OPENING THE COAL BALL WINDOW: RECONSTRUCTING MG/CA RATIOS OF PENNSYLVANIAN SEAWATER USING MG/CA RATIOS OF CRINOID COLUMNALS IN PENNSYLVANIAN COAL BALLS

Pennsylvanian and Early Permian coal balls preserve ephemeral plant organs and have been used to reconstruct swamp communities and paleoclimate, and to understand land plant evolution. Approximately 10% of coal beds that have coal balls also contain marine invertebrate hash, including well-preserved echinoderm (crinoid) columnals. Echinoderms, a mostly marine phylum, produce high-magnesium calcite (HMC) skeletons that track Mg/Ca ratios of seawater through the Phanerozoic. Although a few echinoids, asteroids, ophiuroids and holothuroids tolerate fresh 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. Coal balls for this study come from the Dalton coal in the Wolf Mountain Shale (Palo Pinto Co., TX), and the Calhoun Coal (Berryville, IL); both are Late Pennsylvanian (Kasimovian). Coal balls from both locations contain plant material and crinoid columnals that retain their original labyrinth and rectilinear stereom microstructure.

Crinoids from Dalton and Calhoun coal balls have an average Mg concentration of 10 – 12 mol % MgCO₃ (Dalton: range 6 – 13 mol % MgCO₃, N = 35; Calhoun: range 8 – 16 mol % MgCO₃, N = 85), corresponding to an Mg/Ca seawater ratio of 2.7 – 3.9. 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 crinoids in Dalton and Calhoun coal balls record marine seawater Mg/Ca ratios. Mg/Ca ratios of early HMC cement in Dalton and Calhoun coal balls (7 – 11 mol % MgCO₃, N = 18) suggest that both crinoid columnals and HMC cements formed in seawater and support a marine origin for coal balls. This relationship will be tested in the Late Pennsylvanian (Desmoinesian) Mineral Coal (Rich Hill, MO). If coal balls formed sequentially, as indicated by the presence of cement-filled void spaces in nearly all coal balls, these observations offer strong support for the formation of coal balls in marine swamps from marine water. Early HMC in coal balls, found in paleotropical coals throughout the Pennsylvanian, may provide a detailed stratigraphic record of Mg/Ca ratios in Pennsylvanian seawater.