Abstract:Mäkinen, J. 2014 (January): Sedimentology of an ice lobe margin esker with implications for the deglacial dynamics of the Finnish Lake District lobe trunk.We provide evidence for the subglacial to ice-marginal successive deposition of the Lohtaja-Kivijärvi ice lobe margin esker influenced by the changes in the meltwater delivery and proglacial water depth within the Finnish Lake District lobe trunk during the last deglaciation in Finland. The study is mostly based on the sedimentological data from the 100 km l… Show more
“…Fig. 8.Morphological map (a) and LiDAR DEM of murtoo field arrangement along the hummocky margins of the ice-flow corridors (drumlinized) within the rapidly deglaciated trunk of the Finnish Lake District Ice Lobe (b) (interpretations in (a) by Ahokangas and Mäkinen, 2014). Murtoos occur in corridors in close association with eskers (c) and cut ribbed moraines (d).…”
Section: Resultsmentioning
confidence: 99%
“…Clusters of murtoo fields within the Finnish Lake District ice lobe could reflect the reorganization of the ice flow during deglaciation. The NW cluster of murtoos is related to deglaciation along the trunk of the Finnish Lake District ice lobe with deepening proglacial water (Ahokangas and Mäkinen, 2014).…”
High-resolution digital elevation models of Finland and Sweden based on LiDAR (Light Detection and Ranging) reveal subglacial landforms in great detail. We describe the ice-sheet scale distribution and morphometric characteristics of a glacial landform that is distinctive in morphology and occurs commonly in the central parts of the former Scandinavian Ice Sheet, especially up-ice of the Younger Dryas end moraine zone. We refer to these triangular or V-shaped landforms as murtoos (singular, ‘murtoo’). Murtoos are typically 30–200 m in length and 30–200 m in width with a relief of commonly <5 m. Murtoos have straight and steep edges, a triangular tip oriented parallel to ice-flow direction, and an asymmetric longitudinal profile with a shorter, but steeper down-ice slope. The spatial distribution of murtoos and their geomorphic relation to other landforms indicate that they formed subglacially during times of climate warming and rapid retreat of the Scandinavian Ice Sheet when large amounts of meltwater were delivered to the bed. Murtoos are formed under warm-based ice and may be associated with a non-channelized subglacial hydraulic system that evacuated large discharges of subglacial water.
“…Fig. 8.Morphological map (a) and LiDAR DEM of murtoo field arrangement along the hummocky margins of the ice-flow corridors (drumlinized) within the rapidly deglaciated trunk of the Finnish Lake District Ice Lobe (b) (interpretations in (a) by Ahokangas and Mäkinen, 2014). Murtoos occur in corridors in close association with eskers (c) and cut ribbed moraines (d).…”
Section: Resultsmentioning
confidence: 99%
“…Clusters of murtoo fields within the Finnish Lake District ice lobe could reflect the reorganization of the ice flow during deglaciation. The NW cluster of murtoos is related to deglaciation along the trunk of the Finnish Lake District ice lobe with deepening proglacial water (Ahokangas and Mäkinen, 2014).…”
High-resolution digital elevation models of Finland and Sweden based on LiDAR (Light Detection and Ranging) reveal subglacial landforms in great detail. We describe the ice-sheet scale distribution and morphometric characteristics of a glacial landform that is distinctive in morphology and occurs commonly in the central parts of the former Scandinavian Ice Sheet, especially up-ice of the Younger Dryas end moraine zone. We refer to these triangular or V-shaped landforms as murtoos (singular, ‘murtoo’). Murtoos are typically 30–200 m in length and 30–200 m in width with a relief of commonly <5 m. Murtoos have straight and steep edges, a triangular tip oriented parallel to ice-flow direction, and an asymmetric longitudinal profile with a shorter, but steeper down-ice slope. The spatial distribution of murtoos and their geomorphic relation to other landforms indicate that they formed subglacially during times of climate warming and rapid retreat of the Scandinavian Ice Sheet when large amounts of meltwater were delivered to the bed. Murtoos are formed under warm-based ice and may be associated with a non-channelized subglacial hydraulic system that evacuated large discharges of subglacial water.
