Evidence for a grounding line fan at the onset of a basal channel under the ice shelf of Support Force Glacier, Antarctica, revealed by reflection seismics

Abstract

Abstract. Curvilinear channels on the surface of an ice shelf indicate the presence of large channels at the base. Modelling studies have shown that where these surface expressions intersect the grounding line, they coincide with the likely outflow of subglacial water. An understanding of the initiation and the ice–ocean evolution of the basal channels is required to understand the present behaviour and future dynamics of ice sheets and ice shelves. Here, we present focused active seismic and radar surveys of a basal channel, ∼950 m wide and ∼200 m high, and its upstream continuation beneath Support Force Glacier, which feeds into the Filchner Ice Shelf, West Antarctica. Immediately seaward from the grounding line, below the basal channel, the seismic profiles show an ∼6.75 km long, 3.2 km wide and 200 m thick sedimentary sequence with chaotic to weakly stratified reflections we interpret as a grounding line fan deposited by a subglacial drainage channel directly upstream of the basal channel. Further downstream the seabed has a different character; it consists of harder, stratified consolidated sediments, deposited under different glaciological circumstances, or possibly bedrock. In contrast to the standard perception of a rapid change in ice shelf thickness just downstream of the grounding line, we find a flat topography of the ice shelf base with an almost constant ice thickness gradient along-flow, indicating only little basal melting, but an initial widening of the basal channel, which we ascribe to melting along its flanks. Our findings provide a detailed view of a more complex interaction between the ocean and subglacial hydrology to form basal channels in ice shelves.

On the evolution of an ice shelf melt channel at the base of Filchner Ice Shelf, from observations and viscoelastic modeling

“…Smith et al (2009) found an active subglacial lake at the transition between Academy Glacier and SFG, and also Humbert et al (2018) suggest in the upstream area of SFG a subglacial lake. Hofstede et al (2021a) showed that the subglacial channel appears 7 km upstream of the grounding line increasing its height to 280 m at the grounding line. The location of the channel corresponds with increased subglacial flux found by Humbert et al…”

“…(2018) using a simple routing scheme. The channel formed on the grounded part is most likely the source of a grounding line fan and thus carrying sediments, formed at the seabed under the basal channel Hofstede et al (2021a). Once this topographic feature reaches the ocean, it will focus on the relatively buoyant flow and enhance shear-driven vertical mixing, bringing heat and salt to the ice base leading to higher basal melt rates.…”

“…These terraces are separated by up to 50 m high walls with steep slopes between 40 • and 60 • . Hofstede et al (2021a) found a basal channel on Support Force Glacier at the transition to Filchner Ice Shelf attributed to the outflow of subglacial water. The channel increases in height close to the grounding line and widens afterwards.…”

“…The seismic I, IV and V lines mark the location of active seismic profiles (Hofstede et al, 2021a, b). The background is a hillshade of the Reference Elevation Model of Antarctica (Howat et al, 2018(Howat et al, , 2019 overlaid by the ice flow velocity (Hofstede et al, 2021a). (c) Sketch of a cross-section of the channel with measurement locations on the steepest western surface flank (SW), at the lowest surface elevation (L), on the steepest eastern surface flank (SE) and outside east of the channel (OE).…”

British Antarctic Survey’s Aerogeophysical Data: Releasing 25 Years of Airborne Gravity, Magnetic, and Radar Datasets over Antarctica

“…In 2016-17, ~26,000 line-km of aerogeophysical data were acquired over Filchner and Brunt ice shelves (West Antarctica), as well as over outlet glaciers of English Coast (western Palmer Land, Antarctic Peninsula). Early findings from the 2016-17 aerogeophysical survey revealed subglacial drainage channels beneath Support Force Glacier (Hofstede et al, 2021), provided evidence for a large ~80 x 30 x 6 km mafic intrusion likely resulting from mantle melting during Gondwana break-up (Jordan and Becker, 2018), and helped to delineate the subglacial bathymetry beneath Brunt Ice Shelf (Hodgson et al, 2019).…”