|Giff Miller and Marilyn, Mulan, 2000|
The Australian Outback is known for its hot, dry conditions, but it hasn’t always been that way. Geologists look at sediments around lakes that are now dried out and can tell that in the past, these lakes were filled with water and surrounded by animals and humans. One of the goals of my study of Australian climate has been to figure out the wet-dry cycles that have occurred over the past 120,000 years. Also, I asked the questions: Did early humans have an influence on the amount of rainfall that fell on Australia? Are the current dry conditions in the Outback related to human influences?
When people first came to Australia about 55,000 years ago, the landscape was very different from today. The first Aboriginal settlers came from Indonesia, during a glacial period when sea levels were much lower than they are now. Primitive boats could easily cross the distance between Australia and the northern islands. Within only a few thousand years, people had settled along most of Australia’s vast coastlines as well as the interior that we know today as the Outback, a dry, hot, extreme environment with limited plant and animal life. When humans first arrived, the continent was home to many species of giant marsupials, monotremes (mammals that give birth from eggs like the platypus), birds and lizards. These large beasts are called megafauna. In Australia, the megafauna all went extinct a few thousand years after humans came to the continent.
|Genyornis (left) and emu (right)|
What happened to Australia’s megafauna? Scientists have been studying the causes of major animal extinctions in Australia and North America for many decades. Typically, paleontologists (scientists studying past animals and plants) look for fossils, records of pollen from ancient plants, and study the sediments to figure out the living history of an area. In Australia, there are rich beds of fossils that have been recovered in caves, around lakeshores, and in riverbank sediments. Because Australia has been hot and dry for so long, the sediments from the Outback don’t have pollen in them anymore, so scientists have little idea what kind of plants used to live here.
|Artists rendition of early inhabitants of Australia|
There are a few ideas about what happened to the megafauna. Some scientists believe that they went extinct as the climate changed naturally. They believe that after a cold glacial period, they never came back. Other scientists think that hunting of the megafauna slowly depleted their populations until they no longer could survive. The idea that my colleagues and I have been investigating is whether human use of fire was the main reason for wiping out the megafauna.
How can fire destroy almost all of a country’s large animals? What does fire do to the plants and the climate? Can human actions like burning the landscape, as an aid to hunting and food gathering, change the climate of an entire continent?
|Bush fire, Lake Gregory area, Marilyn in foreground|
Earth’s climate has changed drastically over time. In the past 2 million years, the climate has swung between ice ages and hot houses roughly correlated to subtle differences in how the Earth revolves around the sun. These differences turn out to be cyclical in nature, occurring on periods of 25, 40 or 100 thousand years called Milankovitch cycles. There was very little detailed understanding of the periodicity and magnitude of these cycles until scientists began measuring very small changes in isotope patterns found in carbonates in marine sediment cores. As more and more sediment cores were brought into isotope labs and measured, scientists began to compare and correlate isotope patterns across major ocean basins.
Oxygen isotope patterns in a sediment core are “wiggly lines” shifting as sea ice accumulates and global temperatures increase or decrease. Using radiogenic isotopes, like uranium, some of the cores were dated, then scientists matched the isotope variations in the wiggly lines allowing them to provide a global pattern of climate change over time. It became evident from this work that although climate changed all over the globe at the same time, there were some differences depending on the geographic location of the core. The work I conducted in Australia was concerned with continental scale climate change, which was influenced by global processes but had many more factors influencing it.
Because there are no laminated sediments in the Outback region of Australia, climate science was at a standstill. There needed to be another method for learning about paleo-temperatures, vegetation, and rainfall amounts as a function of geologic time. In the early 1990s, my colleague at the Geophysical Lab, Ed Hare, and Allison Brooks, an archaeologist at George Washington University, had a new idea based on finding beads made from ostrich eggshells in archeological excavations from early human sites in southern Africa. Ed, a geochemist, and Allison worked together and developed techniques for dating the fossil eggshells using amino acid racemization methods described previously.
As opposed to bones or teeth, eggshell holds onto its protein matrix and is not affected by geochemical changes even over hundred of thousands of years. It seemed to be a good bet that fossil eggshell would hold onto its isotope signals as well. Isotope biogeochemists in South Africa experimented with modern ostrich eggshells to see if carbon isotopes in ostrich eggshells hold a signal of the bird’s diet at the time it laid its egg. They did.
|Fossil emu and Genyornis eggshell pieces|
In 1991, Beverly Johnson, then a PhD student with Gifford Miller at the University of Colorado, came to the Geophysical Laboratory for several months to pilot a study on stable carbon and nitrogen isotopes in fossil and modern flightless bird eggshells (e.g., ostriches and emus). The work turned into her PhD dissertation and began a 28-year project that I have continued with Miller. Beverly managed to locate eggshells from farm-raised ostriches in the United States, along with their ostrich chow diet (Johnson et al. 1998). She also field collected ostrich eggshell from South Africa to calibrate the carbon isotope differences between diet, carbonate, and organic shell proteins. Her work on Equus Cave, South Africa, determined changes in vegetation, rainfall, and temperature over the last 17,000 years.
