Geoscience Reference
In-Depth Information
and central and southern Australia (Goudie,
1983
). An early stage is the precipitation
of soft powdery aggregates of calcium carbonate (CaCO
3
) within the profile. The
source of such carbonate may be groundwater, run-off, wind-blown dust or the parent
material. As time progresses, the soft aggregates harden into irregular nodules or
spherical concretions that may eventually occupy more than half of the soil horizon in
which they occur. Later still, the hard nodules become cemented into a single massive
unit (Khadkikar et al.,
2000
). Solution and reprecipitation of carbonate can occur at
any stage. The chemical equations governing solution and precipitation were given in
Chapter 14
(Equations 14.1, 14.2 and 14.3).
Wa t er (H
2
O) combined with carbon dioxide (CO
2
) forms carbonic acid (H
2
CO
3
).
Carbonic acid dissolves calcium carbonate (CaCO
3
) to form calcium bicarbonate in
solution [Ca(HCO
3
)
2
]. An increase in temperature or a decrease in pressure cause the
dissolved carbon dioxide to come out of solution, and calcium carbonate is then pre-
cipitated. Biological activity can also assist in the precipitation of calcium carbonate.
Carbonate coatings can be seen forming today around exposed pine tree roots grow-
ing on sand dunes in Algeria, as well as around acacia tree roots exposed along the
banks of the lower Blue Nile in central Sudan and around River Red Gum (
Eucalyptus
camaldulensis
) roots in the arid Flinders Ranges of South Australia.
Many calcretes are neither pedogenic nor biogenic in origin but have formed as
a result of the subsurface lateral movement of groundwater (Ruellan,
1968
). Such
calcretes are often finely laminated and often occur at shallow depth along the margin
of former lakes and ponds, or else crop out along the scalded margins of fixed dunes
(Williams,
1968a
).
Vertically stacked beds of pedogenic carbonate nodules are found within Pleisto-
cene river levee deposits along the seasonally arid Son Valley in north-central India, as
well as in alluvial fan deposits in semi-arid west-central New South Wales (Williams
et al.,
1991a
). Both examples are indicative of successive phases of alluvial deposition
interspersed with dry phases during which there is seasonal leaching and precipitation
of calcium carbonate below the base of the soil-wetting zone.
One advantage of using calcretes to reconstruct past changes in climate rests on
the relative ease with which they can be dated. Initial dating efforts were confined
to the use of radiocarbon, but problems arose once the 35,000-year upper limit of
conventional radiocarbon (
14
C) dating methods was attained. At this point, even
slight amounts of contamination by both younger and still radioactive
14
C, perhaps
derived from plant rootlets or from old, inert carbon present in groundwater, could
give spurious results, as discussed in
Chapter 6
. This limitation was in part overcome
through the use of uranium-series dating which soon demonstrated that many samples
from Saharan lake calcretes deemed to be very late Pleistocene in age were in fact
of last interglacial age (Causse et al.,
1988
; Fontes et al.,
1992
). The method can
yield reliable ages provided there are no external gains or losses of uranium, that
is, when we are dealing with a closed geochemical system. Furthermore, inputs of
