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 O_2, 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 216–219, no. 0 (10// 2012): 152-61.