Australia: The Land Where Time Began

A biography of the Australian continent 

Kimberly Region, Western Australia, Glaciation in the Late Neoproterozoic - an 17O Record

The authors recently reported depletion of 17O, that was not mass dependent, in sulphate that was deposited following the Marinoan glacial melting at about 635 Ma. Further study linked the Δ17O of barite in Marinoan South China to its sedimentological-Geological context revealing that 17O depletion in sulphate is at its most pronounced at sites near palaeocontinents, which supports the hypothesis that the source of the depletion was atmospheric O2, having been transferred to sulphate by the oxidative weathering of sulphides. For the Marinoan depletion of 17O  host minerals or rocks have been limited to barite, in southern China and western Africa, and sulphates associated with carbonate (CAS) and lenses of limestone within a diamictite (Svalbard). The authors1 suggest that if the 17O depletion in the Marinoan is related to an extraordinary atmospheric condition, the signal should be distributed globally. In the Late Neoproterozoic the Kimberley Region, Western Australia, was close to a continent and it served as a test of the authors'1 hypothesis that depletion of anomalous 17O may be recorded widely in the rock records from this time.

In this paper the authors1 report that in the Moonlight Valley cap dolostones, Texas/Mable Downs (TMD), in the east of the Kimberley region, Western Australia, is depleted anomalously in 17O, with Δ17O values as low as -0.68‰. There is a gradual reduction of the magnitude of the anomaly the overlying Ranford mud-silt sandstones. The geographic distribution of the depleted 17O signal is expanded by this finding, as well as the type of rocks or minerals in which the anomalous sulphates can be present. At Palm Springs (PS) section the CAS in the MV cap dolostones, about 150 km to the south of TMD, and the CAS in the Egan cap dolostones, lack 17O depletion. In the 2 time equivalent, yet spatially different, MV cap dolostones, the presence and absence of 17O anomalies are consistent with the palaeogeography that indicates that TMD was close to the continent at a time when PS was an open sea environment. The Egan cap has distinct δ13C and δ18O values for dolostones and distinct Δ17O, δ34S and δ18O values for CAS, though sharing some of the sedimentological features with that of the MV cap dolostones at TMD, which supports an earlier assignment of the Egan Glaciation to be younger than the Marinoan in the Neoproterozoic.

Sources & Further reading

  1. Bao, Huiming, Zhong-Qiang Chen, and Chuanming Zhou. "An 17o Record of Late Neoproterozoic Glaciation in the Kimberley Region, Western Australia." Precambrian Research 216219, no. 0 (10// 2012): 152-61.

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last Updated 03/07/2013
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading