concept

Euxinia

A more extreme grade of oxygen loss: water that has not only run out of oxygen but accumulated dissolved hydrogen sulfide, which is both toxic and chemically reducing. The Black Sea is the living example. For preservation, euxinia is close to ideal - it suppresses scavengers and simultaneously supplies the reduced sulfur that pyritizes soft tissue. On Earth euxinia is an exceptional state requiring special circumstances; on a world with sulfide in its atmosphere it could be the default.