Palaeogeography, Palaeoclimatology, Palaeoecology
Lead isotopic evidence for an Australian source of aeolian dust to Antarctica at times over the last 170,000 years
Introduction
After Antarctica, Australia is the driest continent. There is ample evidence that during the Last Glacial Maximum (LGM), in particular, the central Australian desert zone expanded towards the coast (Bowler, 1976). This also would have been the case during previous dry phases that coincided with Marine Isotopic Stages (MIS) such as MIS 4, 6, 8 and 10 when major lakes in Australia would have dried up and dune fields were active. On land, an extremely wet period that commenced around 60 ka coincided with what is called the “megalake” phase when many lakes were permanently filled, and that was synchronous with MIS 3 (Bowler et al., 2006). However, by approximately 35 ka, lake levels started to drop and the world entered into the very cold and dry phase [especially Australia; see Miller et al., 1997, Barrows et al., 2000] that culminated at the LGM. During the latter period, dune mobility was extensive throughout Australia and airborne dust would have spread across the continent and into the surrounding oceans and adjacent landmasses.
Jennings (1968) determined that the major direction of the winds in Australia follows an anticlockwise gyre (Jennings, 1968) with two major plumes of aeolian material spreading over the Tasman Sea in the southeast and the eastern Indian Ocean in the northwest (Bowler, 1976). More recently, Sturman and Tapper (2006) have reassessed the characteristic late winter (August) atmospheric transport patterns in the Australasian region and have identified a more complex set of major, near-surface airflow directions for a larger number of locations that pass over the margin of the Australian continent compared to what was previously identified (Fig. 1). Previous studies (Grousset et al., 1992, Basile et al., 1997, Delmonte et al., 2004, Delmonte et al., 2007, Delmonte et al., 2008, Gaiero, 2007) concluded that dust transported to Antarctica (during the LGM) was sourced from South America and primarily from Patagonia, and that dust export from Australia could be excluded. An exception is the recent study of Revel-Rolland et al. (2006), which pointed out that Lake Eyre in central Australia may have contributed to dust deposited at Dome C. A second paper by Marino et al. (2008), based on major elemental composition of aeolian dust recovered from Dome C, concluded that Australia was a possible source of dust during warm phases, notably the Holocene.
It is clear that there is a need to establish the origin of aeolian dust deposited in Antarctica. Has dust always come from the same source or have sources varied through time? Have different sources supplied dust simultaneously? These are important questions because, by determining dust sources, it will become possible to reconstruct atmospheric circulation at high latitudes in the Southern Hemisphere through time, especially over significant periods of climate change.
This paper is an attempt to evaluate those questions based on isotopic evidence obtained from an array of fluvial samples from the extensive Murray Darling Basin of southeastern Australia (Fig. 2, Fig. 3). In addition, the recent access to satellite imageries and meteorological reanalysis data and back/forward trajectory modelling show that, in fact, air masses–some of which carry airborne dust–leave Australia in any possible direction depending on meteorological conditions at the time. It is now clear that some air masses do circumnavigate Antarctica and, therefore, there is potential for Australian dust to reach Patagonia and even the Antarctic mainland (see section below and Fig. 4).
