HR: 1340h
AN: U43A-0825    [Abstracts]
TI: Early Holocene Tephrochronology in West Iceland and its Application for Paleoclimate Studies
AU: * Johannsdottir, G
EM: gudrunb@hi.is
AF: Department of Geosciences, University of Iceland, Sturlugata 7 (Askja), Reykjavik, IS101 Iceland
AU: Thordarson, T
EM: moinui@soest.hawaii.edu
AF: Earth Science Institute, Univerity of Iceland, Sturlugata 7 (Askja), Reykjavik, IS101 Iceland
AU: Thordarson, T
EM: moinui@soest.hawaii.edu
AF: Department of Geology and Geophysics, Univerity of Hawaii at Manoa, 1680 East-West Rd, Honolulu, HI 95622
AU: Geirsdottir, A
EM: age@hi.is
AF: Department of Geosciences, University of Iceland, Sturlugata 7 (Askja), Reykjavik, IS101 Iceland
AB: The climate in Iceland is sensitive to changes in the North Atlantic circulation because of its position at the boundary of the cold East Greenland and warm Irminger currents. This factor along with high sediment accumulation rates (up to 5m per 1000yrs) makes lake sediments in Iceland ideal for studying the timing and magnitude of Holocene climate events. Accurate high resolution age determinations of the sediments support stratigraphic correlations and are critical for paleoclimate and paleoenvironmental research. Explosive basalt to rhyolite volcanism in Iceland has produced numerous widespread tephra fall deposits that are important marker horizons in Holocene sedimentary sequences. Consequently, tephra layers of known age are ideal for stratigraphic correlations and high precision dating. Furthermore, AMS 14C age determinations on lake sediments in Iceland give inaccurate and inconsistent results. Therefore, an accurate tephrochronological record together with continuous paleomagnetic records from these sediment cores underpin the construction of an unambiguous high-resolution chronology across the marine and land based sedimentary environments. We have established the tephra stratigraphy in sediment cores from four lakes lying on a transect from south to northwest Iceland, which collectively provide a near-continuous high resolution tephrochronological record extending as far back as 12 ka BP. The geographic distribution of the lakes as well as their position relative to the active volcanic zones in Iceland allow us to discriminate between marker layers of regional and local importance. Our pilot cores are from Lake Hestvatn (HST) and Lake Hvitarvatn (HVT), which are closest to the active volcanic zones, and as expected contain the highest number of tephra layers. Here we focus on the period from 4 to 12 ka BP. In HST we have identified 44 tephra layers from this period, where 41 layers were produced by the Katla (18), Hekla (15), Grimsvotn (5), and Veidivotn (3) volcanic systems within the Eastern Volcanic Zone (EVZ). Three of the tephra layers come from systems within the Western Volcanic Zone (WVZ). In HVT 23 layers have been identified and all except for 1 are from the EVZ (10 from Katla, 5 from Hekla, 6 from Grimsvotn, 1 from Veidivotn). A total of 83 layers have been analyzed in the cores for the period in question, 77 of which are basaltic, 3 are dacitic and 3 rhyolitic. Most of the layers (94%) are produced by subglacial eruptions at ice-capped volcanoes in the EVZ. The phreatomagmatic origin of the basaltic layers indicates that water to magma interactions, which amplify explosivity of eruptions, play a critical role in forming widespread basaltic tephra layers. Thus far we have identified 7 layers of regional importance; the Vedde tephra (12,000ka), a 9000ka basaltic Hekla layer, a 7500ka BP tholeiitic layer from Veidivotn, H5 (7100ka), the T-tephra layer from Hekla (6600ka) and H4 (4300ka). We have also identified three successive layers at about 10ka formed by eruptions spaced over approximately 100 years. These three layers are identical in major element composition to the Saksunarvatn tephra (10300ka) but which of the three is Saksunarvatn is not known at this stage.
DE: 1105 Quaternary geochronology
DE: 1145 Tephrochronology (8455)
DE: 1605 Abrupt/rapid climate change (4901, 8408)
DE: 1616 Climate variability (1635, 3305, 3309, 4215, 4513)
DE: 8428 Explosive volcanism
SC: Union [U]
MN: Fall Meeting 2005