Geoscience Reference
In-Depth Information
shells may become diagenetically altered after death, and they may lose or gain
uranium from seawater or groundwater, leading to ages that are too young or too old,
respectively. Walker (
2005
, p. 70) considered unaltered coral, clean speleothems and
volcanic rocks to be the most reliable materials for U-series dating, with diagenetically
altered corals, bone, evaporates, calcretes and peat or wood as generally unreliable.
New techniques, whose explanation is beyond the scope of this volume, have enabled
greater precision to be obtained. Precision obtained using alpha-particle spectrometry
can be as high as 1 per cent and can be improved to under 0.5 per cent using
thermal ionisation mass spectrometry (TIMS). Dating speleothems relies increasingly
on precise measurements obtained using multi-collector inductively coupled plasma
mass spectrometers (MC-ICP-MS), and analytical measuring procedures continue to
improve. The useful age range now available from U-series dating is from present up
to 500 ka (
Tabl e 6 . 1
), which makes this a very versatile dating tool for Quaternary
rocks and fossils. As with all dating methods, the wider the array of independent
techniques used at any site, the more likely it is that errors will be detected.
TIMS uranium-thorium dating of the
13
C record from three stalagmites from
caves in southern France and northern Tunisia has revealed synchronous changes with
Chinese stalagmite
18
O records and important differences with SouthernHemisphere
cave records (Genty et al.,
2006
). The same dating method enabled Denniston et al.
(
2007
) to identify intervals of late Holocene aridity shown in the stalagmite record
from a cave in central Missouri. Uranium-series ages obtained on speleothems from
South Africa and Somalia have been used to date wet phases in both regions over the
past 300 ka and, together with
14
C ages spanning the last 35 ka, have shown that when
southern Africa was wet in late glacial times, eastern Africa was dry, and vice versa
during the early Holocene (Brook et al.,
1997
). Vaks et al. (
2007
) used high-precision
MC-ICP-MS measurements to obtain U-Th ages for cave deposits (speleothems)
from the central and southern Negev Desert of Israel to show that the last interval
of sporadically wet climate was from 140 to 110 ka, favouring early modern human
dispersal from north-east Africa into the Levant at that time. Another application of
the U-series method involved U-Pb dating of water-table indicator speleothems (
cave
mammillaries
) from nine sites in the Grand Canyon to reveal that the Grand Canyon
had developed by headward erosion from west to east, with accelerated incision in
the east at around 3.7 Ma (Polyak et al.,
2008
).
6.7 Dating methods based on trapped electrons
6.7.1 Luminescence dating of dunes, loess and other quartz-rich sediments
Aitken has provided detailed accounts of the principles and practice of thermolumin-
escence dating (Aitken,
1985
) and of optical dating methods (Aitken,
1998
), while
Duller (
2008
) has produced a manual on luminescence dating that is specifically aimed
