Molecular indicators of redox and marine photoautotroph composition in the late Middle Ordovician of Iowa, U.S.A.

Saturated and aromatic hydrocarbons were used to evaluate depositional redox conditions and marine photoautotroph contributions to Middle Caradocian strata of the central United States (IA). At the base of the Spechts Ferry Member of the Decorah Formation, ¹³C-enriched aryl isoprenoids, derivatives of green sulfur bacteria, become abundant, indicating the development of photic-zone anoxia. This is coincident with the disappearance of the organic-walled microfossil Gloeocapsomorpha prisca and a marked decrease in the relative abundances of cyanobacterial biomarkers. The development of dysoxic to anoxic conditions and/or associated changes in basin circulation potentially affected the distributions and abundances of these organisms. In the overlying Guttenberg Member, G. prisca-derived organic matter becomes dominant, but relative cyanobacteria abundances remain low. In addition, the percentage total organic carbon is greater than 20%, even though selected biomarker ratios (pristane/phytane ratios greater than 3, and homohopane indices less than 0.5) and the presence of bioturbation indicate that bottom waters were oxygenated. It is suggested that deposition of G. prisca affected both organic matter preservation and depositional redox conditions. Observed variations in redox indicators and marine photoautotroph contributions are associated with changes in siliciclastic deposition, reported macrofauna turnover and with evidence for oceanic cooling and a change in circulation patterns documented in the eastern United States.