Molecular features of fossil organic matter in remains of the Lower Cretaceous fern Weichselia reticulata from Przenosza basement (Poland)

Abstract

Przenosza (Western Carpathians, Poland) is an important palaeobotanical Cretaceous basement site with morphologically well-preserved plant remains. Both the morphology of the macro remains and microscopic examination of isolated plant cuticles reveal species typical of coastal swampy palaeoenvironments, mainly the fern Weichselia reticulata. After isolation of the plant remains, preliminary characterisation of the fossil organic matter was carried out using cross-polarization, magic angle spinning (CP-MAS) 13C Nuclear Magnetic Resonance (13C NMR) spectroscopy and Curie point analytical pyrolysis. The 13C NMR results suggested a type III kerogen from predominantly terrestrial material which was subjected to intense diagenetic and catagenetic changes. The prominent resonance at ca. 130 ppm indicates C- or H-substituted aromatic sub-units and the weak shoulder at ca. 153 ppm suggests removal of oxygen-containing groups from lignin and carbohydrate. The aliphatic carbon signal (ca. 30 ppm) represents about a quarter of the total 13C signal intensity, suggesting a significant contribution from selectively preserved aliphatic macromolecular structures. Curie-point pyrolysis released a range of aromatic compounds and their alkylated derivatives, i.e. benzenes, phenols, naphthalenes, anthracenes/phenanthrenes, fluorenes and pyrenes (probably derived from heavily altered aromatic biomacromolecules such as lignin or tannins, or from secondary aromatisation of diagenetic structures). There was also a series of n-alkane/n-alkene/a,x-alkadiene triplets. These range from C9 to C24 in carbon number, which is consistent with the distributions found in previous studies of thermally altered cutan. Methoxyphenols, catechols, fatty acids, steroids, diterpenes and sesquiterpenes were absent or present only in trace amounts. In contrast to the distributions of aliphatic pyrolysis products, the lack of indicators for selectively preserved lignin, carbohydrate or protein, points to an extensive rearrangement of phenolic constituents into a largely newly formed condensed backbone, where the original lignin, waxes and carbohydrate were removed or became incorporated into a condensed kerogen matrix affected by severe reduction, aromatization and additional cross-linking reactions.