Rhyolitic tephra horizons in northwestern Europe and Iceland from the AD 700s-800s: a potential alternative for dating first human impact

The distribution and geochemistry of four rhyolitic tephra horizons from Iceland dated to the AD 700s-800s is assessed. These include the rhyolitic phase of the Landnam tephra (AD 870s), the AD 860 layer, a previously unrecorded tephra called the GA4-S5 layer (C. AD 700-800) and the Tjornuvik tephra (C. AD 800s). The AD 860 and GA4-85 layers were first found in peat bogs in north Ireland. They are here correlated with equivalent horizons on Iceland which were found below the Landnam tephra (C. AD 870s). This time period is considered important in the North Atlantic region, because it coincides with a phase of human settlement in Iceland and the Faroe Islands. The establishment of a detailed tephrochronology may provide a tool for exact dating of sediment successions and sediments associated with archaeological excavations. Caution must be taken especially on Iceland where the Landnam tephra is often used for dating archaeological sites. This investigation show that several rhyolitic tephra horizons occur close in time to the Landnam tephra, and that mistakes can be made if detailed geochemical analyses are not carried out, especially in areas which are distal to the source of the Landnam tephra (the Veidivotn and Torfajokull volcanic systems, southern Iceland).