T61 Deep Weathering, Ancient Landscapes and Regolith- Dominated Terrains (Posters)

Sunday, 25 September 2016
9:00 AM - 5:30 PM
Exhibit Hall E/F

GEOCHEMICAL CONSTRAINTS ON DURATION OF WEATHERING FOR NEOPROTEROZOIC BALTIC PALEOSOL

Paper number: T61-1
Program: Technical Sessions
Time: 9:00 AM – 5:30 PM

Authors

Abstract

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 K2OCALC, 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 K2OCALC 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.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-277759

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