Rare Antarctic fossils reveal extinction of tundra before full polar climate arrived

(Boston) An international research team in Antarctica led by David Marchant, an associate professor of earth sciences at Boston University, has reported the discovery of exceptionally well-preserved freshwater fossils including mosses, microscopic one-celled algae, known as diatoms, small fresh water crustaceans, and insects that represent the last traces of tundra in the southernmost region of the continent before a dramatic and enduring cooling occurred some 14 million years ago. These rare fossilized terrestrial organisms, from the McMurdo Dry Valleys sector of the Transantarctic Mountains, were uncovered in sediments from a former ice-free lake, which served to pinpoint the inception of modern polar-desert conditions in Antarctica. The fossil discoveries are also the first to be found in the area even though other scientific expeditions have been visiting this southern region since the first expedition more than 100 years ago.

"The fossils and surrounding sediments are extremely well preserved," said Marchant. "They tell us that the landscape has changed very little in 14.1 million years, and that at the time the fossils lived the climate in this sector of Antarctica was similar to that of southern South America today. The organisms died out suddenly by 13.9 million years ago, and since that time interior Antarctica has been in a perpetual deep freeze, with most of the interior ice remaining relatively stable and frozen. The exact cause of this dramatic climatic shift, one of the most significant over the last 65 million years, remains unknown"

Marchant's comments follow the August 4th publication of "Mid-Miocene Cooling and the Extinction of Tundra in Continental Antarctica" in the Proceedings of the National Academy of Science.

Marchant, along with Adam R. Lewis, a former Boston University graduate student (Ph.D,2005) who discovered the fossils while working under Marchant, are the first of 13 authors on the paper. These research collaborators are from other universities in the U.S., Canada, United Kingdom, Sweden, Germany and New Zealand*. The research was funded by the National Science Foundation.**

Like dried museum specimens, these freeze-dried fossils can be rehydrated. They are among the best preserved specimens from this age found anywhere on Earth, noted Marchant. Some species are identical to modern counterparts, and the dominant moss species is indistinguishable from an existing bryophyte (Drepanocladus longifolius). This type of morphologic stability is nearly unheard of in the fossil record.

Between the stems and leaves of the mosses, the researchers found exceptionally well preserved ostracods, tiny crustaceans whose soft parts are not typically found in the fossil record. "In addition to fossils of organisms that inhabited the lake, we have recovered pollen and spores, and a few macroscopic remains of plants and insects that inhabited the lake regions," the paper states.

"Everything about the fossil site, from its geology to the fossils themselves, tells us the climate shift was abrupt, major, and enduring. Its legacy continues to this day," said Marchant, noting that researchers combined evidence from computer modeling, glacial geology, dating of volcanic ashes, and paleoecology to determine the major climate shift centered ~14 million years ago.

The authors maintain that the dramatic and long-lasting climate changes – summer temperatures in the McMurdo Dry Valleys as much as 17 degrees warmer than the present-day average -- are associated with the extinction of tundra and insects, such as beetles and midges.

The paper's conclusion suggests that even when global atmospheric temperatures were warmer than they are now – as they were around 3.5 million years ago, and as they may be in the future due to global warming -- the fossil site remained cold and dry. One of the take-home messages is that climate conditions over a considerable fraction of Antarctica seem impervious to moderate global warming. This finding highlights the potentially complex and non-uniform response of Antarctic ice sheets to future global change.

Source: Boston University