Discovery Suggests New Theory on How Antarctica's Ice Sheet First Formed

An international team of researchers have provided new geologic evidence that defies conventional explanations for how Antarctica's ice sheet first began to form.

Rock samples from the central Scotia Sea near Antarctica reveal the remnants of a now-submerged volcanic arc that, until 12 million years ago, could have blocked the formation of the Antarctic Circumpolar Current (ACC). The ACC is an ocean current flowing around the entire continent that scientists have long believed played a critical role in the initial glaciation of the continent 34 million years ago.

Based on this new discovery, the researchers argue that the onset of the ACC may not be related to the initial glaciation of Antarctica, but rather to the subsequent and well-documented descent of the planet into a much colder "icehouse" glacial state.

"If you had sailed into the Scotia Sea 25 million years ago, you would have seen a scattering of volcanoes rising above the water," said Ian Dalziel, a research professor at the University of Texas at Austin's Institute for Geophysics and professor in the Jackson School of Geosciences. "They would have looked similar to the modern volcanic arc to the east, the South Sandwich Islands."

Using multibeam sonar to map seafloor bathymetry, a process analogous to mapping the topography of the land surface, the team identified seafloor rises in the central Scotia Sea. They also dredged the seafloor at various points on the rises to discover volcanic specimens created from the weathering of volcanic rocks both distinct from normal ocean floor lavas and geochemically identical to those found at the active South Sandwich Islands volcanic arc to the east of the Scotia Sea that forms a barrier to the ACC today.

Using a technique known as argon isotopic dating, the researchers dated the rocks back to a period between about 28 million years ago to approximately 12 million years. Based on these results, the team hypothesizes that an ancient volcanic arc, referred to as the ancestral South Sandwich arc (ASSA), was active in the region during that time and probably much earlier. Furthermore, because the samples were taken from the current seafloor surface and volcanic material accumulates from the bottom up, the researchers infer that much older volcanic rock lies beneath.

Combined with models of how the seafloor sinks vertically with the passage of time, the team posits that the ASSA originally rose above sea level and would have blocked deep ocean currents such as the ACC.