Chemical weathering of mafic rocks in boreal subarctic environment (northwest Russia) under influence of glacial moraine deposits

Chemical weathering of mafic rocks represents substantial sink of atmospheric CO2 yet the mechanismes of this process in high-latitude regions are poorly understood. This work addresses geochemical migration and partitioning of major and trace elements between rock, soil and surface waters during chemical weathering of mafic rocks and soil formation under the influence of a glacial moraine in the northwestern Russian subarctic. We used multidisciplinary approach which included major and trace element chemical analysis, Sr isotopic measurements, and mineralogical structural and microscopic investigations. Quaternary (Pleistocene) deposits were observed even in the deepest horizons of soil profiles and confirm a strong moraine influence as most soils showed podzolic features. Chemical and Sr isotopic analyses revealed a strong impact of the moraine on soil chemistry and mineralogy. Mineralogical studies showed the presence of non-aeolian quartz and zircon in soils which were not linked to the nature of the parental rocks (i.e. mafic and felsic). At the same time, we observed the presence of etch pit corrosion on the surface of zircons and feldspars, and a newly-formed matter adjacent to the surface of primary phases. The chemical index of alteration (CIA) showed weak or no weathering of rocks within the soil profile and the weathering intensity scale confirmed the “felsic” chemical composition of soils developed over mafic and ultramafic rocks. Analysis of Sr radiogenic isotopes demonstrated preferential removal of easily weatherable and less radiogenic minerals over the full depth of soil profile and a more radiogenic signature of the granitic moraine compared to mafic and felsic bedrocks. The composition of most surface waters reflected the weathering of silicate rocks but did not allow for distinguishing purely mafic source. The chemical weathering rates of ultramafic rocks were calculated to be higher than those for the granitic till, but the moraine depositions which dominate this region hide evidence of the real weathering process. This could be due to retention of Mg and Ca in soil due to precipitation of secondary phases such as Mg-vermiculite or divalent cation adsorption on mineral, organic or organo-mineral phases. As a result, weathering rates estimated for the olivinite rock appear to be lower than the weathering rates of mafic rocks reported in other boreal and subarctic regions. Overall, the felsic moraine deposits are capable to sizably decrease the weathering intensity of mafic rocks which should be taken into account for chemical weathering modelling of high-latitude regions subjected to glaciation in the past.