Holocene Formation of Henry Ice Rise, West Antarctica, Inferred From Ice‐Penetrating Radar

Abstract

Ice rises are regions of grounded ice embedded within floating ice shelves. The deformation of ice past them increases the back stress generated by the ice shelf, slowing the flow of the ice sheet. We present ground‐based ice‐penetrating radar data from Henry Ice Rise in the Ronne Ice Shelf, West Antarctica, that indicates regrounding during the Holocene. Relic crevasses and melt synclines are observed upstream of the present‐day grounding line. We conclude that these features formed during a previous flow configuration, from which the grounding line has since advanced to its current position. In agreement with previous work, our observations can be explained if initial grounding of the ice shelf occurred on a bathymetric high, forming an ice rumple that migrated upstream and temporarily ungrounded over the topographic high. The grounding line then advanced, preserving relic basal crevasses in the newly grounded ice. Using a simple ice‐flow model, we simulate the burial of these crevasses. While accounting for uncertainty in accumulation, firn density, radar‐derived depth, ice‐thickening history, initial crevasse height, and glacial isostatic adjustment, we estimate a burial time of 6 ± 2 kyr before present for the oldest relic crevasses, indicating that the ice rise formed at approximately this time. This potentially increased the buttressing generated by the Ronne Ice Shelf, causing thickening and advance of the ice sheet. By dating the formation and providing details of ice‐rise formation, these new results can provide useful constraints on both large‐scale ice‐sheet models and models of ice‐rumple and ice‐rise formation.

Major Ice Sheet Change in the Weddell Sea Sector of West Antarctica Over the Last 5,000 Years

“…HIR consists of two ridges, each approximately 100‐km long, extending to the north and southwest from a peak in surface elevation in the southeast. In contrast to the surface topography, a basal high point is located at the northern end of the northern ridge (Wearing & Kingslake, ). HIR is 400‐ to 800‐m thick.…”

“…Individual dipping reflectors extend vertically 150–200 m into the ice from the base and span up to 2 km horizontally. The isochronal layers intersected by the dipping reflectors decrease in age as one moves toward the grounding line (Wearing & Kingslake, ). This is consistent with these reflectors being relic basal crevasses, formed in a previous flow configuration, when the grounding line was upstream of its present location.…”

“…It is thought that these features form as a result of concentrated basal melting at former grounding lines, as observed at former shear margins on the Siple Coast (Catania et al, ). The spatial distribution of relic basal crevasses and melt synclines (Figure ) suggests ungrounding of the northern basal high, as part of a complete or only partial ungrounding of HIR (Wearing & Kingslake, ). We expect ice rises to remain grounded over their high points if they form during a period of ice sheet retreat (Favier & Pattyn, ), so these observations suggest complete ungrounding, but we cannot rule out partial ungrounding.…”

“…Further englacial reflectors may result from basal sediment inclusion (Siegert et al, 2013;Winter et al, 2019) and from where ice is, or has been, floating to form basal crevasses (Kingslake et al, 2018;Wearing & Kingslake, 2019). In all cases, interpreting past flow from internal layers requires consideration of how changes in time may affect layer arrangements.…”

“…Reviews of Geophysics located at the northern end of the northern ridge (Wearing & Kingslake, 2019). HIR is 400-to 800-m thick.…”