Geochemical and Mineralogical Analyses, Pennsylvanian Kendrick Fauna, Eastern Kentucky: ABSTRACT

Abstract

The Kendrick brachiopods of Pennsylvanian age secreted low-Mg calcite shells with average SR2+ content of 1,140 ppm. The crinoids and rugose corals secreted intermediate-Mg calcite skeletons with average Sr2+ contents of 2,140 ppm and 1,770 ppm, respectively. Conversely, the Kendrick mollusks secreted aragonite shells which contain less than 1,000 ppm Mg2+. However, the average Sr2+ content (4,040 ppm for the cephalopods, 5,210 ppm for the gastropods, and 4,840 ppm for the pelecypods) is higher by a factor of about 2 over the average Sr2+ content of their Holocene counterparts. The brachiopods, gastropods, and pelecypods precipitated calcium carbonate in oxygen (average -4.5 ppt, ^dgr18O, PDB) and carbon (average +1.7 ppt, ^dgr13C, PDB) isotopic equilibrium with ambient Pennsylvanian seawater. The crinoids and rugose corals are light in both ^dgr13C and ^dgr18O by about 5 ppt, relative to isotopic equilibrium values. This isotopic depletion, as in the crinoids' modern counterparts, probably relates to the incorporation of isotopically light metabolic oxygen and carbon at the site of calcification. In contrast, the Kendrick cephalopods apparently precipitated shell aragonite in oxygen isotopic (-4.5 ppt) equilibrium with ambient seawater, whereas their carbon isotopic composition (+0.5 to -5.4 ppt) is controlled pos ibly by kinetic effects. The Fe2+ and Mn2+ contents of the fossil allochems and shale suggest that Kendrick seawater was slightly euxinic. In addition, the Na2+ content and oxygen isotopic composition indicate a slightly higher water temperature and hyposalinity for Kendrick seawater. Diagenetic alteration is limited to occlusion of pore spaces in the fossils and to compaction and cementation of the shale. This process probably occurred in the marine and/or submarine environment. End_of_Article - Last_Page 552------------