143-4 Improving Sand Grain-Shape Analysis Using Focus-Stacking Microphotographs: Application to Depositional-Environment Discrimination

Abstract:

Shapes of clastic grains record transportation and depositional history, as well as post-depositional diagenetic modification. Using elliptic Fourier–principal component analysis (EF–PCA), Suzuki et al. (2015) defined two indices: REF1, a macroscopic measure of grain elongation, and SEF, a microscopic measure of grain-surface smoothness. These indices discriminate sand grains from glacial, fluvial, foreshore, and aeolian environments. However, reproducibility may be affected by image acquisition and particle-outline extraction when incomplete focus obscures fine boundary irregularities. This study aims to improve the reproducibility of EF–PCA and reassess its applicability to depositional-environment discrimination.

We analyzed 250–355 μm quartz grains using focus-stacked two-dimensional images acquired with a digital microscope (VHX-5000, Keyence Co., Osaka, Japan). Elliptic Fourier analysis of binary grain outlines up to the 100th harmonic produced 397 Fourier descriptors. PCA using the variance–covariance matrix yielded REF1, which indicates elongated grains when negative and circular grains when positive. PCA using the correlation matrix yielded SEF, which indicates smoother and more abraded boundaries at higher values.

A reference model was constructed from 1,245 grains from glacial, fluvial, foreshore, and aeolian deposits. REF1 explained 49.20% of the total variance and differed significantly among depositional environments, except between glacial and foreshore grains. SEF explained 30.17% of the amplitude variance and significantly segregated all depositional-environment pairs. Traditional and renewed indices were compared using Kruskal–Wallis tests followed by Dunn’s tests with Holm correction. The same grains photographed using conventional single-focus and focus-stacking microscopy produced different SEF values, indicating improved reproduction of natural microscopic grain boundaries.

On the renewed SEF–REF1 diagram, the four grain types produced distinct clusters. We also applied the diagram to a glacial sand outlier undergoing soil modification at Zmutt Glacier, Switzerland, and a foreshore sand outlier reworked by aeolian abrasion since the Last Glacial Maximum in Tottori, Japan. The former plotted between the glacial and foreshore fields, whereas the latter plotted between the foreshore and aeolian fields. This method therefore discriminates standard grain origins and reveals overprints of different depositional histories. These results demonstrate that focus stacking and digital outline extraction improve the reproducibility of two-dimensional EF–PCA while preserving its ability to evaluate depositional environment and sedimentary history. A remaining concern is that two-dimensional grain characteristics can change with orientation.

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