WHAT CAN WE LEARN FROM ISOTOPES? - Chemical Fundamentals of Geology and Environmental Geoscience - page 198

At 25 °C this fractionation factor has the value 1.0286,
but α calcite/water is found to vary significantly with the
temperature of equilibration. Because of this useful
fact, the δ 18 O values of ancient marine carbonates can
be used to measure temperatures of deposition in the
geological past ('palaeotemperatures'). The calibration
equation in terms of δ 18 O values can be written:

maxima and interglacial periods. The 18 O/ 16 O record in
dated carbonate sediment cores can in principle be
used to monitor both past changes in sea surface tem-
perature and variations in the volumes of polar ice
through geological time, although the details of how
these two factors are disentangled lie beyond the scope
of this chapter.
Antarctic and Greenland ice cores provide an alter-
native isotopic record of late Pleistocene climate
change. Both δ D and δ 18 O in polar ice vary according to
the temperature at which the original snow was

T ° C = 16543 . − . × δ

18

O − δ

18

O

calcite water

(10.10)

2

18

δ 8

1

+× − 013 . δ O O water

calcite

where δ 18 O calcite represents the isotopic composition of
the CO 2 extracted from the fossil calcite, and δ 18 O water
represents the composition of the waters from which
the fossil shell was deposited (relative to PDB and
SMOW standards respectively - Table 10.3).
The very first example of such a study is even now -
60 years on - the most elegant. Urey et al. (1951) drilled
out tiny samples across a cut section of an inch-wide
Jurassic belemnite guard from the Isle of Skye
(Figure 10.11a), and measured the 18 O/ 16 O ratio of each
microsample. Using a thermodynamically derived
temperature calibration, they were able to show that
the creature lived through four summers and four win-
ters (Figure 10.11b), during which time sea temper-
ature varied seasonally between 15° and 20 °C,
superimposed upon a cooling secular trend.
The preservation of such detailed temperature infor-
mation within a single belemnite shell for 150 Ma sug-
gests that carbonate sedimentary successions elsewhere
might prove to be a valuable repository of palaeo-
climate data. This is indeed the case (Zachos et al ., 2001),
although the need to establish the 18 O/ 16 O ratio of the
waters from which the carbonates were deposited - in
order to calculate the temperature in Equation 10.10 -
introduces an element of ambiguity. Recognizing this
problem, Urey et al. (1951) had argued that the Jurassic
sea from which their belemnite grew its calcite guard
had the same oxygen isotope ratio as today's oceans
( δ 18 O = 0.0‰). Yet we now know that - as climate fluc-
tuates over time - seawater 18 O/ 16 O also varies: global
warming causes polar ice with δ 18 O values as low as −50‰
(Figure 10.10b) to melt and mix with the oceans,
thereby lowering mean ocean δ 18 O, while the locking-
up of water in new polar ice during cold periods has
the opposite effect on the oceans. This ice-melt effect
causes δ 18 O seawater to vary by ~1‰ between glacial

(a)

W

S

W

S

W
S
W

24

(b)

20
19

18
17
16
15

0 0.2 0.4 0.6 0.8

Radius/cm

1. 0 1.2 1. 4

Figure 10.11 (a) Cross-section of a Jurassic belemnite guard
showing winter (W) and summer (S) growth rings and
microsampling locations. (b) The measured oxygen-isotope
temperature profile showing seasonal variations during
the life of the belemnite. (Source: Urey et al. (1951).
Reproduced with permission of the Geological Society
of America.)