Biogeochemistry of nautiloids and paleoenvironmental aspects of buckhorn seawater (Pennsylvanian), Southern Oklahoma

Abstract

Sediments in the Buckhorn quarry and adjacent outcrops (southern Oklahoma) are shallow-water carbonates which are intercalated with coals and minor conglomerates. They belong to the Boggy Formation of the Deese Group and are Desmoinesian (Pennsylvanian) in age. Strontium concentrations of shell, septa and cameral deposits of Pennsylvanian cephalopods are variable and fall either in the low range with 0.17–0.22 or the high range with 0.39–0.60 Sr/Ca atomic ratios. These values suggest that extrapallial and/or cameral fluids responsible for the formation of all three cephalopod macrostructures were in chemical disequilibrium with ambient seawater. Thus, the biological discrimination for Sr was well establihed by Pennsylvanian time for nautiloids. The Mg concentrations appear to be controlled at the species/genus levels. In contrast, the Na, Mn and Fe contents of nautiloids are influenced and controlled by the physicochemical conditions of the surrounding seawater. Information obtained from brachiopod and from the least-altered cephalopod samples suggest that Buckhorn seawater was physiochemically similar to that of modern ocean water, and the postulated temperature and salinity values represent the most reasonable intepretations considering paleontological, sedimentological and biogeochemical data. Salinity of the Buckhorn sea, which was within 5°S of the Pennsylvanian equator, ranged from 29 to 34 ppt, with a mean of 32 ppt. This near-normal salinity was complemented by an average water temperature of 36°C (34–38°C) for the shallower, and of 21°C (14–30°C) for the deeper parts of the Buckhorn sea of southern Oklahoma. Overall, water temperatures fluctuated between 14 and 38°C.