A micromorphological study of andosol genesis in Iceland

Abstract

Icelandic Andosols form in climatic and geological conditions that are different from those common for Andosol formation, namely a frigid oceanic climate and the steady aeolian addition of fresh and reworked tephra material. Three representative profiles were sampled in different ecological zones of Iceland for -micromorphological studies. They are classified as Orthidystri-Vitric Andosol, -Dystri-Vitric Andosol and Thaptohistic-Vitric Andosol. The parent material consists entirely of relatively young volcanic ashes, and organic layers. Multiple lithological discontinuities are recognised. Microstratification is observed in thin sections, both in organic and inorganic horizons. The coarse mineral fraction is characterised by variable proportions of pumice, different types of glass, hypo- and holocrystalline pyroclasts and euhedral and subhedral crystals of feldspar and augite. Locally, green-grey volcanic glass shows pellicular alteration to an orange component, considered as palagonite. Some more clayey horizons (described as B horizons in the field) consist of soil aggregates or nodules of various compositions, mixed with pyroclasts covered by a coating of micromass. They may represent locally transported material of older, more evolved soils. The microstratification of both organic and mineral material and the (sub)-horizontal orientation of organ and tissue residues points to a gradual deposition of material, with short interruptions, not disturbed by pedoturbation, especially bioturbation. It means that every sub-layer corresponds to a former soil surface in a temporarily relatively stable environment. A weakly developed lenticular or isoband microstructure, pointing to freeze-thawing phenomena, is observed in several -horizons. Capping, often with reverse sorting, observed between 100-230 cm depth on organ residues in one of the profiles are signs of former frost activity. It is perhaps the first time that such cappings are reported on organic components. The relative pureness of most mineral layers, the freshness of the volcanic components and their sorting indicate in many cases direct deposition during eruptions, rather than a water or air transport over land. © 2008 Springer-Verlag Berlin Heidelberg.