OCEANIC ANOXIA AT THE CRETACEOUS TERTIARY BOUNDARY SUPPORTED BY THE SULFUR ISOTOPIC RECORD

A temporary and drastic increase in S-34/S-32 ratio of whole rock sulfide has been recognized just at the Cretaceous/Tertiary boundary within the continuous marine sedimentary sequence of the Kawaruppu section in eastern Hokkaido. Japan: the apparent extent of fractionation, with respect to inferred contemporaneous seawater sulfate, in the anomaly zone which includes the ''boundary claystone'' at the bottom lies in the range of - 25 +/- 10 parts per thousand., whereas that in the lower and upper siltstone strata is roughly twice as much, giving the values in the range of - 49 +/- 10 parts per thousand. The former is quantitatively equivalent in magnitude with the range of kinetic isotope effect in bacterial dissimilatory sulfate reduction and the latter with the typical variation range as observed in sedimentary-diagenetic sulfides in ordinary oxic marine basins. The present data would strongly support the temporary occurrence of an aerobe-free, anoxic, oceanic sedimentary environment beginning just at the Cretaceous/Tertiary boundary and continuing for approximately 70,000 years, probably as a consequence of a certain catastrophic event responsible for the terminal Cretaceous mass extinction.