Date of Completion

2020

Thesis Type

College of Arts and Science Honors

Department

Geology

First Advisor

Paul Bierman

Keywords

Stream dynamics, X-ray powder diffraction

Abstract

In humid, tropical regions, where chemical weathering rates are high, the relationship between bedrock lithology, stream sediment mineralogy, and stream water chemistry is neither straightforward nor well known. For a country such as Cuba, which has been isolated from much of the world’s geologic community for decades, data are sparse; yet, such data are important for understanding landscape change over time, including deciphering trends in stream water geochemistry and rates of erosion, both natural and human-caused. Here, as part of a collaborative Cuban-American project focused on understanding mass transfer from the island to the ocean, we used quantitative X-ray diffraction to determine the bulk mineral composition of river sediment in two grain sizes. We used this data to determine the relationship between sediment mineralogy and water chemistry, weathering rates, sediment elemental composition, and mapped bedrock geology in 26 central Cuban drainage basins.

Diffraction data show that quartz, feldspar, and calcite are present in most stream sediment samples reflecting the relative abundance of igneous, metamorphic, and carbonate lithologies in central Cuba. Although quartz dominated stream sediment composition across all sampled watersheds (N=26), significant amounts of calcite, swelling clays, feldspars, and amphiboles suggest that the mineralogy of the stream sediments is useful both as a marker for watershed geologic characteristics and as a proxy for both physical and chemical weathering intensity. We find that water quality indicators such as Ca, Mg, DOC, K, and Cl can be deciphered through quantification of weathering-prone minerals such as calcite and weathering products such as swelling clays present in transport-limited streams. Additionally, we suggest that sediment mineralogy may provide insight into water quality parameters over longer time periods than traditional water samples. In combination with other data, mineralogical analysis of stream sediments can be used to determine the lithologic influence on local sediment and water composition in individual watersheds.

Available for download on Saturday, May 08, 2021

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