Tuesday, 11 July 2006

Challenging the Conceptual Model used for Acid Sulfate Soil Mapping on the East Coast of Australia.

Don T. Malcolm1, Shane M. Pointon1, and Colin Ahern2. (1) Queensland Department of Natural Resources and Mines, 80 Meiers Road, Indooroopilly, Queensland, Australia, (2) Queensland Department of Natural Resources, Meiers Rd, Indooroopilly, Australia

Low lying areas of the Australian coastline and many other parts of the world have seen large areas of coastal sediment deposition during interglacial periods when sea levels were higher than those of the present day. Favorable climatic and environmental conditions over time have led to the formation of iron sulfides within these sediments resulting in the development of Acid Sulfate Soils (ASS). Acid sulfate soils have been estimated to underlie approximately 2.3 million hectares of coastal Queensland alone (Powell and Ahern 1999). Programs to identify and map acid sulfate soils at varying scales have occurred in specific areas of coastal Queensland since the late 1990s. Work by Chappell and Shackleton (1986) has shown that sea levels in the Australian region have been higher than present on two occasions over the last 150 000 years. The first was during the Pleistocene Epoch approximately 130 000 years ago and the second and most recent during the Holocene Epoch 4000 to 6000 years ago. Chappell and Shackleton (1986) also showed that sea level dropped as low as 150 metres below present during intervening glacial periods. Whilst remaining totally waterlogged, sulfidic sediments are termed Potential Acid Sulfate Soils (PASS). When drying occurs through drainage or disturbance, iron sulfides react with water and oxygen to produce sulfuric acid thus forming Actual Acid Sulfate Soils (AASS). Dent (1986) describes this as being part of the “ripening” process. Acid sulfate soil ripening embraces all the physical, chemical and biological processes whereby a freshly-deposited mud is transformed to a dry land soil. Physical ripening essentially involves an irreversible loss of water (drying out). In the past it was assumed for the east coast of Australia that any acid sulfate soils which may have formed in sediments of Pleistocene age should have been ripened to a point where they had lost much of their potential and existing acidity (Atkinson et al. 1996). Therefore during mapping programs, if substantial layers of hard indurated sand of Pleistocene age (locally known as coffee rock) were encountered, it was assumed unnecessary to drill any deeper and the area was mapped as non acid sulfate soil. As the ASS mapping program in Queensland progressed, the acquisition of specialized drilling equipment in the early 2000s allowed penetration of these hard coffee rock layers. In the Maroochy Caloundra area of southeast Queensland's Sunshine Coast, unripe sulfidic mud was found in a number of boreholes below indurated sand layers mapped on geological sheets as Pleistocene age. These unripe muds were certainly not spent with respect to potential acidity, as oxidizable sulfur values as high as 1.8 %S were measured. A possible explanation was that the coffee rock layer was of Holocene age rather than Pleistocene. However, subsequent dating on one sample from the indurated sand layer using the Optically Stimulated Luminescence technique (OSL) (Murray and Wintle 2000) returned a date of 190 000 years ± 40 000 years making them clearly Pleistocene deposits. Given this discovery, the previous assumptions were discarded and areas underlain by indurated sands (previously mapped as non acid sulfate soils) were reinvestigated and in some cases remapped as acid sulfate soils. This is particularly important on the Sunshine Coast as canal estates and marina developments commonly disturbed to depths well below indurated sand layers. The authors wish to thank Dr Trevor Graham and Jon Olley for their efforts on the OSL dating.

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