THE ANCIENT SEAWATER ARCHIVE OF ECHINODERMS FROM THE CRITICAL ZONE OF THE LATE PENNSYLVANIAN MIDCONTINENT SEA

Well-preserved echinoderms are used as archives for Mg/Ca ratio of Phanerozoic seawater (Mg/Ca_SW) as their skeletons reflect the Mg content of the seawater in which the animals lived. However, sampling across the Paleozoic is sparse and regional variations in Mg/Ca_SW, especially in epicontinental seas, may complicate reconstructions of global Mg/Ca_SW. Echinoderm-derived Mg/Ca_SW estimate uncertainties fall into four categories: biological, environmental, diagenetic, and procedural. This study examines the Mg/Ca_SW record with regional and temporal sampling across a single epicontinental sea, the Late Pennsylvanian Midcontinent Sea (LPMS) of North America. The echinoderms used in this study come from a range of critical zone depositional and diagenetic environments: coal balls from coastal swamps, shallow marine facies, and siderite nodules from relatively deep-water shales. We use well-preserved echinoderms to explore the stability of estimated Mg/Ca_SW in the mid-to-late Pennsylvanian, and evaluate how differences in depositional environment, diagenetic pathway, and climate effect these estimates. This study includes echinoderms from six Pennsylvanian localities: four coal balls and roof nodules with multiple ossicles from the Kalo Formation of Iowa (Desmoinesian, Midcontinent Shelf); Barnsdall Formation of Oklahoma (Missourian, Anadarko Basin); Mineral Formation of Missouri (Desmoinesian, Midcontinent Shelf) and two single ossicles from the Colony Creek Shale of Texas (Virgilian, Midland Basin) and Necessity Shale of Texas (Virgilian, Midland Basin). With the addition of these localities, we compare Mg/Ca_SW values along the critical zone of a single epicontinental sea (the LPMS) from the early Desmoinesian through the mid-Virgilian, a timespan of approximately 12 m.y., when global models predict increased Mg/Ca_SW, from values of 2.2 to 2.7 in the early Desmoinesian to values of 2.8 to 3.8 in the mid-Virgilian, at the end of the interval. Geochemical analysis yields Mg content ranging from 10.2 – 11.0 mole % MgCO₃ (N = 41 ossicles; 228 points). Estimated Mg/Ca_SWratios are 3.0 – 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 along the critical zone of the equatorial LPMS over 10 million years.