Geology Reference
In-Depth Information
are the bane of many geological and biological
clocks, including radiocarbon. Fortunately, the
decay constants for some of these pairs include
those that correspond to half-lives that are very
useful in neotectonic and climatic investigations.
Because the U-Th series is a more robust
clock than is
14
C, it has been used as a means of
calibrating the
14
C clock beyond the end of the
tree-ring record (Fig. 3.13). In a clever sampling
scheme, Bard
et al.
(1990, 1998) used the
calcium carbonate from submerged corals off
Barbados as a means of obtaining sample pairs
from the same material. The corals contained
both
14
C as some small fraction of the carbon,
and U substituting for Ca in the carbonate lattice.
Accelerator mass spectrometric (AMS) measure-
ments of the
14
C and accurate measurements of
the U-Th revealed a consistent offset between
the two clocks for samples older than the
Holocene. This important result indicates that
ages calculated using
14
C are as much as a few
thousand years too young for samples dating to
about 25 ka.
The substitution of U for Ca in carbonate
lattices makes shells and carbonate coatings in
soils prime targets for U-Th dating. Unfortunately,
not all carbonate materials retain the parents and
the daughters in their lattice through time - that
is, they are not “closed systems.” In particular,
most shells (for instance, typical clams) leak
uranium, making them essentially worthless as
clocks. Luckily, most unrecrystallized corals work
well, both the colonial and solitary types. In
Barbados, Fairbanks (1989) reconstructed past
sea levels using
Acropora palmata
, a coral that
grows within a couple of meters of sea level
(Fig. 2.2). The west coast of North America sports
numerous marine terraces whose ages are
needed in order to determine deformation rates
of the coastline. Because most of the coastline is
at higher latitudes than those at which colonial
corals grow, the target has been instead the
solitary coral
Balanophyllia elegans
. These corals
are difficult to find, as they are only a centimeter
in diameter and look like wagon-wheel
spaghettis. Nonetheless, they have been used by
numerous workers (see review in Muhs, 1992) to
identify which marine terrace was formed during
which sea-level highstand. U-Th has also been
4700
Radiocarbon Calibration
4650
Radiocarbon age:
4600
4470 ± 40 yr (1
σ
)
4550
Calibration
curve:
radiocarbon
ages versus
calendar years
4500
σ
2
4450
4400
Range of
possible
calendar
ages
4350
4300
σ
1
σ
2
4250
4900
5000
5100
5200
5300
5400
Calendar Years (Before Present)
Fig. 3.11
Radiocarbon calibration and ambiguity
of
14
C ages.
The calibration curve of radiocarbon age versus
calendar age is used to estimate the calendar age
represented by a radiocarbon date of 4470
±
40 years.
Despite a 1
σ
uncertainty of only 40 yr in the radiocarbon
date, the range of calendar ages consistent with that
date is
∼
600 yr older and extends across
∼
300 yr (or up
to 400 yr for a 2
σ
uncertainty). This large range of ages
can be narrowed somewhat if either smaller analytical
uncertainties or multiple dates for the same event can
be obtained. Data derived from the CALIB radiocarbon
calibration program of Stuiver
et al.
(2009).
rate within the last couple of millennia. This
tree-ring calibration method is valid to about
10 ka, beyond which another calibration method
is needed. Here we turn to the precise dating of
calcium carbonate deposits (in particular, corals)
by both U-Th dating and AMS with
14
C.
Uranium-thorium (U-Th) dating
The uranium decay series represents a very
trustworthy set of clocks. The series consists of a
set of several isotopes that are generated from
their parents at varying rates and that decay to
their own daughters at yet other rates (Fig. 3.12).
The ultimate parents of the chains are
238
U,
235
U,
and
232
Th. Decays take place at a statistically
steady rate that is independent of temperature,
of the magnetic fields of the Earth and of the
Sun, and of all other environmental factors that
