Gifford Miller Prof. Gifford Miller INSTAAR and Department of Geological Sciences University of Colorado at Boulder Boulder USA If you would like to interview this scientist, please contact Leah Christen. > biographical information > abstract > paper

Biographical Information:

I use the record of the recent geological past, primarily the Late Quaternary, to gain a better understanding of Earth’s climate system. I am based jointly at the Institute of Arctic and Alpine Research (INSTAAR) and the Department of Geological Sciences at the University of Colorado. Although my early research was dominantly in the Polar Regions, I became interested in hot deserts and monsoon systems in the late 1980s while working in North Africa, and have focused on the Australian Summer Monsoon and the footprints of human colonization in Australia for the past 15 years. I continue to maintain an active research program in the Eastern Canadian Arctic and in Iceland, and Co-Chair the PAGES Arctic Program CAPE (Circum-Arctic PaleoEnviornments).

Abstract:

Detecting human impacts on the flora, fauna, and summer monsoon of Pleistocene Australia.

All of Australia’s large mammalian vertebrates became extinct about 50 ka, shortly after human colonization 55 to 50 ka. Between 60 and 40 ka climate was similar to present and not changing rapidly. Consequently, attention has turned toward plausible human mechanisms for the extinction, with proponents for over-hunting, ecosystem change, and introduced disease. To differentiate between these options we rely on isotopic tracers of diet preserved in avian eggshells to track changes in ecosystems before and after human colonization. d13C preserved in eggshell calcite and in intracrystalline organic residues, monitor a bird’s dietary intake in the weeks to months before egg laying. More than 500 dated eggshells from central Australia of the extant, large, flightless Australian emu (Dromaius novaehollandiae), an opportunistic dominantly herbivorous feeder, provide a continuous, 140 ka dietary d13C reconstruction; and additional >400 dated eggshells of the extinct, heavier, Genyornis newtoni, define its dietary intake from 140 ka until its extinction about 50 ka. Less continuous dietary records for both species were developed from two distant regions (>250 additional dated eggshells <130 ka old). Dromaius eggshell dietary d13C reveals an unprecedented reduction in food sources about 50 ka in both carbon reservoirs from all three regions, suggesting conversion from a tree/shrub savannah with occasionally rich grasslands to the modern desert scrub. Genyornis diet everywhere is more restricted than in co-existing Dromaius, implying a more specialized feeding strategy. The dietary shift observed in Dromaius eggshells ca. 50 ka is diagnostic of ecosystem collapse throughout the semi-arid zone shortly after humans colonized Australia, possibly a consequence of systematic burning by early humans. Those animals with feeding strategies that allowed them to adapt to a changed vegetation regime, such as Dromaius, survived; those with restricted dietary sources, such as Genyornis, became extinct. The change in vegetation would have also reduced biosphere-atmosphere interactions that promote penetration of monsoon moisture into the interior. Climate modeling suggests such a shift may reduce monsoon rain in the interior by as much as 50%.

Paper:

Detecting human impacts on the flora, fauna, and summer monsoon of Pleistocene Australia

Gifford H. Miller
Marilyn L. Fogel, Carnegie Institution of Washington, USA
John W. Magee, Australian National University, Australia
Michael K. Gagan, Australian National University, Australia
Beverly J Johnson, Bates College, USA
Simon J. Clarke, Wollongong University, Australia

What was the impact of early human populations when they arrived on new lands? Can we detect their footprints from long ago? For many decades, researchers have postulated that biomass burning by early modern humans modified landscapes across Africa, the Americas and Australia but secure evidence for such impacts has been lacking. And humans have been postulated as the responsible agents for the extinction of most large animals in the Americas and Australia but this action remains hotly debated. We know that the arrival of modern humans in the Americas ~12,000 years ago coincided with the disappearance of most large mammals but temporal coincidence alone cannot be used to demonstrate cause and effect. Climate was also changing rapidly then, as the last ice age came to an end, and abrupt climate change remains a viable alternative scenario.

Australia offers a more tractable setting. Human colonization of Australia occurred much earlier than in the Americas. Occupation was widespread by 45,000 years ago, well before the climatic upheavals at the end of the last glacial cycle. But environmental changes following human arrival in Australia have been difficult to resolve because few continuous environmental records extend back far enough to securely compare conditions before and after human colonization. We provide such a record based on dietary reconstructions of the continent's two largest bird species. The results indicate that human arrival resulted in a profound environmental shift.

We use the proportion of heavy and light carbon isotopes in individually dated eggshells of the Australian emu and the extinct giant flightless bird Genyornis, to reconstruct the proportion of plants with C4 photosynthetic pathway (mainly warm, summer-rainfall grasses) and those using the C3 pathway (most shrubs, trees and winter-watered grasses) in their diets. For 100,000 years before humans arrived, emus ate a wide range of food sources, from purely C3 to purely C4 plants, and all intermediate feeding strategies, confirming its generalist feeding strategy. In contrast, Genyornis living in the same areas always ate a much more restricted diet, indicating a more specialized eater. Their diets show little variability over time, despite known major climate shifts. Then, suddenly, between 50,000 and 45,000 years ago, Genyornis eggshells disappear (extinction) and emu diets exhibit an unprecedented retraction in food sources. Teeth of the marsupial wombat, a grazer, exhibit the same reduction in food sources at the same time as shown by emu eggshells.

Our study of ancient eggshells and teeth supports the controversial theory that early humans caused the extinction of many of Australia's huge animals. We speculate that human firing of landscapes rapidly converted a drought-adapted mosaic of trees, shrubs and nutritious grasslands to the modern fire-adapted desert scrub. Those animals with wide dietary tolerances could adjust to the changed vegetation regime, whereas the picky eaters like Genyornis, with more specialized dietary requirements, were more vulnerable to extinction. The change in vegetation would have also reduced biosphere-atmosphere interactions that promoted penetration of monsoon moisture into the interior. Climate modeling suggests such a shift may have reduced monsoon rain in the interior by as much as 50%.

Figure 1. Nearly intact eggshell of the extinct giant bird Genyornis newtoni, discovered near Port Augusta, South Australia in 2002. Note puncture hole from predator in upper left. The egg has been dated to 60,000 years, about 10,000 years before megafaunal extinction. Photo by G.H. Miller

Figure 2. Wind-deflated surface at Williams Point, southern Lake Eyre, central Australia. The surface shows polygonal gypsum infilling of an old soil, developed on a thick (8 m) deposit of wind-blown sand containing abundant eggshell fragments of the extinct giant bird Genyornis newtoni, as well as of the Australian emu. The surface is dated to about 50,000 years before present. Photo by G.H. Miller

Figure 3. Plant ecosystems in monsoonal Australia burn frequently through natural ignition by lightning strikes during the build up to the wet season, when dry lightning storms are common, but burning is also promoted by human ignition, from both traditional burning practices of Aboriginal Australians and governmental land managers. In Kakadu, a World Heritage Area east of Darwin, ecosystems have adapted to millennia of Aboriginal burning. Consequently, land managers strive to maintain the "natural" vegetation through prescribed burns in consultation with Aboriginal advisors. Land managers deliberately set this fire, 100 km south of Darwin in Sept. 1998. (Photo by G.H. Miller)

Figure 4. Smoldering landscape following a controlled burn near Darwin, northern Australia, in Sept. 1998. (Photo by G.H. Miller)