Quartz Grain Microtextures and Sediment Provenance: Using Scanning Electron Microscopy to Characterize Tropical Highland Sediments from Costa Rica and the Dominican Republic

Microtextures recorded on quartz sand grain surfaces provide evidence of past environment. Environmental processes, such as transport by glacial ice, create unique microtextures on sand grain surfaces that can be observed under high magnification with a scanning electron microscope. These microtextures and their proportions tend to be unique to environment type, allowing investigators to infer the environmental conditions to which sediments have been exposed, for example to distinguish sediments from fluvial versus mass-wasted environments. Microtextural evidence also allows inferences about the history of sediments of unknown origin.

This thesis determines the qualitative and quantitative microtextural fingerprint of glacigenic quartz sand grains deposited by small tropical alpine glaciers in Costa Rica, and compares that fingerprint to the fingerprints of highland Dominican Republic sediments of uncertain genesis, to gauge whether those, individually or grouped, resemble the Costa Rican glacigenic samples.

I selected 18 samples (9 each from Costa Rica and the Dominican Republic) and analyzed a minimum of 100 quartz sand grains per sample using a scanning electron microscope. My sample sizes were dictated by the scale of empirical 99% confidence intervals that would allow meaningful comparison of samples. Analysis using literature-recommended numbers of quartz sand grains would entail such large confidence intervals that practically any results would have been indistinguishable. I recorded the presence or absence of 25 microtextures on each grain, and calculated the percentage of each microtexture’s occurrence in the sample. The percentages constituted the sample’s microtextural fingerprint. As a whole, the Costa Rican fingerprints were very similar to each other, and so were the Dominican Republic fingerprints. Further comparison led me to conclude that the Dominican Republic samples are statistically indistinguishable from the Costa Rican glacierized samples.

This thesis is part of a larger project establishing protocols for distinguishing glacigenic from non-glacigenic sediments, and testing for glacigenicity of sediments in Costa Rica, the Dominican Republic, and elsewhere. My results can be applied in other studies distinguishing tropical highland glacigenic and non-glacial samples. My contribution will hopefully contribute toward completion of the project’s goals, specifically determining the presence or absence of past glaciers in the Dominican Republic.