Siliceous subglacial deposits: archives of subglacial processes during the Last Glacial Maximum

Abstract

We report detailed chemical and isotopic data from a subglacial siliceous deposit on andesitic bedrock recently exposed by glacier retreat. Whereas a single, <1 μm, Si-rich layer covers the highly polished bedrock on the up-glacier (stoss) surfaces, distinct, lithified deposits commonly occur at the lee of small bedrock protuberances, on a scale <0.1 meter. The deposit is millimeters in thickness and consists of laminae tens to hundreds microns thick that differ from one another in color, rock-fragment abundance and chemical composition. Ca-rich laminae that are sufficiently enriched in uranium (~2–50 ppm) to permit U-series isotopic analysis suggest that the subglacial deposit formed 10–20 ka, much earlier than previously assumed. We conclude that (1) the siliceous deposit persisted for at least 10 000 years despite the intervening erosion and weathering, (2) distinct episodes of formation due to significant changes in hydrology and water chemistry are recorded in the deposit, and (3) a siliceous slurry may have existed at the ice-rock interface and influenced the local friction. This work reinforces earlier findings that subglacial chemical deposits can form and persist on geologic time scales and may have implications for the role of the cryosphere in the Earth's geochemical cycles and climate system.

Glacial microsedimentology—a new lens to investigate glacial sediments—a review

“…Tarplee et al, 2011;Cnudde andBoone, 2013. Linch andvan der Meer, 2013;Bendle et al, 2015;Falvard and Paris, 2017;Fouinat et al, 2017;Gehrmann et al, 2017;Paris et al, 2020;Siman-Tov et al, 2021). Recently, Siman-Tov et al ( 2021) have used back-scattered electron images of subglacial deposits to illustrate the nature of siliceous deposits consisting of deposits millimeters in thickness and composed of laminae tens to hundreds of microns in thickness across subglacial bedrock protuberances (Fig.…”