Giff, a former postdoc working with Ed Hare, is a geologist with a natural inclination towards chemistry. His lab at the University of Colorado was housed on the bottom floor of a post-war brick building with aging lab benches. The shelves of his lab were packed with carefully labeled boxes of samples he’d collected in Arctic, African, and Australian localities. A couple of homemade amino acid analyzers hummed away on the benches, churning out measurements of the ratios of D- to L- amino acids (i.e., amino acid racemization) that Giff uses to date most of his samples. He typically has two or three graduate students in his lab at any one time. They keep the instruments going, periodically replacing separation columns and detectors. The lone sink in this lab sits adjacent to a fancy espresso coffee machine that pumps out brew at all hours. A table in the middle of the lab serves as a meeting place for visitors like myself and is a daily gathering place for Giff’s lab group.
Being in Boulder, the Colorado team was usually casually dressed in Patagonia gear or specialized bicycle clothing after they’d taken a 20-mile ride up and down the Flatiron mountains adjacent to the campus. Giff is an especially stylish fellow both at home and in the field. In the 30 years we’ve worked together, he’s transitioned with grace from having almost an eternally youthful appearance to a distinguished salt-and-pepper bearded senior scientist.
As Hare and Brooks work was being completed, Giff was starting a new collaboration with geologist John Magee and others at the Australian National University in Canberra. Emu eggshell and eggshell fragments from the extinct fossil bird Genyornis were plentiful in sand dune deposits throughout much of the arid interior of the Australian continent. Genyornis was a two-meter high flightless bird endemic to Australia. Like the emu, Genyornis was very heavily built and, as a result, were not the fastest movers. The large bird had powerful legs, tiny wings, an enormous beak and hoof-like claws. Fossils of Genyornis bones and eggs have been found throughout the south, west, and east Australia.
|Field vehicles, 2006--We always had two|
It’s not easy to find fossil eggshells—small fragments 1 centimeter in size—spread out over the vastness of the Australian Outback. Giff and John Magee became experts in this “sport”. In the field, Miller is a wizard taking advantage of modern GIS technology to find promising deposits. Year after year, he and others combed sand dunes, head bent down, hands in pockets--slowly walking for hours to find small fragments of eggshells. Wind, over thousands of years, exposes the eggshells leaving them on the sandy surface. In a good day we might find 10 different places with 20 or more eggshell fragments in each. At the end of the day, we always uncorked a good bottle of red wine, cooked a meal of roasted Chook (that is chicken) or lamb over a campfire, and worked on the day’s field notes.
In 1994, Beverly, now a postdoc, Giff, and John Magee joined my family for my first trip to Australia. My son was 3 years old and just recovering from a nasty round of chickenpox. My daughter, 7 years old, was very interested in animals and plants. My husband served as driver, campsite manager, babysitter, and principal ornithologist. We traveled from Adelaide, in Southern Australia, all the way north to Kakadu National Park, almost in Darwin, NT, and then back again, a ~6,000 kilometers roundtrip. All of the fieldwork was in remote regions of the Outback where we pitched a tent and prepared meals over a campfire. I collected plants and soils along the transect, learning to identify new species in the different regions of the continent.
|Evan (foreground), Marilyn, Giff, Bev, Lake Eyre 1994|
That first year we learned a lot about traveling, camping, and doing fieldwork in Australia. When my family split off from the seasoned team led by Giff and John, the sense of adventure was high! Set loose on an entirely new continent with a vehicle that could go anywhere, we took off on the Oodnadatta Track towards Alice Springs. Our first night was spent in a small caravan park in our tent. We ate canned beans for dinner and called it a day. Around midnight, we were awakened by loud voices and shouting from the adjacent Aboriginal community—our first subtle lesson on how this group gets along in Australia.
