A reader recently asked by email why, if cosmic blast materials have been found on the continent in Germany and the Netherlands in Younger Dryas sediments, has nothing been found in the British Isles? Or how about just plain old England, one of the best studied countries on earth?
Elevated iridium values, dated to start of the Younger Dryas cooling event, have been found in sediments deposited at a number of Late Glacial sites in North America and one in Europe. It has been proposed (e.g., Firestone et al., 2007, PNAS 104: 16016-16021) that this widespread iridium enrichment signal is the result of an explosive disintegration of a large extraterrestrial object over North America around 12,900 cal. yr BP, and it is contended that it was this event which instigated the Younger Dryas cooling. This scenario is controversial, and the ‘ET’ explanation of these geochemical signals is not universally accepted. This notwithstanding, we report here the finding of an iridium anomaly in the Allerød-Younger Dryas boundary sediments at Hawks Tor in the southwest of England.
The concentration of iridium and other elements is determined in peat monoliths using ICP-MS, operated in collision-cell mode, and ICP-OES instruments. We find an increase of over 300 % in the iridium concentration measured in the bulk sediment immediately above the Younger Dryas boundary compared with the values found below the transition. The iridium-titanium ratio is used to confirm a lag between the start of the iridium enrichment and the timing of abrupt environmental disruption at the site signalled by decreases in the organic carbon content, and changes the concentrations of potassium, iron and manganese. These geochemical changes coincide with a shift from a humified peat to a minerogenic lithology. By using a new calibration of existing 14C ages, integrated with new AMS dates and optically stimulated luminescence ages, we show that the timing of this iridium enrichment found in southwest England is in agreement with the dates proposed for the iridium enrichment signals previously found in North America and Belgium.
Session: 60 The enigmatic Younger Dryas climatic episode
Authors: William Marshall
Presenter: William Marshall