Paleoclimatic significance of delta D and delta C-13 values in pinon pine needles from packrat middens spanning the last 40,000 years

We compared two approaches to interpreting delta D of cellulose nitrate in pinon pine needles (Pinus edulis) preserved in packrat middens from central New Mexico, USA. One approach was based on linear regression between modem delta D values and climate parameters, and the other on a deterministic is otope model, modified from Craig and Gordon's terminal lake evaporation mod el that assumes steady-state conditions and constant isotope effects. One s uch effect, the net biochemical fractionation factor, was determined for a new species, pinon pine. Regressions showed that delta D values in cellulos e nitrate from annual cohorts of needles (1989-1996) were strongly correlat ed with growing season (May-August) precipitation amount, and delta(13)C va lues in the same samples were correlated with June relative humidity. The d eterministic model reconstructed delta D values of meteoric water used by p lants after constraining relative humidity effects with delta(13)C values; growing season temperatures were estimated via modem correlations with delt a D values of meteoric water. Variations of this modeling approach have bee n applied to tree-ring cellulose before, but not to macrofossil cellulose, and comparisons to empirical relationships have not been provided. Results from fossil pinon needles spanning the last similar to 40,000 years showed no significant trend in delta D values of cellulose nitrate, suggesting eit her no change in the amount of summer precipitation (based on the transfer function) or delta D values of meteoric water or temperature (based on the deterministic model). However, there were significant differences in delta( 13)C values, and therefore relative humidity, between Pleistocene and Holoc ene. Published by Elsevier Science B.V.