The age and chronostratigraphical significance of North Atlantic Ash zone II

Rhyolitic tephra with the geochemical characteristics of North Atlantic Ash Zone (NAAZ) II are described from the giant piston core MD95-2006 from the northeast Atlantic Ocean. Shard size distribution indicates that the tephra represent a wind-sorted, primary air-fall deposit, but with a mode close to 200 μm they are too coarse to be air-fall deposits at the core site, which lies ca. 1000 km from the possible tephra source. Randomly sampled shards reveal a single geochemical population belonging to the Icelandic transitional alkali magma series, suggesting that they are unlikely to represent ice rafted debris derived from Icelandic icebergs. The tephra probably represent air-fall deposits, transported to the core site by sea ice within the northeast Atlantic gyre. The NAAZ II peak coincides with the rapid climate transition (cooling) at the end of interstadial 15, which can be assigned an age of 53 260\textpm2660 yr BP from direct correlation with the Greenland ice-core (GISP2) record. A comparison of the MD95-2006 Neogloboquadrina pachyderma (sinistral) relative abundance and GISP2 δ18O records, relative to the NAAZ II isochron, suggests that this climatic event was synchronous across the North Atlantic. By direct correlation of interstadial maxima between the two records, the GISP2 time-scale is transferred to MD95-2006. Comparison of corrected and calibrated radiocarbon ages derived from monospecific foraminifers with the GISP2 ages at the same stratigraphical horizon suggest major age differences. These probably result from large variations in atmospheric 14C concentration and highlight the significant uncertainties associated with radiocarbon calibration during marine isotope stage 3. Copyright \textcopyright 2004 John Wiley Sons, Ltd.