Practical considerations for the use of pollen δ13C value as a paleoclimate indicator

RATIONALE Workers have shown a correlation between temperature and the pollen d13C value, and therefore suggested using pollen d13C values to reconstruct paleotemperature. To evaluate the potential for pollen d13C values to be used as a paleotemperature proxy, it is essential to quantify the variability in pollen d13C values and to evaluate the effect of temperature on pollen d13C values, in isolation, under controlled environmental conditions. METHODS Pollen was isolated from 146 Hibiscus flowers from 26 plants within a single climate environment to evaluate isotopic variability in pollen d13C values. The nearest leaf (n=82) and flower phloem (n=30) were also sampled to measure the d13C variability in carbon providing the raw material for new growth. To evaluate the correlation between temperature and pollen d13C values, we isolated pollen from 89 Brassica rapa plants grown in controlled growth chambers with temperatures ranging from 17 to 32 degrees C. RESULTS The range in pollen d13C values collected from different flowers on the same Hibiscus plant was large (average?=?1.6 parts per thousand), and could be as much as 3.2 parts per thousand. This amount of variability was similar to that seen between flower-adjacent leaves, and phloem extracted from styles of individual flowers. In controlled growth chamber experiments, we saw no correlation between temperature and the pollen (R2?=?0.005) or leaf (R2?=?0.10) d13C values. CONCLUSIONS We measured large variability in pollen d13C values. When temperature was isolated from other environmental parameters, temperature did not correlate with the pollen d13C value. These results complicate the supposed relationship between temperature and pollen d13C values and caution against using nanogram isotope analytical techniques for characterizing whole-plant individuals. Copyright (c) 2012 John Wiley & Sons, Ltd.