February 28, 2007
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FROM VOLUME 11, NUMBER 12, DECEMBER 1998
A 25,000-Year Tropical Climate History from Bolivian Ice Cores,
L. G. Thompson et al.,Science 282, 1858- 1864 (Dec. 4,
Ice cores taken at the summit of Sajama mountain contained insects and
bark that allowed carbon dating the deepest sample (130 m) at 24,000 to
25,000 BP. Oxygen isotopic ratios decreased between the early Holocene and
the Last Glacial Maximum in agreement with other reported measurements.
The abruptness (both in its beginning and ending) of a warming event
between 15.5 and 14 ka BP suggest that the warming was caused by
atmospheric processes. Sajama ice contains less than one-eighth the dust
of polar cores despite the proximity of volcanoes, indicating greater
precipitation in the tropics than at the poles; the low levels of dust
during the Younger-Dryas period are similar to those in the Antarctic but
unlike those in the Arctic, indicating that the elevated dust levels may
have been a Northern Hemisphere phenomenon.
Climate and Vegetation History of the Midcontinent from 75 to 25 ka:
A Speleothem Record from Crevice Cave, Missouri, USA, J. A. Dorale
et al.,Science 282, 1871-1874 (Dec. 4, 1998).
Oxygen and carbon isotopic ratios were measured in the concentric layers
of four stalagmites. The carbon ratio is sensitive to vegetation
conditions, and the oxygen ratio to meteoric water and thus to
precipitation rates. Dating of the speleothemic material was based on 234U230Th
ratios. Together, these measurements reveal a climate and vegetation
history with submillennial resolution for 75 to 25 ka BP. Over thousands
of years, the regional climate oscillated between cold and warm, and
vegetation responded by alternating among forest, savanna, and grassy
prairie. Prairies predominated during warm periods (peaking between 59 and
55 ka BP), and forests during cold ones (since 55 ka BP).
Abrupt Climate Oscillations During the Last Deglaciation in Central
North America, Z. Yu and U. Eicher, Science 282,
2235-2238 (Dec. 18, 1998).
Examination of the sedimentary records of two inland lakes in Ontario
showed that the same climatic swings that have been detected in North
Atlantic coastal regions (i.e., the Gerzensee-Killarney Oscillation, the
Younger-Dryas, and the Preboreal Oscillation) occurred at the lakesites,
also and with the same chronology. These results indicate that climatic
forcing acted alike in the two regions and that atmospheric processes
played a major role in these oscillations.
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