Mg/Ca Ratios of Crinoid Ossicles and Permineralized Plant Debris from the Late Pennsylvanian Midcontinent Seaway: An Archive for Seawater Mg/Ca

Abstract

Coal balls are carbonate and pyrite concretions that contain permineralized peat, or fossilized plants, preserved at the cellular level, which have been used to reconstruct Pennsylvanian and Early Permian swamp communities and paleoclimate, and understand land plant evolution. Both the exquisite preservation of plant fossils and large amounts of space that was originally empty and is now filled with carbonate cement in coal balls suggest rapid formation in relatively uncompacted peat at the swamp surface. Pennsylvanian coal balls occasionally contain pieces of marine-dwelling invertebrate organisms, including echinoderm ossicles whose skeletons reflect the Mg content of the seawater in which the animals lived. Echinoderms are exclusively marine organisms that cannot tolerate salinity changes: echinoderm fossils in coal balls were probably transported into the swamp by storms. Well-preserved crinoids have been used as an archive of the Mg/Ca ratio of seawater (Mg/CaSW) for the Phanerozoic and as a proxy for marine water presence in swamps. Geological studies coal balls reveal that the first-formed mineral cements are dolomite, high-magnesium calcite (eHMC), and framboidal pyrite, which suggest formation in marine swamps, but could be microbial carbonates precipitated in freshwater swamps. Comparison of Mg/Ca ratios of eHMC in coal balls with Mg/Ca ratios of echinoderm ossicles that retain original microstructure from the same coal balls will enable me to evaluate hypotheses of coal ball formation. Additionally, well-preserved crinoids from a range of depositional and diagenetic environments: coal balls from coastal swamps, shallow marine limestones, and siderite nodules from relatively deep-water shales in the Late Pennsylvanian Midcontinent Sea (LPMS) of North America will be used to determine if there is continuity in the seawater within the LPMS over 10 million years and within basins from different latitudes. In this dissertation I will use paleoecology, carbonate petrology and geochemistry in order to address the questions: (1) What is the depositional environment of Late Paleozoic swamps in which coal balls occur? (2) What is the regional Mg/CaSW of the LPMS using crinoids?