Australia: The Land Where Time Began

A biography of the Australian continent 

Australian Palaeoclimate and Palaeogeography - Jurassic

A number of authors have compiled a large amount of evidence of the Jurassic in Australasia and have interpreted it in terms of the palaeogeography and palaeoclimate in the region (Bradshaw & Yeung, 1990, 1992; McKellar, 1996, 2004, in press; Grant-Mackie et al., 2000), though according to the authors¹ the palaeolatitudinal location of Australia is still not well constrained. Since the publication of the pole path (Embleton, 1981 Fig.3), that was largely conceptual, being based on a few averaged poles (Klootwijk, 1996) indicating that the Australian pole path shows the general outline of an (Late Triassic-) Early Jurassic loop, that was followed by a Late Jurassic-Early Cretaceous loop.

An extensive Triassic-Jurassic loop, the apex of which is around the Triassic-Jurassic boundary, characterises the Indian APWP that is much better established, and is followed by the Jurassic-Early Cretaceous loop with an apex around the Jurassic-Cretaceous boundary. According to the authors¹ it is suggested by the much greater extent of the Triassic-Jurassic Indian loop that the Australian loop apex is presently undefined. As the Australian loop is more extensive than the Jurassic-Early Cretaceous Indian loop suggests that the Indian loop is under-determined. During the Jurassic eastern Gondwana rebounded towards higher southern latitudes, as indicated by the palaeomagnetic data. The Australian and Indian pole paths are at best broadly determined for the Jurassic and don't provide fine detail of the movement to the south of Gondwana (Klootwijk, 1996). The authors¹ suggest that the location of Australia during this period, based on palaeogeographic reconstructions, can be thought of as presenting no more than a general picture. The palynofloristic change from the Araucariacean Phase to the Podocarpacean Phase, with the associated onset of the Microcachryidites Microflora [of Helby et al., 1987] reflects the rotation of Australia to higher, cooler latitudes through the Middle to Late Jurassic.

Some have viewed the Araucariacean Phase to the Podocarpacean Phase transition as a time when the climate cooled (Grant-Mackie et al., 2000), this author being a supporter of the general (global) climate model that had been proposed by Frakes et al. (1992). This time has been recognised as a cooling period (McKellar, 1996, in press), though only in a regional context, based on Australian palynofloral criteria that are similar to those used by Grant-Mackie et al. (2000). It has been proposed that the development of the Argo Abyssal Plain (the proto-Indian Ocean) contributed to a mean sea level rise that continued progressively into the Early Cretaceous (Grant-Mackie et al., 2000). The area occupied by shallow epicontinental seas was said to have increased as a result of this sea level rise, also increasing the extent of the maritime heat sink, heat being transferred to the oceans thereby leading to a cooler world with more equable global temperatures, though the effect was ameliorated by increasing atmospheric CO2 levels that occurred during the Middle and Late Jurassic.

The purported cooling of the climate in east Gondwana that was reflected in the phase transition from Araucariacean to Podocarpacean suggests that much of the palaeolatitudinal change that has been revealed (Klootwijk, 1996) must has occurred over a short period of time in the Late Jurassic, probably after the beginning in the latest Oxfordian of sea floor spreading that occurred on the northwest coast as the Argo Landmass separated from the margin of Australia. In the Northern Hemisphere a northwards shift of the floral zones across Eurasia in the Late Jurassic was interpreted as indicating a general warming trend (Vakhrameev, 1654; Krassilov, 1981; Hallam, 1985; Vajda, 2001). These movements were considered by others to have no direct implications for global change, being relative only to land area, not to palaeolatitude (Ziegler et al., 1993). They indicated that the previous general northward movement had been replaced  by a clockwise southern movement of the continents by the Middle to Late Jurassic, which supported the interpretation for the eastern Gondwanan section (Klootwijk, 1996), and accounted for different climatic trends in opposing hemispheres during the Late Jurassic (McKellar, in press).

 

Sources & Further reading

  1. Turner, S., Bean, L.B., Dettmann, M., McKellar, J. L., McLoughlin, S. & Thulborn, 2009; Australian Jurassic sedimentary and fossil successions: current work and future prospects for marine and non-marine correlation, GFF, Vol. 31, (Pt 1-2, June), pp 49-70. Stockholm, ISSN 1103-5897

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated 18/08/2012 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading