Glaciology: Past ice-shelf collapse in West Antarctica

Abstract

Recent disintegration of the floating ice shelves that fringe the Antarctic Peninsula has provided dramatic evidence of the effects of climate change (Fig. 1). Glaciologists have long regarded such ice shelves as critical to the stability of the Antarctic ice sheet, because they act as a buffer between the grounded, often fast-flowing, parts of the ice sheet and the surrounding ocean, in effect providing a control on the discharge of grounded ice. In the Antarctic Peninsula, satellite-derived observations of glacier velocity have shown that glaciers there accelerated after ice-shelf collapse. Writing in Geology, Jakobsson et al. now present sea-floor images showing that ice-shelf break-up also occurred in Pine Island Bay in West Antarctica during the ice-sheet retreat that occurred before about 12,000 years ago. This finding is notable because it indicates that the larger West Antarctic ice shelves have also collapsed in the past, in this case in an area of the ice sheet that is currently responsible for almost 30% of ice drainage from West Antarctica and that has seen recent thinning and accelerated flow. It is also noteworthy because Jakobsson et al. propose that the icebergs produced during this collapse, and the associated sea-floor features, were strongly influenced by ocean tides. Although such features have been imaged previously in several areas of the Antarctic continental shelf, the new data show them in unprecedented detail. Using geophysical equipment mounted in the hull of an icebreaking research ship, Jakobsson and colleagues carried out a detailed survey of a 4,140-square-kilometre area of the sea floor of Pine Island Bay. Here, a bathy metric trough — Pine Island Trough — extends through the bay, and at the Last Glacial Maximum about 18,000 years ago, it contained a grounded, and probably fast-flowing, glacier. Jakobsson and colleagues’ data reveal surface features in the trough that were produced by icebergs following the break-up of an ice shelf in Pine Island Bay. These landforms comprise linear to curvilinear sets of furrows, oriented along the trough, that are eroded into a relatively flat area of the sea floor and are spaced 150–500 metres apart. Within these furrows, and oriented perpendicular to them, are beautifully preserved, very regular small ridges 1–2 m high, spaced 60–200 m apart. The ridges have a corrugated appearance on the seabed, and bends or kinks are repeated from one ridge to the next. Overall ridge spacing decreases progressively seawards. The authors interpret both the furrows and ridges as a product of the action of icebergs produced during a massive ice-shelf breakout event. Jakobsson et al. propose that the furrows were produced by icebergs grounded on the sea floor and dragged across it, probably owing to the action of wind and/or currents, resulting in the production of a series of longi tudinal scours. The small corrugated ridges are remarkable for the regularity of their spacing, and the authors argue that this implies that a periodic forcing was in operation during their formation. They propose that this forcing was a diurnal tidal cycle, and that the ridges originated from the lifting and settling of ‘mega-icebergs’ on the sea floor in rhythm with the tide. From the well-preserved nature of the ridges and their relatively uniform spacing and length, Jakobsson et al. infer that the mega-icebergs formed part of a much larger, coherent mass of ice that broke off at the grounding line (the transition from grounded ice to a floating ice shelf) and remained upright and largely intact following break-off. Each time the mass of icebergs settled on the sea floor, soft sediment was squeezed up behind, forming a ridge. A series of ridges was therefore produced in the wake of the drifting mass of icebergs before its eventual grounding and fragmentation in shallower water. What caused the disintegration of the Pine Island Bay ice shelf, and what effect did this have on the grounding line? From dating marine sediment cores, Jakobsson et al. propose that breakout of the Pine Island Bay ice shelf occurred before about 12,000 years ago and that ice-shelf break-up may have been driven by some combination of sea-level rise and ocean warming. They further infer that ice-shelf break-up resulted in the retreat of the grounding line in Pine Island Bay. Conceptually, this inference is reasonable by analogy with recent evidence from the Antarctic Peninsula. However, although there is indeed support for recession of grounded ice in Pine Island Bay, direct evidence for a causal connection between the ice-shelf disintegration documented by Jakobsson and colleagues and grounding-line recession in the bay remains inferential and requires further dating control to demonstrate the link conclusively. Admittedly, the acquisition of such dating control is a challenge in Antarctica, where carbonate material, which can be radiocarbon dated to constrain the period of deglaciation, is often sparse in marine sediments.

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