TEPHROCHRONOLOGY AND HOLOCENE SOIL EROSION IN ICELAND

The strengths and limitations of tephrochronology are assessed with reference to Icelandic Holocene soil erosion. Icelandic tephrochronology is based on a system of reference profiles. As a result of differing patterns of volcanic fallout, surface processes, the spatial variability of microtopography and vegetation cover, and changes in these factors through time, single reference profiles in any one region are generally inadequate, so multiple profiles are used. In isolation the physical and chemical characteristics of different tephra layers have different potentials for discrimination. For example, some, or many of the tephras produced by a particular volcanic system (such as Katla) can have similar chemical compositions, meaning that in isolation these characteristics alone have a limited potential for the discrimination and correlation of different tephras. Crucially, effective tephrochronologies do not rely on identifications based on single criteria. Particularly distinctive tephras of known origin and age are used to identify stratigraphically related layers with less notable individual characteristics (such as chemistry that is similar to other tephras produced by the same volcanic system). In Icelandic studies of landscape change tephra layers have been used to determine four key datasets; the rates and types of sediment accumulation for clearly defined periods of time, the geometry of tephra layers, and the microtopography of the surface when the tephra was deposited. It is shown that aeolian soil erosion can be assessed through the use of these records. Accumulation rates and sediment type indicate the nature and intensity of erosion, and the geometry of the tephras indicate whether sources of aeolian sediment (and hence areas of erosion) are local or regional. Microtopography is a key indicator of past environmental processes such as near surface cryoturbation. Tight dating control means that in addition to reconstructing landscape change, probable causal processes can be assessed. Inappropriate land management systems are a principal cause of soil erosion over the last millennium, but climate and other environmental changes have exacerbated their impact.