Date of Award

2016

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

Paul Bierman

Second Advisor

Nicolas Perdrial

Abstract

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.

Language

en

Number of Pages

118 p.

Included in

Geology Commons

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