Evaluating the use of amber in palaeoatmospheric reconstructions: The carbon-isotope variability of modern and Cretaceous conifer resins

Stable carbon-isotope geochemistry of fossilized tree resin (amber) potentially could be a very useful tool to infer the composition of past atmospheres. To test the reliability of amber as a proxy for the atmosphere, we studied the variability of modern resin delta C-13 at both local and global scales. An amber delta C-13 curve was then built for the Cretaceous, a period of abundant resin production, and interpreted in light of data from modern resins. Our data show that hardening changes the pristine delta C-13 value by causing a C-13-depletion in solid resin when compared to fresh liquid-viscous resin, probably due to the loss of C-13-enriched volatiles. Modern resin delta C-13 values vary as a function of physiological and environmental parameters in ways that are similar to those described for leaves and wood. Resin delta C-13 varies between plant species and localities, within the same tree and between different plant tissues by up to 6%, and in general increases with increasing altitudes of the plant-growing site. We show that, as is the case with modern resin, Cretaceous amber delta C-13 has a high variability, generally higher than that of other fossil material. Despite the high natural variability, amber shows a negative 2.5-3% delta C-13 trend from the middle Early Cretaceous to the Maastrichtian that parallels published terrestrial delta C-13 records. This trend mirrors changes in the atmospheric delta C-13 calculated from the delta C-13 and delta O-18 of benthic foraminiferal tests, although the magnitude of the shift is larger in plant material than in the atmosphere. Increasing mean annual precipitation and pO(2) could have enhanced plant carbon-isotope fractionation during the Late Cretaceous, whereas changing pCO(2) levels seem to have had no effect on plant carbon-isotope fractionation. The results of this study suggest that amber is a powerful fossil plant material for palaeoenvironmental and palaeoclimatic reconstructions. Improvement of the resolution of the existing data coupled with more detailed information about botanical source and environmental growing conditions of the fossil plant material will probably allow a more faithful interpretation of amber delta C-13 records and a wider understanding of the composition of the past atmosphere. (C) 2016 Elsevier Ltd. All rights reserved.