Sedimentary signatures of basal ice formation and their preservation in ice-marginal sediments

Abstract

Reconstruction of subglacial processes from sedimentological evidence is central to our understanding of glaciological conditions beneath former ice masses. At Svinafellsjbkull, southeast Iceland, we assess the extent to which two different processes of basal ice formation (regelation and glaciohydraulic supercooling) can be identified from ice-marginal sediments. Our results indicate that the sedimentary characteristics of deposits produced by these two processes can be distinguished from one another and that it may be possible to recognise evidence of these processes in Quaternary sediments and to reconstruct their spatial pervasiveness. Sediments derived from the melting of regelation basal ice have (i) a massive structure; (ii) a sediment matrix (0 to 100) dominated by coarse sand; and (iii) a higher proportion of angular clasts than supercool basal ice and associated sediments. Sediments derived from "supercool" basal ice (i) can be either massive or layered; (ii) tend to have a silt-dominated matrix; and (iii) contain a slightly higher proportion of rounded and well-rounded clasts than 1 regelation basal ice and sediments. Previous studies indicate that the dominance of silt within supercool basal ice may be unique to this process, and hence, supercooling should leave a readily recognisable signature in the sedimentary record. Our results from Svinafellsjbkull lend support to that idea, although we suggest that further work is required to assess whether silt dominance is a process signature diagnostic of supercooling, and in particular, the extent to which subglacial sediment supply determines the sedimentary character of basal ice fades and associated sediments.

Skaftafellsjökull, Iceland: glacial geomorphology recording glacier recession since the Little Ice Age

“…The more restricted pseudostratified outcrops are potentially the rare products of melt-out from supercooled glacier ice, a notable feature of the Icelandic south coast glacier lobes (Cook et al, 2010;Cook, Graham, Swift, Midgeley, & Adam, 2011;Cook, Knight, Richard, Robinson, & Adam, 2007;Cook, Swift, Graham, & Midgley, 2011;Roberts et al, 2002), but predominantly the diamictons display features that are diagnostic of a subglacial traction till genesis such as fissility, compaction and reasonably strong clast macrofabrics (Evans, 2000(Evans, , 2017Evans et al, submitted;Evans, Phillips, Hiemstra, & Auton, 2006;Jónsson et al, 2016).…”

Origin, evolution and dynamic context of a Neoglacial lateral–frontal moraine at Austre Lovénbreen, Svalbard

“…Some of these basal ice characteristics may also be preserved in the moraine sediment as the basal ice melts, offering the potential to reconstruct basal ice characteristics from deglaciated terrain (e.g. Knight et al, 2000;Evans, 2009;Cook et al, 2011).…”

Laboratory observations of sediment entrainment by freezing supercooled water

“…2006). Cook et al . (2011) found that ice‐marginal sediments derived from supercool basal ice could be distinguished from ice‐marginal sediments produced by other subglacial processes, such as regelation, partly because sediments associated with supercooling were dominated by silt.…”

“…1999; Larson et al . 2006) and at glaciers in southern Iceland (Cook et al . 2007, 2010, 2011) have demonstrated that ‘stratified facies’ basal ice (debris‐rich basal ice with a layered appearance) produced by glaciohydraulic supercooling is characterized by high proportions of silt.…”