Quantitative Reconstruction of Sulfur Deposition Using a Mixing Model Based on Sulfur Isotope Ratios in Tree Rings
2015
Ishida, Takuya | Tayasu, Ichiro | Takenaka, Chisato
Quantification of sulfur (S) deposition is critical to deciphering the environmental archive of S in terrestrial ecosystems. Here we propose a mixing model that quantifies S deposition based on the S isotope ratio (δ³⁴S) in tree rings. We collected samples from Japanese cedar (Cryptomeria japonica D. Don) stumps from two sites: one near Yokkaichi City (YOK), which is well known for having the heaviest S air pollution in the world, and one at Inabu-cho (INA) in central Japan, which has been much less affected by air pollution. The δ³⁴S profiles at both sites are consistent with S air pollution and contributions of anthropogenic S. The minimum value in YOK is lower than the δ³⁴S values of anthropogenic S or any other possible source. Because the δ³⁴S in the tree rings is affected by fractionation in the forest ecosystems, we used a mixing model to account for the isotope effects and to distinguish the sources of S. Based on the model results, we infer that the peak of S emissions at YOK occurred sometime between the late 1960s and early 1970s (489 mmol m⁻² yr⁻¹). This estimated value is comparable with the highest reported values in Europe. This is the first quantitative estimate of anthropogenic input of S in forest systems based on δ³⁴S in tree rings. Our results suggest that tree ring data can be used when monitoring stations of atmospheric S are lacking and that estimates of S deposition using δ³⁴S in tree rings will advance our understanding of the local-scale S dynamics and the effect of human activities on it.
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