Preserved chemistry of Cretaceous gymnosperm leaves in volcanic-ash deposits: Baqueró Group, Patagonia, Argentina

Abstract

Gymnosperm foliar compressions belonging to two fossil taxa, Pseudoctenis ornata (Cycadophyta) and Ginkgoites tigrensis (Ginkgophyta), are chemically characterized for the first time by semi-quantitative Fourier Transform Infrared (FTIR) spectroscopy. The specimens collected in the well-known Baqueró Group (Lower Cretaceous, Santa Cruz province, Argentina) are compressions with very well-preserved cuticles. The FTIR data are analyzed along with the available paleoenvironmental data of the Baqueró Group localities in order to interpret the volcanic activity role in the preservation of these foliar compressions and, consequently, their chemical compositions. For the spectrometric study, foliar remains are analyzed into two sample forms: compressions (including coalified mesophyll and cuticle; Cp) and cuticles (Ct). Semi-quantitative data, derived from Cp and Ct spectra, are evaluated by principal component analysis (PCA) analyzing the functional groups preserved in the mesophyll and cuticle of each fossil taxon. In general, the fossil taxa reflect similar chemical compositions between compression and cuticle sample forms revealing that P. ornata and G. tigrensis compressions underwent, during diagenesis and probably post-diagenesis, a natural oxidation process. Considering the known information on the preservation model of Squamastrobus tigrensis, a podocarp recovered also in Baqueró Group, as well as the paleoenvironments and geochemical conditions in the fossiliferous localities along with the FTIR data, two preservation models are here proposed for P. ornata and G. tigrensis. The volcanic activity was most likely involved in the exceptional preservation of the foliar cuticle characters, the transformation of the organic matter, and the oxidation undergone by G. tigrensis and P. ornata foliar remains.