Climate of the Past Discussions www.clim-past-discuss.net 1814-9340 1814-9359 2 4 2006 10.5194/cpd-2-535-2006 http://www.clim-past-discuss.net/2/535/2006/ http://www.clim-past-discuss.net/2/535/2006/cpd-2-535-2006.html http://www.clim-past-discuss.net/2/535/2006/cpd-2-535-2006.pdf 535 562 2006-08-18 Detecting human impacts on the flora, fauna, and summer monsoon of Pleistocene Australia G. H. Miller J. W. Magee M. L. Fogel M. K. Gagan INSTAAR and Dept. of Geological Sciences, University of Colorado, Boulder, CO 80309-0450, USA Department of Earth and Marine Sciences, Australian National University, Canberra, ACT 0200, Australia Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, Washington, DC 20015, USA Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia All of Australia's largest mammalian vertebrates became extinct 50 to 45 ka (thousand years ago), shortly after human colonization. Between 60 and 40 ka Australian 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 utilize isotopic tracers of diet preserved in eggshells of two large, flightless birds to track the status of ecosystems before and after human colonization. &delta;¹³C preserved in their eggshells monitor a bird's dietary intake in the weeks to months before egg-laying. More than 500 dated eggshells from central Australia of the Australian emu (<I>Dromaius novaehollandiae</I>), an opportunistic, dominantly herbivorous feeder, provide a continuous 140 kyr dietary &delta;¹³C reconstruction. More than 350 dated eggshells from the same region of the heavier, extinct, giant bird <I>Genyornis newtoni</I> define its dietary intake from 140 ka until its extinction about 50 ka. Additional dietary records for both species were developed from two distant regions. Dromaius eggshell dietary &delta;¹³C reveals an unprecedented reduction in the bird's food resources about 50 ka, coeval in all three regions, suggesting conversion at that time of a tree/shrub savannah with occasionally rich grasslands to the modern desert scrub. We speculate that ecosystem collapse across the arid and semi-arid zones is a consequence of systematic burning by early humans. Genyornis diet everywhere is more restricted than in co-existing Dromaius, implying a more specialized feeding strategy. These data suggest that generalist feeders, such as Dromaius, were able to adapt to a changed vegetation regime, whereas more specialized feeders, such as Genyornis, became extinct. The altered vegetation may have also impacted Australian climate. Changes in the strength of climate feedbacks linked to vegetation and soil type (moisture recycling, surface roughness, albedo) may have weakened the penetration of monsoon moisture into the continental interior under the new ecosystem. Climate modeling suggests such a shift may have reduced monsoon rain in the interior by as much as 50%.