We compared two approaches to interpreting ??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 ??D values and climate parameters, and the other on a deterministic isotope model, modified from Craig and Gordon's terminal lake evaporation model that assumes steady-state conditions and constant isotope effects. One such effect, the net biochemical fractionation factor, was determined for a new species, pinon pine. Regressions showed that ??D values in cellulose nitrate from annual cohorts of needles (1989-1996) were strongly correlated with growing season (May-August) precipitation amount, and ??13C values in the same samples were correlated with June relative humidity. The deterministic model reconstructed ??D values of meteoric water used by plants after constraining relative humidity effects with ??13C values; growing season temperatures were estimated via modem correlations with ??D values of meteoric water. Variations of this modeling approach have been 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 ~40,000 years showed no significant trend in ??D values of cellulose nitrate, suggesting either no change in the amount of summer precipitation (based on the transfer function) or ??D values of meteoric water or temperature (based on the deterministic model). However, there were significant differences in ??13C values, and therefore relative humidity, between Pleistocene and Holocene.
Additional Publication Details
Paleoclimatic significance of ??D and ??13C values in pinon pine needles from packrat middens spanning the last 40,000 years