PROBLEMS OF GEOCOSMOS

Proceedings of the 7th International Conference "Problems of Geocosmos" (St. Petersburg, Russia, 26-30 May 2008)
Fig.2. Comparison of lake-level fluctuations in the Ziway-Shala Lake in the Sahara–Sahel (Hoelzmann
et al. 1998) and Ethiopian Abhe Lake (Gasse 2000), reflected Indian and African monsoons, with dry spells
due to major Holocene droughts (Gasse 2005).
The storm chronology was inferred by Noren et al. (2002) from terrigenous sedimentations in-wash
layers, which reflect rainfall events of exceptional intensity/duration in the 13 lake drainage basins in the
northeastern United States. The frequency of storm-related floods in the northeastern United States has
varied in regular cycles during the past 13,000 years, with a characteristic millennial periodicity. Maxima of
terrigenous influx coincide with high storminess and flooding episodes in other records from the North
Atlantic area, and with cool periods in Greenland and Europe as recorded in glaciers by Hormes et al.
(2001).
Presently, there is a growing body of evidence that short-lived periodic events have persisted into the
Holocene epoch as for instance the 8200 cal BP and 2800 cal BP cold periods (e.g., Dergachev et al 2004;
Veski et al 2004) together with the perhaps more well-known ‘Little Ice Age’ (Matthews and Briffa 2005).
Neff et al. (2001) presented a high-resolution study of variation in the Indian Ocean monsoon during the
time span from 9600 to 6100 BP derived from oxygen isotope variation (the stable isotopes are used to
provide information concerning climate changes) in a Th/U dated speleothem from Oman. The speleothem
δ 18 O values serve as a proxy for estimating variation in monsoon intensity by measuring past changes in δ 18 O
of monsoon rainfall as recorded in speleothem calcite δ 18 O. In the time span from 8500 to 8000 cal year BP
there are strong peaks at 8400, 8200 and 8000 cal yr BP with the 200-year periodicity (Neff et al. 2001) in
δ 18 O data similar to the pattern of climate change during the Little Ice Age in the past millennium. As was
shown by Fleitmann et al. (2003) from δ 18 O monsoon record in a stalagmite of Qunf Cave in Southern Oman
(17°10’ N, 54°18’ E; 650 m above sea level), between 10,300 and 8000 BP decadal to centennial variations
in monsoon precipitation are in phase with temperature fluctuations recorded in Greenland ice cores. Taking
into account both the stalagmite and GRIP records, decadal scale intervals of reduced monsoon precipitation
(more positive δ 18 O values) correlate with cooling events in Greenland and vice versa, as best expressed at
9100 and 8200 BP.
Olsen (2007) discussed the climate variability based on the Blinden Lake (Denmark) record in
relation to regional and northern hemisphere climate by combining the sedimentological and geochemical
evidence. An estimate of the paleolake water isotope composition (δ 18 Ow) and changes of the lake water
level (ΔW) and thereby also an effective humidity were derived. Figure 3 presents the wavelet power
spectrum of the inferred δ 18 Ow and ΔW values.
Fig. 3. The absolute values of the wavelet coefficient using a morlet wavelet on the δ 18 Оw (lake water
isotope composition) and on ΔW (lake water level) from Blinden Lake (Denmark) sediment.
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