Interrogating trees as archives of environmental sulphur variability

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'Give me a tree and I'll read climate history' is the basic premise of dendrochronology, and to a large extent is perfectly true. Each ring within a tree represents one year of growth and contains within it a record of the prevailing environmental conditions. Trees can thus be age dated, and used as a record of climate change through ring-width analysis and chemical composition. However, chemical interpretations of the nutrient chemistry within each ring have often been subject to much criticism. The basic premise that nutrients which are taken up by the tree are encapsulated to represent environmental conditions for that particular year, is true only for elements which are immobilised from further biological transport accross the width of the tree. We now think we have found such an element which is fixed within the woody tissues during growth and can be used with certainty for environmental reconstruction - this element is sulphur. That sulphur should be the element which is fixed within the annual growth rings is fortuitous given its key role in modulating climate and fantastic potential as an environmental diagnostic tool. The injection of sulphur aerosol into the atmosphere is a key determinant of climate through backscattering and absorption of radiation, and has long been a concern for terrestrial ecology, causing widespread acidification of catchments upon deposition. Two of the key sources of sulphur aerosol injected into the atmosphere are from volcanic and industrial emissions. Both sources can be readily distinguished from background inputs using concentrations and stable isotopes, rendering the sulphur isotopic content of palaeoarchives to be perhaps one of the most important diagnostic elemental signatures available. Whilst the sulphur isotopic content of ice cores has been extensively exploited, the more localised archives such as speleothems and tree rings have largely remained beyond the reach of climate change scientists and we have yet to discover many of the secrets they harbour. Whilst we have initiated and had excellent success with extracting the sulphur record from speleothems, we believe trees will behave as a much more sensitive, readily available record of local atmospheric change over the past millenium. We believe we are now in a position to conduct an intensive research programme to extract the sulphur isotopic composition of trees and forge ahead with the development of such an important local indicator of sulphur forcing and atmospheric change.