Stable Isotopes of Carbon and Sulphur as Indicators of Environmental Change: Past and Present
Newton, Rob, Bottrell, Simon, Journal of the Geological Society
Stable isotopes of carbon (^sup 12^C and ^sup 13^C) and sulphur (^sup 32^S and ^sup 34^S) are fractionated during some of the transformations between reservoirs in the global carbon and sulphur cycles. The main processes controlling the isotopic composition of Earth surface reservoirs are, for carbon, photosynthesis, respiration and equilibration between oceanic dissolved inorganic carbon and atmospheric CO2, and, for sulphur, bacterial sulphate reduction and sulphide reoxidation. Thus the different reservoirs of carbon and sulphur in the global cycles attain characteristic isotopic compositions that can be used as a 'fingerprint' for different carbon and sulphur sources and to track changes in the balance of carbon and sulphur stored in different reservoirs. These isotopic signals can be used both to examine contemporary anthropogenic impacts on the cycling of carbon and sulphur and, where isotopic compositions are faithfully preserved in ancient sediments, to examine historical changes in global geochemical cycles through geological time. A number of case studies are discussed to illustrate how isotopic techniques represent one of the most powerful and important tools available in the study of both the causes and affects of ancient and modern global environmental change.
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Carbon consists of two stable isotopes with masses 12 and 13. The heavier, mass 13, isotope makes up 1.1% of naturally occurring carbon (Nier 1950). Variations in the ratio of ^sup 13^C to ^sup 12^C are usually expressed as per mil ([per thousand] or part per thousand) deviations from the ratio in the Vienna-defined PeeDee Belemnite (V-PDB) standard according to
Natural sulphur consists of four stable isotopes with masses 32, 33, 34 and 36, which have abundances of approximately 95.02, 0.75, 4.21 and 0.02%, respectively (MacNamara & Thode 1950). Most stable isotope fractionation studies utilize the ratio ^sup 34^S/^sup 32^ S and variations in this ratio are usually expressed as per mil ([per thousand] or part per thousand) deviations from the ratio in the Vienna-defined Cañon Diablo Troilite (V-CDT) standard according to
Thus the standard always has a value of zero on the d scale; samples enriched in ^sup 13^C or ^sup 34^S relative to the standard have positive d values and those depleted in ^sup 13^C or ^sup 34^S have negative values.
The subject area covered here has seen a huge expansion in activity since its beginnings in the 1950s. An exhaustive review is impossible and so we have chosen to cover what we feel are the highlights of the research undertaken over this period. To this end we focus on those studies where we feel that the use of stable isotope tracers has provided insights into problems or environmental changes that other techniques could not. These are used as prime examples of the ways in which stable isotope tracers can contribute to our understanding of the processes involved in environmental change. We also focus on the ways that isotopic studies have provided information on past and present perturbations in the global carbon and sulphur cycles.
We begin by concisely reviewing the carbon and sulphur cycles themselves and the processes within them that control the isotopic compositions of the major reservoirs of these elements on Earth. This provides the essential basis for the application of stable isotope studies to understanding of changes occurring within these cycles. The review is then presented as a series of 'case studies' beginning with those from the geological record and moving on to those that deal with anthropogenic environmental change.
The carbon and sulphur cycles, isotope fractionations and isotopic compositions of important reservoirs
Carbon and sulphur are actively cycled through a variety of geological and environmental reservoirs. Both elements exist …
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Publication information: Article title: Stable Isotopes of Carbon and Sulphur as Indicators of Environmental Change: Past and Present. Contributors: Newton, Rob - Author, Bottrell, Simon - Author. Journal title: Journal of the Geological Society. Volume: 164. Publication date: July 2007. Page number: 691+. © Geological Society Publishing House Jan 2009. Provided by ProQuest LLC. All Rights Reserved.