We reached the outskirts of Alice Springs in a few days again setting up a camp with just our tent, no chairs, no tables, no pillows, no nothing. Looking around the caravan park, properly dressed older couples were having afternoon tea on a table laid with a flowered cloth, china teapot, and real teacups. They were seated on sturdy folding chairs in the shade of a gum tree, well out of the red dirt carpeting the caravan park. The next day we drove into Alice to the K-Mart in town and purchased a small folding table, two chairs, some pillows, and proper plates and cups. We stopped off at Woolworth’s supermarket and picked up fresh lamb chops, onions, and meat pies, along with a slab of beer and several bottles of decent red wine. Quickly, we learned that the men take the meat and onions to the Barbie, chat with each other as they cook, and the women remain in their camp with the children.
|Dana and Chris sampling a dead emu, 1994|
The 1994 trip was my first international field expedition. I did not have proper plant import permits from the United States Department of Agriculture, so when we returned to the United States, my plants were confiscated at the border. I learned my lesson. Through contacts at the Smithsonian, I obtained a permit and after a nervous month, all of my samples entered the United States and were delivered safely to my lab. I brought back 250 plant specimens and over the next year we analyzed their carbon and nitrogen isotopic compositions. Grasses in Australia shift from C4 metabolism in the North to mixtures of C4 and C3 grasses in the very south. Acacia species, both shrubs and trees, and eucalyptus trees are C3 plants. In general, the vegetation in Australia has mixtures of grasses, Acacia, and eucalypts with variable proportions of herbaceous C3 plants and chenopods. Our goal was to determine the extent of C4 grasses in an Australian animal’s diet.
Because we were ultimately studying the diets of emu and Genyornis, my field collections centered around two things: the general vegetative landscape and potential emu diet. Emus consume the seeds and flowers of all plants except eucalypts; they sometimes eat whole leaves and consume fruits when available. Without question, emus will also eat any insect or lizard that they can catch, which augments their protein intake substantially during the nesting season, particularly in arid areas. With subsequent trips in 1998, 1999, 2000, 2001, 2006, and 2008, I amassed more than 1000 plant specimens from all over the continent where annual precipitation amounts ranged from less than 100 mm per year to well over 4,000 mm per year. Precipitation affects the ability of plants to fix carbon dioxide during photosynthesis and regulates the cycling of nitrogen containing nutrients in soil.
Fortunately by this time we were using a new elemental analyzer combustion system to measure the carbon and nitrogen isotope values of eggshell organic matter. With a one square centimeter fragment of fossil eggshell, we could obtain a radiocarbon date, an amino acid racemization date, the carbon isotope composition of both organic and inorganic fractions, and the nitrogen isotope composition of the organic fraction. From these measurements we learned the age of the sample, the diet of the bird, the proportion of C3 and C4 plants on the landscape, and the paleo-precipitation and transpiration climate parameters (Johnson et al., 1999). With ever-increasing geographical coverage of Australia’s arid interior based on many months in the field, we assembled ~150,000 year climate records from several places, enabling us to date and determine major ecosystem shifts.
Over several decades Giff and John Magee, along with folks like myself, collected thousands of emu and Genyornis eggshells. Of those collected, I’ve analyzed the stable isotope patterns in about two thousand of them. Together, we’ve worked out how climate changed in Australia over the past 150,000 years. Our work in Australia intersected with one of the major controversies in paleontology today: whether the extinctions of megafauna occurred because of human interactions or because of climate change. In Australia, the timing of the extinction event as well as the arrival of humans was unknown when we began our work. Because each eggshell sample was dated by amino acid racemization, we could determine that Genyornis went extinct about 45,000 years ago throughout Australia. An early criticism of our extinction dates was that our samples were collected only in the Lake Eyre basin. Subsequent fieldwork over decades across western Australia and throughout the Outback proved that we were recording a continent-wide extinction event.
Carbon isotopes in the eggshells opened a new window and revealed much about how the Australia Outback ecosystem changed over time. Prior to the arrival of humans, the carbon isotopes of emu eggshell reflected the full range in potential diet from 100% C3 to 100% C4 vegetation. Genyornis’ diet was less varied than that of co-existing emu and always included a significant component of C4 vegetation (presumably grasses). During the time period that the Genyornis lived, these regions of Australia were probably more temperate grasslands. Earlier estimates by paleontologists had presumed that Genyornis was a browser, eating only leaves from trees. We found this not to be the case. The carbon isotope data throughout the continent portrayed a rich mosaic of vegetation composed of both C3 and C4 plants prior to the arrival of humans and the megafauna extinction.
