Weathering in sub-polar environments: Insights from geochemical analyses of soil samples from the Kerguelen Archipelago

The Kerguelen archipelago is certainly one of the most important sites for studying rock weathering processes in subpolar climates. It is indeed marked by little to no direct anthropogenic impact, by a relatively limited lithological variability (basalts – phonolites / trachytes) but by a significant precipitation variability throughout the archipelago. In order to evaluate the influence of climate gradient on basalt weathering in the Kerguelen Islands soils samples have been collected in different localities along different rainfall and lithology gradients. Field campaigns indicated that soils are poorly developed on bedrocks, except maybe on valley bottom colluvium. Geochemical soil analysis results point out that all the rocks collected show low to intermediate weathering intensity indices (Chemical Index of Alteration ranges from 50-78%) – they also show a systematic loss of Ca and Mg compared with parent rocks linked to the weathering out of plagioclase and ferromagnesian minerals. Although the number of soil profiles studied remains low, the data does not show any major difference in weathering intensity as a function of altitude along slope gradients, nor any significant variation as a function of depth within a profile. Nor do the results show any significant differences in the degree of weathering of rocks according to rainfall in the regions studied. These results will be used to discuss the parameters that might control the intensity of basalt weathering in this geographical area.