Comparing meteoric 10Be, in situ 10Be, and native 9Be across a diverse set of watersheds

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