Geology Reference
In-Depth Information
0.7094
Holocene marine carbonate (HMC)
Pleistocene
Pliocene
Late Miocene
0.7090
Middle Miocene
0.7086
Contribution of
continental erosion
Early Miocene
Late Oligocene
0.7082
Contribution of ocean
ridge hydrothermal activity
Early Oligocene
0.7078
PPl
M
O
E
Pa
0
10
20
30
40
50
60
Age (10
6
years)
Figure 10.7
The correlation between measured
87
Sr/
86
Sr values of Cenozoic marine carbonate sediments and biostratigraphic
age. The abbreviations across the bottom refer to geological epochs (Pa = Palaeocene, E = Eocene, O = Oligocene, M = Miocene,
Pl = Pliocene, P = Pleistocene). (Source: Adapted from DePaolo & Ingram (1985). Reproduced with permission of American
Association for the Advancement of Science.)
the Sr isotope composition of the seawater from which
the shells precipitated. The present oceans are known
to be well mixed with regard to
87
Sr/
86
Sr, but oceanic
87
Sr/
86
Sr has varied significantly through Phanerozoic
time. Figure 10.7 shows how the
87
Sr/
86
Sr values of
marine carbonates have increased consistently (but not
quite linearly) for the last 40 Ma, providing a powerful
dating and stratigraphic correlation tool for Cenozoic
sedimentary rocks, as illustrated by Exercise 10.1.
How has this remarkably regular evolution of sea-
water
87
Sr/
86
Sr during Cenozoic times arisen? Temporal
fluctuations in oceanic
87
Sr/
86
Sr reflect a changing
global balance between the two dominant Sr inputs
into the oceans (Figure 10.7):
weathering and erosion rates considerably, and the
resulting dissolved riverine Sr flux from this region -
with an average
87
Sr/
86
Sr value of about 0.713 - is suf-
ficient to account for most of the steady rise in seawater
87
Sr/
86
Sr shown in Figure 10.7 (Richter
et al.
, 1992).
Surprisingly, much of this Sr flux (>60%) and its radio-
genic signature derives from the weathering of carbon-
ate lithologies rather than silicate rocks (Oliver
et al.
,
2003).
The Sm-Nd radiogenic isotope system
The trace elements samarium (Sm) and neodymium
(Nd) are both 'light' rare-earth elements (LREEs,
Figure 9.9). Although the
α
-decay of
147
Sm to
143
Nd dif-
fers physically from the
β
-decay between
87
Rb and
87
Sr
(Figure 10.1.1), the evolution of the two isotope sys-
tems can be represented by the same algebra and the
two isotope systems are often used together
(Figure 10.9).
The practice developed above for Sr is applicable to
Nd too, ratioing the amount of the radiogenic isotope
143
Nd to a stable Nd isotope, in this case
144
Nd
(Figure 10.1.1). By plotting the
143
Nd/
144
Nd isotope
ratios for a cogenetic suite of rock samples or minerals
• Dissolved Sr from the weathering of continental
landmass, delivered to the oceans by rivers with a
global average
87
Sr/
86
Sr ratio around 0.711; this con-
tribution makes seawater Sr
more
radiogenic.
• Sr leached from ocean-ridge basalts by hydrother-
mal solutions (averaging 0.7045), tending to make
seawater Sr
less
radiogenic.
The past 40 Ma have seen an unusually regular rise in
seawater
87
Sr/
86
Sr. The tectonic uplift of the Himalaya-
Tibet region during this period has increased continental
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