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Mackowiak, Trevor Joseph

Geology

2022

M.S.

As a result of climate change, tropical storms will become more frequent and intense. Resulting changes in river flow dynamics have the potential to change the composition of suspended sediments in crucial tropical river ecosystems, possibly affecting their resiliency. This thesis investigates how changes river discharge and bedrock lithology affected the physiochemical nature of river suspended sediments over a typical year. Suspended sediment samples (n=73) on filter membranes and sediment grab samples (n=18) were collected from the riverbed and riverbank from three watersheds of differing lithology in the Luquillo Mountains, PR (R. Mameyes, R. Icacos, and Q. Sonadora). Suspended sediment samples were analyzed for elemental composition by XRF, mineralogical composition by quantitative XRD, and imaged using SEM-EDX. Sediment grab samples were analyzed for mineralogical composition. By monitoring changes in sediment mineralogical composition as a function of discharge, I determined the sediment loads respond to changes in hydrological input in a typical year. Results showed that bedrock lithology strongly influenced river suspended sediment mineralogy but did not influence its composition. Minerals present in the riverbed and riverbank of all three rivers included quartz, chlorite, biotite, hornblende, albite and gibbsite. Crystalline phases present in the diffractograms of the suspended sediment samples included principally kaolinite, chlorite, and quartz. The fraction of crystalline versus amorphous material was strongly influenced by the dominant lithology of the watershed with the highest number of crystalline phases associated with granodiorite bedrock compared to amorphous material dominating the volcaniclastic watersheds. The nature of crystalline material found across the suspended sediments of the river systems reflected the intense weathering in tropical environments dominated by final weathering products. Minerals found in sediments collected from the riverbed and riverbanks included a variety of primary minerals, while the only primary mineral found in the suspended sediment samples was quartz, the other minerals consisting of chlorite and kaolinite, weathering products of biotite/hornblende and albite respectively. This indicates that the mineralogy of sediments transported in suspension in the three river systems is principally secondary minerals with higher cation exchange capacity than primary parent minerals. Overall, mineralogical and chemical results showed that, bearing quantitative changes upon hydrological events, suspended sediments in all three watersheds returned to baseline composition post storm, suggesting that the three watersheds are resilient to the events recorded that year. While the long-term mineralogical analysis of the evolution of suspended material in the studied rivers proved to provide interesting insights into river response to hydrologic events, it also proved technically challenging as materials in suspension in such pristine rivers are sparse and poorly crystalline