Identification and definition of primary and reworked tephra in Late Glacial and Holocene marine shelf sediments off North Iceland

Esther Ruth Gudmundsdóttir*, Jón Eiríksson, Gudrún Larsen

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

42 Citations (Scopus)

Abstract

Nine tephra layers in marine sediment cores (MD99-2271 and MD99-2275) from the North Icelandic shelf, spanning the Late Glacial and the Holocene, have been investigated to evaluate the effectiveness of methods to detect tephra layers in marine environments, to pinpoint the stratigraphic level of the time signal the tephra layers provide, and to discriminate between primary and reworked tephra layers in a marine environment. These nine tephra layers are the Borrobol-like tephra, Vedde Ash, Askja S tephra, Saksunarvatn ash, and Hekla 5, Hekla 4, Hekla 3, Hekla 1104 and V1477 tephras. The methods used were visual inspection, magnetic susceptibility, X-ray photography, mineralogical counts, grain size and morphological measurements, and microprobe analysis. The results demonstrate that grain size measurements and mineralogical counts are the most effective methods to detect tephra layers in this environment, revealing all nine tephra layers in question. Definition of the tephra layers revealed a 2-3cm diffuse upper boundary in eight of the nine tephra layers and 2-3cm diffuse lower boundary in two tephra layers. Using a multi-parameter approach the stratigraphic position of a tephra layer was determined where the rate of change of the parameters tested was the greatest compared with background values below the tephra. The first attempt to use grain morphology to distinguish between primary and reworked tephra in a marine environment suggests that this method can be effective in verifying whether a tephra layer is primary or reworked. Morphological measurements and microprobe analyses in combination with other methods can be used to identify primary tephra layers securely. The study shows that there is a need to apply a combination of methods to detect, define (the time signal) and discriminate between primary and reworked tephra in marine environments.

Original languageEnglish
Pages (from-to)589-602
Number of pages14
JournalJournal of Quaternary Science
Volume26
Issue number6
DOIs
Publication statusPublished - Aug 2011

Other keywords

  • Morphology
  • Primary
  • Reworked
  • Tephra
  • Tephrochronology
  • Time signal

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