OFR 151.pdf - CRC LEME

An alternative approach is to use the architectural characteristics of leaves (size, type of
margin and stomatal arrangement) and wood anatomy (growth rings) as evidence of past
climate (foliar physiognomic analysis). This allows proxy-climatic information to be
obtained from Cretaceous and Tertiary plant clades that are extinct, or whose NLRs are
unknown. For example, the presence of a 'drip tip' analogous to those found on the leaves of
extant rainforest trees has been used to infer warm and very wet climates (Greenwood 1992,
1994). Uhl and Mosbrugger (1999) propose that leaf venation density has proxy-climatic
value. Whether it is possible to use foliar physiognomic analysis to make reliable quantitative
estimates of past rainfall or temperature continues to be debated (Greenwood and Wing 1995,
Jordan, 1996, 1997b, Wing and Greenwood 1996, Wiemann et al. 1998a, Wilf et al. 1998).
Leaf morphology in sclerophyllous plants is equivocal proxy climatic evidence because of the
convergent effects of low soil nutrient levels (scleromorphy) and dry soils (xeromorphy). Hill
(1998b) concludes that the two can be distinguished via differences in foliar adaptations that
protect the leaf from excessive wetting (cuticular striations, dense covering of trichomes) and
those that reduce water loss (stoma enclosed in pits or within raised structures, revolute leaf
margins). However, plants with xeromorphic characters are inherently unlikely to occur at
sites conducive to fossilisation.
Modern relationships between climate and wood anatomical features in North American,
South American, African and Malaysian floras have been used to develop a method for
inferring past climates from fossil wood (Wiemann et al. 1998b). More recently Falcon-Lang
(2000) has challenged the uncritical use of growth rings in fossil conifer wood as a tool for
reconstructing seasonality, citing a strong inverse relationship between leaf longevity and the
growth ring architecture ('ring markedness'). Other recent studies have investigated the value
of phytoliths in identifying Nothofagus (Lophozonia) in the Tertiary fossil record (Whang and
Hill 1995) and reviewed the use of fungal germlings as evidence for past humidity (Wells and
Hill 1993).
2.4.2 Other proxy-climatic evidence
Away from the continental margin basins, erosion and/or prolonged periods of non-deposition
or deep weathering of the regolith have destroyed much of the organic matter deposited
during the Cretaceous and Tertiary. Hence the continental landscape largely is a palimpsest
of erosional and weathering features developed under varying climatic regimes, especially in
the cratonic regions of central and Western Australia.
Most erosional features of the landscape are difficult to use as proxy-climatic data for several
reasons: (1) Landforms may or may not change abruptly at thresholds in the climatic
continuum (Chappell, 1983). (2) In many regions, climate-driven processes have changed in
intensity rather than in character during the geologic past. (3) The time(s) of formation of
most landforms almost always are poorly constrained.
Using a mathematical model in which climate fluctuated sinusoidally between wet and dry
states, Rinaldo, et al. (1995) found that both end states left detectable geomorphologic
signatures in the landscape only when there was no active tectonic uplift. Where uplift had
occurred, the topography was found to track the prevailing climate, but only those features
developed during the wet state were likely to be preserved. Probable examples are the
dissected pediments and cemented soil horizons (duricrust) now found across much of inland
Australia. A more favourable situation exists in aggradational terrain where mineralogy,
lithostratigraphy, stratal patterns (facies architecture) and animal remains reflect past climates
although, once again, such evidence is often difficult to date.
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