A two-photon fluorescence micro-spectroscopy method for chemotaxonomic characterization of fossil resins

Amber, a fossilized chemical defensive exudate, is important for understanding ancient plant-insect/pathogen interactions, yet its botanical origins, especially for Cretaceous amber, remain enigmatic. Here, we develop and apply a fluorescence micro-spectroscopy approach within the context of a larger study on the provenance of amber.

Emission spectra were collected from 21 amber, copal, and Defaunation resin samples at six excitation wavelengths. The emission spectra at all excitation wavelengths were deconvolved using simultaneous fitting, with each emission spectrum modeled as a linear combination of four Gaussian components.

Differences between Gaussian means separated the samples into five groups. Cross-referencing these groupings with prior studies on their botanical origins indicate that they corresponded broadly to ambers from well-defined conifer families, including the Araucariaceae (group 1 associated with Wollemia and Araucaria, groups 2 and 3 associated with Agathis) and Cupressaceae (group 4), and Baltic amber (group 5) with unresolved taxonomic affinity.

Fluorescence micro-spectroscopy is a useful approach for characterizing fossil resin in a reproducible and nondestructive manner. Variations in the fluorescence properties of resins do not correspond to variations in age or maturity but rather to plant origin. Therefore, this approach provides an additional line of evidence for inferring the botanical origins of amber.