naturally occurring radionuclides. The decay of
187 Re produces
187 Os and the decay of
190
Pt produces
186 Os; changes in Os isotopic composition that
arise from these decay schemes are commonly reported as variations in
187
Os/
188 Os and
186
Os/
188 Os,
respectively (Table 3). The long half-life and low
isotopic abundance of
190 Pt restricts the range of
186 Os/
188
Os variations in most natural materials,
making these isotopic analyses extremely challenging. Significant
186 Os/
188
Os variability in marine
deposits has yet to be documented. However, available Pt and Os concentration data from metalliferous
sediments and marine manganese nodules demonstrate that these deposits have Pt/Os ratios among the
highest measured in terrestrial materials. These data
suggest that the Pt-Os decay scheme may be exploited in the future as a tool for dating these deposits, and provide the impetus for further
investigating the geochemical processes that are responsible for producing the large Pt/Os ratio variation observed in marine deposits.
In contrast to the Pt-Os decay scheme, the Re-Os
system gives rise to large variations in
187 Os/
188 Os of
marine deposits and is currently the subject of vigorous investigation. This work is motivated by two
fundamentally important attributes of Os geochemistry; the relatively short marine residence time
of Os, and the record of past variations in the
187 Os/
188
Os of sea water preserved in marine sediments. Direct analyses of sea water do not yield
evidence of any resolvable difference in the
187 Os/
188 Os ratio between different ocean basins,
but higher precision analyses of Mn crust surfaces do
suggest that the
187
Os/
188 Os of the Atlantic ocean
may be slightly larger than in the Indian or Pacific
basins. This isotopic contrast is extremely small
compared to the large range in
187 Os/
188 Os of
sources of Os supplied to the ocean (Table 4). The
nearly homogeneous character of modern sea water
relative to oceanic inputs implies that the marine
residence time of Os is poised close to the mixing
time of the oceans. Spatial variations in
187 Os/
188
Os
of modern sea water are also small compared to the
record of temporal variations in sea water
187 Os/
188 Os preserved in marine sediments. Past
variations in the
187
Os/
188
Os of sea water provide a
globally integrated record of Os input to ocean that
can be exploited to make inferences about the geologic history of chemical weathering, and to identify
extraterrestrial impacts in the sedimentary record.
Detailed discussion of the marine Os isotope record
is beyond the scope of this review.
The PGEs as Tracers of Extraterrestrial Material in
Marine Sediments
Very large concentrations of the PGEs in extraterrestrial material relative to the average upper
crustal material (Table 1) make the PGEs valuable
indicators of the presence of particulate extraterrestrial material in marine sediments. For example,
addition of 0.01% by weight chondritic material to a
sediment with average crustal Ir and Os concentrations would roughly double the concentrations of
these elements in the mixture relative to that of the
starting material. Ir is more widely exploited than Os
as a tracer of particulate extraterrestrial material
because methods for low level Ir analysis were established earlier and are more widely available. The
global Ir enrichment at the Cretaceous–Tertiary
boundary, and the subsequent identification of a
major extraterrestrial impact crater, provide the best
known example of this type of research. Ir data have
been applied in a similar manner to study numerous
other event horizons in the geologic record. Other
PGE analyses can be integrated into these studies to
provide additional constraints on PGE source. For
example, the Pt/Ir ratios typical of upper crustal
material are roughly 10 times larger than in chondrites (Table 1). This type of contrast in element
ratios can be used to help evaluate whether elevated
Ir concentrations are truly related to an extraterrestrial PGE source, or are the result of natural enrichment of PGEs from the ambient environment.
Many studies motivated by the controversy surrounding the interpretation of the Ir anomaly at the
250
200
150
100
50
0
Depth (m)
Range of measured concentration in the open ocean (n= 5)
Ir (fmol kg
_ 1 )
0
1
2
3
4
5
6
7
Figure 4 Dissolved Ir profile from the Baltic Sea plotted with the
range of Ir concentrations reported for analyses of open ocean
samples. Baltic Sea samples were filtered prior to acidification,
open-ocean data were acidified and unfiltered. The abrupt
increase in dissolved Ir at 150 m depth in the Baltic Sea profile
coincides with complete depletion of dissolved oxygen. Anbar et
al. (1996) Science 273: 1524.
