VOLCANIC AEROSOL RECORDS AND TEPHROCHRONOLOGY OF THE SUMMIT, GREENLAND, ICE CORES

The recently collected Greenland Ice Sheet Project 2 (GISP2) and Green land Ice Core Project ice cores from Summit, Greenland, provide length y and highly resolved records of the deposition of both the aerosol (H 2SO4) and silicate (tephra) components of past volcanism. Both types o f data are very beneficial in developing the hemispheric to global chr onology of explosive volcanism and evaluating the entire volcanism-cli mate system. The continuous time series of volcanic SO42- for the last 110,000 years show a strong relationship between periods of increased volcanism and periods of climatic change. The greatest number of volc anic SO42- signals, many of very high magnitude, occur during and afte r the final stages of deglaciation (6000-17,000 years ago), possibly r eflecting the increased crustal stresses that occur with changing volu mes of continental ice sheets and with the subsequent changes in the v olume of water in ocean basins (sea level change). The increase in the number of volcanic SO42- signals at 27,000-36,000 and 79,000-85,000 y ears ago may be related to initial ice sheet growth prior to the glaci al maximum and prior to the beginning of the last period of glaciation , respectively. A comparison of the electrical conductivity of the GIS P2 core with that of the volcanic SO42- record for the Holocene indica tes that only about half of the larger volcanic signals are coincident in the two records. Other volcanic acids besides H2SO4 and other SO42 - sources can complicate the comparisons, although the threshold level picked to make such comparisons is especially critical. Tephra has be en found in both cores with a composition similar to that originating from the Vatnaoldur eruption that produced the Settlement Layer in Ice land (mid-A.D. 870s), from the Icelandic eruption that produced the Sa ksunarvatn ash (similar to 10,300 years ago), and from the Icelandic e ruption(s) that produced the Z2 ash zone in North Atlantic marine core s (similar to 52,700 years ago). The presence of these layers provides absolute time lines for correlation between the two cores and for cor relation with proxy records from marine sediment cores and terrestrial deposits containing these same tephras. The presence of both rhyoliti c and basaltic shards in the Z2 ash in the GISP2 core and the composit ion of the basaltic grains lend support to multiple Icelandic sources (Torfajokull area and Katla) for the Z2 layer. Deposition of the Z2 la yer occurs at the beginning of a stadial event, further reflecting the possibility of a volcanic triggering by the effects of changing clima tic conditions.