Reconstructing the rise of recent coastal anoxia; molybdenum in ChesapeakeBay sediments

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.