Characterization of glacial silt and clay using automated mineralogy

Abstract

Glacial erosion produces vast quantities of fine-grained sediment that has a far-reaching impact on Earth surface processes. To gain a better understanding of the production of glacial silt and clay, we use automated mineralogy to quantify the microstructure and mineralogy of rock and sediment samples from 20 basins in the St. Elias Mountains, Yukon, Canada. Sediments were collected from proglacial streams, while rock samples were collected from ice marginal outcrops and fragmented using electrical pulse disaggregation. For both rock fragments and sediments, we observe a log-normal distribution of grain sizes and a sub-micrometer terminal grain size. We find that the abrasion of silt and clay results in both rounding and the exploitation of through-going fractures. The abundance of inter- versus intragranular fractures depends on mineralogy and size. Unlike the relatively larger grains, where crushing and abrasion are thought to exploit and produce discrete populations of grain sizes, the comminution of fines leads to a grain size, composition and rounding that is continuously distributed across size, and highly dependent on source-rock properties.

Variations in Hard‐Bedded Topography Beneath Glaciers

“…The PCA is conducted on the compiled parameter data set of all the DEMs. Plotting the scores of the first two PCs on a Cartesian plane helps identify morphological distinctions of the proglacial areas by clustering the scores of the DEM subsection that show similar morphologies (e.g., Crompton et al, 2019).…”

Insights into glacial processes from micromorphology of silt-sized sediment

“…Images captured with the 10× objective were exported because this magnification preferentially captures finer grain sizes, including the silt-sized range dominant in MPDs, compared to the 0.5× objective. The lower detection limit for the 10× objective is 0.8 µm; to avoid sampling bias skewed towards that threshold or towards clay minerals, images of grains finer than 2.4 µm were excluded from analysis (Crompton et al, 2019).…”

“…Subglacial processes responsible for generating glacial silts and the "terminal grain size mode", or the smallest silt size to which a grain can be comminuted based on its mineralogical structure, have been explored through field observations and controlled experiments (e.g., Vagners, 1971, 1972;Iverson et al, 1996;Crompton et al, 2019). These studies have largely agreed that abrasion is a widespread process in subglacial environments (Alley et al, 2019) driving comminution by exploiting weaknesses in the mineral fabric of larger grains (Haldorsen, 1981;Crompton et al, 2019) and that the microtextural signatures of abrasion on grain surfaces include different step and fracture types (e.g., Mahaney, 2002;Passchier et al, 2021).…”

“…Subglacial processes responsible for generating glacial silts and the "terminal grain size mode", or the smallest silt size to which a grain can be comminuted based on its mineralogical structure, have been explored through field observations and controlled experiments (e.g., Vagners, 1971, 1972;Iverson et al, 1996;Crompton et al, 2019). These studies have largely agreed that abrasion is a widespread process in subglacial environments (Alley et al, 2019) driving comminution by exploiting weaknesses in the mineral fabric of larger grains (Haldorsen, 1981;Crompton et al, 2019) and that the microtextural signatures of abrasion on grain surfaces include different step and fracture types (e.g., Mahaney, 2002;Passchier et al, 2021). Furthermore, abrasion beneath glacial ice has been credited with rounding grain shape (Hart, 2006;Rose and Hart, 2008), which is consistent with the degrees of solidity and circularity in our results, particularly for those most mature (i.e., those having undergone reworking by multiple glacial advance and retreat cycles) sediments from West Antarctica (Figs.…”

Counting a pot of gold: A till golden standard (AuGC-1)

“…During the last few years, several automated techniques have been developed to identify (Maitre et al, 2019) and count minerals in HMC (Lougheed et al, 2020;Cook et al, 2017;Crompton et al, 2019) and therefore limit the potential for human error while also decreasing sample processing time. Despite such improvements, there is still a lack of quantitative quality control of gold grain counting; no reference material exists to monitor the quality and uncertainties of automated and manual techniques.…”