The last refuge of Antarctica's forests

[n2doc]

By Scott Johnson | Published about 4 hours ago

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.

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http://arstechnica.com/science/news/2011/06/why-so-cold-the-demise-of-antarcticas-forests.ars