Identification of Primary Tephra Layers on the SW Icelandic Shelf

Marine sedimentary deposits off Iceland often provide the sole witness for explosive Icelandic volcanic eruptions, in cases where glaciation and erosion hampered the formation/preservation of deposits on land. However, interpretation of the marine record is not straightforward: The depositional environment of Iceland can produce reworked tephra layers which may not be easily distinguished from primary ones. Furthermore, a considerable time delay up to centuries may be involved between deposition of primary and reworked tephra from the same eruption in the same stratigraphic succession. \n \n\tHere, we present results of a case study on sections taken from three marine gravity cores collected from the southwestern Icelandic shelf during the P457 cruise of R/V Poseidon, to better resolve the eruptive record and to evaluate methods for the identification of primary tephra layers in this complex environment. We employed visual macroscopic and microscopic inspection, and major element compositions and 2D shape parameter analysis of tephra shards, derived from electron microprobe analysis and images. We correlated layers in the core sections to two events of the volcanic systems Katla and Veidivötn-Bárdarbunga, respectively, and constrained their depositional histories. Major element signatures and characteristic bimodality of rhyolite and basalt identify the Katla layers as the ~ 12.1ka BP Vedde Ash, the Veidivötn-Bárdarbunga layers having formed ~ 300 years later. The depositional modes involve primary fallout, rafting of tephra by sea ice and/or icebergs, and background sedimentation of remobilised shards both from the layer-forming eruption and from earlier events. Our results demonstrate the power of a combined-method approach for the identification of primary tephra, in particular when reconstructing complex depositional histories such as those of the late Quaternary Icelandic shelf.