Geoscience Reference
In-Depth Information
contained within the rock (
biostratigraphy
). McGowran (
2005
) provides a scholarly
and comprehensive overview of the merits and pitfalls of biostratigraphy, with a par-
ticular focus on Cenozoic marine microfossils. Relative ages were obtained according
to the standard geological principles of
superposition
and
cross-cutting relations.
In
the case of superposition, if bed A overlies bed B, then it is younger, unless the beds
have been overturned as a result of folding or faulting. If bed P cuts through bed
A, then it is younger than A. These two fundamental principles were already clearly
recognised by the Scottish polymath James Hutton (
1795
) more than two centuries
ago, but they did not become widely acknowledged until the Scottish lawyer-geologist
Charles Lyell published his three volume
Principles of Geology
some thirty years later
(Lyell,
1830
-
1833
). Other relative dating methods include using weathering rinds on
individual rocks, lichen patches, soils of different degrees of 'maturity', depth and
degree of bedrock weathering, or even prehistoric stone tool assemblages, all of which
can provide useful preliminary information, but none of which are capable of yielding
ages that are both accurate and precise. For this we need to use methods capable of
providing an absolute age, some of which are listed in
Tabl e 6 . 1
.
Tabl e 6 . 1
summarises the more common dating methods used in the reconstruction
of climatic change in deserts and desert margins, together with their range and preci-
sion. They can be grouped into six broad categories. The first category - correlation
methods - includes geomagnetic dating, chemical fingerprinting of volcanic ash beds
(tephrochronology) and correlation of marine isotope stages inferred from variations
in the stable oxygen isotopic composition of marine foraminifera, calibrated against
the astronomical orbital time scales. All three methods require independent calibration
using absolute dating techniques. The second category of dating methods is where
the unstable parent isotope undergoes radioactive decay to produce a stable daugh-
ter isotope. Radiocarbon dating, potassium-argon dating and argon-argon dating all
fall within this group of widely used dating methods. The third category involves
using isotopes in which there is disequilibrium between the parent and the daughter
radioisotopes. All forms of uranium-series dating methods are included within this
group. The fourth category involves the use of electrons trapped within the lattice
structure of certain common minerals (luminescence methods) and tooth enamel or
bone apatite (electron spin resonance). Category five comprises certain cosmogenic
isotopes, and the best-known method in category six is amino acid racemisation dat-
ing. Each method has its own inherent precision and time range, and it is always
advisable to use as many independent dating methods as possible. We expand on
these points in the next section.
6.3 Atoms, isotopes and radiometric dating
Tabl e 6 . 1
lists six broad categories of dating techniques that have all been used in the
reconstruction of environmental and climatic change in deserts. Correlation methods
