Geology Reference
In-Depth Information
10.4
The half-life of
14
C is 5730 years; calculate its decay
constant. Charcoal from charred tree remains pres-
erved in a volcanic pyroclastic flow shows an
average
β
-
count rate of 5.8 disintegrations per
minute (dpm) per gram of sample, whereas car-
bon extracted from living vegetation today pro-
duces a
14
C count rate of 13.56 dpm per gram.
Calculate the age of the volcanic deposit.
10.5
Analysis of a Neogene
12
marine limestone core gives
a δ
18
O
calcite
value of −1.3 ± 0.1‰. Calculate (includ-
ing ± analytical uncertainty) the temperature of the
sea from which it was deposited, assuming sea-
water isotopic composition at the time of deposition
was the same as today. How much additional uncer-
tainty in the temperature value arises from not
knowing the actual seawater δ
18
O at that time?
10.6
Spot the incorrect statements among the following,
and explain how they are wrong:
(a) Every
87
Rb nucleus that decays becomes an
87
Sr nucleus.
(b) Every
87
Sr atom in a rock is the product of the
decay of an
87
Rb nucleus in that rock.
(c)
87
Sr/
86
Sr grows more rapidly in continental
crust than in the mantle.
(d)
143
Nd/
144
Nd grows more rapidly in continen-
tal crust than in the mantle.
(e) Radiocarbon provides an important dating
technique for all artefacts and rocks.
(f) The fractionation of stable isotopes in Nature
is always mass-dependent.
12
Neogene refers to Miocene and Pliocene epochs lasting from
22 Ma to 1 Ma ago, during which period Antarctic ice sheets
had become established.
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