Utah Geochemist Helps Date Extinction in Australia

UNIVERSITY OF UTAH MEDIA RELEASE

Embargoed by the journal Science for release at noon MDT Thurs. June 7, 2001

Contacts: --Linda Ayliffe, geochemist -- office (801) 587-7865, laboratories (801) 581-4271 and (801) 585-6671. If no answer, send message to [email protected]--Lee Siegel, science news specialist -- (801) 581-8993, cellular (801) 244-5399, [email protected]

Utah Geochemist Helps Date Extinction in Australia;Disappearance of Big Animals 46,400 Years AgoTied to Early Humans, Not Drought

June 7, 2001 -- A team of Australian scientists -- including a University of Utah geochemist -- has made the best estimate yet for the date of a mass extinction that wiped out most of Australia's large reptiles, birds and mammals, including some early kangaroos.

"We found that the disappearance of the large animals in Australia seemed to occur in a very short time interval 46,000 years ago, which is 10,000 to 15,000 years after the arrival of humans," said Linda Ayliffe, a postdoctoral researcher at the university in Salt Lake City. "It suggests the extinction was related to the arrival of humans."

The study, published in the June 8 issue of the journal Science, dates the mass extinction of "megafauna" -- larger animals weighing more than 100 pounds (45 kilograms) -- to about 46,400 years ago, with a range of uncertainty from about 39,800 years ago to 51,200 years ago.

The timing of the die-off means it was not caused -- as some scientists have argued -- by an Ice Age drought when glaciers were at their peak or maximum about 19,000 to 23,000 years ago and conditions were colder and drier, the research team concluded. Ayliffe noted that large animals had survived earlier glacial periods.

The researchers -- led by Richard G. Roberts of the University of Melbourne -- said they could not determine if the mass extinction was caused by other climate changes, by a hunting "blitzkrieg" -- the German term for a fast, violent offensive -- in which humans arrived in Australia and quickly wiped out large animals, or by slower destruction of habitat by early humans.

Ayliffe doubts the early human population was large enough to hunt large animals to extinction quickly, meaning in hundreds of years. She believes early humans -- nomadic hunter-gatherers who were the ancestors of modern aborigines -- gradually destroyed animal habitat by setting fires to drive game into the open, and that the habitat loss ultimately led to mass extinction over thousands of years.

"There is evidence of increased frequency of fires at the time of extinction," she said. A small number of people "can set fire to a large area of land, which renders that area unusable for the animals."

The researchers said the extinction killed all Australian land mammals, reptiles and birds weighing more than 220 pounds (100 kilograms) and six of seven genera -- the category above species -- with animals weighing more than 100 pounds.

Ayliffe said extinct creatures included giant snakes and lizards, large ostrich-like flightless birds and a wide variety of marsupials -- animals that develop in their mother's pouches. Modern marsupials include kangaroos, koalas, wombats and possums. Marsupials that went extinct 46,000 years ago in Australia included elephant-sized creatures, so-called marsupial lions and marsupial rhinos and a number of kangaroo species, Ayliffe said.

A native of Port Pirie in the state of South Australia, Ayliffe earned her doctorate at Australian National University (ANU) in Canberra. Before taking the Utah job in July 2000, she was a postdoctoral researcher at the University of Manchester, England, and ANU, then worked as a research associate at the Laboratory of Sciences of the Climate and the Environment outside Paris.

It was there that Ayliffe measured the radioactive decay of uranium-234 into thorium-230 to determine dates of many samples in the study.

The researchers studied fossils of large animals and the rocks in which they were buried at 27 sites across Australia and one in West Papua (also known as Irian Jaya), which was connected to Australia when sea levels were low during glacial periods. The fossil sites mainly were dunes near lakes or the sea, river or swamp sediments, and sediments in caves. Many of the fossils were in museums, so researchers had to go back to their collection sites to get rock samples that could be dated.

Ayliffe said the date of the Australian extinction has been debated for years, largely because there are relatively few places where sediments and fossils from that time were preserved. A series of estimates based only on one to a few species or fossil sites had placed the extinction anywhere from 20,000 to 40,000 years ago.

"People were thinking the timing of this extinction was about 20,000 years ago, and therefore had something to do with the last glacial maximum," she said.

The extinction date estimated by the new study is much more reliable because "it is the first study that's looked at many different species across all environments -- desert, coastal fringe, semi-arid zones and tropical forest," Ayliffe said.

In addition, some previous estimates were based on dating carbon-14 in sediments. That dating method can go back only to about 40,000 years ago, and incorrect procedures fouled some of the earlier attempts to date the mass extinction.

Two methods were used in the new study to date sediments in which fossils were buried and thus estimate the time of the mass extinction.

The first method -- optical dating -- measures the faint luminescence emitted by quartz, feldspar and certain other minerals in sediments. The faint glow happens because the minerals are hit by radioactive particles in the ground, "exciting" electrons in the minerals. Sunlight wipes out this excitation. So the amount of glow reveals the length of time since the sediments were last exposed to sunlight, which was when they were deposited.

Ayliffe used the other dating method for her part of the study. Uranium-234, which can dissolve in water, decays radioactively to become thorium-230, which is insoluble. So water dripping in limestone caves contains uranium-230 but not thorium. When calcium carbonate in the water starts to crystallize in cave sediments and deposits such as stalagmites, thorium-230 can begin to accumulate as the uranium-234 decays. By measuring the ratio thorium-230 to uranium-234, Ayliffe determined the age of cave sediments and thus the time of death for animals buried in them.

She said many of the animals died by stumbling into sinkhole-like "pitfall traps" on the ground above the caves.

The one genus of large animal that survived the extinction included large kangaroos.

"They're pretty fast and they cover a lot of ground," Ayliffe said. "They could have gotten away from hunters and migrated to unburned habitat."

Since arriving at the University of Utah, Ayliffe has worked with geochemist Thure Cerling and other scientists on a study of how Earth's prehistoric climate and ecology were affected by changing levels of carbon dioxide in the atmosphere. The study may shed light on future changes due to global warming caused by industrial emissions of carbon dioxide.

In addition to Ayliffe and Roberts, the team of geochemists, paleontologists and archeologists who conducted the study included Timothy Flannery of the South Australian Museum, Adelaide; Hiroyuki Yoshida of University of Melbourne; Jon Olley of Australia's Commonwealth Scientific and Industry Research Organization (CSIRO), Canberra; Gavin Prideaux of the University of California, Riverside; Geoff Laslett, of CSIRO, Melbourne; Alexander Baynes of the Western Australian Museum, Perth; M.A. Smith of the National Museum of Australia, Canberra; Rhys Jones of ANU; and Barton L. Smith of La Trobe University, Melbourne.

University of Utah Public Relations201 S Presidents Circle, Room 308Salt Lake City, Utah 84112-9017(801) 581-6773 fax: 585-3350