5 Isotopic Composition of Seawater 147
sedimentss 9 The authors' original conclusion has remained unchanged but not
unchallenged. Their preferred interpretation for the high concentrations of the
platinum group elements was that they had arrived from the cosmos and had been
spread across the World following the impact of a meteorite, as some meteorites
were known to be especially rich in these platinum group elements. Discoveries of
shocked quartz, microspherules of possible tektite origin, and possibly even the
smoking gun itself: a crater off Mexico, have since strengthened this hypothesis.
We shall however concern ourselves mostly with the isotopic evidence for impact,
bearing in mind that simultaneity of events does not prove a causal link between
them.
It was believed at the time that the isotopic ratio of osmium at this level might
clinch the argument. The reasons for this have been touched upon earlier in this
section and relate to the extremely low, 'primitive' 187Os/tS6Os isotopic ratios of
extraterrestrial material (due to their low initial Re/Os ratios). If the Ir at the
boundary layers derived from impact then the Os isotopic ratios in the same ought
to be low, around 1.
12
10
u~ 8
0
Ia3
r
4
o LL44-GPC3
(Pegram et at,. 1992)
= Leg 92 Data (This Work)
J
0
""'Continentat "~
I' "End Member" ;
Present Oay 5eawater
Mantle
"End Memioer"
,.3
0 '
, t
0.700
0.705
g
0
d~
i'i
Cenozoic Seawater Minimum
-------Globat Riverine Sr Input
L
l
. . . .
[
. . . .
I
,
9
9
0.710
0.7is
0.720
87Sr / 8+Sr
0.72 5
Fig. 5.40. The range of Os isotope ratios vs. Sr isotope ratio for seawater through the
Cenozoic era (Ravizza 1993). It is clear that seawater Os isotope ratio has fluctuated freely
between the two end members whereas Sr isotope ratio has remained relatively
conservativee 9 Is Os responding to relative rates of black shale weathering rates and
cosmogenic input?
sedimentss 9 The authors' original conclusion has remained unchanged but not
unchallenged. Their preferred interpretation for the high concentrations of the
platinum group elements was that they had arrived from the cosmos and had been
spread across the World following the impact of a meteorite, as some meteorites
were known to be especially rich in these platinum group elements. Discoveries of
shocked quartz, microspherules of possible tektite origin, and possibly even the
smoking gun itself: a crater off Mexico, have since strengthened this hypothesis.
We shall however concern ourselves mostly with the isotopic evidence for impact,
bearing in mind that simultaneity of events does not prove a causal link between
them.
It was believed at the time that the isotopic ratio of osmium at this level might
clinch the argument. The reasons for this have been touched upon earlier in this
section and relate to the extremely low, 'primitive' 187Os/tS6Os isotopic ratios of
extraterrestrial material (due to their low initial Re/Os ratios). If the Ir at the
boundary layers derived from impact then the Os isotopic ratios in the same ought
to be low, around 1.
12
10
u~ 8
0
Ia3
r
4
o LL44-GPC3
(Pegram et at,. 1992)
= Leg 92 Data (This Work)
J
0
""'Continentat "~
I' "End Member" ;
Present Oay 5eawater
Mantle
"End Memioer"
,.3
0 '
, t
0.700
0.705
g
0
d~
i'i
Cenozoic Seawater Minimum
-------Globat Riverine Sr Input
L
l
. . . .
[
. . . .
I
,
9
9
0.710
0.7is
0.720
87Sr / 8+Sr
0.72 5
Fig. 5.40. The range of Os isotope ratios vs. Sr isotope ratio for seawater through the
Cenozoic era (Ravizza 1993). It is clear that seawater Os isotope ratio has fluctuated freely
between the two end members whereas Sr isotope ratio has remained relatively
conservativee 9 Is Os responding to relative rates of black shale weathering rates and
cosmogenic input?