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 δ¹³C of fossil material as proxies 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 molecular and isotopic composition, using pyrolysis gas chromatography – mass spectrometry (MS), and both elemental analyzer and laser ablation isotope ratio MS, respectively. We find that saponification removes extractable and ester-bound lipids, which increases the δ¹³C value of the sporomorphs. Treatment with HF and HCl removed most hydrolysable polysaccharides and proteins, causing a drop in δ¹³C values. Acetolysis produced aromatic-rich residual sporomorphs with the lowest δ¹³C 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 δ¹³C values, we advise the use of HF-HCl and a lipid removal step other than acetolysis as palynological treatment, as acetolysis treats the material non-uniformly. Lastly, laser ablation-IRMS shows promise for targeted isotopic analysis of individual specimens of various types of palynomorphs.