34 PLATINUM GROUP ELEMENTS AND THEIR ISOTOPES IN THE OCEAN
187 Re produces
187 Os and the decay of
190
Pt produces
186 Os; changes in Os isotopic composition that
arise from these decay schemes are commonly reported as variations in
187
Os/
188 Os and
186
Os/
188 Os,
respectively (Table 3). The long half-life and low
isotopic abundance of
190 Pt restricts the range of
186 Os/
188
Os variations in most natural materials,
making these isotopic analyses extremely challenging. Significant
186 Os/
188
Os variability in marine
deposits has yet to be documented. However, available Pt and Os concentration data from metalliferous
sediments and marine manganese nodules demonstrate that these deposits have Pt/Os ratios among the
highest measured in terrestrial materials. These data
suggest that the Pt-Os decay scheme may be exploited in the future as a tool for dating these deposits, and provide the impetus for further
investigating the geochemical processes that are responsible for producing the large Pt/Os ratio variation observed in marine deposits.
In contrast to the Pt-Os decay scheme, the Re-Os
system gives rise to large variations in
187 Os/
188 Os of
marine deposits and is currently the subject of vigorous investigation. This work is motivated by two
fundamentally important attributes of Os geochemistry; the relatively short marine residence time
of Os, and the record of past variations in the
187 Os/
188
Os of sea water preserved in marine sediments. Direct analyses of sea water do not yield
evidence of any resolvable difference in the
187 Os/
188 Os ratio between different ocean basins,
but higher precision analyses of Mn crust surfaces do
suggest that the
187
Os/
188 Os of the Atlantic ocean
may be slightly larger than in the Indian or Pacific
basins. This isotopic contrast is extremely small
compared to the large range in
187 Os/
188 Os of
sources of Os supplied to the ocean (Table 4). The
nearly homogeneous character of modern sea water
relative to oceanic inputs implies that the marine
residence time of Os is poised close to the mixing
time of the oceans. Spatial variations in
187 Os/
188
Os
of modern sea water are also small compared to the
record of temporal variations in sea water
187 Os/
188 Os preserved in marine sediments. Past
variations in the
187
Os/
188
Os of sea water provide a
globally integrated record of Os input to ocean that
can be exploited to make inferences about the geologic history of chemical weathering, and to identify
extraterrestrial impacts in the sedimentary record.
Detailed discussion of the marine Os isotope record
is beyond the scope of this review.
The PGEs as Tracers of Extraterrestrial Material in
Marine Sediments
Very large concentrations of the PGEs in extraterrestrial material relative to the average upper
crustal material (Table 1) make the PGEs valuable
indicators of the presence of particulate extraterrestrial material in marine sediments. For example,
addition of 0.01% by weight chondritic material to a
sediment with average crustal Ir and Os concentrations would roughly double the concentrations of
these elements in the mixture relative to that of the
starting material. Ir is more widely exploited than Os
as a tracer of particulate extraterrestrial material
because methods for low level Ir analysis were established earlier and are more widely available. The
global Ir enrichment at the Cretaceous–Tertiary
boundary, and the subsequent identification of a
major extraterrestrial impact crater, provide the best
known example of this type of research. Ir data have
been applied in a similar manner to study numerous
other event horizons in the geologic record. Other
PGE analyses can be integrated into these studies to
provide additional constraints on PGE source. For
example, the Pt/Ir ratios typical of upper crustal
material are roughly 10 times larger than in chondrites (Table 1). This type of contrast in element
ratios can be used to help evaluate whether elevated
Ir concentrations are truly related to an extraterrestrial PGE source, or are the result of natural enrichment of PGEs from the ambient environment.
Many studies motivated by the controversy surrounding the interpretation of the Ir anomaly at the
250
200
150
100
50
0
Depth (m)
Range of measured concentration in the open ocean (n= 5)
Ir (fmol kg
_ 1 )
0
1
2
3
4
5
6
7
Figure 4 Dissolved Ir profile from the Baltic Sea plotted with the
range of Ir concentrations reported for analyses of open ocean
samples. Baltic Sea samples were filtered prior to acidification,
open-ocean data were acidified and unfiltered. The abrupt
increase in dissolved Ir at 150 m depth in the Baltic Sea profile
coincides with complete depletion of dissolved oxygen. Anbar et
al. (1996) Science 273: 1524.
34 PLATINUM GROUP ELEMENTS AND THEIR ISOTOPES IN THE OCEAN