The Quaternary and Australian aridity

There is evidence that by 14 mya the rate of ice rafting greatly increased in the Northern Hemisphere marking the onset of glaciations and ice sheet development in Greenland which continued to 5 mya (Eyles, 2008). At around 3 mya renewed cooling initiated commencement of glacial/interglacial cycles associated with Milankovitch orbital forcing. Initiation of Northern Hemisphere Glaciation (NHG) has been attributed variously to the closure of the Panama seaway (Kennett, 1977, Rogers and Santosh, 2004, Bartoli et al., 2005, Huber and Nof, 2006), the development of the ENSO system (Molnar and Cane, 2002, Fedorov et al., 2006), the uplift of the Tibetan Plateau (Qiang et al., 2001), and the continued restriction of the Indonesian Throughflow (Fedorov et al., 2006). Generally the contemporary view espouses the involvement of a complex mix of forcing mechanisms such as, evolving tropical ocean/atmospheric interactions, biogeochemical cycles, orbital forcing and ice albedo, creating step-wise shifts in global climate (Fedorov et al., 2006). The overwhelming impression is of a complex amalgam of interacting processes forcing climatic amplifications, teleconnections and cascading changes over time and space globally, leading ultimately to an irreversible „climatic crash‟ around 2.75 mya. Thenceforth glacial/interglacial cycles of the Quaternary exerted major influence on the development of the current Australian arid zone.

Climatic fluctuations of the Quaternary have been instrumental in driving erosive and depositional processes that have been responsible for the current expression of landscape forms and features evident in the dunes, lakes, rivers and soils in arid Australia. Following the

relatively rapid development and growth of northern hemisphere continental ice caps, in conjunction with development of sea ice around Antarctica around 2.5 mya, the global climate entered a period of glacial/interglacial cycles (Williams, 2000). The decisive factor modulating these cycles was the development of the NHG. These glacial/interglacial cycles, marked by the waxing and waning of ice caps and related rise and fall of earth‟s oceans, were driven primarily by periodic orbital forcing related to periodic shifts in the earth‟s orbital geometry. These changes in eccentricity (shape of earth‟s orbit, 100 ka cycles), obliquity (tilt of earth‟s axis, 41 ka cycles) and precession (seasonal insolation), affect the distribution and amount of incident solar energy received by the earth (Milankovitch frequencies) (Zachos et al., 2001). Thus, superimposed on the general global cooling trend through the Tertiary, were the extreme oscillations of the glacial/interglacial phases of the Quaternary. Initially these „Milankovitch frequencies‟ oscillated around ~41 ka from 1.8 to 0.7 mya but have shifted to ~100 ka cycles since 0.7 mya (Williams, 2000). Kershaw et al (2003a) posit that the trend to drier and more variable climates was initiated around 350, 000 ka.

There was a marked decrease in fluvial activity in the Cooper Creek from around 300,000 ka, reflected in the retreating shorelines of Lake Eyre and gradual desiccation of the landscape (Rust and Nanson, 1991, Nanson et al., 1992, Croke et al., 1999, Nanson and Price, 1998). This trend was thought to have been driven by changes in oceanic circulation principally associated with the alteration of land-sea configurations in the Indonesian region. This further enhanced the WPWP which in turn intensified the influence of the ENSO system (Kershaw et al., 2003a). Sedimentary sequences in the LEB record these significant climatic shifts from wetter to dryer periods throughout the Quaternary in synch with glacial/interglacial cycles, along with a general weakening of the summer monsoon and increasing influence and activity of the ENSO system. Aeolian, fluviatile and lacustrine sedimentary sequences directly record the palaeohydrology of the LEB (Hesse et al., 2004). These cycles in central Australia were reflected not in advancing and retreating glaciers but in oscillating periods of moisture availability and restriction (Kershaw et al., 2003b). Climatic cycles have driven hydrological changes which in turn have controlled the character and rates of sedimentation within the study area.

Most importantly for the development of the current soil cover of the Cooper Creek floodplain a marked hydrological transition from a previous high energy, high coarse sand load system to a lower energy, mud dominated system, occurred around 110 ka (Rust and Nanson, 1986, Nanson et al., 1998).

Teleconnections between the ENSO, ASM and STHP systems have resulted in amplification of climatic variability in the Australian arid and semi-arid zones, exerting a profound effect on hydrological, geomorphological and hence floodplain dynamics of the study area.

From a biological perspective the increasing trend to aridity was exacerbated by an increasing variability in climate, imposing selective pressure for biological characteristics of

persistence and resilience. Biota has been driven to adapt to an „excess‟ of variability by developing a range of diverse life history strategies. Due to the temporally and spatially stochastic nature of rainfall events, fauna have adapted by evolving opportunistic reproductive strategies, rather than following a seasonal pattern of reproduction. When conditions are suitable reproduction occurs. Nomadism is common amongst avifauna responding to spatially stochastic rainfall events (Nicholls, 1989). Plants have adapted by developing a range of strategies such as drought tolerance or avoidance (Nicholls, 1991).

Commencing about 25,000 ka, the Last Glacial Maximum (LGM) marked the onset of the last major arid phase. The overall trend to increasing aridity over the Quaternary reached maximum expression around 18,000 ka. Within the study area in particular, this arid phase resulted in falling lake levels and increasing salinity. Lake beds were exposed to seasonal wetting and drying, resulting in the flocculation of clay minerals caused by efflorescing salts, especially halite and gypsum. These desiccated clay pellets were subsequently deflated by high- velocity winds onto the lee side of lakes, forming extensive gypsum and clay-rich lunettes (Bowler, 1976, Ludbrook, 1980). Within the study area these environments are commonly encountered and are characterized by the presence of halophytic and gypsophillous vegetation.

During increasingly severe glacial periods, falling sea levels and lower sea surface temperatures, in conjunction with greatly enhanced continentality, directly caused a weakening of the monsoon system. This in turn dramatically reduced hydrological flows down the inland river systems. These cold dry periods inhibited vegetation cover and enhanced wind erosion. These were the periods of dune building in the centre of Australia. Conversely, during the wetter warmer interglacials the summer monsoons were more effective, as higher sea level and warmer oceans facilitated deeper incursion of warm moist air into the continent. Material from the rivers, deposited during the wet interglacial periods, was deflated from river channel to floodplain, forming light coloured source bordering dunes (Twidale and Wopfner, 1990). Ultimately, over many glacial/interglacial cycles, these increasing aeolian deposits were constantly reworked and deflated further distances from their floodplain environment, becoming redder in hue with age, gradually evolving into the extensive sand dune field deserts of the Lake Eyre Basin (Wasson, 1982, Wasson, 1983). Recent research has determined that these sand deserts began to form 1 mya (Fujioka et al., 2009).