OFR 151.pdf - CRC LEME

SECTION 5 (EARLY CRETACEOUS CLIMATES)
5.1 Global backdrop
Phenomena shaping Early Cretaceous climate and vegetation on the global scale include: (1)
changes in the area and relative position of the continents, which in turn altered oceanic
circulation patterns (Frakes et al. 1987, Tarling 1997); (2) subdued orography, which may or
may not have modified atmospheric circulation patterns; and (3) changes in relative sea level
(isostatic and changes driven by variations in sea floor spreading rates). Cretaceous floral
provinces have been reviewed by Herngreen and Chlonova (1981) and Srivastava (1994).
One not surprising consequence of the complex changes in palaeogeography is the continual
challenging of existing climatic reconstructions. For example, calcareous nannofossils from
high latitudes in the Northern Hemisphere indicate global 'ice house' conditions existed during
the Early Valanginian, contrary to the widely held view that there were warm equable
conditions in polar regions (Mutterlose and Kessels 2000). Ocean-wide anoxic events in the
Late Barremian to Early Aptian (Bralower et al. 1994) almost certainly had terrestrial
ramifications since high rates of carbon burial in marine sediments will have lead to
drawdown of atmospheric carbon dioxide and reduced oxygen concentrations in bottom
water. The global increase in the production of oceanic crust during the mid Cretaceous
coincided with abundant volcanism in the Pacific Basin (Larson and Kincaid 1996).
In order to simulate observed temperature patterns in the mid Cretaceous, GCMs require
atmospheric ρCO2 to be four times the present-day value (Berner 1990, Moore et al. 1992,
Barron et al. 1995). Volcanism provides a convenient mechanism for this increase but the
explanation is under challenge (Sellwood et al. 1994, Heller et al. 1996, Cowling 1999,
Sadler and Grattan 1999, Kump 2000). For example, high CO2 concentrations could be due
to a change in deepwater circulation, which in turn altered the flux of warm, saline water from
low to high latitudes (MacLeod and Huber 1996). Climatic sensitivity experiments
(references in Barron and Peterson 1991, Barron et al. 1995) predict that even small increases
in poleward ocean heat transport will lead to a significant increase in temperatures at the poles
and a decrease in temperatures at the equator.
Whichever explanation(s) prove to be correct, the point remains that Cretaceous Earth was
very different from the present. Notable differences are: (1) a broad zone of largely nonseasonal
climates stretching from palaeolatitudes 32 0 N to 32 0 S (Creber and Chaloner 1985)
and (2) plant productivity reached its maximum in middle to high latitudes, a region
encompassing Australia, which was located at palaeolatitudes of 65 0 -78 0 during the Late
Jurassic to mid Cretaceous (Veevers et al. 1991). Frakes and Francis (1988) note evidence of
ice rafting on many continents at mid-high palaeolatitude, including Australia. In contrast,
landmasses in the palaeotropics experienced arid conditions (Hallam 1984, 1985, Horrell
1991).
5.2 Australian backdrop
For much of the last 140-200 million years, Australia has been part of much larger continental
landmasses, first as part of the Super Continent Pangaea, then part of East Gondwana. Thin
coals confirm that rainfall and temperatures were adequate to support mire communities at
palaeolatitudes >70 0 S in southeastern Australia. Petrified tree trunks provide evidence for
relatively mild climates in polar latitudes in West Antarctica, then part of West Gondwana
(Jefferson 1992, del Valle et al. 1997).
59