Two new Icelandic tephra markers: The Hekla O tephra layer, 6060 cal. yr BP, and Hekla DH tephra layer, ∼6650 cal. yr BP. Land-sea correlation of mid-Holocene tephra markers

A sedimentary basin within the Tjornes Fracture Zone on the north Icelandic shelf has been investigated with the purpose of locating tephra layers for dating and stratigraphical correlations of terrestrial and marine archives. Two new mid-Holocene tephra markers originating from the Hekla volcano in Iceland have been identified in the marine sediment core MD99-2275 retrieved from the basin east of the island of Grimsey. These tephra layers have been correlated to their terrestrial counterparts, the Hekla O tephra and the Hekla DH (Dark Horse) tephra. The Hekla O tephra has been dated to 6060 +/- 115 cal. yr BP by four AMS C-14 dates from two terrestrial sites in Iceland. An age estimate of 6650 cal. yr BP has been calculated (SAR, soil accumulation rate) for the Hekla DH tephra from three soil sections in Iceland. Preliminary volume estimates of the Hekla O and Hekla DH tephra layers are similar to 1 km (3) and similar to 0.8 km(3), respectively. These two tephra layers enhance the age control between the Hekla 4 and Hekla 5 tephra markers in the shelf sediments previously unrestrained by terrestrially dated tephra markers and are a valuable addition to the tephrochronology of Iceland and the north Icelandic shelf.