OFR 151.pdf - CRC LEME

climates. For example, grass species utilising the C4 photosynthesis pathway are most
numerous in regions where summers are hot and wet. Their relative abundance declines in
regions where mean annual temperatures and/or summer rainfall are relatively reduced
(Hattersley 1983, 1987). Connin et al. (1998) have used δ 13 C variation in herbivore tooth
enamel to evaluate patterns of C4 plant abundance, and therefore infer trends in summer
rainfall, in the southwestern United States.
3. Minerals
A number of minerals form under highly specific conditions and these (or their casts) can
provide direct or indirect information on past climates.
An example is glendonite, which provide unequivocal evidence of seasonal freezing of
seawater in the Eromanga Basin during the Early Cretaceous (Frakes et al. 1995). Gibson et
al. (2000) have used a spike in kaolinite-dominated mineral assemblages to infer intensified
weathering due to increased precipitation and temperatures on the north-east Atlantic coast of
the United States during the Late Paleocene. Other widely used environmental indicators
include evaporite minerals that form onshore only under arid and semi-arid conditions, e.g.
anhydrite, gypsum and halite (cf. Bowler 1976). These are at the dry end of a spectrum of
mineral indicators whose wet end are lignites and coal (Rees et al. 1999). Between these end
members are clays such as illite, which indicate weathering under temperate conditions, and
smectite, which indicates weathering under warm and semi-arid conditions. Uranium-lead
dating of zircons has been used to provenance sands reworked into Quaternary sand dunes
(Pell et al. 1997) and the same technique can be applied to pre-Quaternary contexts (B.
Pillans pers. comm.).
4. Palaeosols
Fossil soils (palaeosols, paleosols) developed across former landsurfaces are indirect
evidence of past climates and climatic change (Catt et al. 2000). Recent reviews of the
terminology and taxonomy of palaeosols include Nettleton et al. (2000) and Reuter (2000).
In Australia, considerable attention has been paid to the local conditions under which major
cementing minerals in duricrust are transported by, and precipitated from, circulating
groundwater. Examples are iron/aluminium sesquioxides (ferricrete), secondary silica
(silcrete) and carbonates (calcrete) (Arakel 1991, Bourman 1993, Anand 1997).
These and related studies indicate: (1) Ferricrete is best developed under climates with a
seasonally variable rainfall (Milnes et al. 1985, Butt 1981, cited in Clarke 1994). (2) Silcrete
requires acid-weathering conditions within the soil but otherwise cannot be linked to specific
environmental conditions (Milnes and Twidale 1983). (3) Gibbsite, one of the major minerals
found in bauxite, implies mean annual temperatures were above 22 ° C, unless drainage and
parent rock characteristics were unusually favourable (references in: Price et al. 1997, Price
1998, Taylor 1998). (4) Calcrete may form under sub-humid to semi-arid climates although
one form (pedogenic calcrete) preferentially occurs in winter rainfall regions such as
southwestern Western Australia whilst the other form (groundwater calcrete) preferentially
occurs in regions receiving summer rainfall. (5) Saprolite can develop under almost any
humid climatic regime although rates of 'deep weathering' will vary. More generally,
Christopherson (1997) proposes that heavily weathered soils with distinctive iron and
aluminium oxide horizons (oxisols) reflect seasonal- and non-seasonal wet tropical climates,
respectively. Ekart et al. (1999) have used palaeosol carbonates to estimate atmospheric
ρCO2 levels over the past 400 million years.
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