MISSISSIPPIAN OCEAN CHEMISTRY INFERRED FROM ISOTOPIC AND ELEMENTAL PROXIES

To test the theory that oceanic Mg/Ca chemistry has varied during the Phanerozoic and was controlled primary carbonate mineralogies between aragonite-dominated and calcite-dominated “seas” (Sandberg 1983, Stanley and Hardie 1998, Lowenstein et al. 2001), a study was undertaken to assess Mississippian oceanic Mg/Ca. Marine cements offer a unique opportunity for testing this hypothesis, as they are direct abiotic precipitates from seawater. This contrasts with biotic calcites that are subject to poorly constrained vital effects and evaporite inclusions for which brine evolution must be defined. In addition, the Mississippian is at the end of a putative “calcite sea” as well as a period of faunal recovery after the Frasnian/Famennian mass extinction.

The Early Mississippian (Osagean) Muleshoe Bioherm (Lake Valley Fm, Alamogordo, NM) contains a diverse fauna in addition to micrite, crinoid syntaxial cements, and radiaxial fibrous marine cements. Stable isotope (δ¹⁸O, δ¹³C) and cathodoluminescence characteristics were used to elucidate the diagenetic history of the bioherm and to identify the least diagenetically altered specimens for elemental analyses. Stable isotopes covary between endpoints of –6.04‰ (δ¹⁸O_VPDB, +3.83‰ δ¹³C_VPDB and –1.28‰ δ¹⁸O_VPDB,, +4.92‰ δ¹³C_VPDB. Mg/Ca (mol/mol) varies between 0.010 and 0.020 and Sr/Ca (mmol/mol) between 0.015 and 0.035. Sr/Ca and Mg/Ca covary with m=0.0176 (i.e. Sr/Mg) and r²=0.67. This Sr/Mg is of similar magnitude to those reported by Cicero and Lohmann (2001) for other Paleozoic units.

Using a range of D_Sr=0.02 to 0.3 (Carpenter et al. 1991) and the average Sr/Ca from the most positive δ¹⁸O, cements, the Sr/Ca (mmol/mol) for Mississippian seawater averages between 8.90 and 0.59, in comparison to the modern value of 8.8. Using a D_Mg=0.0318 of Dickson (2004), the Mg/Ca (mol/mol) for Mississippian seawater averages 0.301 (using the most positive δ¹⁸O crinoids) to 0.306 (using the most positive δ¹⁸O cements). For comparison, the modern value is 5.1. Thus, either crinoidal material is subject to negligible vital effects (Dickson 2004) or both phases have suffered equally during diagenesis. While Mg/Ca of 0.3 is compatible with the Early Mississippian being a time of “calcite seas”, this is lower than the Phanerozoic Mg/Ca minimum of 1.0 illustrated by Hardie (1996).