Catastrophic chemical events in the history of the ocean

Abstract

Catastrophic chemical events are characterised by sharp rises in δ³⁴S in the surface of the whole world ocean, and by greater overshoots locally. Three events are recognised and named for the formation in which they are most sharply displayed. The sharpness of the rise in δ³⁴S suggests that the sulphide deposition necessary to explain it must have been accumulating residual high-δ³⁴S seawater for some tens of millions of years out of contact with the surface ocean. A modified geological model is presented: brine generated by evaporite deposition is stored in deeps of a mediterranean basin; underneath the brine, pyrite precipitation builds a store of brine heavy in ${\text{δ}}^{\text{34}}{\text{S}}_{{\text{so}}_{\text{4}}}$, whose corresponding buildup of ${\text{δ}}^{\text{38}}{\text{O}}_{{\text{so}}_{\text{4}}}$ may balance the decrease of ${\text{δ}}^{\text{38}}{\text{O}}_{{\text{so}}_{\text{4}}}$ from evaporite deposition. Catastrophic mixing of the brine and the surface ocean, initiated by destruction of the storing basin, is the source of the sharp rise in the sulphur isotope age curve detected world-wide in evaporites. These events have important implications not only for modelling of the chemical history of the ocean, atmosphere, and sediments, but also for the explanation of faunal crises, and the many aspects of geology that depend on the composition and circulation of the oceans and their peripheral basins.