Identifying moraine surfaces with similar histories using lichen size distributions and the u2 statistic, southeast iceland

Abstract

Identifying moraine surfaces with similar histories using lichen size distributions and the U 2 statistic, southeast Iceland. Geogr. Ann., 90 A (2): 151-164.ABSTRACT. Moraine ridges are commonly used to identify past glacier ice margins and so infer glacier mass balance changes in response to climatic variability. However, differences in the form of past ice margins and post-depositional modification of moraine surfaces can complicate these geomorphic records. As a result, simple relationships, such as distance from current ice margin, or linear alignments, may not necessarily indicate moraines deposited contemporaneously. These disturbances can also modify the size distribution of lichen populations, providing a distinctive signature for surfaces with similar histories and a means of identifying contemporaneous moraine surfaces. In this paper, statistical analysis of lichen size distributions is used to identify moraine surfaces with similar histories from complex suites of Little Ice Age moraine fragments in the proglacial areas of Skálafellsjökull (including Sultartungnajökull) and Heinabergsjökull, southeast Iceland. The analysis is based on a novel use of the goodness-of-fit statistic, Watson's U 2 , which provides a measure of 'closeness' between two sample distributions. Moraine fragments with similar histories are identified using cluster analysis of the U 2 closeness values. The spatial pattern of the clustered moraines suggests three distinct phases of moraine deposition at Skálafellsjökull and Heinabergsjökull, four phases at Sultartungnajökull and a digitate planform margin at Heinabergsjökull. These spatial patterns are corroborated with tephrochronology. The success of the U 2 statistical analysis in identifying surfaces with similar histories using lichen size distributions suggests that the technique may be useful in augmenting lichenometric surface dating as well as differentiating between other surfaces that support lichen populations, such as rock avalanche deposits.

Rock surface micro−roughness, Schmidt hammer rebound and weathering rind thickness within LIA Skálafellsjökull foreland, SE Iceland

“…This may be explained by exfoliation of basaltic surfaces on older moraines (Etienne 2002) leading to smoothening of rock sur− faces (Dąbski and Tittenbrun 2013) or by diversified structure and texture of stud− ied rocks, which is frequently impossible to notice in the field. Therefore, it is in− ferred that the indices used are unsuitable for precise dating of glacial landforms created since the Little Ice Age, and they do not provide additional arguments in the ongoing discussion about timing of LIA maximum in Iceland (Evans et al 1999;Bradwell 2001Bradwell , 2004Dąbski 2002Dąbski , 2007Dąbski , 2010McKinzey et al 2004;McKinzey et al 2005;Bradwell et al 2006;Orwin et al 2008;Chenet et al 2011;Kirkbride and Winkler 2012). Matthews and Owen (2008) working on gneiss boulders within LIA maximum of Storbreen (Norway) found that Schmidt hammer R−values obtained from sur− faces colonized by Lecidea auriculata lichens gradually decreases from the glacier margin to moraines 30-50 years old, supporting the notion of a rapid but relatively short−term proglacial weathering.…”

“…However, this was questioned by McKinzey et al (2005) who critically assessed the historical data and concluded that LIA glacier maximum occurred in S Iceland in the late 18 th to early 19 th cen− tury. There are lichenometrical datings of the outermost Skálafellsjökull moraines pointing at the end of the 19 th century (Gordon and Sharp1983;Evans et al 1999), but other date the oldest moraines rather to the mid 19 th century (McKinzey et al 2004;, depending on the lichenometrical technique used, which is also discussed in Orwin et al (2008) andBradwell (2009). Moraines of Skálafellsjökull were in contact with moraines of Heinabergs− jökull and Fláajökull ( Fig.…”

“…Consecutively, the study aimed to determine the rate of weathering in freshly deglaciated terrains, as it is an important issue in a light of contemporary retreat of mountain glaciers and paraglacial modification of landscapes in alpine and polar regions. Furthermore, there is an on−going discussion about the timing of LIA in southern Iceland (Evans et al 1999;Bradwell 2001Bradwell , 2004Dąbski 2002Dąbski , 2007Dąbski , 2010McKinzey et al 2004;McKinzey et al 2005;Bradwell et al 2006;Orwin et al 2008;Chenet et al , 2011Kirkbride and Winkler 2012), there− fore, developing tools for relative dating of glacial landforms seems an urging ne− cessity. The marginal zone of Skálafellsjökull constitutes a full LIA and recent moraine sequence, similar to that of Fláajökull, allowing easy access to the oldest as well as the youngest parts of moraines.…”

Lichenometric dating: a commentary, in the light of some recent statistical studies

“…The authors use the U 2 statistic to group lichen populations and, after numerous statistical steps (e.g. observation ranking, cluster analysis and similarity matrices), to assign relative ages to recent glacial deposits and highlight complex depositional histories (Orwin et al . 2008).…”

The Acheron rock avalanche deposit, Canterbury, New Zealand: age and implications for dating landslides

“…Orwin et al 2008); some of these uncertainties are related to post-emplacement disturbance that can also reset weathering rinds. We outline some of these later (see Discussion, p. 13).…”