Australia: The Land Where Time Began

A biography of the Australian continent 

The Last Interglacial – Inland Vegetation

There is a paucity of direct data on the vegetation of the last interglacial. The long pollen sequence in a marine core, Fr10/95 GC17, obtained from near Exmouth Gulf in northwestern Australia is the only such sequence available. This is a record of fire, vegetation and climate for the last 100 ka (van der Kaars & De Deckker, 2002; van der Kaars, De Deckker & Gingele, 2006). As a result of wind patterns Fr10/95 GC17 provides a strategic record of the vegetation that was present across adjacent areas of northern Australia, though it was taken 60 km from shore. The pollen in the core indicate the presence of eucalypt woodlands in the northern part of the arid zone that were grass-rich 100 ka-85 ka, followed by a shift to arid open woodland with chenopods and Callitris, the native dryland conifer. It is suggested by reconstructions of palaeoclimate (van der Kaars, De Deckker & Gingele, 2006) that after 81.5 ka regional rainfall was about 440 mm/annum (p.a.), which brought the area within the arid zone. The northern bound of the arid zone of the present is set as 500 mm p.a. Carbon isotope records from sediment cores from Lake Gregory that had been dated with OSL corroborates the record from Fr10/95 GC17 (Pack et al., 2003), and also indicate a long-term trend towards an more arid climate.

A phytolith sequence obtained from the Puritjarra Rockshelter in Central Australia (Bowdery, 1998; Smith, 2009b) indicate that during the last interglacial (MIS 5) local grasslands were well established, with grass values approaching modern levels at about 96 ka. The landscape of the last interglacial is suggested by the evidence to be open grassland vegetation and dunes that were active, at least seasonally, analogous to the conditions in the area around the rockshelter at the present.

Deflation of the floor and periodic dissolution the salt crust of Lake Eyre have frustrated attempts to obtain a long, continuous pollen record from the lake bed. A core obtained from Lake Madigan, LE8_2/2, is poorly dated and provides a broken record of the changes of vegetation, though it does indicate an important component of regional vegetation prior to 30 ka was Callitris (Luly, 2001). At Punkrakadarinna Waterhole on the Warburton River pollen has been recovered from the Upper Katapiri sediments (Hesse, Luly & Magee, 2005), indicating significant areas of open vegetation comprised of Poaceae and Asteraceae (grass and daisies) similar to the vegetation of the area at the present, were included in the interglacial environment.

An alternative record of the palaeovegetation of the Lake Eyre region over the last 65 ka is provided by carbon isotopes in fossil emu eggshell (Dromaius novaehollandiae) (Johnson et al., 1999). It is shown by these isotopes that C4 grasses, that are associated with summer rain, were more abundant in the early part of the record, 65-45 ka, following which they declined, the implication being that the Australian summer monsoon was more effective at that time than it is now.    

Overall, the picture is one of open arid or semi-arid vegetation extending across much of the interior during the last interglacial, with a shift towards a more dryland taxa, which included towards the end of MIS 5 Callitris and chenopods, though it is clear that even after MIS 5, summer monsoon rainfall was more effective in the interior than at the present until it collapsed at 45 ka.

Sources & Further reading

1. Smith, Mike, 2013, The Archaeology of Australia’s Deserts, Cambridge University Press