QUANTIFYING PARTICLE SHAPE BY IMAGE ANALYSIS: APPLICATION TO NATURAL SEDIMENT SAMPLES IN VARIOUS CLIMATE SYSTEMS

Particle shape is an important parameter for sedimentary studies because it relates to weathering and transport, which can in turn relate to environmental conditions such as climate. However, particle shape is commonly assessed in geology using qualitative descriptors. Here, we employ automated quantitative shape analyses on mud- and sand-sized particles collected from fluvial systems in different climatic regions to assess shape changes potentially related to climate and/or transport.

Samples were collected from fluvial systems in four contrasting climates: hot-arid (southeastern California), hot-humid (eastern Puerto Rico), cold-arid (proglacial streams of the Dry Valleys, Antarctica), and cold-humid (Austerdalen proglacial stream, Norway). Samples were collected at regular intervals along the fluvial/alluvial transects beginning at the headwaters and extending ~15 km downstream. Samples were processed to remove carbonate and organic matter, and sieved to isolate various size fractions. Mud and sand samples were analyzed using the Malvern Morphologi3 instrument— a microscope system that enables static image analysis of hundreds to thousands of individual particles. This instrument captures 2D images, so provides information on size parameters such as circle-equivalent diameter, length, width, perimeter, and area, as well as shape parameters such as circularity and convexity. We compared given size fractions from different climates, and compared proximal-medial-distal samples from individual climates. Results provide quantitative constraints on shape differences that relate to climate and transport, even for very fine-grained sand and mud size fractions. For ancient sedimentary rocks that can be effectively disaggregated for particle shape analysis, this approach could provide insight on paleoclimatic interpretations.