Geochemical Fingerprints of Ginkgoales Across the Triassic-Jurassic Boundary of Greenland

Abstract

Premise of research. Geochemical fingerprinting of fossil plants is a relatively new research field complementing morphological analyses and providing information for paleoenvironmental interpretations. Ginkgoales contains a single extant species but was diverse through the Mesozoic and is an excellent target for biochemical analyses. Methodology. Cuticles derived from fresh and fallen autumn leaves of extant Ginkgo biloba and seven fossil ginkgoalean leaf taxa, one seed fern taxon, and two taxa with bennettitalean affinity were analyzed by infrared (IR) microspectroscopy at the D7 beamline in the MAX IV synchrotron laboratory, Sweden. The fossil material derives from Triassic and Jurassic successions of Greenland. Spectral data sets were compared and evaluated by hierarchical cluster analysis (HCA) and principal component analysis performed on vector-normalized, first-derivative IR absorption spectra. Pivotal results. The IR absorption spectra of the fossil leaves all reveal signatures that clearly indicate the presence of organic compounds. Spectra of the extant G. biloba leaves reveal the presence of aliphatic chains, aromatic and ester carbonyl functional groups from polymer cutin and other waxy compounds, and polysaccharides. Interestingly, both the extant autumn leaves and the fossil specimens reveal the presence of carboxyl/ketone molecules, suggesting that chemical alterations during the initial stages of decomposition are preserved through fossilization. Two major subclusters were identified through HCA of the fossil spectra. Conclusions. Consistent chemical IR signatures, specific for each fossil taxon are present in cuticles, and sufficient molecular content is preserved in key regions to reflect the plants' original chemical signatures. The alterations of the organic compounds are initiated as soon as the leaves are shed, with loss of proteins and increased ester and carboxyl/ketone compound production in the fallen leaves. We further show that the groupings of taxa reflect a combination of phylogeny and environmental conditions related to the end-Triassic event.