I,.',8 Peter Stille and Graham Shields
Luck and Turekian (1983) measured 787Os/186Os ratios at Stevns Klint fish clay,
Denmark and Ratan Basin. Colorado yielding 1.654 +I- 0.004 and 1.29
respectively. Note these values are indeed lower than the lowest values measured
by Pegram et al. (1992) or Ravizza (1993) for the succeeding 60 Ma (Fig. 5.38).
Fig. 5.40 displays the range of Os isotopic ratios of mantle material relative to the
continental crust. Clearly the values obtained by Luck and Turekian from the K-T
boundary lie below the Cenozoic minimum but are still within a range covered by
mantle material, i.e. 1-1.2. Thus, the evidence would also tend to support another
theory concerning the mass extinction, that massive volcanic activity had
somehow destabilized the climate and sea-level by producing huge quantities of
aerosols and lavas. In this case, the Ir and Os could derive directly from the
mantle. Such a large influx of mantle derived material with characteristically
primitive isotopic signatures is not supported by Sr isotope evidence, which
actually shows anomalously high ratios around the K/'T boundary. These results
can be interpreted as the result of impact related, acid rain leaching of soils and
rocks with high 87Sr/86Sr, typical for clays, for example.
Recently further work has been carried out at the K-T boundary by PeukerEhrenbrink et al. (1995). They analyzed sediments (both leachates and bulk rock)
across the K-T boundary in the South Pacific where the boundary can be
recognized not only on the basis of nannoplankton diversity turnover but also by
the,presence of shocked quartz and a pronounced Ir peak. Their work can be seen
in Figs. 5.41 and 5.37. Iridium concentrations are clearly much higher at the K/T
boundary at DSDP 596 in the Pacific ocean and are especially high in the zone of
shocked quartz crystals. Os, both leachable and bulk, is also enriched at the
boundary, whereas Os isotope ratios mirror these trends, being lowermost in the
sample richest in osmium and iridium.
It may also be noticed that low Os isotope ratios preceded the actual boundary.
If these ratios were truly representative of seawater, it would certainly lessen the
credibility of any impact signature. The authors argue convincingly that this
smearing of the signal may be the result of bioturbation, something which is
additionally supported by the presence of shocked quartz up to t m below the K-T
boundary. The low Os isotope ratios in samples whose concentrations are very
high at the K-T boundary is perfectly consistent with an impact scenario.
Estimates range around 5 * 1011g for the amount of cosmogenic Os that might
have been released after the impact which would correspond to the fluviatile input
of 6 Ma. Such a huge input of Os would necessarily have had an enormous and
immediate effect on the Os isotopic system. Such large amounts would explain the
sluggish recovery of the seawater Os isotope ratio after the boundary, but only
provided the residence time of Os is long (over 1 Ma), although this does not
appear to be the case with estimates on the order of 10 ~ years (Peuker-Ehrenbrink
et al. 1997) and only if low Os isotope ratios below the boundary are an artefact of
bioturbation. More work clearly needs to be carried out to separate cosmogenic Os
from hydrogenous and detrital Os for isotopic measurement but the Re-Os and Ir
evidence still remains at the heart of the K-T impact hypothesis.
Luck and Turekian (1983) measured 787Os/186Os ratios at Stevns Klint fish clay,
Denmark and Ratan Basin. Colorado yielding 1.654 +I- 0.004 and 1.29
respectively. Note these values are indeed lower than the lowest values measured
by Pegram et al. (1992) or Ravizza (1993) for the succeeding 60 Ma (Fig. 5.38).
Fig. 5.40 displays the range of Os isotopic ratios of mantle material relative to the
continental crust. Clearly the values obtained by Luck and Turekian from the K-T
boundary lie below the Cenozoic minimum but are still within a range covered by
mantle material, i.e. 1-1.2. Thus, the evidence would also tend to support another
theory concerning the mass extinction, that massive volcanic activity had
somehow destabilized the climate and sea-level by producing huge quantities of
aerosols and lavas. In this case, the Ir and Os could derive directly from the
mantle. Such a large influx of mantle derived material with characteristically
primitive isotopic signatures is not supported by Sr isotope evidence, which
actually shows anomalously high ratios around the K/'T boundary. These results
can be interpreted as the result of impact related, acid rain leaching of soils and
rocks with high 87Sr/86Sr, typical for clays, for example.
Recently further work has been carried out at the K-T boundary by PeukerEhrenbrink et al. (1995). They analyzed sediments (both leachates and bulk rock)
across the K-T boundary in the South Pacific where the boundary can be
recognized not only on the basis of nannoplankton diversity turnover but also by
the,presence of shocked quartz and a pronounced Ir peak. Their work can be seen
in Figs. 5.41 and 5.37. Iridium concentrations are clearly much higher at the K/T
boundary at DSDP 596 in the Pacific ocean and are especially high in the zone of
shocked quartz crystals. Os, both leachable and bulk, is also enriched at the
boundary, whereas Os isotope ratios mirror these trends, being lowermost in the
sample richest in osmium and iridium.
It may also be noticed that low Os isotope ratios preceded the actual boundary.
If these ratios were truly representative of seawater, it would certainly lessen the
credibility of any impact signature. The authors argue convincingly that this
smearing of the signal may be the result of bioturbation, something which is
additionally supported by the presence of shocked quartz up to t m below the K-T
boundary. The low Os isotope ratios in samples whose concentrations are very
high at the K-T boundary is perfectly consistent with an impact scenario.
Estimates range around 5 * 1011g for the amount of cosmogenic Os that might
have been released after the impact which would correspond to the fluviatile input
of 6 Ma. Such a huge input of Os would necessarily have had an enormous and
immediate effect on the Os isotopic system. Such large amounts would explain the
sluggish recovery of the seawater Os isotope ratio after the boundary, but only
provided the residence time of Os is long (over 1 Ma), although this does not
appear to be the case with estimates on the order of 10 ~ years (Peuker-Ehrenbrink
et al. 1997) and only if low Os isotope ratios below the boundary are an artefact of
bioturbation. More work clearly needs to be carried out to separate cosmogenic Os
from hydrogenous and detrital Os for isotopic measurement but the Re-Os and Ir
evidence still remains at the heart of the K-T impact hypothesis.