date: Fri, 03 Apr 1998 16:02:58 +0200
from: H�kan grudd <gruddatXYZxyzgeo.su.se>
subject: Dropstones and tree rings
I have just been listening to a very interesting seminar on dropstones from
caves in South Africa and Gotland (Island east of Sweden). Both are works
by graduate students. In the African dropstone annual varves have been
identified and crosdated with tree rings. The width of the varves correlate
with annual precipitation (semiarid climate) and tree-ring width. The
maximum gray tone in each varve has been correlated with temperature. The
correlation values for the calibration period is very good! The student
presented a temperature curve for the last c. 6500 years. It looks really
interesting: a generally warm period in the first part, drops down to
generally lower vaules at around 2000 BC, a big peak centered around 400
BC! Little bit varmer with a lot of variability in Medieval times, a drop
in 14th century with cold temperature and very little variation for some
hundreds of years (Little Ice Age?) and the again higher in the last two
The dropstone from Gotland had dating problems. They tried to crossdate
with Bartholin's pine chronologies without success.
Karin Holmgren, who has been supervising this work and herself made her
thesis on dropstones and climate, have very little knowledge in
dendro-methods and seeks co-operation. The world seems filled with
prospects! Do you know anything about tree-ring chronologies from
north-eastern South Africa?
PS. Any luck in finding my keys?