4 Isotope Geochemistry in the Environment
61
are enriched in the heavier, radiogenic isotope 187Os and mantle rocks contain
more of the lighter isotope m6Os. Osmium is mined from ultramafic, magmatic
complexes and so anthropogenically derived Os ought to contain more of the
lighter Os isotope than normal crustal rocks. In these, the first studies of their
kind, Ag and Os concentrations and Os isotopic ratios were measured in short
sediment cores from a series of stations close to the mouth of Boston Harbor and
in the Cape Cod Bay, Massuchussetts. Two samples of sewage sludge were also
analyzed in order to define the chemical characteristics of the metal source.
Sludges from the two different sewage deposits and one treatment plant
contain osmium contents up to 10-40 times higher than background (i.e. pristine,
pre-anthropogenic influence, marine sediments). Their Os isotope ratios are also
very low and virtually indistinguishable from ore rocks as would be expected from
anthropogenic Os. Os isotopic signatures from pristine sediments, sewage
influenced sediments and sewage sludge can be compared in Table 4.4.
l,Z 7~
~2.10N
t,,.I.7*N
,
I
Cape
i. o
', 9 11
,..."
".. It.
* 8
3 ,.-'
,.::.-'--...
112 ".
~A.SSACHUSETTS BAY'". ""
g"
Ss
,
t. "
i" Banle,
":
]2.
~
l S
~ - -
9
g ,,CO
,
ape Cod "~
\
31
|
Fig. 4.10. Sampling locations for Os isotopic analysis as a pollutant tracer. (Ravizza and
Bothner 1996)