Online

Campbell, Mae Kate

Geology

2021

M.S.

Knowledge of denudation rates over geologic timescales provides important insight into the processes that govern soil formation, the regulation of Earth's climate, and the evolution of landscapes. Accurately establishing long-term denudation rates is also key to understanding how human actions have altered the rates and patterns of erosion over time. While cosmogenic nuclides are often used to measure long-term denudation rates at the basin scale, these rates can be unrepresentative of total landscape denudation in areas where significant mass loss occurs through rock dissolution. In tropical landscapes, mass loss by solution often represents a significant portion of total landscape mass loss. We compare long-term sediment generation rates derived from measurements of in-situ cosmogenic nuclides 26Al and 10Be in stream sediment to rock dissolution rates from stream solute fluxes in order to characterize the pace and processes of denudation in the humid, tropical landscape of central Cuba. We use the ratio of 26Al to 10Be to explore the exposure histories of surface materials within watersheds. We also compare long-term sediment generation rates to sediment yield data collected during the height of industrialized agriculture in Cuba, to evaluate how human actions have affected the pace of sediment generation over time. Long-term sediment generation rates derived from measurements of 10Be for 24 basins in central Cuba range from 3.7-182 tons km-2 year-1 (mean = 61.1), while rock dissolution rates range from 23.9-153.7 tons km-2 year-1 (mean = 84.3). Rock dissolution rates are higher than cosmogenic-based erosion rates in 15 of the 22 basins in which both measurements were made. Sediment generation rates are slowest in basins underlain by sedimentary rocks and highest in basins underlain by metamorphic rocks, while the opposite trend was observed for rock dissolution rates, demonstrating a lithologic control on the rates and processes of denudation in the Cuban landscape. Low 26Al/10Be ratios (2.89-5.09) in 5 samples provides evidence for complex exposure histories. Sediment generation rates are lower on average than erosion rates calculated from island-wide sediment yield data collected during the peak of Soviet-assisted agricultural production, suggesting that modern sediment generation rates in Cuba have increased above long-term averages, likely due to agriculture. We find that relying on cosmogenic nuclide-based sediment generation rates alone underestimate total landscape denudation in central Cuba. The discrepancy between high rock dissolution rates and low sediment generation rates suggests that significant mass loss is occurring by solution at depths below significant production of cosmogenic nuclides (> 3m). 26Al/10Be ratios in basins with the highest disagreement (9-32X) between fast rock dissolution rates and slow sediment generation rates are consistent with expected nuclide concentrations and ratios calculated assuming long-term exposure within a deep mixed surface layer -- providing additional evidence for rock dissolution at depth. We suggest that measuring rock dissolution rates alongside cosmogenic-derived sediment generation rates can be used to set minimum and maximum bounds on total landscape denudation in areas where mass loss at depth plays a large role in total landscape denudation. These data emphasize the importance of accounting for mass loss at depth when interpreting cosmogenic nuclide measurements in tropical landscapes.