Not sure what category to file this post under but Ancient History will do as not many of them come up. The subject is actually dating – and we all know that tree rings and ice cores play a major role in anchoring ancient history. We also know that Vesuvius erupted and buried a Roman town and historians know what date that happened – but do the ice cores know the date? Two links to go to – www.clim-past-discuss.net/8/5429/2012/cpd-8-5429-2012.pdf and Mike Baillie's response at www.clim-past-discuss.net/8/C2434/2012/cpd-8-C2434-2012.pdf . Within the flow of argument it emerges that an acidity signal in Greenland ice cores, thought to be volcanic, was dated to AD79/80 (see also Clausen et al, 1997) on the basis it is known, from historical records of the Romans, that Vesuvius erupted in that year. Baillie says Vesuvius, in the grand scheme of things, was not such a big volcano, and it was a long way from Greenland. Can we be sure it was Vesuvius. The whole argument, as Baillie laments, is somewhat circular – they are arguing for lack of knowledge of other volcanoes that might be alternatively associated with the ice core. However, the known historical date of Vesuvius actually provides a powerful synchronism, and one that has not really been challenged. It anchors the ice core data. The uncertainty is therefore a necessary evil – yet, is it historically accurate. It also emerges that Vesuvius was chosen as the prime candidate before the benefit of tephra analysis was available. In other words, it was an educated guess. Wonderful. Baillie argues that it is 7 years amiss, but may it have been even further apart? We don't know. Do they?