Stable-isotope stratigraphy of brachiopods from Pennsylvanian shales in Texas

Abstract

Stable-isotope stratigraphies have been generated for brachiopods from five Late Pennsylvanian sections (upper Winchell cycle, Finis shale, Colony Creek shale, Necessity cycle, and Wayland shale) in north-central Texas to examine the relationship between isotopic composition, paleoenvironment, and sea-level change. Nonluminscent calcite from more than 300 specimens of 4 species of brachiopod ( Composita subtilita, Crurithyris planoconvexa, Eridmatus texana, Neospirifer cameratus ) was analyzed. Sedimentological and paleontological data are used as evidence for paleoenvironmental change. Higher δ 18 O values of brachiopod shells occur in intervals of greatest paleodepth as indicated by fossil content and lithology. These higher δ 18 O values reflect a decrease in bottom temperature with increased water depth. Normalized for paleodepth, the δ 18 O values of the brachiopods are similar in all of the units except the Wayland shale, for which they are ∼0.5‰ higher. This suggests either cooler temperatures or greater continental ice volume during the time of Wayland deposition. Brachiopod δ 13 C values do not correlate with paleodepth. The δ 13 C values of Composita are consistently higher than those of Crurithyris and Eridmatus . Composita δ 13 C values are similar in the upper Winchell cycle, Necessity cycle, and Wayland shale, averaging 4.8‰ to 4.9‰. In contrast, Crurithyris δ 13 C values range from 2.5‰ in the Necessity cycle to 4.4‰ in the Wayland shale; Eridmatus values are also lowest in the Necessity cycle (3.4‰) and greatest in the Wayland shale (3.9‰). These differences between species do not correlate with susceptibility to diagenesis and appear to reflect a pore-water influence on semi-infaunal ( Crurithyris ) forms. The δ 13 C values of Composita shells suggest that the Late Pennsylvanian ocean off eastern Laurussia was enriched in 13 C by about 1‰ relative to the ocean off western Laurussia.