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The dynamics of mountain erosion : cirque growth slows as landscapes age.

Barr, Iestyn D. and Ely, Jeremy C. and Spagnolo, Matteo and Evans, Ian S. and Tomkins, Matt D. (2019) 'The dynamics of mountain erosion : cirque growth slows as landscapes age.', Earth surface processes and landforms., 44 (13). pp. 2628-2637.

Abstract

Glacial cirques are widely used palaeoenvironmental indicators, and are key to understanding the role of glaciers in shaping mountain topography. However, notable uncertainty persists regarding the rate and timing of cirque erosion. In order to address this uncertainty, we analyse the dimensions of 2208 cirques in Britain and Ireland and model ice accumulation to investigate the degree of coupling between glacier occupation times and cirque growth. Results indicate that during the last ~120 ka, cirques were glacier‐free for an average of 52.0 ± 21.2 ka (43 ± 18%); occupied by small (largely cirque‐confined) glaciers for 16.2 ± 9.9 ka (14 ± 8%); and occupied by large glaciers, including ice sheets, for 51.8 ± 18.6 ka (43 ± 16%). Over the entire Quaternary (i.e., 2.6 Ma), we estimate that cirques were glacier‐free for 1.1 ± 0.5 Ma; occupied by small glaciers for 0.3 ± 0.2 Ma; and occupied by large glaciers for 1.1 ± 0.4 Ma. Comparing occupation times to cirque depths, and calculating required erosion rates reveals that continuous cirque growth during glacier occupation is unlikely. Instead, we propose that cirques attained much of their size during the first occupation of a non‐glacially sculpted landscape (perhaps during the timeframe of a single glacial cycle). During subsequent glacier occupations, cirque growth may have slowed considerably, with the highest rates of subglacial erosion focused during periods of marginal (small glacier) glaciation. We propose comparatively slow rates of growth following initial cirque development because a ‘least resistance’ shape is formed, and as cirques deepen, sediment becomes trapped subglacially, partly protecting the bedrock from subsequent erosion. In support of the idea of rapid cirque growth, we present evidence from northern British Columbia, where cirques of comparable size to those in Britain and Ireland developed in less than 140 ka.

Item Type:Article
Full text:(AM) Accepted Manuscript
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Status:Peer-reviewed
Publisher Web site:https://doi.org/10.1002/esp.4688
Publisher statement:This is the accepted version of the following article: Barr, Iestyn D., Ely, Jeremy C., Spagnolo, Matteo, Evans, Ian S. & Tomkins, Matt D. (2019). The dynamics of mountain erosion: cirque growth slows as landscapes age. Earth Surface Processes and Landforms 44(13): 2628-2637 which has been published in final form at https://doi.org/10.1002/esp.4688. This article may be used for non-commercial purposes in accordance With Wiley Terms and Conditions for self-archiving.
Date accepted:18 June 2019
Date deposited:02 July 2019
Date of first online publication:30 July 2019
Date first made open access:30 July 2020

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