We know Antarctica as an unfathomably cold wasteland, suitable for little beyond penguins and foolhardy researchers. But it hasn’t always been like that. At times in the distant past, plate tectonics and warmer climates have combined to cover Antarctica with lush forests, dinosaurs, and even marsupials. A paper published in PNAS details the final transition from habitable continent to the inhospitable ice cap that has developed over the past 40 million years. Its results describe the ecosystems that survived in the last unglaciated corner of Antarctica before the ice sheet swept away the last remnants of terrestrial life.

To accomplish this, the team behind the paper set out to read the history recorded in the sediment around the Antarctic Peninsula (Antarctica’s "tail"), which would have been the last piece of the continent to be covered by an ice sheet. They used seismic imaging of the sedimentary layers offshore to identify locations with sediment from the desired age range.

Changes in sediment characteristics (like grain size, shape, and mineralogy) allowed the researchers to infer changes in the extent of glaciation on land. The core sites were far enough offshore that only the smallest grains of silt and clay were carried by the currents. This means that anything the size of a sand grain or larger had to be carried by icebergs, which dropped their payloads as they melted. The type of clay mineral present can also describe the weathering the grains experienced. On top of that climatic information, the researchers also analyzed pollen grains in the core, which painted a picture of the terrestrial ecosystems inhabiting the peninsula.

The records indicate there were four distinct periods during this transition. The first, between 37 million and 34 million years ago, shows the initiation of alpine glaciers high in the mountains and the presence of ecosystems similar to those currently present in Southern Patagonia in South America. Sea ice formed seasonally along the coast, and diverse forests of angiosperms and conifers grew on land. From 34 million to 23 million years ago, the angiosperms largely disappeared as the climate cooled, leaving the land dominated by conifers and tundra.

By 13 million years ago, the tundra had all but disappeared as the ice sheet began to encroach on the peninsula and climate continued to cool. The peninsula was completely overridden by the ice sheet around 4 million years ago, as the last refuges of green gave way to the white, frozen expanse. We have limited knowledge of the animals that populated Antarctica at that time, but we certainly know why they died out.

Apart from an interesting history of a hard-to-imagine land, these results may help us understand what caused the growth of the Antarctic Ice Sheet. Two mechanisms have been proposed. There was a sharp decrease in atmospheric carbon dioxide between 35 million and 30 million years ago: it dropped from between 1,000 and 2,000 parts-per-million towards concentrations below 500 ppm, where they have stayed to the present. The disconnection of Antarctica from South America, which opened the Drake Passage, also occurred during this period. The passage led to the development of the Circum-Antarctic Current, which isolates Antarctica, leaving it surrounded by cold air and water and preventing warm, equatorial water from transporting heat toward the pole.

The onset of glaciation in Antarctica does coincide with the large decrease in carbon dioxide, but the cooling continued long after the carbon dioxide concentration became relatively stable. This points to the gradual tectonic opening of the Drake Passage as the likely driver behind the large-scale expansion of the ice sheet that eventually engulfed the entire continent, erasing the terrestrial ecosystems which once painted Antarctica green.

PNAS, 2011. DOI: 10.1073/pnas.1014885108 (About DOIs).

Listing image by Photo by John Spooner