Tephrochronology, landscape and population: impacts of plague on medieval Iceland
Streeter, Richard Thomas
This thesis examines the extent to which geomorphological change in sub-arctic landscapes may be driven by rapid declines in population over timescales of decades to centuries. Demographic decline driven by disease in pastoral agricultural systems is expected to alter patterns of land use. Using a chronology with 20 visible dated tephra layers from AD 870 to present, 2625 tephra layers were identified in 200 sediment profiles. Rates of sediment accumulation dated by tephra provide a record of erosion in Skaftártunga, South Iceland. The scale of enquiry is that of individual landholdings (5–10 km2) over decades to centuries; in order to tackle questions of resilience and change within coupled socio-ecological systems larger and smaller spatial scales (regions of 400 km2 and individual sediment profiles) and longer and shorter temporal scales (2.6 ka and years to decades) are also considered. The novel application of photogrammetric techniques to recording stratigraphic sections increases the frequency of measurement from tens to hundreds per stratigraphic unit and the resolution from ±2.5 mm to ±1 mm. This technique improves the accuracy of representative measures of sediment accumulation and their use in measuring landscape change. Two little known 15th century AD Grímsvötn tephras are mapped and dated to AD 1432±5 and AD 1457± 5 using sediment accumulation rates. A period of landscape stability from AD 1389–1597 is consistent with reduced grazing pressure due to population declines of more than 30% after plague in AD 1402–1404 and AD 1494. Climatic deterioration from AD 1450-1500 does not increase erosion as much as expected; this may be due to decreased grazing pressure after population decline in the 15th century. Increased erosion from AD 935–1262 is related to woodland clearance and increases in sediment accumulation post AD 1625 are related to climatic cooling during the Little Ice age and the migration of erosion fronts into deep lowland sediments.