GEOCHEMICAL, PETROGRAPHIC AND PALEOENVIRONMENTAL CONSTRAINTS ON COAL-BALL FORMATION

Coal balls are carbonate and pyrite concretions that formed in Pennsylvanian and early Permian peat swamps and preserve ancient plants in cellular detail. Although coal-ball plant fossils inform our understanding of Late Paleozoic plant evolution and paleoecology, their environment of formation remains controversial. In North American coal balls the first widespread cement is high-Mg calcite with average Sr/Ca mmol/mol values between 0.7 and 1, and Na/Ca mmol/mol values > 1, indicating formation from marine pore water. This high-Mg calcite takes the form of large (40-60 µm x 250 µm) polycrystals, and fills large spaces in coal balls, ranging in size from cm-scale hollow stems, aerenchymatous roots, and seed cavities to the lumena of plant cells and tracheids, 40 – 60 µm in diameter. The size and shape of early high-Mg polycrystals suggests formation due to degassing, as in travertine deposits and similar to the formation of beach rock on Grand Caymen. In coal balls, high-Mg calcite polycrystals record the porosity of the peat in which they formed.

The next generation of carbonate is low-Mg calcite, some with Na/Ca mmol/mol values ≤ 0.5, indicative of formation in fresh water, placing coal-ball formation in the freshwater-marine mixing zone of Pennsylvanian and early Permian coastal peat swamps. This low-Mg calcite rims individual high-Mg calcite polycrystals and permineralizes cell walls and decayed peat with small (< 40 µm) interparticle pore spaces. Low Mg-calcite in coal balls precipitated as micrite and subsequently altered to fine crystalline (∼100 µm) anhedral calcite. Close association of some low-Mg calcite with organic carbon in coal balls suggests that these are microbial carbonates.

Because coal balls mix marine high-Mg calcite and fresh-water low-Mg calcite on the micron-scale, stable oxygen isotopic analyses have yielded freshwater or brackish marine isotopic signals. The oxygen stable isotopic signature of coal-ball calcite appears to depend on the ratio of high-Mg to low-Mg calcite in the sample.