Thursday, 8 December 2011

More information on the tough Australian climate during the Late Pleistocene

As I promised this blog entry is focusing on the changing climate in Australia during the Late Pleistocene.

This article is an overall review of the arid conditions in Australia in the Late Quaternary period (Hesse et al 2004). The basic conclusion they come to is that aridity has played a dominant role in the last few glacial cycles in Australia. This is especially clear in northern and central Australia where Hesse et al (2004) pose that lowered temperatures caused reduced monsoonal rainfall caused the drying of lakes and rivers, sparser vegetation coverage, more active sand dunes and increased dust advection. All of which I will give evidence of below.

Temperatures were thought to be 9 degrees lower during the LGM than they are today. This is demonstrated through the LGM wetland on Brachnina Creek in Flinders Ranges where there is evidence of high effective moisture during the LGM without the influence of snow melt (Williams et al 2001). This is because the low temperatures suppress evaporation to more than composite with the decrease in summer tropical rainstorms and allow growth of damp swamp vegetation.

The summer monsoon bringing moisture to northern and central Australia is not apparent from evidence during the LGM. And evidence shows there was only increased monsoonal activity from 14ka onwards, way after the lower rainfall interval of the LGM demonstrated by the increased rate of sand dune building. General linear dune field patters were dated and indicated there was a period of lake-shore dune building during the LGM. This lake shore dune building indicates increased volumes of sand resulting from arid and dry conditions (Chen et al 1995). Also several sites in northern Australia, from the Kimberley to Cape York Peninsula show increased sand dune building ensuing from sand blown from the exposed continental shelf (Lees 1992). In the Lake Eyre basin, linear sand dune building was widespread by 30ka.

The vegetation composition also saw a big shift during the Late Pleistocene. A marine core from the offshore Cape range Peninsula in north-western Australia demonstrated a large shift to drier vegetation during the LGM, with wetter vegetation re-establishing again after 14ka (van der Kaars and De Deckker 2002). Pollen records show during the LGM there was an increased coverage of shrubland (Callitris-Chenopodiaceae) which is indicative of a drier climate (van der Kaars and De Deckker 2002).

Maximum clay lunette formation occurs as lakes dry out. This lunette building was very high in the LGM and marks a transition to dry stable lake floor from the previous permanent fresh lakes (Page and Nanson 1996). Also the dust flux was a massive 3 times higher in the LGM than in the previous intervals.

So as far as northern and central Australia go, the pattern seems very clear, decreased temperatures caused less monsoon rainfall caused a series of knock-on effects and shifted the climate to a more arid condition. However in south-eastern Australia, the pattern is less clear. Although the vegetation was sparser and there was high sand dune activity during the glacial, there was also records of increased runoff into lakes and rivers, the exact opposite to northern Australia, and also to what would be expected in an arid interval. This may have been due to a greater effective moisture during the LGM. Now as stated before this could have been because of decreased temperatures, but also because of changes in runoff threshold of precipitation, amount of precipitation, humidity or vegetation cover.

So Hesse et al (2004) points to a much more arid climate in the Late Pleistocene era in Australia, which was especially intense during the LGM when most of the megafaunal species went extinct. This obviously supports the climate change hypothesis, with the climate becoming especially severe when the majority of the species went extinct. This could also fit to Horton’s hypothesis based on concentric habitats and free water availability. He claims that the 26,000- 15,000 years ago, approximately at the same time as the LGM, a threshold was crossed where the climate pushed the environment and the existing species over the edge where they no longer had available water or food (see earlier blog for more info). So the last transition to a glacial period was the last catalyst in a long series of changing climates. Although human predation may have killed some of the species at the time, they weren’t the main catalyst in their final demise.

No comments:

Post a Comment