Geoscience Reference
In-Depth Information
and fruit are often beautifully preserved even many millions of years after they died
(Hill,
1994a
; Hill,
1994b
), as the silicified leaves of the late Cenozoic flora in the
Upper Lachlan Valley of eastern Australia described by von Ettingshausen (
1888
)
more than a century ago attest. Plant macrofossils and charcoal in archaeological sites
provide a partial glimpse into the contemporary prehistoric flora.
A particularly useful form of plant macrofossils are those preserved in packrat
or stick-nest rat middens in the drier parts of northern Mexico and the south-west
United States (Van Devender and Spaulding,
1979
; Betancourt,
1990
; Betancourt
et al.,
1990a
;Betancourtetal.,
1990b
;Cole,
1990
; Spaulding,
1990
; Van Devender,
1990a
; Van Devender,
1990b
), as well as central Australia (Pearson and Dodson,
1993
;
McCarthy et al.,
1996
; Pearson,
1999
). In the deserts of the south-west United States,
packrat middens provide a record of environmental change spanning the last 40 ka;
in central Australia and the arid Flinders Ranges of South Australia, the stick-nest rat
(
Leporillus
spp.) middens investigated so far do not extend back any earlier than the
Holocene.
The pollen record preserved in the faecal pellets of
Leporillus
middens from two
sites in arid western Australia spanning the last 1,150 years provides information about
local vegetation and dietary preferences, in contrast to the regional signal from playa
lakes, and suggests a less wooded vegetation cover between 0.9 and 0.3 ka (Pearson
and Dodson,
1993
). Eight
Leporillus
middens from the arid northern Flinders Ranges
indicate wetter conditions and more widespread woodlands between 8.8 and 5.3 ka
(McCarthy et al.,
1996
). The dominance of halophytes (salt-tolerant plants) at the
Pleistocene-Holocene transition may indicate continued aridity or a change in rainfall
seasonality or more local influences.
The late Quaternary packrat midden record from the arid south-west of the United
States and northern Mexico sheds new light on certain critical aspects of desert
biogeography and will therefore be discussed here in some detail. The Chihuahuan
Desert is an inland continental desert bounded to the west by the Sierra Madre
Occidental, to the east by the Sierra Madre Oriental, to the south by the highlands of
the Mexican Plateau and to the north by the Rocky Mountains (Van Devender,
1990a
).
Analysis of 220 packrat middens with 259 associated AMS
14
C ages has provided a
remarkably detailed picture of vegetation change in this very arid desert during the
last 40 ka. Apart from in the lowest parts of the desert, which remained arid, the early
Wisconsin climates may have been somewhat wetter than they were in the middle
Wisconsin at 31 ka, with more humid conditions during full glacial times at 22 ka.
The existence of C
4
perennial grasses indicates rainfall in late spring or summer,
when temperatures were relatively warm. The overall LGM climate was mild with
few winter freezes, cool summers and higher rainfall throughout the desert. None of
the evidence supports the model of a cold, dry LGM climate proposed by Galloway
(
1970
;
1983
) and Brakenridge (
1978
). The demise of the winter rainfall regime took
place after 9 to 8 ka, when the modern climatic regime began to be established, and
