By Cliff Ollier
A new book has been produced on the coast of South Australia (1). It is essentially about coastal geomorphology and includes much that I shall not mention here such as tectonics, coastal processes, coastal management and human impacts on coasts. This brief note presents, mainly in their own words, what they have to say about aspects of carbonate production.
The South Australian coast originated with the separation of Antarctica and Australia about 43 million years ago, but the continental margin is well to the south of the present coast. During higher sea levels of the Late Eocene (37 to 34 Ma), the coast stood several hundred kilometres landward of its present location.
Some parts of the South Australian coast are exceptionally stable while other parts have moved up and down, to make a series of fault blocks. The Last Interglacial (130-118 ka ago) shoreline stood ~2 m higher than present sea level on the stable parts. Sediments deposited at that time are called the Glanville Formation. Fringing the coast of Eyre Peninsula on the stable Gawler Craton, the Glanville Formation lies at 2 m above present sea level but it has been tectonically depressed (-7 m at Port Adelaide), uplifted on Fleurieu Peninsula (+10 m at Normanville; +6 m at Victor Harbor), depressed at Goolwa (-0.8 m) in the Murray Estuary and progressively uplifted on the Coorong Coastal Plain to a maximum of +18 m near Mount Gambier.
The Glanville Formation contains inter-tidal, warm water fauna, including the mollusc Anadara trapezia (now locally extinct) which shows this higher than present shoreline was warmer than the present. At the height of the Last Glacial Maximum, some 22,000 years ago, sea level was about 125 m below the present level. Large areas of continental shelf were exposed as dry land, and wind blew sediments inland. After that sea level rose and reached present levels about 7,000 years ago.
THE CARBONATE SAND FACTORY
Apart from bedrock areas the coasts are dominated by carbonate sands in beaches and dunes. At many points in the book the authors refer to the ‘carbonate factory’ that produces it, and even claim it is “… part of the world’s largest temperate carbonate factory”.
“The sources of the sediment for these coastal successions were the Lacepede and Bonney Shelves, which represent a subtidal carbonate factory with the prolific growth of calcareous marine invertebrates such as molluscs, bryozoans, coralline algae, echinoids, and foraminifers. The post mortem attrition of these organisms leads to the formation of sand-sized sedimentary particles of calcium carbonate”.
“In Gulf St Vincent seagrasses thrive in the subtidal and intertidal shallow warm waters, with extremely productive calcareous algae, foraminifers and molluscan organisms manufacturing vast amounts of calcareous sediment. The accumulation of these … sediments has generated the resultant wide intertidal and supratidal flats visible today by causing the shoreline to aggrade (build up) and prograde (build seawards)”.
“The dominant processes of coastal development in the protected, tidally dominated waters of the northern Spencer Gulf are related to the massive production and accumulation of biogenic skeletal carbonate fragments derived from coralline algae, foraminifers, molluscs and bryozoans; the site is a ‘major carbonate factory’, sequestering much CO2.”
Furthermore aeolianite dunes also depend on the process. “The aeolianites are primarily composed of marine carbonate, reflecting the absence of streams delivering terrestrial, quartzose sediments to the coast”.
“The sand-sized particles of calcium carbonate derived from the mechanical abrasion of marine invertebrates were entrained landwards by inner-shelf currents, and subsequently brought on land by aeolian processes to form thick dune deposits, which are interbedded with relict soil profiles (paleosols) that formed during periods of lower sea levels. The presence of aeolianite reflects the prolonged history of aridity of the Australian continent and the paucity of terrigenous-clastic sediment delivered to much of the coastline of southern Australia”.
“The coastline of South Australia is part of the world’s largest aeolianite (dune limestone) temperate sedimentary carbonate province, which extends from western Victoria to north of Shark Bay, Western Australia. The aeolianite deposits attest to the high calcium carbonate bioproductivity of the surrounding continental shelf environments”.
And the carbonate sand is important even offshore:
“It has been demonstrated that the Murray Canyons Group [submarine canyons] is still being actively modified, with sediment transport to the oceanic deeps more than 100 km from the foot slope. Given the lack of river sediment supply, the marine carbonate ‘factories’ of the Lacepede Shelf provide the main supply of sediments for turbidity currents. Initially, canyon erosion involved mainly land-sourced sediment, with later cutting predominantly by biogenic carbonates produced on the continental shelf”.
In the debates about Global Warming, now usually referred to with the less specific title Climate Change, the alarmists place much emphasis on the role of carbon dioxide. First its role as a greenhouse gas was supposed to cause warming. Then to demonise it further it was supposed to cause ‘acidification’ of the oceans, despite the fact that the oceans have been alkaline throughout their existence on Earth. We are supposed to reduce the production of anthropogenic carbon dioxide to ‘protect’ the ocean.
The ‘carbonate factories’ described here show that carbon dioxide is vital for the production of carbonate sands and ultimately limestone, and of course for the formation of the past and presents coasts. It is part of the big geochemical picture: carbon dioxide originates from volcanic eruptions and is fixed by limestone formation. Trying to reduce the carbon dioxide content of the ocean by reducing emissions by human activities is not only futile, but if it could be done it would have harmful consequences on all the carbonate fixing animals and plants in the ocean, and ultimately on the shape of the continent.
(1) Coastal Landscapes of South Australia
by Robert P. Bourman, Colin V. Murray-Wallace and Nick Harvey
$132.00 | 2016 | Paperback | 978-1-925261-20-2 | 420 pp
FREE | 2016 | Ebook (PDF) | 978-1-925261-21-9 | 420 pp