Conceptual Models of 1200 years of Icelandic Soil Erosion Reconstructed Using Tephrochronology

With reference to 18 tephra isochrones, we present six reconstructions of landscapes in South Iceland at precise times through the last 1200 years and develop three related models of soil erosion. Before the late ninth century A.D., the landscapes of Iceland were without people and resilient to natural processes. The initial impact of human settlement in the ninth century AD was most profound in ecologically marginal areas, where major anthropogenic modifi cations of the ecology drove geomorphological change. In the uplands, overgrazing contributed to the formation of a dense patchwork of breaks in the vegetation cover where soil erosion developed and resulted in the rapid denudation of large areas. As the upland soils were shallow (generally <0.5 m), the overall impact of erosion on total aeolian sediment fl uxes before AD 1510 was modest. Later erosion of the deeper lowland soils (generally >2 m) involved a lower spatial density of eroding fronts and a slower loss of soil cover, but a much greater movement of sediment. Land-management strategies, changes in species patterns of plant communities, extreme weather events, and climate changes have combined in differing degrees to initiate and drive rates of soil erosion. Sensitivity to change and the crossing of erosion thresholds has varied through time. The record of soil erosion has major implications for both archaeology and contemporary land management. School of GeoSciences, University of Edinburgh, Edinburgh EH8 9XP, Scotland. Department of Geology and Geography, University of Iceland, 101 Reykjavík, Iceland. School of Biological and Environmental Sciences, University of Stirling, Stirling FK9 4LA, Scotland. Corresponding author [email protected]. Introduction Iceland provides rare opportunities to assess human impacts on soil erosion and landscape change. Before the Norse settlement, or Landnám of the ninth century A.D., there is no evidence of people in Iceland (Buckland et al. 1995, Fridriksson 1994). As a result, it is possible to identify environmental records from long periods of the Holocene, during climates similar to, warmer, and cooler than today that have no anthropogenic components (Caseldine 1987; Dugmore 1987, 1989; Stötter 1991). Contrast can be drawn with records from the last 12 centuries when the changing climates of the “Medieval Warm Period” (Grove and Switsur 1994) and “Little Ice Age” (Grove 1988) have been interwoven with extensive human impacts on the landscape (Arnalds 1987, Runolfsson 1978, Thomson and Simpson 2007). At present, however, chronologically precise and spatially explicit models of long-term landscape change in Iceland are lacking. Spatial and temporal patterns of tephra deposition provide one means of creating detailed models of change that can be tested and used to develop understanding of the interplay of different processes over diverse landscapes through century-millennia timescales. In this paper, we focus on a district of southern Iceland and use 18 tephra isochrones to develop six reconstructions of Icelandic landscapes at precise times through the last 1200 years. Landscape change in Iceland The Norse colonists introduced herbivorous mammals to Iceland for the fi rst time, rapidly building up populations of sheep, goats, pigs, cattle, and horses (Amorosi et al. 1997). Woodland and scrub were cleared, and fi eld systems established (Vésteinsson 1998). Up to the 20 century, livestock grazed all year round, and farms had access to common summer pastures extending up to 500–600 m above sea level (Fridriksson 1973, Thoroddsen 1919). Vegetation cover in Iceland (103,000 km) has diminished signifi cantly since early Norse settlement. Today it is about 28% of the island’s area (LMI 1993) as opposed to much more extensive presettlement vegetation cover estimated to be between 54% (Ólafsdóttir et al. 2001) and 65% (Thorsteinsson 1986) of total land area. The composition of the vegetation cover has also greatly changed; estimates of woodland coverage at the time of settlement vary from 15,000 km (14.5%)–40,000 km (39%) (Bergthorsson 1996, Bjarnason 1974, Einarsson 1962, Olafsdottir et al. 2001, Sigurdsson 1977, Thorarinsson 1961, Thorsteinsson 1986), whereas present woodland coverage is 1% (LMI 1993). Changes in the species composition of plant communities, their distribution, and overall vegetative cover, have been related to enhanced soil erosion, increased aeolian sediment fl uxes, slope instability, and hydrological changes (e.g., Arnalds 1987; Arnalds et al. 2001a; Dugmore et al. 2000; Einarsson 1961, 1963; Gísladóttir 1998; Hallsdóttir 1987; Haraldsson 1981; Ólafsdóttir et al. 2001; Thorarinsson 1961; Thorsteinsson 1986, 2001). The changes in both the extent and nature of vegetation cover have been attributed to direct or indirect anthropogenic effects acting in combination with unfavorable climate and erodable soils. 2009 2:XX–XX Journal of the North Atlantic