A Biogeochemical Comparison of Fossil (Carboniferous) and Modern Crustose Red Algae

The nature of the contribution of the various types of algae to sedimentary organic matter continues to be a topic of research interest. Crustose red algae have however received less attention than other types. The fossil calcareous red algae (Rhodophyta) analyzed in this study are two relatively unrecrystallized specimens of Parachaetetes (Family Solenoporacea) from the lower part of the Ste. Genevieve Formation (Carboniferous, Visean) in Union County, Illinois, USA. They occurred in the patch reef phase of a small carbonate mudmound-patchreef. The three modern specimens (collected and identified by F. Collier) are the crustose algae Lithothamnion, Clathromorphum and Phymatolithon, (Family Corallinacea, Subfamily Melobesoidea), from a rocky intertidal area near Cape May, Massachusetts, USA. The pyrolyzates of the three (dried, unextracted) modern algal specimens exhibited strong similarities, including a predominance of alkylbenzenes, phenols and indoles. Polysaccharide pyrolysis products are also important, as would be expected. The minor dipeptides detected indicate the presence of proteins. A variety of simpler nitrogen compounds, including pyrroles and benzonitriles may be derived from proteins or from more resistant macromolecular structures. The long-chain alkylnitrile is also noteworthy. The fossil specimens of Parachaetetes (pyrolyzed at 610°C after HCl digestion to remove carbonates and thermodesorption at 310° for 20 sec. to remove bitumen) yielded relatively abundant monoaromatic hydrocarbons and phenols. While their distributions are different, the importance of monoaromatic and phenolic compounds in both the Carboniferous and modern specimens demonstrates a significant similarity. The fossil specimen\u27s pyrolyzate also contains a variety of minor nitrogen compounds, some of which are also found in the modern samples and all of which are unusual in the pyrolyzates of such ancient (ca. 340 Ma) organic matter. The C16 and C18 alkylnitriles are relatively important, but normal hydrocarbons are not, except for the C15 and C17 n-alkanes. Coincidentally, the C17 n-alkane is the only significant normal hydrocarbon in the modern sample. In this study we have found significant chemical similarities between the pyrolyzates of specimens of modern and fossil coralline red algae. The encrustation of the organic matter of Parachaetetes by calcite greatly enhanced its preservation, in spite of the specimens\u27 great geologic age