Dispersal, source and glass chemistry of a third tephra unit within Iceland's Settlement Layer

Tephra layers may be studied to understand the dynamics and sequence of past explosive volcanism. In Iceland, the distinctive Settlement Layer (AD ~877) underlies almost all archaeological evidence for the Norse Settlement and is preserved in soils across Iceland and as cryptotephra overseas. It comprises an olive‐green, tholeiitic basalt unit erupted from the Vatnaöldur crater row (within the Veiðivötn fissure swarm) and a white transitional alkali rhyolite unit from the Hrafntinnuhraun craters (within the Torfajökull complex). This paper describes a previously undocumented, dark‐grey tephra unit that we discovered in the Búland landholding (Skaftárhreppur). Electron probe micro‐analyses determined that this tephra unit is a bimodal transitional alkali basalt. Although it was deposited less than a year before the Vatnaöldur unit, it is unclear whether it also preceded the Hrafntinnuhraun unit. Nonetheless, we use the unit's thickness and composition to infer the likely origin to be Skyggnishlíðarvötn, a triplet of maar craters SW of the Torfajökull complex. The major‐/minor‐oxide composition of tephra collected from within the crater rim of the Skyggnishlíðarvötn maars (~45 km WNW of Búland) is indistinguishable from that of the unit we recorded near Búland. These findings demonstrate that volcanic unrest in the Veiðivötn fissure swarm is associated with explosive basaltic volcanism in the Torfajökull complex. The Skyggnishlíðarvötn phase may well have acted as a precursor to the impending Settlement eruption sequence.