GEOCHEMICAL CONSTRAINTS ON DURATION OF WEATHERING FOR NEOPROTEROZOIC BALTIC PALEOSOL

Time duration of pedogenesis is one of the most elusive parameters in studies of paleoweathering, especially for deep-time paleosols with limited or unavailable geochronological control. The Neoproterozoic Baltic paleosol (600-560 Ma) developed on a major disconformity in the eroded Paleo- to Mesoproterozoic crystalline basement of the former Baltica continent in northwest Russia, Estonia, Latvia and Lithuania. Mean Annual Precipitation (1500-1600 mm yr-1: CIA-K) and Mean Annual Temperature (10-15 ^oC: PPM1.0) were estimated using bulk geochemistry for three deep weathering profiles sampled from drillcores. Mass-balance calculations assumed immobile Al₂O₃ during weathering, because it shows less variability with depth, on a relative percentage basis, than TiO₂ or Zr. Protoliths (parent materials) were chosen for each drillcore profile based on evaluation of Harker plots, as well as trends exhibited on A-C*N-K diagrams. A correction for K-metasomatism (additions) for each profile was applied and defined as K₂O_CALC, before calculating mass flux of K₂O. Calculations for pCO₂ follow the thermodynamic method of Sheldon, with evaluation of total mass flux for CaO, MgO, Na₂O, and K₂O_CALC during weathering of each protolith material. Iterative calculations were then performed varying time duration of pedogenesis, while assuming all other parameters remained constant; when considering acceptable values of paleoatmospheric pCO₂ based on earlier studies (1.0 < x PIAL< 10.0), a minimum of 3.0 x 10⁵ and a maximum of 5.7 x 10⁶ yrs weathering duration is indicated, which is consistent with previous interpretations that the Baltic paleosol is a deep paleo-Oxisol. This long weathering duration estimate, using our approach here, does not require invoking a “transient Neoproterozoic greenhouse event” to explain the observed depth (to 40 m) and intensity of weathering, especially given newer paleolatitudinal reconstructions. Sensitivity testing of a “modern” granite weathering profile suggests that this is an appropriate procedure for saprolite-protolith systems with thin or no soil cover. Sensitivity of the pCO₂ model to weathering duration can therefore be used for interpreting paleoweathering profiles in which pCO₂ can been constrained independently by other studies.