5 Isotopic Composition of Seawater 143
strongly enriched in the crust. As a result, the crust with t87Rel t86Os ratios of
--400, displays markedly higher ISTOs/186Os ratios of 10 to 15, while the mantle
shows IS7Os/186Os ratios of around 1. These two sources form the two end
members for the interpretation of the osmium isotopic composition in the ocean at
any one time. Cosmogenic material also yields low ratios as low as the mantle.
Commonly cosmogenic material is not considered in mass balances but as the
concentration of osmium is relatively high in meteoritic material this ought to be
considered as a third source although the solubility of this cosmogenic osmium in
seawater is poorly understood,
The measurement of Os isotope ratios suffers the same problems of small
concentrations in nature as many of the radiogenic isotope systems discussed so
far. However, new isotope techniques involving the production of negative ions
instead of positive ions has made successful analysis possible at low
concentrations. Average concentrations of both Os and Re in the Earth-Moon
system surface rocks are on average 1000-100,000 times less than Rb or St!
Concentrations of Os are likewise minute in seawater and no successful direct
measurements of Os isotopic ratios have been reported in the literature. Instead, as
with Pb. researchers have turned to authigenic mineral phases, which are likely to
have precipitated in chemical equilibrium with seawater. For example,
ferromanganese nodules may contain Os in the hundreds of ppt range. Os may
also be preferentially incorporated into organic matter.
Luck and Turekian { 1983) were the first to link the Os isotopic composition of
ferromanganese nodules to seawater. These authors measured Os isotopic ratios
from several of the World's oceans and were able to establish significant
differences from place to place. As Os and Re are unlikely to be fractionated by
any simple kinetic or biogenic process due to their high masses, these results
could be taken to indicate that Os has a short residence time in seawater, like Nd,
Pb or Ce. However. it was first necessary to demonstrate beyond doubt that the Os
leachable from Fe-Mn nodules is truly representative of "" hydrogenous osmium ".
That is to say, osmium that derives from seawater and not from detritus or from
material which has undergone exchange with detritus.
If the residence time of Os is indeed much longer than the ocean mixing time,
then this system carries potential as a stratigraphic tool in the same way as St.
AIternatively, a shorter residence time would suggest potential as a possible tracer
of seawater circulation patterns cf. Nd, Pb, and Ce.
Further research on the isotopic composition of hydrogenous Os from organicrich sediments distributed widely across the globe produced less ambigous results
(Ravizza and Turekian 1992L They discovered that all samples measured, yielded
an isotopic ratio of around 8.6, and so the residence time could in fact be longer
than had been assumed and indeed this appears to be the case. Up to only a year
ago the residence time of Os in seawater was unknown. However, recent studies
have constrained it to around 104 years which is about an order of magnitude
longer than the mixing time of the oceans (Peuker-Ehrenbrink et al. 1997).
strongly enriched in the crust. As a result, the crust with t87Rel t86Os ratios of
--400, displays markedly higher ISTOs/186Os ratios of 10 to 15, while the mantle
shows IS7Os/186Os ratios of around 1. These two sources form the two end
members for the interpretation of the osmium isotopic composition in the ocean at
any one time. Cosmogenic material also yields low ratios as low as the mantle.
Commonly cosmogenic material is not considered in mass balances but as the
concentration of osmium is relatively high in meteoritic material this ought to be
considered as a third source although the solubility of this cosmogenic osmium in
seawater is poorly understood,
The measurement of Os isotope ratios suffers the same problems of small
concentrations in nature as many of the radiogenic isotope systems discussed so
far. However, new isotope techniques involving the production of negative ions
instead of positive ions has made successful analysis possible at low
concentrations. Average concentrations of both Os and Re in the Earth-Moon
system surface rocks are on average 1000-100,000 times less than Rb or St!
Concentrations of Os are likewise minute in seawater and no successful direct
measurements of Os isotopic ratios have been reported in the literature. Instead, as
with Pb. researchers have turned to authigenic mineral phases, which are likely to
have precipitated in chemical equilibrium with seawater. For example,
ferromanganese nodules may contain Os in the hundreds of ppt range. Os may
also be preferentially incorporated into organic matter.
Luck and Turekian { 1983) were the first to link the Os isotopic composition of
ferromanganese nodules to seawater. These authors measured Os isotopic ratios
from several of the World's oceans and were able to establish significant
differences from place to place. As Os and Re are unlikely to be fractionated by
any simple kinetic or biogenic process due to their high masses, these results
could be taken to indicate that Os has a short residence time in seawater, like Nd,
Pb or Ce. However. it was first necessary to demonstrate beyond doubt that the Os
leachable from Fe-Mn nodules is truly representative of "" hydrogenous osmium ".
That is to say, osmium that derives from seawater and not from detritus or from
material which has undergone exchange with detritus.
If the residence time of Os is indeed much longer than the ocean mixing time,
then this system carries potential as a stratigraphic tool in the same way as St.
AIternatively, a shorter residence time would suggest potential as a possible tracer
of seawater circulation patterns cf. Nd, Pb, and Ce.
Further research on the isotopic composition of hydrogenous Os from organicrich sediments distributed widely across the globe produced less ambigous results
(Ravizza and Turekian 1992L They discovered that all samples measured, yielded
an isotopic ratio of around 8.6, and so the residence time could in fact be longer
than had been assumed and indeed this appears to be the case. Up to only a year
ago the residence time of Os in seawater was unknown. However, recent studies
have constrained it to around 104 years which is about an order of magnitude
longer than the mixing time of the oceans (Peuker-Ehrenbrink et al. 1997).