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

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. Plain Language Summary We present results from an ice-penetrating radar survey on a grounded section of ice (Henry Ice Rise) within the floating Ronne Ice Shelf in West Antarctica. The radar reveals preserved crevasses at the base of the ice that are now inactive and signs of melting from contact with the ocean, both in areas that are grounded today. This implies that these areas were part of the floating ice shelf in the past. Using the distribution and size of relic crevasses within the ice, we date the formation of this grounded area to approximately 6 kyr before present. Our date has an associated uncertainty of 2 kyr, primarily due to uncertainty in the rate of snowfall and the size of the crevasses when they first formed. The grounding of this area most likely allowed the West Antarctic Ice Sheet to grow to its present-day configuration.