Impact of chemical treatments on the molecular and stable carbon isotopic composition of sporomorphs

Abstract

The stable carbon isotope composition (δ13C) of plants and algae is influenced by environmental factors, such as pCO2, water availability, and altitude. To effectively use the δ 13C of fossil material as a proxy for these parameters, it is essential to understand the chemical and isotopic effects of diagenesis and conventional chemical treatments. In this study, we subject various species of pollen and spores of higher plants to different chemical treatments simulating diagenesis and chemical alteration in the environment as well as palynological processing. We analyze changes in the molecular and isotopic composition using pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) and both elemental analyzer-isotope ratio mass spectrometry (EA-IRMS) and laser ablation-isotope ratio mass spectrometry (LA-IRMS), respectively. We find that saponification removes extractable and ester-bound lipids, which increases the δ 13C value of the sporomorphs. Treatment with HF and HCl (HF-HCl) removed most hydrolyzable polysaccharides and proteins, causing a drop in δ 13C values. Acetolysis produced aromatic-rich residual sporomorphs with the lowest δ 13C values compared to other treatments, likely representing the diagenetically resistant sporopollenin polymer. These findings imply a successional depletion of 13C during fossil maturation, where aliphatic lipids are diagenetically removed in the process, until only the relatively 13C-depleted sporopollenin remains. To adequately compare fossil and extant sporomorph δ 13C values, we advise the use of HF-HCl and a lipid removal step other than acetolysis as the palynological treatment, as acetolysis treats the material non-uniformly. Lastly, LA-IRMS shows promise for targeted isotopic analysis of individual specimens of various types of palynomorphs.