5 Isotopic Composition of Seawater
93
lh-O3
0')
0.709&
0.7090
0.7086
0.7082
0.7078
~Marine
carbonate from the Hotocene
~
pl,~,istocene
Ptiocen?
~aj~_ M_ioj.._,_ _
~XI~ Mid - Miocene
"e~__Earty
Miocene
' ~ Late Oligo~.~
MPll
0
.,~ ...AP 'O
I, o
,l
~
I 'p"
1 M"
~'
~o
3'0
,.b
~o' ~o
70
Age (106years)
Fig. 5.3. The precise 87Sr/86Sr isotopic analyses of DePaolo and Ingrain (1985) on
stratigraphically well constrained marine carbonates allowed the reconstruction of a single
clear trend in Tertiary seawater isotopic variation for the first time.
l
"i
Miocene
Ptei. ~tloc
Late . I
middle
0.7092 ~
....
\ 4
I
"%%%* ~-~,. ~'%- -,%
0.7090 '" " - - ' ' T ~ ' ,
03 0.7088
~
"
0") 0.7086 -
L
... ~,,,
-.,,x
16
20
2/.
0.7084
0.7082
0
z,
8
12
Age (Ma)
Fig. 5.4. 261 Sr isotopic analyses on planktonic foraminifera allowed Hodell et al. (1991)
to constrain Sr isotopic evolution in the Neogene ocean very precisely. Such secular trends
permit isotope microstratigraphy. Subsequent studies replicated these results within the
error permitted by the analytic technique.