A coupled natural immobilisation mechanism for mercury and selenium in deep-sea sediments

In the succession of redox conditions encountered with increasing depth in sediments, the first major redox change is the oxic/post-oxic boundary. The geochemical behaviour of Hg is investigated in three different deep-sea situations where this boundary has been localised within a narrow depth zone for a sustained period (thousands of years) because of changes in sedimentary accumulation conditions. From previous work it is known that a variety of redox-sensitive elements form diagenetic peak concentrations above and below this boundary. This work shows for the first time that Hg also develops sharp peaks immediately into post-oxic conditions in two different situations where sediments containing trace pyrite have been re-oxidised. The Hg peaks are always closely associated with corresponding Se peaks, and the diagenetic concentrations of both elements are persistent over millions of years on subsequent burial into more reducing conditions. There is an apparent offset in the locations of Hg and Se peaks observed in a continuously accumulated case where Se uptake from bottom waters occurs independently of pyrite formation or re-oxidation, which may be a consequence of a widely spread Se peak. It is proposed that formation of the HgSe species tiemannite is involved, by analogy with selenium ore occurrences and the other elements found immobilised along with Se and Hg in the cases studied.