Sedimentary processes at the base of a West Antarctic ice stream; constraints from textural and compositional properties of subglacial debris

Abstract

Samples of sediments from beneath Ice Stream B (at camp UpB), West Antarctica, provide the first opportunity to study the relationship between sediment properties and physical conditions in a sub-ice-stream environment. Piston coring in holes bored by hot-water drilling yielded five 1-3 m long, undisturbed subglacial sediment cores. We analyzed granulometry, composition, and particle morphology in these cores. The UpB cores are composed of a clay-rich, unsorted diamicton containing rare marine diatoms. Sedimentary particles in these cores bear no evidence of the recent crushing or abrasion that is common in other subglacial sedimentary environments. The presence of reworked diatoms and their state of preservation, as well as the relative spatial homogeneity of this diamicton, suggest that the UpB cores sampled a several-meter-thick till layer and not in situ glacimarine sediments. The till does incorporate material recycled from the subjacent poorly indurated Tertiary glacimarine sediments of the Ross Sea sedimentary basin, which extends beneath this part of the West Antarctic Ice Sheet. We propose that the lack of significant comminution in the UpB till is ultimately due to its setting over these easily erodible, clayrich source sediments. The resulting fine-grained till matrix inhibits glacial comminution, because it facilitates buildup of high pore-water pressures and hinders interparticle stress concentrations. Our observations are consistent with the conjecture that subglacial deformation of weak, fine-grained tills does not produce significant comminution of till debris (Elson 1988). Based on our findings, we hypothesize that extensive layers of weak till may develop preferentially where ice overrides preexisting, poorly indurated, fine-grained sediments. Since such weak till layers create a permissive condition for ice streaming, subglacial geology may have an indirect but strong control over the location, extent, and basal mechanics of ice streams.

Subglacial Sediments as a Control on the Onset and Location of two Siple Coast Ice Streams, West Antarctica

“…A maximum Dt RMS = 43 ms observed at an offset of 11,500 m, which is approximately 0.85% of the total travel time, while most of the residuals are less than 10 ms. Studinger et al, 2001]. Several authors have also placed emphasis on the close tie between the onset of streaming ice flow and a step in subglacial topography [McIntyre, 1985;Joughin et al, 1999;Hulbe et al, 2000], and our results are in agreement with this interpretation, revealing an indirect influence of subglacial topography on the initiation of streaming ice flow along the upper reaches of ice streams C and D. Seismic evidence points to depressions in bedrock topography that have been filled by subglacial sediments, probably from marine deposition in a preglacial setting [Scherer et al, 1998;Tulaczyk et al, 1998]. Ice flow velocity increase in the flow direction of C1B occurs with no identifiable step in bed topography (Figure 5), and the enhanced velocity of ice stream D occurs over a subtle low in basal topography (Figure 6).…”

“…The black dashed box to the top of the map is the location of the aerogeophysical survey of Bell et al [1998], and the white dashed line is the seismic refraction study of Anandakrishnan et al [1998]. The study region of Studinger et al [2001] covers most of this map, and the aerogeophysical survey of Blankenship et al [2001] covers the catchments of ice streams B and C. Other seismic and borehole experiments performed along the Siple Coast are located closer to the grounding line along the ice streams Rooney et al, 1987aRooney et al, , 1987bRooney et al, , 1991Engelhardt et al, 1990;Munson and Bentley, 1992;Engelhardt and Kamb, 1998;Tulaczyk et al, 1998;Kamb, 2001].…”

“…The underlying questions are how strongly subglacial geology influences the transition from slow flow via internal deformation to streaming ice flow due to rapid basal motion, and how far the potential for ice streaming extends into the inland ice reservoir of West Antarctica. Previous work has shown that streaming ice in West Antarctica is underlain by sedimentary packages Rooney et al, 1991;Smith, 1997;Anandakrishnan et al, 1998;Bell et al, 1998;Doake et al, 2001;Studinger et al, 2001;Anandakrishnan, 2003;Vaughan et al, 2003], that a soft bed is present immediately beneath the ice to allow rapid basal motion [Engelhardt et al, 1990;Engelhardt and Kamb, 1998;Tulaczyk et al, 1998;Kamb, 2001], and that subglacial sediments appear to shape the inland extent of streaming ice flow [Anandakrishnan et al, 1998;Bell et al, 1998;Blankenship et al, 2001;Studinger et al, 2001]. However, much geological heterogeneity has been found across West Antarctica, indicating that the mechanisms for producing streaming ice flow may vary.…”

“…These fast flowing ice streams are conspicuous in satellite images [Stephenson and Bindschadler, 1990;Bindschadler, 1991, 1993;Hodge and Doppelhammer, 1996] and ice velocity maps [Joughin et al, 1999[Joughin et al, , 2002, with well-defined lateral margins, and extend several hundred kilometers into the ice sheet. Seismic imaging Rooney et al, 1991;Anandakrishnan et al, 1998] and borehole analysis [Engelhardt et al, 1990;Tulaczyk et al, 1998;Kamb, 2001] have shown that, in places, streaming ice is coincident with subglacial sediments. The upper meters of these sediments are soft till, with the potential for active deformation Engelhardt et al, 1990].…”

Basal till and subglacial conditions at the base of the Upper Odra ice lobe (southern Poland) during the Odranian (Saalian) Glaciation

“…For ex am ple, rapid move ment of Ice Stream B in West Antarctica was ini tially at trib uted to the pres ence of thick, un con sol i dated, strongly sat u rated de formed de pos its at the ice sheet base (Alley et al, 1986(Alley et al, , 1987, but sub se quent stud ies have shown that the basal move ment of the ice stream is con cen trated within only a very thin layer of sub strate (cf. Engelhardt and Kamb, 1998;Tulaczyk et al, 1998). Brown et al (1987) pointed out that the Puget Lobe of the Pleis to cene Cordilleran Ice Sheet moved mainly by slid ing and that only lo cal de for ma tion was caused by clasts plough ing the sub strate.…”