Sulphur isotopic composition in the Palaeogene coal of Japan

Abstract

Sulphur isotopic compositions of organic and inorganic sulphur are reported for 39 coal samples from three major coalfields of Japan. The δ 34S values are from 2.7 to 21.1% and from 2.6 to 20.3% for organic and pyritic sulphur respectively, in low-sulphur coal beds ( < 1% total sulphur). The δ 34S value of organic sulphur of the Kado coal (2.7–3.9%), which was formed in a freshwater environment, are probably the δ 34S values of the primary sulphur in the Palaoegene (Eocene) coals of Japan. The δ 34S values show a downward decrease in the low-sulphur coal beds in the Ishikari and Miike coalfields. In contrast, the δ 34S values are from 0.2 to 30.6% and from −9.6 to 43.1% for organic and pyritic sulphur respectively, in high-sulphur coal beds (> 1% total sulphur) in the Miike coalfield, and show three distinctive vertical variations: (1) a little downward and remarkable downward increase in organic and pyritic sulphur respectively; (2) a large vertical variation having the maximum value near the middle of the bed in both organic and pyritic sulphur, and (3) a downward decrease associating major fractionations near the top and bottom of the bed in both organic and pyritic sulphur. Both organic and pyritic sulphur, even in the low-sulphur coal beds except for the Kado coal, probably originated from the reduction of seawater sulfate, which infiltrated from above and partially from below the bed. Two major stages of sulphur incorporation were inferred; the early stage occurred at the peat-forming stage and the late stage occurred after burial at a few hundred meters depth. A distinctive relationship between the δ 34S values and the sulphur contents is observed among individual coal samples. In the low-sulphur coals, organic sulphur is enriched in δ 34S relative to pyritic sulphur and is more abundant than pyritic sulphur. In high-sulphur coals, three types are recognized: (1) when the δ 34S values of both organic and pyritic sulphur are over 20%, organic sulphur is depleted in 34S relative to pyritic sulphur, and is more abundant than pyritic sulphur; (2) when the δ 34S values of organic sulphur are over 20% but those of pyritic sulphur are not, organic sulphur is enriched in 34S relative to pyritic sulphur, and is more abundant than pyritic sulphur; and (3) when the δ 34S values of both organic and pyritic sulphur are less than 20%, organic sulphur is enriched in 34S relative to pyritic sulphur, but no general pattern in the sulphur contents exists.