Monday, November 30, 2009
Roughly 15,000 years ago, at the end of the last ice age, North America's vast assemblage of large animals — including such iconic creatures as mammoths, mastodons, camels, horses, ground sloths and giant beavers — began their precipitous slide to extinction.
And when their populations crashed, emptying a land whose diversity of large animals equaled or surpassed Africa's wildlife-rich Serengeti plains then or now, an entirely novel ecosystem emerged as broadleaved trees once kept in check by huge numbers of big herbivores claimed the landscape. Soon after, the accumulation of woody debris sparked a dramatic increase in the prevalence of wildfire, another key shaper of landscapes.
This new picture of the ecological upheaval of the North American landscape just after the retreat of the ice sheets is detailed in a study published in the journal Science. The study, led by researchers from the University of Wisconsin-Madison, uses fossil pollen, charcoal and dung fungus spores to paint a picture of a post-ice age terrain different from anything in the world today.
The work is important because it is "the clearest evidence to date that the extinction of a broad guild of animals had effects on other parts of these ancient ecosystems," says John W. Williams, a UW-Madison professor of geography and an expert on ancient climates and ecosystems who is the study's senior author. What's more, he says, the detailing of changes on the ice age landscape following the crash of keystone animal populations can provide critical insight into the broader effects of animals disappearing from modern landscapes.
The study was led by Jacquelyn Gill, a graduate student in Williams' lab. Other co-authors are Stephen T. Jackson of the University of Wyoming, Katherine B. Lininger of UW-Madison and Guy S. Robinson of Fordham University.
The new work, says Gill, informs but does not resolve the debate over what caused the extinction of 34 genera or groups of large animals, including icons of the ice age such as elephant like mastodons and ground sloths the size of sport utility vehicles. "Our data are not consistent with a rapid, 'blitzkrieg' overkill of large animals by humans," notes Gill, nor was their decline due to a loss of habitat.
However, the work does seem to rule out a recent hypothesis that a meteor or comet impact some 12.9 thousand years ago was responsible for the extinction of ice age North America's signature large animals.
The study was conducted using lake sediment cores obtained from Appleman Lake in Indiana, as well as data obtained previously by Robinson from sites in New York. Gill, Williams and their colleagues used pollen, charcoal and the spores of a dung fungus that requires passage through a mammalian intestinal tract to complete its life cycle to reconstruct a picture of sweeping change to the ice age environment. The decline of North America's signature ice age mammals was a gradual process, the Wisconsin researchers explain, taking about 1,000 years. The decline in the huge numbers of ice age animals is preserved in the fossil record when the fungal spores disappear from the record altogether: "About 13.8 thousand years ago, the number of spores drops dramatically. They're barely in the record anymore," Gill explains.
Like detectives reconstructing a crime scene, the group's use of dung fungus spores helps establish a precise sequence of events, showing that the crash of ice age megafauna began before plant communities started to change and before fires appeared widely on the landscape.
"The data suggest that the megafaunal decline and extinction began at the Appleman Lake site sometime between 14.8 thousand and 13.7 thousand years ago and preceded major shifts in plant community composition and the frequency of fire," notes Williams.
Absent the large herbivores that kept them in check, such tree species as black ash, elm and ironwood began to colonize a landscape dominated by coniferous trees such as spruce and larch. The resulting mix of boreal and temperate trees formed a plant community unlike any observed today.
"As soon as herbivores drop off the landscape, we see different plant communities," Gill explains, noting that mastodon herds and other large animals occupied a parkland like landscape, typified by large open spaces and patches of forest and swamp. "Our data suggest that these trees would have been abundant sooner if the herbivores hadn't been there to eat them."
While both the extinction of North America's ice age megafauna and the sweeping change to the landscape are well-documented phenomena, there was, until now, no detailed chronology of the events that remade the continent's biological communities beginning about 14.8 thousand years ago. Establishing that the disappearance of mammoths, giant beavers, ground sloths and other large animals preceded the massive change in plant communities, promises scientists critical new insight into the dynamics of extinction and its pervasive influence on a given landscape.
(Photo: Barry Roal Carlsen)
University of Wisconsin-Madison
Much of our planet's mineral wealth was deposited billions of years ago when Earth's chemical cycles were different from today's. Using geochemical clues from rocks nearly 3 billion years old, a group of scientists including Andrey Bekker and Doug Rumble from the Carnegie Institution have made the surprising discovery that the creation of economically important nickel ore deposits was linked to sulfur in the ancient oxygen-poor atmosphere.
