Hi All Recently we have been using the oxygen isotope ratios of a cellulose derivative (Phenylglucosazone, see Sternberg et al. (2007) Geochimica Cosmochimica Acta 71: 2463-2473) as a proxy for environmental water. This oxygen isotope ratio of the derivative is much more fateful to the oxygen isotope ratio of environmental waters. We have done a tree ring sequence with very promising results. However, we need more tree ring sequences to confirm the validity of this method. Here is what we need: A tree ring sequence (7 or more years) having more than 25mg of cellulose per annual ring (It would be easier if you have extracted the cellulose already). Each of the year must have a recorded annual amount-weighed average oxygen isotope ratios of precipitation. If you would like to contribute to this effort, please contact me directly. Thanks. Leo.