Small Catchment Scale Molybdenum Isotope Balance and its Implications for Global Molybdenum Isotope Cycling

Abstract

The mass balance of molybdenum (Mo) was studied in the Strengbach catchment. Monitoring of rainfall, vegetation, and soil characteristics in this 0.8 km 2 catchment was started decades ago. We present Mo concentrations and isotope compositions of about 60 samples including bedrock types, perennial springs, soil profiles, roots and leaves, and the outflowing brook. Both stream waters and bedrocks have Mo concentrations at least one order of magnitude lower than global averages. The Mo isotope composition of topsoils, foliage, litter, and roots is rather homogeneous. Net biological fractionation is thus subordinate to differences in the Mo sources. Efficient Mo recycling from organic litter to plants keeps Mo bioavailable. The Mo and Sr isotope data, are used to identify the source(s) of Mo and Sr and their (transient) storage within the catchment. The resulting best model identifies rock weathering and seawater derived aerosol as the principal Mo sources. Moreover, soil in the Strengbach catchment has reached steady state for Mo (the time constant to achieve soil steady state is calculated to be in in the order of 50 years) where the Mo isotope compositions of fluxes to and from the catchments soil are identical.