2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 10
Presentation Time: 10:15 AM


HASIUK, Franciszek Józef and LOHMANN, Kyger C., Department of Geological Sciences, University of Michigan, 2534 CC Little Building, 1100 N University Ave, Ann Arbor, MI 48109-1005, franek@umich.edu

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‰ (δ18OVPDB, +3.83‰ δ13CVPDB and –1.28‰ δ18OVPDB,, +4.92‰ δ13CVPDB. 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 DSr=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 DMg=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).