MODELS PREDICT VOLATILITY, ECHINODERMS SUGGEST STABILITY: PENNSYLVANIAN Mg/Ca RATIOS IN THE WESTERN MIDCONTINENT SEA

Well-preserved echinoderms have been used as archives for Mg/Ca ratio of Phanerozoic seawater (Mg/Ca_SW) because 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. Dickson based his original Mg/Ca_SW approximation for the Pennsylvanian on 10 echinoderm ossicles from four localities, all on the western side of the Late Paleozoic Epicontinental Sea (LPMS), which covered North America. We measured the Mg/Ca_C of well-preserved echinoderm ossicles from six new localities the same region (TX, OK, MO, IA) and found remarkably consistent Mg/Ca_SW values for the mid-to-late Pennsylvanian, an interval of ~12 m.y. Further, our average Mg/Ca_SW value (3.0 – 3.8 mol/mol) closely matches that of Dickson (2.8 – 3.7 mol/mol).

Echinoderm-derived Mg/Ca_SW estimate uncertainties fall into four categories: biological, environmental, diagenetic, and procedural. Of these, undiagnosed diagenetic alteration has the greatest potential to introduce error. To estimate ancient Mg/Ca_SW, we used back-scattered electron (BSE) images to identify well-preserved ossicles, and wavelength dispersive x-ray spectroscopy (WDS) to gather Mg/Ca_C data, collecting 7-10 points per ossicle. Many power functions and partition coefficients have been proposed to predict Mg/Ca_SW from Mg/Ca_C, leading to a wide range in estimates. Reliable Mg/Ca_C measurements allow comparison of results from different studies. Our best-preserved ossicles come from siderite and calcite concretions, which are often overlooked in Mg/Ca_SW studies. Despite coming from a wide variety of environments and depths (shallow water adjacent to coal swamps, subaerially exposed reefs, and deep-water shale), all samples had similar Mg/Ca_SW, suggesting that the LPMS had relatively uniform water depth. However environmental variation tied to temperature and depth will affect estimated Mg/Ca_SW. Ossicles in our study likely came from crinoids, assessing variation in Mg/Ca_C due to species affiliation (the vital effect) will require study of modern and ancient crinoids. Ossicles in our study certainly came from different species.