans, LA 70118 United States
AU: Marcantonio, F
EM: fmarcan@tulane.edu
AF: Tulane University, Department of Earth and Environmental Sciences, New Orleans, LA 70118 United States
AU: McKee, B A
EM: bmckee@tulane.edu
AF: Tulane University, Department of Earth and Environmental Sciences, New Orleans, LA 70118 United States
AU: Stewart, C M
EM: cstewar2@tulane.edu
AF: Tulane University, Department of Earth and Environmental Sciences, New Orleans, LA 70118 United States
AB: The world's 25 largest river systems contribute nearly 50% of all freshwater to the global ocean and carry large quantities of dissolved trace metals annually. Trace metal concentrations in these systems show large variances on seasonal time scales. In order to constrain the causes of these variations, consistent sampling on sub-seasonal time intervals is essential. Here, we focus on the Mississippi River, the seventh largest river in the world in terms of freshwater discharge and the third largest in terms of drainage basin area. Biweekly sampling of the lower Mississippi River at New Orleans was performed from January 2003 to August 2004. Uranium concentrations and $^{234}$U/$^{238}$U activity ratios were measured for the dissolved component ($<$0.2 $\mu$m-fraction) of river water. Over the course of this study, dissolved U activity ratios spanned a range of about 25%, from 1.23 to 1.60. Dissolved U concentrations ranged from 0.28 to 1.06 ppb. The relationship between concentrations, activity ratios, and lower river discharge is complicated, and no clear pattern is observed on both biweekly and seasonal timescales. However, there does seem to be a relationship between the larger seasonal trends in the lower Mississippi River and variations in the discharge of its upstream tributaries. To constrain this relationship, we have sampled water from the Missouri River, the upper Mississippi River above the confluence with the Missouri, the Ohio River, and the Arkansas River in February, April, and August of 2004. For the upstream samples measured thus far, the highest dissolved uranium concentrations are observed for the Missouri River at 2.02 ppb, while the lowest are found in the Ohio River at 0.38 ppb. Dissolved $^{234}$U/$^{238}$U activity ratios are as unique for each tributary and vary from 1.36 in the Ohio River to 1.51 in the Missouri River. A preliminary mass balance analysis reveals that the lower river uranium activity ratios are controlled simply by the quantity and isotope signature of the waters discharged from the upstream tributaries. A discussion of the implications of this work for global ocean budgets of uranium will be presented.
DE: 1806 Chemistry of fresh water
DE: 1836 Hydrologic budget (1655)
DE: 1860 Runoff and streamflow
DE: 1886 Weathering (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting