The Allerod-Younger Dryas iridium anomaly: comet impact or volcanism

Recent scientific reports have proposed a comet or meteorite struck the Earth about 12,900 years ago causing widespread vegetation burning. This lead to the extinction of many large mammals in North America such as the mammoth and sabre toothed cats and consequently effected some of earliest colonizing human cultures of that continent. It is further proposed that the meteorite impact caused a rapid cooling of the Northern hemisphere climate and a return to glacial conditions in northern latitudes which lasted for about 1,400 years. Part of the evidence presented for this idea is an excess of Iridium, relative to background values, found at one particular horizon and which is associated with an increased abundance of organic matter or charcoal. This is similar to the Iridium anomaly associated with the Chicxulub meteorite impact which occurred 65 million years ago and remains one of the best documented meteorite impacts within the geological record. However the evidence presented for a comet strike has been contested by other scientists and the topic remains the subject of much controversy. It is also recognized that large volcanic eruptions may effect climate in the short term and we suggest volcanism as an alternative explanation. This research aims to clarify the position by using a simple diagnostic test capable of distinguishing a meteoritic from a volcanic signature. Meteorites are found to have higher abundances of the platinum group elements (Iridium, Platinium, Palladium, Rhodium, Ruthenium, Rhenium and Osmium) than ordinary crustal rocks and in proportions similar to those of the most primitive meteorite groups, thought to be representative of material from which the solar system formed. In contrast volcanic processes tend to produce a more variable abundance profile for the platinum group elements because of the different chemical behaviour and properties of each element within this group. We aim to measure the platinum group element abundances in material collected from the organic rich layers, with a previously reported iridium anomaly, from across North America, Europe and from sediment drillcores in the North Atlantic. In addition we will also search for other diagnostic evidence of a meteorite impact such as shocked quartz, enhanced siderophile element abundance and nickel-rich spinels.