MG/CA RATIOS OF CRINOID COLUMNALS IN PENNSYLVANIAN COAL BALLS: IMPLICATIONS FOR RECONSTRUCTING MG/CA RATIOS OF PENNSYLVANIAN SEAWATER

Pennsylvanian and Early Permian coal balls preserve ephemeral plant organs and have been used both to reconstruct swamp communities and paleoclimate, and to understand land plant evolution. Approximately 10% of coals that have coal balls also contain marine invertebrate hash, including well-preserved echinoderm (crinoid) columnals. Echinoderms, a largely marine phylum, produce high-magnesium calcite (HMC) skeletons that have been used to trace the Mg/Ca ratios of seawater through the Phanerozoic. Although some echinoids, asteroids, ophiuroids and holothuroids tolerate hyposaline water, living crinoids are stenohaline. Here we compare the Mg/Ca ratios of crinoid columnals and early carbonate cements from the same coal balls to test the hypothesis that coal balls are marine concretions. The coal balls used in this study come from the Dalton coal in the Wolf Mountain Shale (Texas), which is Late Pennsylvanian (Missourian) in age. Dalton coal balls contain plant material and crinoid columnals that retain their original labyrinth and rectilinear stereom microstructure.

Crinoids from the Dalton coal have an average Mg concentration of 10 mol % MgCO₃ (range 6 – 13 mol % MgCO₃, N = 35), corresponding to an Mg/Ca seawater ratio of 2.9 – 3.5. These values fall within the range of values reported by Dickson for contemporaneous crinoid ossicles from marine limestones in North America, 9.9 – 12.5 mol % MgCO₃, corresponding to Mg/Ca ratios of 2.8 – 3.8, and indicate that crinoid debris in Dalton coal balls record marine seawater Mg/Ca ratios. The Mg concentration of HMC cement in Dalton coal balls range between 9.9 – 12.5 mol % MgCO₃, corresponding to an Mg/Ca ratio of 2.8 – 3.8. Overlap between these the Mg/Ca ratios of early HMC cement and crinoid ossicles from Dalton coal balls suggests that both crinoid columnals and HMC cements formed in sea water and supports a marine origin for coal balls. This relationship can be tested in two other coals, the Calhoun (Late Pennsylvanian) and Amburgy (Early Pennsylvanian), which have crinoids in coal balls (Calhoun) or in close proximity to coal balls (Amburgy). Coal balls, which occur commonly in Pennsylvanian coals from North America and the Donets Basin, which are stratigraphically constrained may provide additional data to reconstruct the Mg/Ca ratios of Pennsylvanian seas.