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Global Climate Change Digest
A Guide to Information on Greenhouse Gases and Ozone Depletion
Published July 1988 through June 1999

FROM VOLUME 11, NUMBER 12, DECEMBER 1998

PROFESSIONAL PUBLICATIONS...
PALEOCLIMATE RECONSTRUCTIONS

Item #d98dec11

“A 25,000-Year Tropical Climate History from Bolivian Ice Cores,” L. G. Thompson et al.,Science 282, 1858- 1864 (Dec. 4, 1998).

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.

Item #d98dec12

“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 234U–230Th 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).

Item #d98dec13

“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.