Development of a buried paleosol on silica-undersaturated mafic lava (Fogo Island, Cape Verde): An Fe speciation study using Mössbauer spectroscopy and magnetization

Abstract

A brown paleosol developed on mafic lava and overlain by lapilli (Campanas profile, northern part of the Fogo Island, Cape Verde archipelago) was studied. The detailed iron speciation obtained by Mössbauer spectroscopy and magnetization confirmed that oxidation is a major chemical paleoweathering mechanism, as it still is in present day soils of these semi-arid islands. The thermal effects on the paleo-surficial due to the overlying lapilli deposit do not appear to have been significant since only an increase of the Fe3+ oxide particles size is observed. Nevertheless no biological remains could be found in this brown paleosol, which may be mainly attributed to the paleoenvironmental conditions and/or to low atmospheric inputs/eolian transport from continental Africa. Maghemite is the predominant Fe oxide in the profile. Magnetite slightly oxidized is still present in all samples of the profile. No positive correlation was found between Cr and As distribution and nano-sized oxides: Cr is gradually depleted upwards following the gradual weathering increase of the silicates, and there is no trend in the As content variation along the profile. This may be related to the fact that nano-sized maghemite and magnetite do not adsorb Cr and As as readily as nano-sized hematite, which could only be detected in trace amounts. In these oxidative weathering conditions, Cr may be significantly lost from mafic lavas, mobilized and transported as dissolved and/or particulate matter in rivers and groundwater. In the first weathering stages a significant increase of the mafic index of alteration for oxidizing environments (MIA(O)) and an increase of the Fe3+ in oxide contents occur; in the upper levels of the profile, MIA(O) does not vary significantly, but Fe oxidation within the silicates structure increases. The combination of MIA(O) and the Fe oxidation degree, particularly the Fe3+/(total Fe) fraction in silicates, appears to be a good approach for an accurate evaluation of the alteration/weathering degree of paleoweathered mafic lava in semi-arid and oxidizing environment.