Warm-season dead zones-volumes of coastal water containing too little O(2) to support macrofauna-are a growing global menace. Trace elements that are deposited in sediments in response to reducing or sulfidic conditions can provide proxy records for reconstructing dead zone evolution. Based on relative enrichment in reduced vs oxidized marine sediments, Re seems promising as a dead zone proxy. Here, Re is determined by isotope dilution mass spectrometry in sediments underlying the summertime dead zone in Chesapeake Bay. Contrary to expectation, Re becomes only modestly (∼2-fold) elevated during the 20th century and fails to track the historic record of summertime O(2) depletion. Rhenium enrichments are watershed-specific and apparently controlled by anthropogenic sources, not by redox-linked authigenic processes. In contrast, Mo enrichments do track historic O(2) depletion. Three factors cause redox control to override anthropogenic control in the case of Mo: relative to weathering fluxes, anthropogenic Mo fluxes are weaker than Re fluxes; during anoxic periods, Mn refluxing amplifies Mo but not Re concentrations near the sediment surface; and high pore water sulfide-polysulfide promotes Mo fixation in pyrite while promoting formation of organo-Re adducts; the latter are too mobile and reactive to preserve a reliable historic record under seasonally fluctuating redox conditions.
Because the isotopic composition of anthropogenic Os is
normally distinctive in comparison to continental crust
and is precisely measurable, this platinum-group element
is attractive as a tracer of transport pathways for
contaminated sediments in estuaries. Evidence herein and
elsewhere suggest that biomedical research institutions
are the chief source of anthropogenic Os. In the Chesapeake
Bay region, uncontaminated sediments bear a crustal 187Os/188Os signature of 0.73 ± 0.10. Slightly higher 187Os/188Os
ratios occur in Re-rich Coastal Plain deposits due to post-Miocene 187Re decay. The upper Susquehanna Basin
yields sediments also with higher 187Os/188Os. Beginning in
the late 1970s, this signal was overprinted by a low 187Os/188Os (anthropogenic) source in the lower Susquehanna Basin.
In the vicinity of Baltimore, which is a major center of
heavy industry as well as biomedical research, anthropogenic
Os has been found only in sediments impacted by the
principal wastewater treatment plant. Surprisingly, a mid-Bay site distant from anthropogenic sources contains
the strongest anthropogenic Os signal in the data set, having
received anthropogenic Os sporadically since the mid-20th Century. Transport of particles to this site overrode
the northward flowing bottom currents. Finding anthropogenic
Os at this site cautions that other particle-borne substances,
including hazardous ones, could be dispersed broadly
in this estuary.
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