Sporadic, direct observations over a 50 yr period inadequately characterize
the history of seasonal hypoxia and anoxia in Chesapeake Bay, a large estu
ary threatened by eutrophication. Here, we undertake a reconstruction of 20
th century oxygen depletion in this estuary using Mo concentrations in Pb-2
10-dated sediments; Cu concentrations are used to control for anthropogenic
influences. Cores from the central channel display mild Mo enrichments abo
ve crustal backgrounds (up to 5 mug/g) and strong Cu enrichments (up to 35
mug/g). Temporally, Cu enrichment (mostly anthropogenic) began earlier and
stabilized in the last two thirds of the 20th century. In contrast, Mo enri
chment has grown during the last two thirds of the century. Molybdenum enri
chment is mostly hydrogenic, except in a section of the channel that receiv
es additional Mo from erosion of Early Miocene shore deposits. Two geochemi
cal mechanisms promote Mo enrichment: manganese refluxing concentrates diss
olved MoO42- at the sediment-water interface and sulfide substitution into
MoO42- produces thiomolybdates, which can be fixed by particles. The Mo enr
ichment mechanisms operate primarily during periods when bottom waters are
anoxic and thiomolybdate formation can occur near the sediment-water interf
ace. This implies a temporal coupling between water-column anoxia and Mo fi
xation even though fixation occurs only within sediments. The Mo enrichment
profiles suggest that Chesapeake Bay has experienced growing O-2 depletion
since the first half of the 20th century, but especially after 1960. Copyr
ight (C) 2001 Elsevier Science Ltd.