Sedimentation associated with Antarctic Peninsula ice shelves: Implications for palaeoenvironmental reconstructions of glacimarine sediments

Journal of the Geological Society, May 2002 by Evans, J, Pudsey, C J

Sediment gravity flows contributes to deposition in the deep Larsen Inlet trough and on the shelf immediately beyond the trough mouth. Facies include massive diamicton, diamicton with normal grading of the coarse-tail (i.e. grades from clastrich diamicton to muddy diamicton) and massive diamicton with rare aligned clasts and mud-clasts, all possessing relatively abrupt contacts (Fig. 3). These characteristics indicate deposition from cohesive debris flows (diamicton facies) (Walker 1992). Mass flows on the shelf (VC270; Fig. 3) are inferred to derive from remobilization of unstable grounding line sediment, with interbedded massive diamicton and dropstone mud facies representing periods of ice-shelf rafting.

Debris flows in Larsen Inlet originate from subglacial and sub-ice-shelf sediment deposited on the flanks of the trough or directly from a grounding line stabilized at the trough margin.

Ice distal glacimarine/open marine facies

Massive, bioturbated terrigenous silty clay and diatomaceous mud dominate sedimentation in the distal ice-shelf environment (Figs 2 & 3). This setting is established as the grounding line recedes towards the coast with progressive deglaciation.

Transition from terrigenous bioturbated muds to diatombearing bioturbated muds immediately overlying the iceproximal facies (Figs 2 & 3) may not actually signify the disappearance of the ice shelf, as diatoms can be advected beneath ice shelves from areas of seasonally open water by surface currents. Rather, the transition to diatom-bearing muddy facies indicates establishment of open-marine conditions close to the ice-shelf edge. Pudsey & Evans (2001) date the onset of seasonally open water on the continental shelf near to site VC267 at 11 14C ka BP. However, whether the onset of deposition of the diatom-bearing mud is synchronous across the region is uncertain in the absence of reliable radiocarbon ages SW of Prince Gustav Channel.

The presence of limited quantities of ice-rafted debris within these muddy facies forming thin units of dropstone mud, diatom-bearing gravelly mud, and diamicton (e.g. VC249; Figs 2 & 3), indicates passive rain-out of basal debris from the ice shelf. Basal freezing is observed under Antarctic ice shelves (e.g. Zotikov et al. 1980; Jenkins & Doake 1991) and this mechanism could prevent the release of debris allowing it to be transported far beyond the grounding-line (Drewry & Cooper 1981; Powell 1984; Pedley et al 1988). Alternatively, the coarse debris may indicate iceberg rafting in an open-marine setting following ice-shelf retreat. Evidence from the nearby Prince Gustav Channel (Fig. 1) indicates that the ice shelf there disappeared during the mid-Holocene climactic optimum (5-2 ka) (Pudsey & Evans 2001). Gravelly surface facies in VC257, VC249, VC247 and VC267 more than likely result from iceberg rafting associated with the recent break-up of the ice shelf in Larsen-A (Figs 2 & 3).

Discussion and wider significance

This study shows that diamicton and gravel facies dominate the proximal glacimarine environments of Antarctic Peninsula ice shelves, with deposition of mainly fine-grained facies in the more distal ice-shelf and open-marine settings. Meltwater discharge and deposition of subaqueous outwash have virtually no role in sedimentation. The sediment facies are similar to the proximal, ice-shelf rafted diamicton and dropstone mud described from the small floating outlet glacier in the Ross Sea (Powell et al. 1996). There is also an absence of meltwaterderived facies. However, the sediment facies of this study are more heterogeneous than those described previously from Late Pleistocene-Holocene Antarctic ice shelves (e.g. Domack & Harris 1998; Domack et al. 1998, 1999). In the Ross Sea and Prydz Bay, ice-proximal sedimentation includes deposition of a granulated facies inferred to have been deposited by sub-iceshelf rain-out of pelletized till matrix and terrigenous coarsegrained debris. Bottom currents contribute to sedimentation, but sediment gravity flow deposits are rare. Coarse-grained, ice-proximal granulated facies are overlain by fine-grained, distal ice-shelf glacimarine muds and diatom-rich muds with a current-transported sand and silt component. These differences in facies and depositional style between the Antarctic Peninsula and the Ross Sea and Prydz Bay reflect regional variations in physiographic, bathymetric and oceanographic factors rather than glaciological regime, which is consistently polar.