ECHINODERM OSSICLES IN PENNSYLVANIAN COAL BALLS: A PROXY FOR SEAWATER MG/CA RATIOS

Coal balls are carbonate and pyrite concretions that contain permineralized plant matter, with exceptional preservation of plant anatomy, which have been used to reconstruct of Pennsylvanian and Permian plant communities. Some coal balls also contain well-preserved, marine shell hash, which is syndepositional with the paleotropical peat. Early diagenetic primary dolomite and high-magnesium calcite (HMC) cement in coal balls indicate that they precipitated from marine water. Early diagenetic framboidal pyrite in coal balls supports the marine hypothesis, as does the presence of marine invertebrate hash, including echinoderm ossicles. Echinoderm ossicles that retain original microstructures can record the Mg/Ca ratio of seawater at the time of biomineralization, and have been used as a proxy for the Mg/Ca ratio of Phanerozoic seawater.

In Late Pennsylvanian (Kasimovian) coal balls from the Dalton coal in the Wolf Mountain Shale of Texas, early diagenetic HMC crystals contain 9.4 – 11.1 mol % MgCO₃. These HMC crystals have low-magnesium calcite rims due to freshwater diagenesis that followed the precipitation of marine HMC cement. As a result, their stable oxygen isotopic signatures (δ¹⁸O = -6.32 to -2.42‰) reflect mixing between marine and freshwater, assuming that the average oxygen isotopic ratios of unaltered Kasimovian brachiopods (-2.0‰) reflect seawater values. Echinoderm ossicles in Dalton coal balls appear to retain original microstructure, and may preserve the Mg/Ca ratio of Late Pennsylvanian seawater. Abiotic HMC cement from Dalton coal balls has Mg/Ca ratios very similar to those of contemporaneous echinoderm ossicles (9.9 – 12.5 mol % MgCO₃), predicting an Mg/Ca ratio of 2.8 – 3.8 for Late Paleozoic seawater. Coal balls occur commonly in Pennsylvanian coals from the mid-continent of North America and the Donets Basin. They are more common than echinoderm ossicles that preserve original microstructure, and equally well dated. If echinoderm ossicles and abiotic HMC from Dalton coal balls have similar mol % MgCO₃ values, yielding similar Mg/Ca ratios for Late Pennsylvanian seawater, we may be able to use the coal-ball record to reconstruct the history of Mg/Ca ratios in Late Pennsylvanian seas.