Weathering of tephra and the formation of pedogenic minerals in young Andosols, South East Iceland

Frequent tephra deposition and a steady influx of aeolian material of multiple origins, dominate soil formation in Iceland. Little is known about the weathering behaviour, mineral formation and alteration of tephra and Icelandic Andosols after tephra deposition. Two sampling sites in the volcanically active area south of Vatnajökull in South East Iceland were selected according to the presence of two distinctive tephra layers; a light coloured, rhyolitic, tephra from the Öræfajökull eruption in 1362 CE and a black, basaltic, Veiðivötn tephra from 1477 CE. Through a combination of physical, chemical and mineralogical analyses, the present research improves the understanding of changes in properties, weathering processes and mineralogy in Andosols after the deposition of tephra and aeolian material. It adds significant knowledge about the impact of explosive volcanic eruptions and consequences of heavy tephra fall and erosion on soil development in Iceland in particular, but may have wide-ranging influence on the management of volcanic soils worldwide. Both pedons can be classified as Andosols, showing silandic and vitric soil properties. The soils were acidic and dominated by sand. Fe_o/Fe_d ratios above 0.75 in all soils indicated a low degree of soil development. The major portion of the clay size particles mainly derived from poorly crystalline and amorphous constituents (e.g. allophane and ferrihydrite). In spite of the low soil age (less than 650 years) and the prevailing cool climatic conditions, we observed signs of pedogenesis and the presence of secondary clay minerals in both, soils and tephra. It was mainly secondary chlorite, which could be verified. Usually it takes more time under given environmental conditions for clay minerals to form and alter. The phyllosilicates we found are supposedly attributed to aeolian influx of material from older, more weathered, more developed eroded surfaces of unknown origin and chemical composition, maybe from sources outside Iceland. This and the particular local site conditions (e.g. moisture, soil temperature, SOM), rather than the primary composition of the parent material, are the driving factors in the development of the investigated Andosols and the alteration of minerals. Additionally, the high volcanic activity in Iceland constantly provides new tephra to the soils.