Climate of the Past
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2007
10.5194/cp-3-463-2007
http://www.clim-past.net/3/463/2007/
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http://www.clim-past.net/3/463/2007/cp-3-463-2007.pdf
463
473
2007-08-06
Detecting human impacts on the flora, fauna, and summer monsoon of Pleistocene Australia
G. H. Miller
gmiller@colorado.edu
J. W. Magee
M. L. Fogel
M. K. Gagan
INSTAAR and 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 Inst. of Washington, 5152 Broad Branch Road, NW, Washington DC, 20015-1305, USA
Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
The moisture balance across northern and central Australia is dominated by
changes in the strength of the Australian Summer Monsoon. Lake-level records
that record changes in monsoon strength on orbital timescales are most
consistent with a Northern Hemisphere insolation control on monsoon
strength, a result consistent with recent modeling studies. A weak Holocene
monsoon relative to monsoon strength 65–60 ka, despite stronger forcing,
suggests a changed monsoon regime after 60 ka. Shortly after 60 ka humans
colonized Australia and all of Australia's largest mammals became extinct.
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. More than 800 dated eggshells of the Australian emu
(<i>Dromaius novaehollandiae</i>), an opportunistic, dominantly
herbivorous feeder, provide a 140-kyr dietary reconstruction that reveals
unprecedented reduction in the bird's food resources about 50 ka, coeval in
three distant regions. These data suggest a tree/shrub savannah with
occasionally rich grasslands was converted abruptly to the modern desert
scrub. The diet of the heavier, extinct <i>Genyornis newtoni</i>, derived
from >550 dated eggshells, was more restricted than in co-existing
<i>Dromaius</i>, implying a more specialized feeding strategy. We suggest
that generalist feeders, such as <i>Dromaius</i>, were able to adapt to a
changed vegetation regime, whereas more specialized feeders, such as
<i>Genyornis</i>, became extinct. We speculate that ecosystem collapse
across arid and semi-arid zones was a consequence of systematic burning by
early humans. We also suggest that altered climate feedbacks linked to
changes in vegetation may have weakened the penetration of monsoon moisture
into the continental interior, explaining the failure of the Holocene
monsoon. Climate modeling suggests a vegetation shift may reduce monsoon
rain in the interior by as much as 50%.
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