The importance of mineral determinations to PROFILE base cation weathering release rates: A case study
Authors: Casetou-Gustafson Sophie , Akselsson Cecilia , Hillier Stephen , Olsson Bengt A. 
Published: January 2019

Abstract. This study explored the influence of uncertainty in quantitative mineralogy on PROFILE base cation (Ca, Mg, K, Na) weathering rates obtained using normative mineralogy compared to those obtained using measured mineralogy, which was taken as a reference. Weathering rates were determined for two sites, one in Northern (Flakaliden) and one in Southern (Asa) Sweden. At each site, 3–4 soil profiles were analyzed at 10 cm depth intervals. Normative quantitative mineralogy was calculated from geochemical data and qualitative mineral data with the Analysis to Mineralogy program (A2M) using two sets of qualitative mineralogical data inputs to A2M: A site-specific mineralogy determined from X-ray powder diffraction (XRPD) analyses, and regional mineralogy, representing the assumed mineral identity and compositions for larger geographical areas in Sweden. For the site-specific mineral input the precise elemental compositions of minerals were determined by microprobe analysis, whereas for the regional mineralogy the compositions were as assumed in previous studies. A2M does not provide a unique mineralogical solution and one thousand random mineralogical solutions were calculated by A2M for each soil unit in order to include the full space of quantitative mineralogies in model outcome, all equally possible. A corresponding number of PROFILE runs were made to estimate weathering rates. The contribution of individual minerals to the release of base cations was also quantified by using a version of PROFILE which outputs this detail. A discrepancy between weathering rates calculated from XRPD data (WXRPD) and weathering rates based on A2M (WA2M) was only considered significant if the former was outside the full range of the latter. Arithmetic means of WA2M were generally in relatively close agreement with WXRPD. The hypothesis that using site-specific instead of regional mineralogy will improve the confidence in mineral data input to PROFILE was supported for Flakaliden. However, at Asa, site-specific mineralogies reduced the discrepancy for Na between WA2M and WXRPD but produced larger and significant discrepancies for K, Ca and Mg. For Ca and Mg the differences between weathering rates based on different mineralogies could be explained by differences in the content of some specific Ca- and Mg-bearing minerals, in particular amphibole, apatite, pyroxene and illite. It was concluded that improving the precision in the content of those minerals would reduce weathering uncertainties. High uncertainties in mineralogy, due for example to different A2M assumptions, had surprisingly low effect on the weathering from Na- and K-bearing minerals. This can be explained by the fact that the weathering rate constants for the minerals involved, e.g. K-feldspar and micas, are similar in PROFILE. Improving the description of the dissolution rate kinetics of the plagioclase mineral group as well as major K-bearing minerals (K-feldspars and micas) should be of particular importance to future weathering estimates.