A quasi-annual record of time-transgressive esker formation: implications for ice-sheet reconstruction and subglacial hydrology

Abstract

We identify and map chains of esker beads (series of aligned mounds) up to 15 m high and on average ∼ 65 m wide in central Nunavut, Canada, from the high-resolution (2 m) ArcticDEM. Based on the close 1 : 1 association with regularly spaced, sharp-crested ridges interpreted as De Geer moraines, we interpret the esker beads to be quasi-annual ice-marginal deposits formed time-transgressively at the mouth of subglacial conduits during deglaciation. Esker beads therefore preserve a high-resolution record of ice-margin retreat and subglacial hydrology. The well-organised beaded esker network implies that subglacial channelised drainage was relatively fixed in space and through time. Downstream esker bead spacing constrains the typical pace of deglaciation in central Nunavut between 8.1 and 6.8 cal kyr BP to 165–370 m yr−1, although with short periods of more rapid retreat (> 400 m yr−1). Under our time-transgressive interpretation, the lateral spacing of the observed eskers provides a true measure of subglacial conduit spacing for testing mathematical models of subglacial hydrology. Esker beads also record the volume of sediment deposited from conduits in each melt season, thus providing a minimum bound on annual sediment fluxes, which is in the range of 103–104 m3 yr−1 in each 6–10 km wide subglacial conduit catchment. We suggest that the prevalence of esker beads across this predominantly marine-terminating sector of the Laurentide Ice Sheet is a result of sediment fluxes that were unable to backfill conduits at a rate faster than ice-margin retreat. Conversely, we hypothesise that esker ridges form when sediment backfilling of the subglacial conduit outpaced retreat, resulting in headward esker growth close to but behind the margin. The implication, in accordance with recent modelling results, is that eskers in general record a composite signature of ice-marginal drainage rather than a temporal snapshot of ice-sheet-wide subglacial drainage.