Geoscience Reference
In-Depth Information
wet during late glacial times and dry during the early Holocene. Southern Africa was
apparently wetter at 202-186, 50-43, 38-35, 31-29, 26-21 and 19-14 ka. Earlier work
in this region showed substantial uncertainties associated with some of the younger
ages based on radiocarbon dating, so some recalibration was deemed necessary.
In north-east Brazil, Wang et al. (
2004
) used a combination of speleothems and tufa
deposits to provide a record of wet phases during the past 210 ka. They found that wet
periods in this presently semi-arid region coincided with times of weak East Asian
summer monsoons in China, cold periods in Greenland, phases of iceberg discharge
in the North Atlantic (Heinrich Events) and reduced run-off into the Cariaco Basin
off the coast of Venezuela. They concluded that the wet intervals probably reflected
southward displacement of the Intertropical Convergence Zone, leading to forest
expansion and the creation of a forest corridor between the Atlantic and Amazonian
rainforests. Earlier work on speleothems and travertines/tufas that date back to around
400 ka in semi-arid north-east Brazil (Auler and Smart,
2001
) had shown that during
the LGM, the regional water-table was 13
1mhigher than it is today (and even higher
during MIS 6), in contrast to pollen evidence of a drier glacial climate in Amazonia
and southern Brazil. These studies demonstrated that there was considerable regional
variation in precipitation in this region during glacial times.
±
14.5 Conclusion
Technical advances in extracting climatic signals from speleothems pioneered in the
1970s were strengthened thanks to improvements in dating methods perfected during
the late 1980s and onwards. These allowed very precise ages to be obtained from cave
stalagmites and have helped extend the practical dating range back to about half a
million years. Analysis of the stable isotopes of oxygen and carbon has allowed past
changes in precipitation and in surface plant cover to be determined with reasonable
quantitative accuracy. Additional analysis of fluid inclusions within speleothems,
notably of the ratio of deuterium to hydrogen, has allowed an independent estimate of
past temperature. The high-resolution records from Israel, peninsular Arabia, north-
east Brazil and China have provided invaluable insights into possible links between
summer monsoon fluctuations, temperature changes in the North Atlantic and, at finer
time scales, changes in solar intensity. However, it is worth sounding a note of caution.
An annually laminated stalagmite fromMechara just south of the Ethiopian Rift Valley
in south-east Ethiopia grew for around 440 years from 5,023 yr BP onwards (Asrat
et al.,
2007
). Comparison of different climate proxies, such as
13
C,
234
U/
238
U,
annual growth rate and the fluorescence index, revealed a variety of responses to a
single climate forcing, confirming the need for caution in interpreting climate signals
from speleothem records.
Comparisons between precise speleothem chronologies of certain critical episodes,
such as the Younger Dryas cold event and the Bølling/Allerød warm interstadial phase,
18
O,
