It has been proposed that Phanerozoic variation in the Mg/Ca ratio of seawater has resulted in variation in the primary mineralogies of shallow-water carbonates between aragonite- and calcite-dominated "seas." To test this hypothesis, this study estimates the Mississippian oceanic Mg/Ca and Sr/Ca ratios from the composition of marine carbonate cements and crinoidal carbonate, both of which have been suggested to faithfully record seawater chemistry.
The early Mississippian (Tournaisian–Visean) Muleshoe Mound (Lake Valley Fm, Alamogordo, NM, U.S.A.) lends itself well to this endeavor because it contains abundant crinoids and marine carbonate cement. Stable isotope and elemental analyses, as well as cathodoluminescence characteristics, elucidate the diagenetic history of the bioherm and identify the least diagenetically altered materials for reconstruction of paleoseawater chemistry. Isotopic analyses document two diagenetic trends, one meteoric and one burial, diverging from values similar to literature estimates of Mississippian marine calcite (−1.5‰ δ18O, +4.5‰ δ13C). The burial diagenetic trend follows from this origin to more negative oxygen values (−7‰ δ18O, +3‰ δ13C). The meteoric diagenetic trend is defined by a meteoric calcite line at ~ −3‰ δ18O. Similarity of non-luminescent marine cement and non-luminescent crinoid isotopic and elemental compositions suggests that crinoidal carbonate was completely replaced by cement in the marine environment. Average elemental ratios for least altered marine cements are: Mg/Ca = 11 mmol/mol; Sr/Ca = 0.2 mmol/mol; Mn/Ca = 0.2 mmol/mol, and Fe/Ca = 0.4 mmol/mol.
Early Mississippian seawater is estimated to have had a Mg/Ca ratio of ~ 0.2–0.3 mol/mol. This Mg/Ca ratio is compatible with the early Mississippian being an interval of "calcite sea" conditions and compares well with the Devonian estimate from marine cements, 0.5, though it is 5–10 times lower than estimates from mass balance modeling. The Sr/Ca for early Mississippian seawater was between 0.5 and 0.9 mmol/mol, which is much lower than previous estimates of ~ 6 mmol/mol from brachiopod calcite and argues for the development of a Sr sink (aragonite?) in the latest Devonian to earliest Mississippian.