Australia: The Land Where Time Began

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Late Cryogenian Extreme Ocean Anoxia Recorded in Reefal Carbonates in Southern Australia

A great change took place on the surface of the earth and in marine environments, which included extensive climate variability, widespread oxygenation in the oceans, ad was accompanied y the rise of animal life during the Neoproterozoic. The timing of the oxygenation of the oceans has remained uncertain, however, particularly with regard to the seas of the Cryogenian, where there were long periods of disruption caused by glaciation on a global scale. In the Northern Adelaide Fold Belt, Australia, reef complexes from the interglacial Cryogenian contain abundant primary marine dolomite cements. There are well-preserved textural and growth zonation in these cements, which indicates they preserve their original marine chemistry and Hood & Wallace say they can be used as geochemical proxies for the palaeoceanography of the late Cryogenian. Significant geochemical gradients with depth have been revealed by analysis of marine cements recovered from peritidal nearshore facies, shallow platform facies and deep framework facies of the reef complexes. The shallow and deep cements have very high concentrations of iron, while the nearshore cements have a low content of iron and commonly have inclusions of iron-oxide. Rare earth elements are present in highest contraptions in the cements from deeper facies, whereas chalcophile elements, such as copper, cadmium, lead, zinc, etc. are most abundant in the nearshore cements. Profiles of rare earth elements are unusual, with convex profiles being found negligible Cerium/Cerium* anomalies and positive Europium/Europium* anomalies.

This carbonate geochemistry, which is constrained by sedimentology, provides a window into the ocean chemistry and structure of the interglacial Cryogenian ocean. Pronounced chemical stratification in the ocean during the late Cryogenian is revealed by the marine cements. At the surface of the ocean there was a thin veneer of oxic surface waters, in peritidal facies, with seawater being increasingly anoxic and iron-rich seawater at depth. It is suggested by the distribution of elements such as copper and cadmium, which are strongly chalocphilic (an element that forms sulphide minerals if enough sulphur is available), across the chemocline that although ferruginous, some dissolved sulphide was probably contained in deeper waters that were anoxic. A ferro-sulphidic ocean is described by these conditions and it encompasses some of the most extreme anoxia that has been documented for the Late Precambrian. Disruption of the ocean system on such a large scale during the Neoproterozoic is suggested by a return to Archaean-like ocean conditions at this time. 

 

Hood, A. v. S. and M. W. Wallace (2015). "Extreme ocean anoxia during the Late Cryogenian recorded in reefal carbonates of Southern Australia." Precambrian Research 261(0): 96-111.

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading