Borehole imagery of meteoric and marine ice layers in the Amery Ice Shelf, East Antarctica

Abstract

A real-time video camera probe was deployed in a hot-water drilled borehole through the Amery Ice Shelf, East Antarctica, where a total ice thickness of 480 m included at least 200 m of basal marine ice. Down-looking and side-looking digital video footage showed a striking transition from white bubbly meteoric ice above to dark marine ice below, but the transition was neither microscopically sharp nor flat, indicating the uneven nature (at centimetre scale) of the ice-shelf base upstream where the marine ice first started to accrete. Marine ice features were imaged including platelet structures, cell inclusions, entrained particles, and the interface with sea water at the base. The cells are assumed to be entrained sea water, and were present throughout the lower 100-150 m of the marine ice column, becoming larger and more prevalent as the lower surface was approached until, near the base, they became channels large enough that the camera field of view could not contain them. Platelets in the marine ice at depth appeared to be as large as 1-2 cm in diameter. Particles were visible in the borehole meltwater; probably marine and mineral particles liberated by the drill, but their distribution varied with depth.

Evolution of a supercooled Ice Shelf Water plume with an actively growing subice platelet matrix

“…Platelet ice, a tangible example of this connection, is formed from sea water in the subice shelf environment, and thus blurs the distinction between the regimes. Observations beneath the Amery Ice Shelf [Craven et al, 2005[Craven et al, , 2009 show that marine ice layers are likely to have similar fine-scale structure to the subice platelet matrix observed in considerable detail beneath the sea ice of McMurdo Sound. That being the case, the boundary layer interactions beneath them would be similarly complex.…”

Roles of marine ice, rheology, and fracture in the flow and stability of the Brunt/Stancomb‐Wills Ice Shelf

“…Over the longer term, the highly heterogeneous structure of the ice shelf, and the role revealed here that marine ice could play as part of the mélange in keeping its mechanical integrity and shaping its flow regime also exposes a possible vulnerability. Both the rate of formation of frazil crystals [ Jenkins and Bombosch , 1995; Tison et al , 2001; Khazendar and Jenkins , 2003], and the structural integrity of marine ice layers that result from their accretion, are highly sensitive to the properties of ocean water [ Craven et al , 2005]. In particular, ice shelf basal melting rates are very sensitive to small changes in temperature.…”

“…Several Antarctic ice shelves have been shown to be composed of marine ice, several tens of meters in thickness, at many locations [e.g., Morgan , 1972; Oerter et al , 1994; Fricker et al , 2001]. Distinct from meteoric or sea ice, marine ice forms when frazil ice crystals accumulate at the base of an ice shelf as part of the thermohaline circulation in the underlying cavity [ Jenkins and Bombosch , 1995] and subsequently consolidate into layers, the lower of which can be highly porous [ Engelhardt and Determann , 1987; Craven et al , 2005]. Fracture, itself a critical aspect of ice shelf stability, is often affected by marine ice, which can fill rifts and bottom crevasses [ Khazendar et al , 2001; Khazendar and Jenkins , 2003].…”

Turbulent heat transfer as a control of platelet ice growth in supercooled under-ice ocean boundary layers

“…1) under ice shelves is difficult to observe directly (Craven et al, 2005), but similar effects are readily observed beneath nearby sea ice where it is called platelet ice (e.g. Robinson et al, 2014;Hughes et al, 2014;Hoppmann et al, 2015;Langhorne et al, 2015).…”

“…In this way, fresh glacial ice near the grounding line can be transformed to marine ice (Langhorne, 2008). Evidence from icebergs (Kipfstuhl et al, 1992), borehole (Craven et al, 2005) and radar studies (Engelhardt and Determann, 1987;Robin et al, 1983;Holland et al, 2009) indicate that marine ice can reach appreciable thicknesses, and that the ice pump is active under shelves where the water entering the cavity is near freezing.…”