Geochemistry of Holocene cryptotephras from the North Iceland Shelf (MD99-2269): intercalibration with radiocarbon and palaeomagnetic chronostratigraphies

Abstract

Tephra-grain counting, morphology and geochemistry in the 150–1000 μm size fraction were used to identify 22 individual tephra horizons and one visible tephra layer (Saksunarvatn) from postglacial sediment of the north Iceland shelf. Core MD99-2269 (66°37.53‘N, 20°51.16‘W, water depth 365 m, length 2533 cm) from the Reykjafjardaráll basin is constrained at the base by the Vedde tephra (c. 12 000 cal. yr BP). Geochemical analyses and stratigraphical position of the 22 tephra horizons were used to identify the following cryptotephras: KOL-1372 (AD 1372), Hekla 1104 (AD 1104), Snæfellsnes 1 (c. 1790 cal. yr BP), Hekla 3 (c. 2970 cal. yr BP), Hekla 4 (c. 4230 cal. yr BP), TV 5 (c. 6860 cal. yr BP) and Suduroy (c. 8070 cal. yr BP). A tephra of the same geochemistry as KOL-1372 was found near the base of the core and is here named KOL3-2269 (>11 000 cal. yr BP). Eleven additional cryptotephras were geochemically typed to their respective Icelandic volcanic centres of origin and their ages constrained. Four tephra horizons had a very heterogeneous geochemical signal and may reflect secondary rather than primary deposition although we speculate whether the heterogeneity may be due to mixing of distinct cryptotephras within our sample. The cryptotephras and one visible tephra layer (Saksunarvatn) were intercalibrated with the combined MD99-2269 and MD99-2322 (67°08.18‘N, 30°49.67‘W, 714 m water depth, length 2617 cm) palaeomagnetic secular variation record, in depth and on a calibrated radiocarbon chronology, providing a fundamental chronological and stratigraphic template for future studies of environmental and volcanic variability for the Icelandic region, eastern North Atlantic and NW Europe.