Dear Dr. Turney,
Although I myself is not an expert in this and specialize in 
carbonate isotopic comp of sediments and speleothems (paleocl.),
I have a knowlege of a mid-continental site containing tree stumps 
and trunks buried in a sand deposit. They estimated to be 15-36 Ka. 
The place is near lake Baikal, Siberia. C14 had been done I guess on 
some of them and they have (I beleive) some sections taken already. 
If you are interested, I will dig up an e-mail address of a person 
there and forward it to you (all).
Sincerely, Pavel.

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Pavel Romashkin
Stable Isotope Lab, Department of Geological Sciences
University of South Carolina, Columbia SC 29208 USA
ph (803)777-7668 fax (803)777-6610