These ancient ores -- specifically iron-nickel sulfide deposits -- yield 10% of the world's annual nickel production. They formed for the most part between two and three billion years ago when hot magmas erupted on the ocean floor. Yet scientists have puzzled over the origin of the rich deposits. The ore minerals require sulfur to form, but neither seawater nor the magmas hosting the ores were thought to be rich enough in sulfur for this to happen.
"These nickel deposits have sulfur in them arising from an atmospheric cycle in ancient times. The isotopic signal is of an anoxic atmosphere," says Rumble of Carnegie's Geophysical Laboratory, a co-author of the paper appearing in the November 20 issue of Science.
Rumble, with lead author Andrey Bekker (formerly Carnegie Fellow and now at the University of Manitoba), and four other colleagues used advanced geochemical techniques to analyze rock samples from major ore deposits in Australia and Canada. They found that to help produce the ancient deposits, sulfur atoms made a complicated journey from volcanic eruptions, to the atmosphere, to seawater, to hot springs on the ocean floor, and finally to molten, ore-producing magmas.
The key evidence came from a form of sulfur known as sulfur-33, an isotope in which atoms contain one more neutron than "normal" sulfur (sulfur-32). Both isotopes act the same in most chemical reactions, but reactions in the atmosphere in which sulfur dioxide gas molecules are split by ultraviolet light (UV) rays cause the isotopes to be sorted or "fractionated" into different reaction products, creating isotopic anomalies.
"If there is too much oxygen in the atmosphere then not enough UV gets through and these reactions can't happen," says Rumble. "So if you find these sulfur isotope anomalies in rocks of a certain age, you have information about the oxygen level in the atmosphere."
By linking the rich nickel ores with the ancient atmosphere, the anomalies in the rock samples also answer the long-standing question regarding the source of the sulfur in the ore minerals. Knowing this will help geologists track down new ore deposits, says Rumble, because the presence of sulfur and other chemical factors determine whether or not a deposit will form.
"Ore deposits are a tiny fraction of a percent of the Earth's surface, yet economically they are incredibly important. Modern society cannot exist without specialized metals and alloys," he says. "But it's all a matter of local geological circumstance whether you have a bonanza -- or a bust."
Researchers William Jungers, Ph.D., and Karen Baab, Ph.D. studied the skeletal remains of a female (LB1), nicknamed "Little Lady of Flores" or "Flo" to confirm the evolutionary path of the hobbit species. The specimen was remarkably complete and included skull, jaw, arms, legs, hands, and feet that provided researchers with integrated information from an individual fossil.
The cranial capacity of LB1 was just over 400 cm, making it more similar to the brains of a chimpanzee or bipedal "ape-men" of East and South Africa. The skull and jawbone features are much more primitive looking than any normal modern human. Statistical analysis of skull shapes show modern humans cluster together in one group, microcephalic humans in another and the hobbit along with ancient hominins in a third.
Due to the relative completeness of fossil remains for LB1, the scientists were able to reconstruct a reliable body design that was unlike any modern human. The thigh bone and shin bone of LB1 are much shorter than modern humans including Central African pygmies, South African KhoeSan (formerly known as 'bushmen") and "negrito" pygmies from the Andaman Islands and the Philippines. Some researchers speculate this could represent an evolutionary reversal correlated with "island dwarfing." "It is difficult to believe an evolutionary change would lead to less economical movement," said Dr. Jungers. "It makes little sense that this species re-evolved shorter thighs and legs because long hind limbs improve bipedal walking. We suspect that these are primitive retentions instead."
Further analysis of the remains using a regression equation developed by Dr. Jungers indicates that LB1 was approximately 106 cm tall (3 feet, 6 inches)—far smaller than the modern pygmies whose adults grow to less than 150 cm (4 feet, 11 inches). A scatterplot depicts LB1 far outside the range of Southeast Asian and African pygmies in both absolute height and body mass indices. "Attempts to dismiss the hobbits as pathological people have failed repeatedly because the medical diagnoses of dwarfing syndromes and microcephaly bear no resemblance to the unique anatomy of Homo floresiensis," noted Dr. Baab.