After Genyornis disappeared from the fossil record, the carbon isotopes of emu eggshell shifted dramatically to more C3 vegetation indicating that there was considerably less plant diversity in their habitat. We termed this an “ecosystem collapse,” where diminished grasslands could no longer sustain megafauna like Genyornis and Diprotodon that relied on grass. Diprotodon, the first fossil mammal described from Australia (Owen, 1838) was a large wombat-like marsupial that was widespread across the continent when humans arrived. The widespread ecosystem collapse--meaning vegetation change--requires large-scale phenomena to drive the continental shift in vegetation composition.
Because of their speedy extinction around 45,000 years ago, many believe persistent hunting and egg raiding by early aboriginal settlers led to the large bird’s disappearance. Others maintain that climate change and other natural factors also played a role. Scientists from our team found that Genyornis’ in south and eastern parts of Australia were probably subjected to extreme climate change due to early aboriginal burning. Based on our data, we found that Genyornis always needed fresh grass in its diet. When grasslands are burned, the edible tasty species of grass are damaged and no longer grow. Hard, spiny grasses, called Spinifex, are less tasty and may not have been able to support the large-bodied Genyornis.
We proposed that human use of fire might well have caused the collapse. Vegetation in Australia today is principally fire-adapted, as it is in Africa and arid regions of North America. Fire-adapted plants regrow rapidly after fire, and some even require fire for seed germination. Early humans practiced landscape burning to clear the ground to enable easier movement, for hunting, and for communicating between distant groups of people. Many of Australia’s plants, such as Eucalyptus and Acacia species, can tolerate being burnt once every 25-30 years. They regrow from the burnt stumps and within a few years are fully re-established (Latz, 1995). The grasses also are fire-adapted, but if fires occur more frequently, for example every 5-10 years, more palatable grasses disappear and are replaced by the spinifex grasses.
Spinifex grasses are much less palatable for native animal species, such as emus and kangaroos, even though these plants are widespread. In addition to changing the type of vegetation, frequent burning can result in lower soil organic matter contents, which in turn decreases the soil’s ability to hold moisture. Soil moisture affects how far the Asian monsoon can penetrate into the Australian outback. Lower soil moisture results in lower rainfall in the continental interior. A decrease in annual rainfall affects the type of vegetation, including both fire-adapted and drought-adapted plant species. It can be a vicious feedback loop.
Eggshells from emus and Genyornis have given us some independent answers from what geologists have learned from lake sediments. Emus are known omnivores—they eat mostly plants but when they are laying their eggs, they need extra protein, which they get from insects and lizards. Watch an emu foraging along the roadside and you’ll see them pecking at bushes and darting their long necks at passing grasshoppers. The protein in their diet contains the element nitrogen. It turns out that nitrogen and its isotopic forms (14N and 15N) in plants, insects, and lizards are related to the amount of rainfall over the last growing season. Our team has collected plants and insects from all over the Australian Outback from 1994 to 2010. We learned that plants, especially growing in the drier places like north of Port Augusta, have more 15N in them than plants growing in wetter areas, like near the Top End of Northern Australia.
|Marilyn and emu, West Australia, 2010|
As our team worked around the country over the years, we collected modern emu eggshells, often from the same places where we collected the plants. It turns out that the nitrogen isotopes in the eggshells showed nearly the same relationship to rainfall, as did the plants, but with a small difference. Basically, you are what you eat! Because we know how rainfall affects plants and modern eggshells, we used this information to figure out wet and dry periods from nitrogen isotopes in the fossil eggshells. We found that we could identify certain places that were wetter on the continent than others. We also saw that the Outback became drier over time, although there were swings in Wet-Dry throughout our study period. Today, the Outback is significantly drier than it was in the years prior to the Megafauna Extinction event, 45,000 years ago. Changes from European settlements and ranching probably have served to create an even drier Outback than there might have been otherwise.
After several years collecting eggshells in west Australia, Giff led a more diverse team to study Genyornis eggshells that were found in clusters, suggesting they’d been burnt in a campfire—by humans. We used our chemical methods to discern if the eggshells had been burned and found that a preponderance of them in this area were indeed subjected to fire. The archeologists on the team confirmed that these were campfires made by early Aboriginal settlers. Our conclusion was that the eggs, not the large Genyornis adults, were an easy target for humans and probably contributed significantly in this area of Australia to the gigantic bird’s extinction.
|Burnt eggshells--human "hunting"|
After nearly 30 years of study, our team of Giff Miller, John Magee, and I have come to the conclusion that both climate change and human activities had direct influence on the extinction of the megafauna. In most complex problems such as ecosystem changes over thousands of years, multiple factors contribute to extinctions and environmental changes. The stable isotope analyses provided very plausible data to support these conclusions.
|Western Australia site of human campfires|