Alexandra E. Kirshner

Geological record of ice shelf break-up and grounding line retreat, Pine Island Bay, West Antarctica

The catastrophic break-ups of the floating Larsen A and B ice shelves (Antarctica) in 1995 and 2002 and associated acceleration of glaciers that flowed into these ice shelves were among the most dramatic glaciological events observed in historical time. This raises a question about the larger West Antarctic ice shelves. Do these shelves, with their much greater glacial discharge, have a history of collapse? Here we describe features from the seafloor in Pine Island Bay, West Antarctica, which we interpret as having been formed during a massive ice shelf break-up and associated grounding line retreat. This evidence exists in the form of seafloor landforms that we argue were produced daily as a consequence of tidally influenced motion of mega-icebergs maintained upright in an iceberg armada produced from the disintegrating ice shelf and retreating grounding line. The break-up occurred prior to ca. 12 ka and was likely a response to rapid sea-level rise or ocean warming at that time.

Progressive Cenozoic cooling and the demise of Antarctica’s last refugium

The Antarctic Peninsula is considered to be the last region of Antarctica to have been fully glaciated as a result of Cenozoic climatic cooling. As such, it was likely the last refugium for plants and animals that had inhabited the continent since it separated from the Gondwana supercontinent. Drill cores and seismic data acquired during two cruises (SHALDRIL I and II) in the northernmost Peninsula region yield a record that, when combined with existing data, indicates progressive cooling and associated changes in terrestrial vegetation over the course of the past 37 million years. Mountain glaciation began in the latest Eocene (approximately 37–34 Ma), contemporaneous with glaciation elsewhere on the continent and a reduction in atmospheric CO2 concentrations. This climate cooling was accompanied by a decrease in diversity of the angiosperm-dominated vegetation that inhabited the northern peninsula during the Eocene. A mosaic of southern beech and conifer-dominated woodlands and tundra continued to occupy the region during the Oligocene (approximately 34–23 Ma). By the middle Miocene (approximately 16–11.6 Ma), localized pockets of limited tundra still existed at least until 12.8 Ma. The transition from temperate, alpine glaciation to a dynamic, polythermal ice sheet took place during the middle Miocene. The northernmost Peninsula was overridden by an ice sheet in the early Pliocene (approximately 5.3–3.6 Ma). The long cooling history of the peninsula is consistent with the extended timescales of tectonic evolution of the Antarctic margin, involving the opening of ocean passageways and associated establishment of circumpolar circulation.

Constraints on Antarctic Ice Sheet configuration during and following the Last Glacial Maximum and its episodic contribution to sea-level rise

Abstract Marine geological studies provide a record of diachronous expansion and retreat of the Antarctic Peninsula Ice Sheet, West Antarctic Ice Sheet and East Antarctic Ice Sheet during the past c. 30 000 cal yr BP. Retreat of these ice sheets and Antarctica’s contribution to sea-level rise was largely complete by the early Holocene. Estimates of ice sheet thickness, based on maximum grounding depths, range from 640 to 1640 m on the inner continental shelf. Grounding depths on the outer continental shelf equate to minimum thicknesses of 410–950 m. Geomorphic features indicate that retreat from the continental shelf was mostly step-wise around the continent, a result of the different factors that control ice sheet behaviour and the degree to which these factors vary regionally. Thus, the nature of post-LGM (Last Glacial Maximum) sea-level rise was episodic and believed to have been punctuated by rapid pulses triggered by individual ice stream collapse. Most of these pulses would have been of sub-metre magnitudes and below the resolution of existing sea-level curves, but they would have had significant impact on coastal evolution, especially along low-gradient coasts.