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Biogeochemistry of Sulfur Isotopes

Danish Center for Earth System Science (DCESS) and Institute of Biology, Odense University, SDU, Campusvej 55, 5230 Odense M, Denmark

The first 20% of the full text of this article appears below.

	INTRODUCTION

 
Sulfur, with an atomic weight of 32.06, has four stable isotopes. By far the most abundant is ³²S, representing around 95% of the total sulfur on Earth. The next most abundant isotope is ³⁴S, followed by ³³S, and finally ³⁶S is the least abundant contributing only 0.0136% to the total (Table 1). The natural abundances of sulfur isotopes, however, vary from these values as a result of biological and inorganic reactions involving the chemical transformation of sulfur compounds. For thermodynamic reasons, the relative abundance of sulfur isotopes can vary between coexisting sulfur phases. This is because lighter masses partition more of the total bond energy into vibrational rather than translational modes. Bonds with a higher vibrational energy are also more easily broken which is why lighter isotopes are generally enriched in the reaction products in chemical reactions with associated fractionation. Thus, for a nonreversible chemical reaction, as often occurs in biological systems, independent reactions may be written for the transformation of the light, L, and heavy, H, isotopes of reactant, A, to product, B (Eqns. 1 and 2).

(1)

(2)

Each of these reactions has associated rate constants, k_H and k_L, and as described above, k_H is generally less than k_L, yielding an enrichment of the lighter isotope in the product. Fractionations associated with a unidirectional process are referred to as kinetic fractionations.

Fractionations can also occur between two chemical species at equilibrium. The basis for equilibrium fractionations is thermodynamic and, as with kinetic fractionations, is related to mass-dependent differences in bond energies between light and heavy isotopes. The generalized isotope equilibrium between two chemical species is presented in Equation (3).

(3)

From Equation (3) an equilibrium constant, K_eq, may be defined as:

(4)

If x and y are unity, then K_eq is identical to . . . [Full Text of this Article]

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