The carbon isotope geochemistry of goethite (α-FeOOH) in ironstone of the Upper Ordovician Neda Formation, Wisconsin, USA: Implications for early Paleozoic continental environments

Abstract

Carbon isotope exchange experiments and data from natural samples in a state of isotopic disequilibrium indicate that the apparent Fe(CO 3)OH component in natural goethites is a closed system. These results support the solid solution model for Fe(CO 3)OH in goethite. The carbon isotope geochemistry of the oolitic Neda Formation ironstone at occurrences in Wisconsin and Iowa is consistent with goethite formation in a Late Ordovician subaerial weathering environment. δ 13 C values of the Fe(CO 3)OH component in Neda Formation goethite indicate that organic matter was being oxidized to produce CO 2 in the ancient weathering profile. The δ 13 C value of this organic matter was about −27‰. At depths greater than about 20 cm, the partial pressure of CO 2 in the Late Ordovician weathering profile was 5.6 times larger than the P CO 2 of the Earth's atmosphere at that time. This high “soil” CO 2 partial pressure and its origin in the oxidation of organic matter suggest that there was substantial biological activity on continental land surfaces prior to the widespread colonization by vascular plants. It indicates a possible role for biological activity in the chemical weathering of continents in the early Paleozoic.