Geoscience Reference
In-Depth Information
234
U
values in speleothem calcite from Fort Stanton Cave in southern New Mexico as an
index of effective precipitation, together with ages obtained from rim pools in the Big
Room of Carlsbad Cavern. They discovered that following a moist climatic interlude,
a very severe drought was evident in the arid American Southwest from just before
14.5 until around 12.9 ka, coeval with the warm Bølling/Allerød interval evident in
the Greenland ice core records from 14.6 to 12.8 ka. They pointed out that the last
appearance of sixteen out of thirty-five mammal genera that became extinct between
13.8 and 11.4 ka coincided with this 1,500-year drought in the American Southwest
and took place before the arrival of the Clovis hunters in this region. It also occurred
before the controverted cometary impact that Firestone et al. (
2007
) proposed as the
reason for the demise of the large late Pleistocene mammals of North America.
An important archive that has been used to reconstruct the late Quaternary ecolo-
gical and climatic history of the deserts of northern Mexico and the arid south-west of
the United States is that provided by packrat middens (Betancourt,
1990
a; Betancourt
et al.,
1990a
; Betancourt et al.,
1990b
; Spaulding,
1990
; Van Devender,
1990a
;Van
Devender,
1990b
). In the very arid Chihuahuan Desert, 220 packrat middens and
259 associated AMS
14
C ages have revealed a 40 ka history of vegetation change. The
lowest parts of the desert remained arid throughout this time, but at higher elevations,
the macroflora preserved in the middens show that early Wisconsin climates were
somewhat wetter than those in the middle Wisconsin at around 31 ka, with humid
climates during full glacial times at about 22 ka. The presence of C
4
perennial grasses
points to rainfall in late spring or summer, when temperatures were relatively warm.
Van Devender (
1990a
) concluded that the LGM climate was comparatively mild,
with few winter freezes, cool summers and higher-than-present rainfall throughout
the desert. This conclusion runs counter to the earlier views of Galloway (
1970
;
1983
)
and Brakenridge (
1978
), who argued for a cold, dry LGM climate. The winter rainfall
regime ended sometime after 9-8 ka, with the establishment of the modern climatic
regime by about 4,000 years ago. Van Devender (
1990a
) noted that the response to
climate changes varied with the species, so the resulting plant communities were
always in a state of flux and never attained equilibrium, a conclusion reached earlier
by Davis (
1976
;
1986
) from her studies of postglacial deciduous forest change in
North America. A more recent record of Holocene environmental change comes from
the
13
Cand
of climatic changes. For example, Polyak et al. (
2012
) used the
13
C composition of two stalagmites from a cave in central Missouri, which show
positive excursions at 3.5 and 1.2-0.9 ka that are consistent with more arid climates
in the semi-arid Great Plains at those times (Denniston et al.,
2007
).
20.8 Conclusion
The Cenozoic cooling and desiccation of North America was closely associated
with the tectonic movements that caused uplift of the Rockies along the west of the
continent, faulting and subsidence in the Basin and Range Province, and progressive
