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 HE is investigated in three different deep-sea si
tuations where this boundary has been localised within a narrow depth zone
for a sustained period (thousands of years) because of changes in sedimenta
ry accumulation conditions. From previous work it is known that a variety o
f redox-sensitive elements form diagenetic peak concentrations above and be
low this boundary. This work shows for the first time that Hg also develops
sharp peaks immediately into post-oxic conditions in two different situati
ons where sediments containing trace pyrite have been re-oxidised. The Hg p
eaks are always closely associated with corresponding Se peaks, and the dia
genetic concentrations of both elements are persistent over millions of yea
rs on subsequent burial into more reducing conditions. There is an apparent
offset in the locations of Hg and Se peaks observed in a continuously accu
mulated case where Se uptake from bottom waters occurs independently of pyr
ite formation or re-oxidation, which may be a consequence of a widely sprea
d Se peak. It is proposed that formation of the HgSe species tiemannite is
involved, by analogy with selenium ore occurrences and the other elements f
ound immobilised along with Se and Hg in the cases studied. Copyright (C) 1
999 Elsevier Science Ltd.