Dear Darren,
 I am looking at carbon isotopes within plant material during the last
glacial-interglacial transition (14-9 ka BP).  Although this is nowhere near
as old as your samples the isotopes from terrestrial plant organs appear to
be giving consistent shifts within several sites across NW Europe during this
period.  This suggests 1) that diagenesis of the material at least on this
time scale is not significant and 2) that some common forcing is causing
these shifts.  With regards to the latter my review of the literature
suggests that the main climatic control is relative humidity (though some
support a temperature control which is of course autocorrelated to relative
humidity).  Some of the papers that appear to support this are:
Figge and White,1995, Global Biogeochemical Cycles, 9, 391-403
Saurer and Siegenthaler, 1989, Dendrochronologia, 7, 9-13
Stuiver and Braziunas, 1987, Nature, 328, 58-60
White et al., 1994, Nature, 367, 463-469
Winter et al, 1982, J. of Experimental Botany, 33, 88-91

I'm testing this hypothesis at the moment.  I've collected Carex fruits from
a variety of species in Britain and am receiving more from within Europe.
These have been collected close to meteorological stations and the carbon
isotope ratio determined will be compared to relative humidity and other met.
variables.

 Also it would be worth contacting Dave Beerling at Sheffield as hes working
on carbon isotopes in plant material on the geological timescale.  His e-mail
is [log in to unmask]

 Best wishes,
 Chris Turney,
 Centre for Quaternary Research,        Tel.: 01784 443563
 Geography Dept.,                       Fax: 01784 472836
 Royal Holloway,                        e-mail: [log in to unmask]
 University of London,
 Egham,
 Surrey,
 TW20 0EX,
 UK