Mineralogy and chemistry of the lower Pennsylvanian Kendrick fauna, eastern Kentucky: 2. Stable isotopes

Abstract

Carbon- and oxygen-isotope analyses of unaltered fossil shells and skeletons from the Kendrick shale show that the brachiopods, gastropods and pelecypods precipitated calcium carbonate, calcite and aragonite, in isotopic equilibrium with ambient Pennsylvanian seawater. The crinoids of the Kendrick shale incorporate metabolic carbon and oxygen into their calcite skeletons, as do their modern counterparts and are depleted both in 13C and 18O by ∼5‰ (PDB) relative to calcite precipitated in isotopic equilibrium with contemporaneous seawater. Similar to the crinoids, the Kendrick rugose corals precipitated calcite coralla in carbon- and oxygen-isotopic disequilibrium with ambient contemporaneous seawater, and the average 18O depletion of 5‰ is similar to that for the Kendrick crinoid. In contrast, the cephalopods of the Kendrick precipitated shell aragonite in oxygen-isotopic equilibrium with ambient seawater, whereas their 13C content is controlled possibly by the growth rate of the individual specimen. The grey-brown shale of the Kendrick exhibits an 18O range of ∼5.4 to −7.0‰ (PDB), whereas the 13C range of +0.3 to −8.0‰ (PDB) for the shale is greater by a factor of 5. This depletion and wide range in carbon isotopes is due to the incorporation of isotopically depleted marine calcite cement in the interstitial pores which was formed from CO 2 produced by the oxidation of organic matter.