Tephrochronological Investigation of Mid- to Late Holocene Acid Signals in Greenland Ice Cores

Volcanic acidity peaks provide critical reference horizons that enable the linkage of multiple ice cores in Greenland and have aided the construction of the Greenland Ice Core Chronology 2005 (GICC05). The source eruption of these acid layers can only be established though geochemical characterization of associated tephras, a process which then enables the correlation of all sedimentary records containing the same tephra at fixed points in time and, in the case of prehistoric eruptions, a refinement of the age of the volcanic event. Here we present results of a study that examined sub-sections of the Dye-3, GRIP and NGRIP ice cores, the main objectives of which were to isolate tephras in order to: 1) confirm the robustness of the ice core chronology by locating tephras from Katmai AD 1912, Laki AD 1783, Öraefajökull AD 1362, Hekla AD 1104, Eldgjá ~AD 930s, and Vesuvius AD 79; 2) use the ice core chronology to ascertain precise ages for the prehistoric eruptions of Thera ~17th century BC and Hekla 4 ~2300 BC; 3) determine the source of volcanic signals at AD 1259 and ~AD 536, both of which have counterparts in Antarctic ice; and 4) compare the relationship of tephra layers and other volcanic products in the ice. Major element geochemistry of isolated tephras has been characterised using single-shard EPMA. We report the discovery of the Katmai and Öraefajökull tephras in the NGRIP and GRIP cores respectively, confirming the annually-resolved ice core chronology for the last 700 years. The Katmai tephra precedes the peak levels in ECM, Cl- and SO4-- associated with this eruption, with sulphates peaking last. The Öraefajökull tephra is not accompanied by any notable acid signal. Although the remaining targeted events were not found, we identified tephras from 14 other eruptions, including the so-called "AD860B" tephra in NGRIP, a widespread isochron found across NW Europe to which a GICC05 date of AD 847 can now be assigned. Three tephras were present in NGRIP in ice dating to ~1644/1643 BC, ~1641/1639 BC and ~1629/1628 BC, the first two of which are closely comparable in their major element composition with published data from an Aniakchak eruption of similar date. Only the 1641/1639 BC tephra corresponds directly with a rise in ECM and sulphate. The source of the remaining tephras has not yet been established. We examine the implications of our findings for the understanding of volcanic records in ice cores.