EARLY DIAGENETIC HIGH MAGNESIUM CALCITE AND PRIMARY DOLOMITE SUGGEST THAT COAL-BALL PLANTS GREW IN MARINE MIRES

Coal balls are carbonate and pyrite permineralizations of peat that contain three-dimensional plant fossils, preserved at the cellular level. Coal balls occur in Pennsylvanian and earliest Permian paleotropical coals, and have been used to reconstruct ancient mire plants and plant communities in exquisite detail. Despite the wealth of paleobotanical and paleoecological studies focused on coal balls, their depositional environment remains controversial. The nearest living relatives of coal-ball plants (modern lycopsids, sphenopsids, marratialean ferns and conifers) grow in fresh water. Likewise the stable oxygen isotopes of coal-ball carbonate (δ¹⁸O = -16 to -3 per mil) suggest freshwater. Nonetheless, the widespread occurrence of marine invertebrates and early diagenetic framboidal pyrite in coal balls suggests a marine origin.

Petrographic and geochemical (microprobe) analysis of coal-ball carbonates in Pennsylvanian coals from the mid-continent of North America (Western Interior Basin, West Pangaea) and the Ruhr and Donets Basins (East Pangaea) indicate that the first-formed carbonate is either radaxial, non-stochiometric dolomite or high magnesium calcite (9 – 17 mol % MgCO₃), suggesting precipitation in marine water. Although both primary dolomite and high magnesium calcite can form in lacustrine settings, these minerals usually form in lakes in carbonate terranes that experience significant evaporation. Yet paleotropical coal-ball mires, which are under- and overlain by siliciclastic sediments, would have required wet climatic conditions for peat accumulation in fresh water. Pervasive freshwater diagenesis, with low magnesium calcite enveloping individual grains, results in most coal-ball carbonates having a freshwater or mixed isotopic signature.

The realization that coal-ball mires grew in salt water will change our understanding of Pennsylvanian plant communities. The modern analogue of coal-ball mires would be mangroves, among the most productive modern land-plant communities, rather than tropical freshwater mires, among the least productive plant communities in the ever-wet tropics. Because marine peat accumulation depends on the rate of sea level rise rather than rainfall continuity, Late Paleozoic coals with coal balls might not indicate ever-wet paleoclimate.