Implications of Ordovician (≈460 Myr) marine cement for constraining seawater temperature and atmospheric pCO2

The marine diagenetic history of the Effna buildups (Virginia, lower Caradocian, ≈460 Myr) is atypical of Phanerozoic carbonates and reflects variable redox conditions in the semi-restricted Appalachian foreland basin. Fibrous marine cements are the focus of this study. Least-altered fibrous calcite from the Effna Formation has a translucent appearance in transmitted light and a 2–3 mol% MgCO3 composition; the latter is similar to values (2–5 mol%) from translucent fibrous calcite in other Caradocian units [Holston (Tennessee) and Kullsberg (Sweden) formations]. Some fibrous calcite is overlain by hardgrounds that in places have an irregular micro-topography with sharp overhangs indicative of syn-depositional dissolution. Geochemical evidence supporting the marine dissolution includes δ18O values from altered fibrous calcite that are more positive (up to −3.9‰ PDB) than coeval least-altered fibrous calcite from Holston buildups in Tennessee (up to −4.8 to −5.8‰ PDB). Syn-depositional dissolution was likely associated with sulfide oxidation along an oxic–anoxic interface and not due to upwelling of cold bottom water in the Appalachian foredeep. Faunal and lithologic evidence suggests that oxic surface waters had a normal salinity and a water temperature conducive for metazoan growth (<33°C). This paper constrains the temperature of near-surface seawater in the Appalachian foreland basin (22–33°C) and atmospheric pCO2 (>14 present atmospheric level) during the early Caradocian.