Desert surfaces: pavements, patterned ground, varnishes and crusts
275
investigations reveal that the varnish is superimposed on, and clearly separated from, the
rocky substrate, supporting the theory of an external origin of the constituents.
The most popular hypothesis is based on a biogenic origin, in which Mn enrichment is
due to fixing by mixotrophic bacteria which oxidise Mn 2+ to Mn 4+, along with clay
cementation which traps other elements. This then repeats itself, producing an accretion
and subsequent lamination of varnish, which in turn implies cycles of humidity and aridity.
At the present time the bacteria that fixes the Mn is believed to be Metallogenium
personatum, the oldest known living organism (Oberlander, 1994). Others propose fungi
and cyanobacterias as fixing organisms (Krumbein and Jens, 1981). Some advocate purely
physico-chemical processes (Engle and Sharp, 1958; Elvidge and Moore, 1979). Mn is
more soluble than Fe and in desert environments, Eh values are high, indicating elevated
oxidation, and pH is alkaline as a consequence of the low levels of leaching. According to
these authors, variations in oxidation-reduction and the basicity of the medium can give
rise to varnishes. It is clear that an intense debate exists concerning the origin of varnishes;
it is very possible that the conjunction of biotic activity with the physico-chemical
variations could be the route to follow in future experiments.
4.3. Environmental implications
The first studies of varnishes emphasised their climatic significance and indicated that a
more humid climate than the present was needed for their formation. This implied that
existing varnishes were all relict (see Dorn, 1998). The studies also argued for cycles of
patination, whereby the varnish is eroded and a new one created.
Currently, with modem high-resolution techniques, the varnish laminations which form
microlayers with distinct compositions are interpreted climatically. Periods of reduced Mn
accumulation correspond to arid or interpluvial phases. Periods where the Mn/Fe
relationship is increased indicate more humid, pluvial or lacustrine phases, where aeolian
deflation diminishes because of expansion of the lakes (Figure 12.20). In many places,
however, the varnish sections offer an unconvincing register of palaeoclimatic
oscillations, and are more likely to indicate that the processes of varnish formation are
very complex (Oberlander, 1994).
5. Duricrusts
5.1. Introduction
One of the characteristics of arid zones is the presence of surface and subsurface crusts of
differing chemical composition, such as calcareous, siliceous and gypsum crusts. It is also
possible to encounter laterites, formed in environments of much greater precipitation,
which are mostly found on desert margins. They extend over significant areas in arid zones
and some, thanks to their enhanced hardness, are resistant to erosion and are situated in
elevated areas. Furthermore, they are good indicators of palaeoenvironmental conditions
and an understanding of Quaternary crusts is very useful in the interpretation of past
geological stages (Goudie and Pye, 1983). They can also be of economic interest, for
275
investigations reveal that the varnish is superimposed on, and clearly separated from, the
rocky substrate, supporting the theory of an external origin of the constituents.
The most popular hypothesis is based on a biogenic origin, in which Mn enrichment is
due to fixing by mixotrophic bacteria which oxidise Mn 2+ to Mn 4+, along with clay
cementation which traps other elements. This then repeats itself, producing an accretion
and subsequent lamination of varnish, which in turn implies cycles of humidity and aridity.
At the present time the bacteria that fixes the Mn is believed to be Metallogenium
personatum, the oldest known living organism (Oberlander, 1994). Others propose fungi
and cyanobacterias as fixing organisms (Krumbein and Jens, 1981). Some advocate purely
physico-chemical processes (Engle and Sharp, 1958; Elvidge and Moore, 1979). Mn is
more soluble than Fe and in desert environments, Eh values are high, indicating elevated
oxidation, and pH is alkaline as a consequence of the low levels of leaching. According to
these authors, variations in oxidation-reduction and the basicity of the medium can give
rise to varnishes. It is clear that an intense debate exists concerning the origin of varnishes;
it is very possible that the conjunction of biotic activity with the physico-chemical
variations could be the route to follow in future experiments.
4.3. Environmental implications
The first studies of varnishes emphasised their climatic significance and indicated that a
more humid climate than the present was needed for their formation. This implied that
existing varnishes were all relict (see Dorn, 1998). The studies also argued for cycles of
patination, whereby the varnish is eroded and a new one created.
Currently, with modem high-resolution techniques, the varnish laminations which form
microlayers with distinct compositions are interpreted climatically. Periods of reduced Mn
accumulation correspond to arid or interpluvial phases. Periods where the Mn/Fe
relationship is increased indicate more humid, pluvial or lacustrine phases, where aeolian
deflation diminishes because of expansion of the lakes (Figure 12.20). In many places,
however, the varnish sections offer an unconvincing register of palaeoclimatic
oscillations, and are more likely to indicate that the processes of varnish formation are
very complex (Oberlander, 1994).
5. Duricrusts
5.1. Introduction
One of the characteristics of arid zones is the presence of surface and subsurface crusts of
differing chemical composition, such as calcareous, siliceous and gypsum crusts. It is also
possible to encounter laterites, formed in environments of much greater precipitation,
which are mostly found on desert margins. They extend over significant areas in arid zones
and some, thanks to their enhanced hardness, are resistant to erosion and are situated in
elevated areas. Furthermore, they are good indicators of palaeoenvironmental conditions
and an understanding of Quaternary crusts is very useful in the interpretation of past
geological stages (Goudie and Pye, 1983). They can also be of economic interest, for