Oxisolic processes and geochemical constraints on duration of weathering for Neoproterozoic Baltic paleosol

A transient greenhouse warming event during the late Neoproterozoic has been invoked to explain the incongruence of cooler temperatures and intense weathering at high latitudes. We explore this further by examining three deep weathering profiles of the Neoproterozoic Baltic paleosol (600-560 Ma). These profiles have an Oxisolic horizon dominated by quartz, kaolinite, phlogopite/vermiculite, hematite and goethite. Mass-balance calculations (assuming immobile Al2O3) for the newly characterized Erra profile studied here showed loss of 25-50% SiO2 and MgO, complete loss of CaO and Na2O, 25-50% loss of K2O, and 25-100% gain in Fe2O3 in the upper Oxisolic horizon. pH was estimated as moderately acid to very acidic (4.5-6) during weathering using a new paleo-pH proxy. MAP (1500-1600 mm yr−1: CIA-K) and MAT (13-15(+) °C: PPM1.0) were also estimated using bulk geochemical proxies. Calculations for pCO2 follow an earlier thermodynamic method with evaluation of total mass flux for CaO, MgO, Na2O, and K2OCALC (corrected for K metasomatism) during weathering of each protolith material. When considering a range of acceptable values of paleoatmospheric pCO2 based on earlier studies, a minimum of 1.88 × 105 and a maximum of 1.02 × 106 yr weathering duration is indicated, consistent with previous interpretations that the Baltic paleosol is a deep paleo-Oxisol. This long weathering duration estimate does not require a “transient Neoproterozoic greenhouse event” to explain the depth (to 30 m) and intensity of weathering, especially given newer paleolatitudinal reconstructions.