ASSESSING MARINE INFLUENCE IN CARBONIFEROUS COAL BALL FORMATION WITH STRONTIUM ISOTOPES

Coal balls are the diagenetically altered remains of plants living in Carboniferous (~300 Ma) swamps. Sections of the peat swamp are permineralized with calcium carbonate and can preserve cellular level detail of plant material. The University of Illinois houses the most comprehensive collection of coal balls in the world, the Tom L. Phillips collection, consisting of over 500,000 coal ball peels. These coal balls also present an opportunity to understand their formation, which is poorly constrained. Previous work has suggested that marine water is necessary for the formation (Raymond and Phillips, 1983), while others found that coal balls formed under the influence of meteoric water as the degassing of carbon dioxide drives pH higher (Siewers and Phillips, 2015). We tested whether marine or meteoric water was required for coal ball formation by examining strontium stable isotopes in a stratigraphic section of the Herrin Coal. If strontium values were similar to the Carboniferous marine strontium isotope curve, then marine water was likely inundating coal swamps during formation, while if strontium values differed, then the coal balls were formed in meteoric water conditions.

We analyzed strontium isotopes from 14 coal balls from the Phillips Collection. Strontium isotope values were largely consistent throughout the stratigraphic section and greater than seawater concentrations throughout the Carboniferous. This suggests that an influx of strontium was delivered to peat swamps via the erosion of shales enriched in strontium, likely from the Central Pangean Mountains. This would indicate that coal balls were forming in the presence of meteoric water sourced from the Laurentian highlands. An alternate explanation is that terrigenous clays were included within the calcium carbonate of the coal ball and are increasing the measured strontium values. We are currently reanalysing our samples with a new technique to exclude any terrigenous clays and distinguish between these two sources of strontium.