Effect of mineralogy and pedoclimatic variations on Ni and Cr distribution in serpentine soils under temperate climate

Abstract

Part of the soils occurring in the serpentinite Szklary Massif (SW Poland) have developed in Quaternary under temperate climate on the parent rock not affected by chemical weathering. In studied parent ultrabasite, Ni occurs in Cr-magnetite (up to 0.96 wt.% of NiO), forsterite (up to 0.55 wt.% of NiO), iddingsite (up to 0.99 wt.% of NiO), and serpentine (up to 0.55 wt.% of NiO), while Cr is bounded with Cr-magnetites (up to 21.77 wt.% of Cr 2O 3) and clinochlores (up to 1.68 wt.% of Cr 2O 3). Cr-magnetites are highly resistant to weathering while other primary minerals, more susceptible to weathering release parts of Ni and Cr which are trapped in authigenic phases. The latter, however, contain less Ni and Cr than corresponding primary minerals: clayous matrix 0.37 wt.% of NiO and 0.08 wt.% of Cr 2O 3, smectites 0.30 wt.% of NiO and 0.07 wt.% of Cr 2O 3 and mixture of oxy-hydroxides and clay minerals 0.36 wt.% of NiO and 0.14 wt.% of Cr 2O 3. At the mineral scale, smectite plays an important role in the trapping of Ni. At the profile scale, Ni and Cr are moderately diluted upwards the soil profile and Cr is generally less mobile than Ni. The soil formed in Szklary is chemically different than the analogous serpentine soils occurring more to the south in Europe (French Massif Central, Vosges Mountains and Ligurian Apennines, NW Italy). The difference is due to participation of fluvioglacial material produced by Quaternary continental glaciacion in Poland. However, similar vertical relative variations of Ni and Cr were found in all compared soils in spite of differences in parent rock composition, allogenic contaminations and degree of soil development. Comparison with other well-drained pedons located in similar uphill positions shows a climatic control of the Cr and Ni vertical distribution in serpentine soils. Humid mountainous variant of temperate climate induces leaching of Mg and subsequent lack in smectite neoformation which lead to the release of Ni. Ni is less mobile in the colder and dryer climate of Poland. Moreover, some primary minerals which are generally considered as not stable under soil conditions (olivine, serpentine and chlorite) are still present even in the A horizon of the studied Polish pedon. Their resistance is due to slower rates of weathering related to the climate but also due to former hydrothermal processes affecting the parent rock fabric.