Paleoclimatic significance of δD and δ13C values in piñon pine needles from packrat middens spanning the last 40,000 years

We compared two approaches to interpreting δD of cellulose nitrate in piñon pine needles (Pinus edulis) preserved in packrat middens from central New Mexico, USA. One approach was based on linear regression between modern δD values and climate parameters, and the other on a deterministic isotope model, modified from Craig and Gordonʹs 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, piñon pine. Regressions showed that δD values in cellulose nitrate from annual cohorts of needles (1989–1996) were strongly correlated with growing season (May–August) precipitation amount, and δ13C values in the same samples were correlated with June relative humidity. The deterministic model reconstructed δD values of meteoric water used by plants after constraining relative humidity effects with δ13C values; growing season temperatures were estimated via modern correlations with δD 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 piñon needles spanning the last ∼40,000 years showed no significant trend in δD values of cellulose nitrate, suggesting either no change in the amount of summer precipitation (based on the transfer function) or δD values of meteoric water or temperature (based on the deterministic model). However, there were significant differences in δ13C values, and therefore relative humidity, between Pleistocene and Holocene.