A negative correlation between depth and the sulphur content and δ34S ratio in organic matter is used to study diagenesis in the freshwater lignites of the Hula Basin. The samples were taken along a 2450 m thick sequence ranging from Miocene to Recent.
A between six and ninefold enrichment in organic sulphur in these lignites compared to that in their living precursors suggests diagenetic control. Since all the water sources of the basin are low in sulphate, oxidation of organic matter must be the main source for this secondary enrichment. The released sulphur reacts with marls interlayered with the lignites, to form gypsum and is taken into the remaining organic matter, probably as ester sulphate. Heavy sulphur isotope composition of the plant material, the gypsum, the humic acids of the lignites at relatively shallow depths and the water sources feeding the basin support this supposition. Negative δ34S in the lowermost samples indicate, on the other hand, that further enrichment in sulphur through bacterial sulphate reduction occurs also. The parallel trend of decrease in sulphur content and δ34S is explained by the breakdown of ester sulphate-containing compounds with increased maturation and the sulphur derived through bacterial reduction of the ester sulphates and the gypsum is taken into the organic matter.
The transformation of humic acids into kerogens at the very beginning of the maturation stage (up to c. 0.3% Ro) involves only a partial breakdown of ester sulphate-containing compounds, as evidenced by the decrease in sulphur content and an insignificant change in δ34S. Further transformation at somewhat higher maturation levels (c. 0.5% Ro) involves the final breakdown of the ester sulphate-containing compounds and the introduction of bacterially reduced sulphur, probably in a free radical mechanism.
The diagenetic enrichment of organic sulphur in such coals is the sole evidence that the high amounts of organic matter preserved in them actually represent only a fraction of the total organic matter originally formed in the basin.