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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