A GRANULAR APPROACH TO SEAWATER MG/CA RATIOS OF THE LATE PENNSYLVANIAN MIDCONTINENT SEA

Well-preserved crinoids have been used to predict the Mg/Ca ratio of Phanerozoic seawater (Mg/Ca_SW), because crinoid skeletons reflect the Mg content of the seawater in which the animals lived. However, this method ignores species-level differences in crinoid Mg/Ca ratios (Mg/Ca_C), and regional variations in the Mg/Ca_SW within epicontinental seas. Current understanding of Pennsylvanian Mg/Ca_SW ratios comes from four localities reported by Dickson. All lay in the Late Pennsylvanian Midcontinent Sea (LPMS) of North America; however, only the Virgilian Holder Formation in the Orogrande Basin came from a known stratigraphic unit.

This study examines the Mg/Ca_SW record for the LPMS using Desmoinesian to Virgillian samples from the Midland, Anadarko and Midcontinent basins. All of the crinoids used in this study preserve stereom microstructure. They come from a range of depositional and diagenetic environments: coal balls from coastal swamps, shallow marine limestones, and siderite nodules from relatively deep-water shales. We use both a partition coefficient^* D^CMg = 0.03757 and power partition function^† Mg/Ca_C = 0.0516Mg/Ca_SW^0.668 to estimate the Mg/Ca_SW ratio of LPMS seawater.

Pennsylvanian crinoids from the LPMS studied by Dickson contained 9.9 – 12.5 mole % MgCO₃, corresponding to Mg/Ca_SW of 2.9 – 3.8^* and 3.1 – 4.6^† mol/mol. This study includes crinoids from the: Kalo Formation, IA (Desmoinesian, coal ball); Dalton coal, TX (Missourian, coal ball); Barnsdall Formation, OK (Missourian, siderite nodule in marine shale); and Colony Creek Formation, TX (Virgillian, marine shale). Geochemical analysis yields Mg content ranging from 10.2 – 11.0 mole % MgCO₃ (N = 39 ossicles; 193 points). Estimated Mg/Ca_SW ratios are 3.0 – 3.3^* and 3.5 – 3.8^† mol/mol. These ratios fall well within the range of previously reported Mg/Ca_SW for the LPMS, suggesting that the Mg content of seawater was stable within the equatorial LPMS over 10 million years.