Rock Coating and Weathering-Rind Development at the Edge of Retreating Glaciers: An Initial Study

Abstract

Six different electron microscope techniques imaged and analyzed boulder and bedrock surfaces with glacially polished textures collected from the margins of the Athabasca Glacier in Canada, Bunger Oasis of Antarctica, Franz Josef Glacier in New Zealand, a Greenland outlet glacier, the Ngozumpa Glacier in Nepal, and the Middle Palisade Glacier in California. The purpose of this pilot investigation involves developing a better understanding of both rock decay and rock coating development at the edge of retreating glaciers. Our hope is that others in the Association of Pacific Coast Geographers will test some of the findings of this research in their study areas. In research on rock decay, five different samples from the six contexts revealed the presence of weathering rinds with varying degrees of porosity ranging from 0.3 to 7.1 percent. An often-made assumption, inconsistent with these data, is that recently exposed glaciated rocks exist in a fresh or unaltered state and that progressive decay can be monitored by measurement of weathering-rind thicknesses or Schmidt Hammer measurements from this "initial state. " The observed variability in weathering-rind porosity thus creates an additional uncertainty factor that could be incorporated into future studies of rock decay over time. Several different types of rock coatings occurred on the studied surfaces, including a newly recognized type of Mg-rich coating, iron films, rock varnish, fungal mats, and silica glaze-where silica glaze was the most commonly observed rock coating. The remobilized constituents of silica glaze, rock varnish, and iron also migrate into the underlying weathering rind to produce case hardening. Studies of rock varnish chemistry revealed evidence of twentieth-and twenty-first-century anthropogenic lead contamination in the upper micron of varnishes at the Greenland and Middle Palisade Glacier sites.

Rock varnish revisited

“…(iv) I have not found the same correlation between varnish type and climate as Macholdt et al (2017). For example, their Type I occurs in Antarctica (Dorn et al, 1992), Greenland (Dorn and Jeong, 2018), Mediterranean climate of Southern California (Dorn, 2019), and along the Orinoco River where von Humboldt initiated the scholarly study of rock varnish (Dorn et al, 2012b; Von Humboldt, 1812).…”