Australia: The Land Where Time Began

A biography of the Australian continent 

Early Holocene – Climatic and Environmental Changes about 11.5-8 cal. ka BP

A number of rainforest taxa disappear from the Lake Euramoo sequence between 12.6 and 9.6 cal. ka BP, which included Agathis and Podocarpus, and it is at this time that a peak representation of Casuarina is recorded (Haberle, 2005). Turney et al. suggest this may represent a reversal towards climatic conditions that are drier. As there are continuous appearances of rainforest taxa in the record it suggests that disappearances of taxa may represent changes in canopy dominance through competitive advantage instead of a reversal of climatic conditions that necessarily restricted the advancement of rainforest into the area.

A maximum rainforest increase is recorded at Lake Euramoo between 9.6-8.7 cal. ka BP that was led by taxa that are typical of lower montane forests. Turney et al. suggest it most likely represents the establishment locally of closed canopy rainforest under the influence of increasing precipitation at the site. High precipitation is also suggested at Chillagoe between 10-7 ka by δ¹⁸O values, which supports a regional event. The establishment of rainforest occurs within a 900 year period, which is comparable to the period of time recorded at of other sites on the Atherton Tablelands where the transition from dominance of sclerophyll woodlands to dominance of rainforest has been established to have occurred in a period of time between about 400 and 1,000 years (Hiscock & Kershaw, 1992). The gradual exclusion of sclerophyll woodlands prior to the development peak of lower montane rainforests at 7.3 cal. ka BP was involved in the final phase of development of rainforest. In spite of the persistent occurrence of fires that appear to have maintained local patches of sclerophyll woodland, that may have retarded the encroachment of rainforest around the site, local dominance of rainforest was achieved.

A younger surface on which dunes of Holocene age could develop are suggested by Turney et al. to have been formed by the reworking of part of the dunefield by palaeochannels of the Todd River (Hollands et al., 2006). A characteristic of most of the dunes in the Simpson Desert is asymmetrical cross-sections that have steeper eastern flanks. It appears this morphology was acquired in the Late Pleistocene since the LGM. It has remained unclear what climate conditions caused this tendency to lean towards the east. It is suggested by Turney et al. that a shift in the prevailing winds to more westerly some time after the primary north-south strike of the dunes was defined, which thereby formed a steeper eastern lee-side or slip-face. This could explain those at Birdsville where the prevailing winds of the present intersect the dunes tangentially from the west (Nanson et al., 1992a). What is important, however, is that it would have required some of these dunes, those that have an eroded core of Pleistocene age on their eastern flanks, to have migrated into the prevailing wind. The solution of this issue is not helped by the dunes at Finke; their eastern flanks face the prevailing wind, and have been shown to have done so for at least the last 5 ka (Nanson et al., 1995). A final problem is that at Camel Flat both the oldest and youngest linear dunes are eastwardly asymmetrical, though the prevailing wind direction is almost perfectly aligned with the strike of the youngest dunes and with the older dunes it approaches tangentially the eastern face which is steeper. To interpret the occurrence and possible chronology of wind shifts on the basis of asymmetry of linear dunes has remained fraught with problems.

About 12 ka something that approached the phase of fluvial activity in coastal eastern Australia of the present began, the Nambucca Phase, during which there were much lower flows than those of the Late Pleistocene, though they were certainly more pronounced than those of the present. Alluvial deposits are scarce as the catchments were well forested and therefore shed water that was largely sediment free (Cohen et al., in press). A period of sustained fire appears to have taken place at Gooches Swamp between 11-10 cal. ka BP, which contrasts with the situation in the Termination.

At Tower Hill it is inferred there was a climatic reversal between 12.5 and 10.9 cal. ka BP, with percentages of Casuarinaceae pollen that were lower, and higher values of grass, together with a limited, though persistent Chenopodiaceae pollen presence (Turney et al., 2006; Fig. 3). After 10.9 cal. ka BP high erosion began which is indicated by the onset of the input of clastic material from the sides of the catchment (Turney et al., 2006). It appears this coincided with conditions that were wetter, as is reflected in the increase of arboreal development, that appear to have prevailed in the Early Holocene (D’Costa et al., 1989).

It is indicated by the results from Law Dome that the high accumulation regime of the present, about 0.7 m yr^-1 ice equivalent, was established at some time after about 7 ka, and following an increase of approximately 80 % from the Early to mid-Holocene. Thorough the Early Holocene the change between the 2 modes of accumulation occurred progressively, which possibly reflects changes in the atmospheric content of moisture and circulation that were driven by insolation. The overall effect appears to have been a shift towards increased cyclonic activity (van Ommen et al., 2004), which resulted in a contrasting precipitation regime to those that were recorded by other cores in the interior of the continent. It is suggested the largest difference between the core from Law Dome and other sequences in Antarctica, however, is the period after 11 ka, during which there was prolonged warming before reaching a maximum in the Early Holocene at around 9.5 ka (Morgan et al., 2002).

Sources & Further reading

Turney, C. S. M., S. Haberle, D. Fink, A. P. Kershaw, M. Barbetti, T. T. Barrows, M. Black, T. J. Cohen, T. Corrège, P. P. Hesse, Q. Hua, R. Johnston, V. Morgan, P. Moss, G. Nanson, T. van Ommen, S. Rule, N. J. Williams, J. X. Zhao, D. D'Costa, Y. X. Feng, M. Gagan, S. Mooney and Q. Xia (2006). "Integration of ice-core, marine and terrestrial records for the Australian Last Glacial Maximum and Termination: a contribution from the OZ INTIMATE group." Journal of Quaternary Science 21(7): 751-761.