Online

Greene, Emily Sophie

Geology

2016

MS

The cosmogenic nuclide ¹⁰Be is a tool for quantifying earth surface processes that occur on millennial timescales. ¹⁰Be is produced in the atmosphere (meteoric ¹⁰Be) or in mineral grains (in situ ¹⁰Be). Well-understood nuclear physics, physical mixing processes, and the denudation of regolith control concentrations of in situ ¹⁰Be; in contrast, a combination of geomorphic, pedogenic, geochemical, and biological processes influence meteoric ¹⁰Be concentrations. Some have hypothesized that meteoric ¹⁰Be can be used as a tracer of sediment movement if meteoric ¹⁰Be is normalized against the concentration of native ⁹Be in grain coatings. This study aims to better understand Be dynamics in river sediment systems by further characterizing a large dataset of fluvial sediments (202 total samples from 7 study areas) that have previously been analyzed for in situ and meteoric ¹⁰Be. I determined ⁹Be and major element compositions of grain coatings (as the acid-extractable fraction) and grains (by total digestion) of fluvial sediments. I compiled the emical data with characteristics of sample watersheds that I acquired using ArcGIS and with meteoric and in situ ¹⁰Be data from previous studies. With this dataset, I performed a statistical analysis testing relationships between the concentration of meteoric ¹⁰Be and ⁹Be in acid-extractable grain coatings, meteoric ¹⁰Be/⁹Be ratios, ⁹Be concentrations in mineral grains, watershed characteristics, and major element compositions of fluvial sediment grains and grain coatings. I calculated meteoric ¹⁰Be/⁹Be-derived denudation rates using a published mass balance model and compared them to in situ ¹⁰Be-derived denudation rates. Though this thesis focuses on fluvial sediment samples, I also measured ⁹Be concentrations of soil, suspended sediment, and glacial lake sediment samples with known meteoric ¹⁰Be or in situ ¹⁰Be concentrations, which can be used in future studies of ⁹Be and ¹⁰Be dynamics. I find that meteoric ¹⁰Be and ⁹Be concentrations in grain coatings are significantly influenced by geochemical and geomorphic conditions in watersheds. HCl-extracted ⁹Be is significantly correlated to total meteoric ¹⁰Be concentrations in all but one study area, suggesting that meteoric ¹⁰Be and ⁹Be are well mixed in most, but not all, soil systems. Trends in meteoric ¹⁰Be do not mirror trends in in situ ¹⁰Be. Though normalizing meteoric ¹⁰Be against ⁹Be concentrations improves the correlation between meteoric ¹⁰Be and in situ ¹⁰Be in fluvial sediments, the spatial variation in parent ⁹Be concentrations and meteoric ¹⁰Be delivery rates, combined with the observation that meteoric ¹⁰Be and ⁹Be are not always well mixed, makes it difficult to interpret changes in meteoric ¹⁰Be/⁹Be across study areas. A mass balance model for deriving meteoric ¹⁰Be/⁹Be denudation rates helps control for some variation in ⁹Be concentrations and meteoric ¹⁰Be delivery rates across study areas, but uncertainties in quantifying these variables for each watershed introduce noise into the correlations between meteoric ¹⁰Be/⁹Be -derived denudation rates and ¹⁰Beis-derived denudation rates. When considering all samples, meteoric ¹⁰Be/⁹Be-derived and ¹⁰Beis-derived denudation rates are significantly correlated and have similar central tendencies. However, the ¹⁰Bemet/⁹Bereactive -derived measure is less sensitive to changes in denudation than the ¹⁰Beis -derived measure.