40Ar/39Ar Age of the Thorsmörk ignimbrite, Iceland and correlation with North Atlantic Z2 ash: Cross-calibration of ~60,000 counted annual layers in the Greenland ice cores

Air fallout volcanic ash layers are important in time scale studies because they provide widespread, essentially instantaneous, time markers in sedimentary sequences, including ice cores. Tephrochronology offers the possibility of tying together terrestrial and marine records for a better understanding of past climatic and ocean circulation systems and for studying leads and lags between them. However, distal airborne volcanic ash (hundreds to thousands of kilometers from the eruptive site), deposited in deep sedimentary basins with high-resolution paleoenvironmental and paleoclimate records, are not usually amenable to 40Ar/39Ar dating because of the paucity of crystals and the fine-grained nature of the deposit (< 100 microns). In such cases, geochemical 'fingerprinting' can be used to correlate proximal terrestrial tephra deposits, which have been dated by radiometric methods, with distally equivalent fine-grained volcanic ash in marine sediments and ice cores. This combined approach provides a numerically based chronological framework in which to place the paleoenvironmental record. The peralkaline Thorsmörk ignimbrite, the largest Pleistocene rhyolite eruption on Iceland, has been proposed as the source of widespread Ash Zone 2 ash, found in North Atlantic sediments and the Greenland ice cores and deposited shortly after (~ 420 yr) the onset of Greenland interstadial 15 at ~ 56 ka (marine isotopic stage 3). Here we present ongoing single crystal (anorthoclase) laser 40Ar/39Ar age determinations for the Thorsmörk ignimbrite. In combination with geochemical data for the Z2 ash and Thorsmörk ignimbrite we discuss the proposed correlation of these proximal and distal volcanic deposits. We then examine the possibility of using the 40Ar/39 age for the Thorsmörk ignimbrite as a cross-calibration check on the Greenland ice core chronology, derived by counting annually deposited ice layers.