Evidence for floatation or near floatation in the mouth of Kamb Ice Stream, West Antarctica, prior to stagnation

Abstract

[1] Ice-penetrating radar profiles reveal distinctive internal structure within the flat ice terrains that bound the lower reaches of the now stagnant Kamb Ice Stream, West Antarctica; slightly warped but continuous upper layers overlie extensive deep line diffractors located at a uniform depth several hundreds of meters above the bed. On the basis of their orientation and morphology we interpret the line diffractors to be the tips of basal crevasses. At the boundary between these terrains and the inter-ice stream ridges, internal layers are strongly downwarped, and the deepest layers are truncated at the bed. Results from simple kinematic ice flow modeling indicate that the observed layer pattern requires a cumulative basal melt of 120-350 m over a time period that is less than 300 years. Fields of basal crevasses are typically observed in ice shelves; however, isolated basal crevasses can also be found in grounded ice when the subglacial water pressure is close to overburden. On the basis of the presence of these crevasses we argue that these terrains were once floating or near floating in the past. We combine model results with estimates of recent ice thickness change to suggest that floatation was transitory and the region may have become fully grounded roughly 200 years prior to stagnation of Kamb Ice Stream.

Century‐scale discharge stagnation and reactivation of the Ross ice streams, West Antarctica

“… Jezek [1984] mapped the spatial pattern of basal reflectance downstream of Crary Ice Rise and estimated grounding line retreat past the ice rise within the last 1000 years on the basis of the pattern of reflection strength within the ice shelf. Catania et al [2006] imaged downwarped internal stratigraphy in the low‐relief ice terrain on the downstream end of the ridge between KIS and WIS and interpreted them as evidence of a grounding line well inland of its present location within the last few hundred years. Together, these observations suggest 100s of km of grounding line retreat and readvance on century timescales.…”

“…Those authors suggested that the low‐relief ice plain could allow grounding line retreat rates up to 20 times faster than the observed rate, a range that embraces the rates generated in our simulations. It is possible that the upstream incursion of the KIS grounding line suggested by Catania et al [2006] is a result of WIS stagnation. Our models do not, at present, extend far enough upstream to verify this suggestion.…”

“…Consider the sequence of events leading to the recent stagnation of the KIS outlet. Five hundred years ago, a few decades prior to the reactivation of WIS, the KIS grounding line is suggested to have been about 100 km upstream of its present location [ Catania et al , 2006]. One hundred and fifty years later, what must have been a grounded northern KIS margin jumped inward, forming the Duckfoot and marking the start of ice stream stagnation [ Jacobel et al , 2000; Retzlaff and Bentley , 1993].…”

“…Seismically imaged sedimentary wedges on the sea floor seaward of the RIS front have been interpreted as grounding line features deposited at sites where a retreating grounding line paused and held a steady position before resuming retreat [ Mosola and Anderson , 2003]. Deformed ice–internal stratigraphy upstream of the present‐day Kamb Ice Stream (KIS, formerly named C) grounding line suggests that the grounding line of this currently quiescent outlet was upstream of its present location within the last few hundred years [ Catania et al , 2006]. The implication of these observations is that grounding line retreat since the LGM has been episodic, with transient stable stands and local readvance events.…”

Low post-glacial rebound rates in the Weddell Sea due to Late Holocene ice-sheet readvance

“…These include time-varying perturbations in local sea level (Gomez et al, 2012), accumulation, ice viscosity (Schoof, 2007), dynamism of bed friction (Sergienko and Hindmarsh, 2013) and changes in the stabilizing effect of the surrounding ice shelves (Gudmundsson, 2013;Wright et al, 2014) through basal melting induced by ocean temperature changes (Pollard and DeConto, 2009;Hellmer et al, 2012). Whether the readvance proposed in our revised model is due to external forcing (e.g., less warm water penetrating under the FRIS) or internal dynamics (e.g., GIA uplift leading to bed shallowing and grounding line readvance) is difficult to resolve by ice-sheet modelling owing to the sensitivity of grounding-line motion to melt, but either process could have operated here (Wright et al, 2014) or in other areas of the WAIS such as the Amundsen Sea embayment and the Ross Sea (Bindschadler et al, 1990;Catania et al, 2006). The possibility that some of these grounding lines might currently be advancing has implications for forecasting their response to warming associated with global change, as the initiation of unstable retreat would require changes in controls such as sub-ice shelf melt (Wright et al, 2014).…”