Constructing a tephrochronology framework for the last interglacial - glacial transition

Natural archives, such as ice-cores and sediment records from the oceans and lakes provide considerable evidence for the pattern of past climatic and environmental changes. Detailed investigations of such records are fundamental for improving our understanding of natural climatic events particularly as human activities are thought to be altering the global climate system. The last warm episode or interglacial period in particular, (over 100,000 years ago) serves as the closest analogue for understanding the mechanisms, timing and environmental responses that may lead to a glaciated Earth at the end of the current warm episode. Reconstructing the sequence and timing of past climatic events, however, is hampered by large dating errors and the lack of suitable dating techniques. Matching and comparing climatic records from the oceans, continents and ice-sheets in order to understand the environmental responses to climatic events are therefore problematic. A key tool for enabling the synchronisation and direct comparison of different natural archives is volcanic ash. Instantaneous deposition and dispersal of volcanic ash following an explosive eruption often occurs over large geographical areas and these are incorporated as distinct marker layers within different natural archives. In recent years, it has become clear that these ash deposits can actually be traced over thousands of kilometres from volcanic sources, therefore, providing a key technique for the precise correlation of natural archives. A detailed record or framework giving the age and chemical composition of past volcanic eruptions is essential for the successful employment of this technique. The only available archive with the necessary temporal resolution and independent dating control to provide this information is the Greenland ice-sheet. This archive is likely to contain ash deposits from nearby sources such as Iceland and even more distal sources such as Alaska or low-latitude volcanoes. Tracing volcanic events from such sources within the Greenland ice-sheet will enable their precise dating as the ice accumulates in annual layers. More often than not, however, these layers cannot be identified with the naked eye due to the low concentration of ash present in the ice-core material. These horizons however, can be detected through analysis of the chemical signature of the ice itself. A specially-adopted technique will be applied to use these chemical data to pinpoint and extract volcanic ash particles within the ice-core material between 123,000 and 70,000 years ago. Geochemical analysis of each layer will provide a fingerprint of each eruption to identify its volcanic source and to establish links to the same deposit identified in other geological records. In addition to the traditional geochemical technique (electron microprobe) that provides 9 chemical elements, a new method will be employed to provide an additional 30 chemical elements. This technique has as yet, not been used to analyse some of the small volcanic particles contained within the ice-core - but has the potential to advance the specific identification of volcanic sources and differentiation of volcanic ash deposits. Constructing a framework of this kind will provide a detailed record of the timing and frequency of volcanic activity during the end of the last interglacial and the beginning of the last cold episode. This scheme will be of long-term significance for the precise correlation of natural archives to compare the timing of climatic events and the environmental responses in widely separated localities at a critical time in the Earth's history. A better understanding of the pattern of natural climatic events will indeed assist us to predict the likely triggers and sequence of events that may be looming at the end of the current warm episode.