Modern analogs reveal the origin of Carboniferous coal balls
Abstract Coal balls are calcareous peats with cellular permineralization invaluable for understanding the anatomy of Pennsylvanian and Permian fossil plants. Two distinct kinds of coal balls are here recognized in both Holocene and Pennsylvanian calcareous Histosols. Respirogenic calcite coal balls have arrays of calcite δ18O and δ13C like those of desert soil calcic horizons reflecting isotopic composition of CO2 gas from an aerobic microbiome. Methanogenic calcite coal balls in contrast have invariant δ18O for a range of δ13C, and formed with anaerobic microbiomes in soil solutions with bicarbonate formed by methane oxidation and sugar fermentation. Respirogenic coal balls are described from Holocene peats in Eight Mile Creek South Australia, and noted from Carboniferous coals near Penistone, Yorkshire. Methanogenic coal balls are described from Carboniferous coals at Berryville (Illinois) and Steubenville (Ohio), Paleocene lignites of Sutton (Alaska), Eocene lignites of Axel Heiberg Island (Nunavut), Pleistocene peats of Konya (Turkey), and Holocene peats of Gramigne di Bando (Italy). Soils and paleosols with coal balls are neither common nor extinct, but were formed by two distinct soil microbiomes.