DIAGENETIC ALTERATION OF CARBONATE PHASES IN A RHIZOCRETION FROM A WELL DEVELOPED PALEOSOL IN THE UPPER PENNINGTON FORMATION, POUND GAP KENTUCKY

     Paleosols can provide valuable information about past climates and depositional environments. Although carbon isotopic analysis of pedogenic carbonate nodules is a powerful tool to obtain paleo-atmospheric pCO_2, it is necessary to acquire samples from unaltered or little altered carbonate phases within soil carbonate rhizoliths and nodules.  Isotopic and elemental analysis of several carbonate phases in a large, 8cm diameter, rhizocretion from the Mississippian upper Pennington Formation, at Pound Gap, Kentucky, provide evidence of minimal alteration of  pedogenic phases by later diagenetic fluids which precipitated calcite spar in cracks penetrating the rhizocretion.

     Petrographic analysis revealed seven areas of interest, four complex micritic or micro-sparatic pedogenic phases and three equant, crack-fill, calcite spar phases.  Cathodoluminescence revealed that micritic and crack-fill phases represented discrete depositional phases.  If later diagenetic fluids, which precipitated the crack-fill phases, altered the pedogenic micritic phases, then geochemical analysis should reveal mixing trends between micritic phases and crack fill phases.

    Geochemical and isotopic analysis of individual phases revealed distinct differences between the crack-fill phases and the pedogenic micritic phases.  Isotope values for the micritic phases cluster within typical Carboniferous, meteoric water ranges, -7.46‰ < d¹⁸O < -5.49‰, mean of -6.51‰ (PDB), -6.62‰ >  d¹³C > 4.98‰, mean of -6.18‰ (PDB).  In contrast, the crack-fill phases are variable with, d¹⁸O from -13.08‰ to -5.54‰ and d¹³C from -7.86‰ to -0.94‰.  Mn and Fe quantities in the micritic phases varied widely.  Mn ranged between 2085ppm and 25856ppm.  Minimum Fe quantities ranged from 11 wt. % to 29 wt. %.  The crack fill spar phases had less variation in both Fe and Mn and Fe quantities were much lower than the micritic phases.  Mn ranged from 7186ppm to 11491ppm and Fe ranged between 1.0 wt. % and 1.9 wt. %.  The micritic phases have a distinct composition from the crack-fill spars and appear to be genetically related.  In contrast, the crack-fill spars are not genetically related and do not appear to have altered the micritic/micro-spar  phases, suggesting the micritic/micro-spar phases could yield estimates for paleo-atmospheric pCO₂ reconstruction.