Section snippets
Atmospheric circulation over the Southern Ocean and Antarctica
Over the Antarctic continent, low level winds are dominated by downslope katabatic flows from the elevated interior towards the coast. Topography is also important in determining the final strength of the wind; some areas of confluence result in very high wind speeds. The low level flow at the continental margin is generally anticyclonic, a result of Coriolis forces, and the meridional circulation. At levels above 500 hPa, the situation is reversed and a generally cyclonic circulation exists
Provenance of dust in Antarctica
Until recently, the concept that Patagonia is the major source of dust in Antarctic ice cores has become entrenched in the literature, based mostly on Sr and Nd isotopic compositions (see Basile et al., 1997, Delmonte et al., 2004, Delmonte et al., 2007, Delmonte et al., 2008, Gaiero, 2007). This concept has been backed by several modellers (viz. Lunt and Valdes, 2002) despite the fact that Joussaume (1993) had already strongly argued that Australia today is the most significant dust source in
Climatology of the 1884 to 1908 period, a long period of drought in Australia
The Law Dome ice core samples with Pb isotopic compositions characteristic of Broken Hill were deposited at a time when the most extreme negative Southern Oscillation Index (SOI) values for the past century of − 42.2 and − 42.6 occurred in 1896 and 1905. This coincided with severe El Nino events that resulted in the prolonged extreme drought conditions over southern Australia, and hence, enhanced occurrence of dust raising. Whilst instrumental observations of the atmospheric circulation are
Murray–Darling Basin
The Murray Darling Basin (MDB) covers 1,073,000 km2 of southeast Australia and drains 14% of the Australian landmass. The Basin extends over 2 climatic zones, being influenced by the tropical monsoon in the North and the westerlies in the South, resulting in highly variable and episodic river discharge and sediment transport. The Murray and Darling Rivers and their tributaries travel through a variety of geological formations, each of which have their peculiarities and specific geochemical and
Fluvial clays
The Murray and Darling rivers and their tributaries and anabranches were sampled at 26 locations throughout the basin (Fig. 2, Fig. 3). Emphasis was laid on obtaining clayey material. Preferred sampling sites were deposits of suspended matter from previous flooding events, such as dried-out mudpools in river beds and bank sediments, which contain a sequence of previous flooding events. Such samples should therefore yield an average composition of the fine-grained fraction of the regolith within
Pb isotopes
Pb isotopic compositions provide a time-integrated record of U, Th, and Pb relative abundances, and are therefore useful for fingerprinting crustal terranes and the provenance of marine and terrestrial sediments (Chow and Patterson, 1962, Gulson, 1986, McLennan et al., 1993, Winter et al., 1997). Pb isotope signatures of aeolian dust can be used to trace regional and global patterns of atmospheric transport, and to document contributions of anthropogenic Pb in a variety of modern environments (
Pb isotope evidence
It is difficult to argue with great confidence that the MDB is a source of aeolian dust over the last 217 Ka because of the error bars justifiably placed on the EPICA Dome C samples by Vallelonga et al. (2005). Nevertheless, there are several ice-core samples that have 208Pb/207Pb and 206Pb/207Pb which plot very close to MDB values. In particular, examination of Fig. 9 shows that the dust deposited at Dome C at 6.9 Ka [at least one of the 2 samples], at 33.7 Ka, at 47.2 Ka, and one sample at 122.8
Conclusions
We argue here that SE Australia, and in particular the dry Darling sub-basin, as well as central Australia [viz. the Lake Eyre region] are a source of airborne dust to parts of Antarctica as well as to Patagonia. This dust pathway appears to be activated by changes in Southern Hemisphere circulation, namely a poleward migration of the subtropical ridge and the westerlies, together with an increased frequency of blocking high pressure systems. It is during interglacials, such as today, and in
Acknowledgements
Funding for the Pb isotopes was provided to PDD by a grant from the Murray Darling Basin Commission. Some aspects of the research were also supported by ARC Discovery grant 0772180 awarded to PDD and colleagues. PDD is also grateful to Dr B. Delmonte who provided unpublished data and for numerous discussions at an early stage of the manuscript preparation. Mrs J. Shelley also provided much help with referencing and also commenting on the manuscripts during its many stages. Goodwin's work on
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2020, Global and Planetary ChangeCitation Excerpt :The lower panel in this Fig. 14 displays composite satellite images showing clouds and associated fronts that are clearly visible. The band of dust identified by Alan Wain in De Deckker et al. (2010) using a multilayered HYSPLIT trajectory. The band of dust visible in the middle panel for April 3 occurs ahead of the cold front (red arrow in the upper panel for that day, and the yellow arrow in the bottom panel for that day.