Isotopic evidence in tree rings for historical changes in atmospheric sulfur sources

Little is understood about the usefulness of sulfur isotopic ratios (delta S-34) in tree rings because the sulfur content in rings is generally insufficient for analysis using conventional methods. We present delta S-34 values of the water-soluble and the organically bound sulfur fractions in rings of coniferous trees grown in Japan, analyzed using a large-volume oxygen bomb. Comparing the delta S-34 values of the organically bound fraction in tree rings with past atmospheric sulfur concentrations and with those of their sources, we find clear evidence that the delta S-34 values of the organically bound fraction in the rings are dependent upon the values of the atmospheric sulfur sources. The evidence suggests that the delta S-34 values in tree rings are a useful chronological proxy for evaluating possible causes of past atmospheric sulfur pollution.