Implications of Ordovician (≈460 Myr) marine cement for constraining seawater temperature and atmospheric pCO_2

摘要

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% MgCO_3 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 δ~(18)O 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 ℃). This paper constrains the temperature of near-surface seawater in the Appalachian foreland basin (22-33 ℃) and atmospheric pCO_2 (>14 present atmospheric level) during the early Caradocian.