“…The Bothnian Sea retreat sequence that we identify here is not entirely consistent with this conventional view. 'sublobes', Ahokangas & M€ akinen 2014). 1).…”
Section: Regional Framework For a Bothnian Sea Ice Streammentioning
confidence: 99%
“…Rather, any large ice lobe that may have existed in the Bothnian Sea was dissected by spatially and temporally variable flow behaviour (cf. 'sublobes', Ahokangas & M€ akinen 2014). While the early retreat geometry of the Bothnian Sea may have encompassed a broadly divergent ice-sheet sector, the ice stream event recorded by flowset A post-dates the 'Baltic Sea lobe' configuration (sensu Punkari 1980Punkari , 1994Boulton et al 2001Boulton et al , 2009, and mobilized only a local corridor within the Bothnian Sea, rather than the whole basin.…”
Section: Regional Framework For a Bothnian Sea Ice Streammentioning
The Gulf of Bothnia hosted a variety of palaeo‐glaciodynamic environments throughout the growth and decay of the last Fennoscandian Ice Sheet, from the main ice‐sheet divide to a major corridor of marine‐ and lacustrine‐based deglaciation. Ice streaming through the Bothnian and Baltic basins has been widely assumed, and the damming and drainage of the huge proglacial Baltic Ice Lake has been implicated in major regional and hemispheric climate changes. However, the dynamics of palaeo‐ice flow and retreat in this large marine sector have until now been inferred only indirectly, from terrestrial, peripheral evidence. Recent acquisition of high‐resolution multibeam bathymetry opens these basins up, for the first time, to direct investigation of their glacial footprint and palaeo‐ice sheet behaviour. Here we report on a rich glacial landform record: in particular, a palaeo‐ice stream pathway, abundant traces of high subglacial meltwater volumes, and widespread basal crevasse squeeze ridges. The Bothnian Sea ice stream is a narrow flow corridor that was directed southward through the basin to a terminal zone in the south‐central Bothnian Sea. It was activated after initial margin retreat across the Åland sill and into the Bothnian basin, and the exclusive association of the ice‐stream pathway with crevasse squeeze ridges leads us to interpret a short‐lived stream event, under high extension, followed by rapid crevasse‐triggered break‐up. We link this event with a c. 150‐year ice‐rafted debris signal in peripheral varved records, at c. 10.67 cal. ka BP. Furthermore, the extensive glacifluvial system throughout the Bothnian Sea calls for considerable input of surface meltwater. We interpret strongly atmospherically driven retreat of this marine‐based ice‐sheet sector.
“…Two principal hypotheses have been put forward for the formation of esker beads in the literature: (1) deposition at a retreat margin, with time-transgressive formation by sequential deposition of sediment debouching from subglacial conduits into a low energy subaqueous environment such as a lake or sea (e.g. Banerjee & McDonald, 1975;Rust & Romanelli, 1975;Cheel & Rust, 1986;Warren & Ashley, 1994;Mäkinen, 2003;Ahokangas & Mäkinen, 2014); and (2) entirely subglacial deposition with 185 synchronous formation during periodic separation of the glacier from its bed causing sediment-rich water to spill laterally out from the main subglacial conduit (esker ridge) into neighbouring subglacial cavities (e.g. Gorrell & Shaw, 1991).…”
Section: A Model For Quasi-annual Deposition Of Esker Beads In An Icementioning
Abstract. We identify and map chains of esker beads (series of aligned mounds) up to 15 m high and on average ~ 65 m wide across central Nunavut, Canada from the high-resolution (2 m) ArcticDEM. Based on the close one-to-one 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 7.2 and 6 ka 14C 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 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 the prevalence of esker beads across this predominantly marine terminating sector of the former Laurentide Ice Sheet is a result of sediment fluxes that were unable to backfill conduits at a rate faster than ice-margin retreat. Esker ridges, conversely, are hypothesised to 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.
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