Semester of Graduation

August 2025

Degree

Master of Science (MS)

Department

Geology and Geophysics

Document Type

Thesis

Abstract

Understanding river dynamics is crucial to being able to interpret the geological history preserved in sediment deposited on continental margins, deltas, and alluvial plains. Sediment buffering can be a key control on the sediment flux to the ocean and involves sediment storage and recycling in flood plains. The efficiency of the transport varies depending on climatic and more recently anthropogenic influences. Sediment moving through meandering streams may be temporarily deposited in a point bar on the convex side of a meandering river. Meanders are sometimes cut off, creating an oxbow lake and filled in. The area of study, False River, is an oxbow lake and point bar of the Mississippi River located north of Baton Rouge, while Lake St. John is an oxbow lake located 120 km further north. Another core was collected close to the Mississippi south of New Orleans. Together these represent an archive of the composition of Mississippi River sediment spanning more than 3000 years. The sediments were collected and analyzed for major elements, Sr and Nd isotopes, as well as detrital zircon U-Pb geochronology. Grain size is a major influence in controlling provenance because suspended sediment is transported rapidly through the river compared to coarse-grained material which travels more slowly as bedload. Grain size also controls major element geochemistry as well as Sr isotopes. Chemical weathering shows a positive correlation with sediment alteration through time, but little correlation between the Chemical Index of Alteration (CIA) proxy and Sr isotopes. εNd values are minimally affected by chemical weathering, with notable variations in the past 100 years linked to human activity, indicative of a loss of strong recycling in the lower reaches. This study highlights the changing composition of the river, showing some variability before levee construction which suggests that levees may suppress sediment dilution. The provenance of the river was found to have a long-term change in erosion patterns with a growing influence on older basement sources like the Superior Province. Despite some variations during the Roman and Medieval Warm periods and the Little Ice Age, this indicates there is little recycling and buffering due to the construction of levees. However, there was limited evidence of stronger buffering prior to anthropogenic influences.

Date

7-30-2025

Committee Chair

Guangsheng Zhuang

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