92
Peter Stille and Graham Shields
the time-scale of the whole age of the Earth (of. Fig. 5.24), initial research showed
quickly that this was not the case during the Phanerozoic and that the seawater Sr
isotope ratio had fluctuated ~ignificantly in the geologic past depending on which
source of Sr was more dominant at any one particular time.
The most important sources of dissolved Sr in the sea are considered to be I)
products of erosion carried by rivers and the wind, 2) ancient and recent marine
c~bonate dissolution on the seafloor, and 3) exchange with hydrothermal sources
on the ocean floor Peterman et al. (1970) showed in an isotope study on carbonate
fossil fragments that the Sr isotopic ratio of seawater underwent strong
fluctuations during the Phanerozoic era. The authors explained these fluctuations
by invoking variable input of crustat Sr with high 87Sr/86~qr ratios. Increases in
seawater 87Sr/86Sr would not necessarily correlate with enhanced continental
weathering but would also reflect the Sr isotopic ratios (ages and initial Rb/Sr
ratios) of the crustal rocks undergoing weathering. Veizer and Compston (1974)
came to similar conclusions. Clauer (1976) considered that "sea-floor spreading"
and thus the opening of the ocean basins at various times had also strongly
influenced seawater 87Sr/86St.
Fig. 5.2. shou,s Ihe results of 786 measurements of bulk marine carbonates of
Pbanerozoic age (Burke et al. 1982). The broad temporal trends in seawater
87Sr/8.6Sr ratio implied in these data have been confirmed by every, study since.
(/1
o Tt00
~, ~J
;, , ,~; 'i J
a7o9o ,~,i I 1
\~,~, : 'I
'1',~ L[
o :~ao
~',X,,"-~ j ',
o.7070
I ! .~ ~ '-" "O
II 1
I I ,
! I I
0.7060 ~Tetliar: Crelac,
0
100
/
i f
i\
i I
~
t
I I i s
Jurass Trias Perm. Carbon Devon
2OO
300
~00
Million
yeors
/
" ;'/f;Silut Or~k:, CamOr.
500
Fig. 5.2. Temporal variations of the Sr isotope ratio in seawater based on 786 isotopic
analyses of marine carbonate rocks. The dashed lines show the extent of 95% of the data
poinls. (Burke et al.. t 982)
Peter Stille and Graham Shields
the time-scale of the whole age of the Earth (of. Fig. 5.24), initial research showed
quickly that this was not the case during the Phanerozoic and that the seawater Sr
isotope ratio had fluctuated ~ignificantly in the geologic past depending on which
source of Sr was more dominant at any one particular time.
The most important sources of dissolved Sr in the sea are considered to be I)
products of erosion carried by rivers and the wind, 2) ancient and recent marine
c~bonate dissolution on the seafloor, and 3) exchange with hydrothermal sources
on the ocean floor Peterman et al. (1970) showed in an isotope study on carbonate
fossil fragments that the Sr isotopic ratio of seawater underwent strong
fluctuations during the Phanerozoic era. The authors explained these fluctuations
by invoking variable input of crustat Sr with high 87Sr/86~qr ratios. Increases in
seawater 87Sr/86Sr would not necessarily correlate with enhanced continental
weathering but would also reflect the Sr isotopic ratios (ages and initial Rb/Sr
ratios) of the crustal rocks undergoing weathering. Veizer and Compston (1974)
came to similar conclusions. Clauer (1976) considered that "sea-floor spreading"
and thus the opening of the ocean basins at various times had also strongly
influenced seawater 87Sr/86St.
Fig. 5.2. shou,s Ihe results of 786 measurements of bulk marine carbonates of
Pbanerozoic age (Burke et al. 1982). The broad temporal trends in seawater
87Sr/8.6Sr ratio implied in these data have been confirmed by every, study since.
(/1
o Tt00
~, ~J
;, , ,~; 'i J
a7o9o ,~,i I 1
\~,~, : 'I
'1',~ L[
o :~ao
~',X,,"-~ j ',
o.7070
I ! .~ ~ '-" "O
II 1
I I ,
! I I
0.7060 ~Tetliar: Crelac,
0
100
/
i f
i\
i I
~
t
I I i s
Jurass Trias Perm. Carbon Devon
2OO
300
~00
Million
yeors
/
" ;'/f;Silut Or~k:, CamOr.
500
Fig. 5.2. Temporal variations of the Sr isotope ratio in seawater based on 786 isotopic
analyses of marine carbonate rocks. The dashed lines show the extent of 95% of the data
poinls. (Burke et al.. t 982)