Micromorphological characteristics reflecting soil-forming processes during Albeluvisol development in S Norway .

Abstract

This paper presents micromorphological observations of the only two Albeluvisol chronosequences to have been reported in the international literature so far. These observations are combined with existing profile morphological and soil chemical data in order to identify the major processes involved in the development of Albeluvisols. The study area is located in the counties Vestfold and Østfold on the western and eastern sides of the Oslofjord, S Norway. The region is characterized by continuous glacio-isostatic uplift over the Holocene, and hence the age of the land surface increases continuously from the beach towards the higher elevations. Twelve soil pits in loamy marine sediments were investigated, six each in Vestfold and Østfold; in addition, three samples of fresh sediments were taken from the shoreline. Results of this study suggest that as soon as the land surface is raised above sea level, drainage of the coarse pores and aeration of the upper part of the young soils leads to five major processes: i) development of deep desiccation cracks, forming a polygonal pattern; ii) compaction, taking place as soon as the land surface reaches an elevation above sea level that leads to drainage of the coarse pores; iii) pyrite oxidation, releasing sulfuric acid; iv) rapid decarbonatization of the originally calcareous sediments through carbonate dissolution by acids from pyrite and iron oxidation; v) precipitation of iron hypocoatings and coatings in the capillary fringe. The next morphological change, also taking place within less than 2.1 ka, is horizon differentiation into Ah, Eg and Btg horizons due to the limited water permeability of the fine-textured sediments. Eg horizons, for example, become lighter in colour with time. The process leading to the next morphological change in the soil profiles is clay illuviation, which is also already present in the 2.1 ka-old soil. Soil pH in the upper part of the E horizon of this soil is already too low for significant clay mobilization. Clay illuviation is still active in all soils studied, but the upper boundary of the clay mobilization zone is at 20-50 cm depth. Progressive clay illuviation is recorded by the increasing thickness of clay coatings and proportion of voids having clay coatings. Clay mobilization and iron co-eluviation in the upper parts of the Eg horizons cease within less than 2.1 ka, whereas weathering and formation of clay minerals and iron oxides continue, leading to formation of a BE horizon in the upper part of the Eg horizon. Albeluvic tongues start to form after 4.6-6.2 ka. They develop preferably along the desiccation cracks. Albeluvic material is washed into the cracks, and enhanced leaching of bases and clay eluviation takes place in the cracks. As both processes proceed, the albeluvic tongues get longer and wider. Clayey intercalations occur in the Stagnic Albeluvisols of the sequence, and the following concept is suggested to explain their genesis: after snow melt or a rainy period infiltrating water arrives at the lower end of an albeluvic tongue, the tongue fills up with water, and perched water also accumulates on top of the dense Btg horizon. Water, carrying suspended clay, penetrates under pressure from the tongue into the Btg horizon, where additional clay is mobilized. The clay settles when the velocity of the water decreases, forming clayey intercalations in the dense matrix of the Btg horizon.