Illitization in a Paleozoic, peat-forming environment as evidence for biogenic potassium accumulation

350 210 Stroud Water Research Center

Gill, S., and Yemane, K. 1999. Earth and Planetary Science Letters 170(3):327–334.

doi: 10.1016/S0012-821X(99)00102-8


Potassium enrichment is common in surface horizons of many paleosols and has been attributed to the formation of authigenic illite from smectites during burial diagenesis. In such cases, the presence of primary smectite is key to the illitization process, which then results in potassium enrichment as a byproduct. Potassium enrichment in paleosols where no primary smectite was present, however, requires a different process of accumulation. A Lower Pennsylvanian, tropical Ultisol from the Southern Anthracite Field in Pennsylvania is enriched in potassium in the surface horizon and formed in an environment unfavorable for smectite accumulation. Kaolinite dominates clay mineral fractions of soils that form in such highly weathering environments. In the absence of primary smectite to scavenge potassium during illitization, there should be no potassium enrichment during burial diagenesis. However, in Lykens Valley paleosol, the pattern of potassium concentration, organic matter distribution and authigenic illite growth suggests that organic matter may have provided an important source of potassium for illite formation. Illitization of kaolinite in the organic-rich surface horizon is complete but, illite coexists with kaolinite in the B horizon. Within the paleosol matrix, illitization is more complete in areas of high organic concentration where organic fragments often have illite haloes with illite whorls growing from their edges. We propose that the potassium enrichment both in the soil profile and in the overlying coal is the result of the luxury uptake of potassium by peat-forming plants (nutrient intake at rates exceeding metabolic requirements). Further, potassium immobilized by adsorption on the peat provided interlayer cations for illite authigenesis to begin. If, as we suggest, luxury nutrient uptake during peat formation sequestered sufficient potassium to promote later illite formation, the pattern and extent of illitization within the paleosol profile reflects early biogenic nutrient accumulation. Thus, illite, in some paleosols, may represent fossil evidence for a biological process: the luxury uptake of potassium.