An O-17 record of Neoproterozoic snowball Earth in Kimberley, Western Australia

Non-mass-dependently 17O-depleted signatures have recently been found in sulfate associated with the aftermath of Marinoan glacial meltdown at ca. 635 million years ago. The anomaly was proposed to be inherited from atmospheric O2 via oxidative weathering of sulfides. An extremely high pCO2 atmosphere could produce such an anomaly. The finding has become one of the strongest lines of evidence supporting a "hard" snowball Earth hypothesis. Further studies linking the Δ17O of barite to sedimentological-geological context in Marinoan South China confirm that the 17O-depleted sulfate was derived from oxidative weathering and the anomalous 17O signal was detectable at sites close to paleo-continental runoffs while diluted and often undetectable in the open oceans. On the other hand, host minerals or rocks for the Marinoan 17O anomalies have been limited to barite (South China and West Africa) and carbonate-associated sulfate (CAS) in limestone lenses within the diamictite (Svalbard). If the 17O depletion is indeed closely related to an extraordinary atmospheric condition associated with the great Marinoan meltdown, the signal should be global in its distribution. Kimberley region of Western Australia was close to a continent in late Neoproterozoic, according to Rodinia reconstruction and regional geology. A confirmation of this anomalous signal in Australia is a critical test to our hypothesis. We report here that the Δ17O of carbonate-associated sulfate reaches as low as -0.68% in the dolostones draping the Neoproterozoic Moonlight-Valley (MLV) diamictite, Texas/Mabel Downs, Kimberley, Western Australia. The magnitude of the 17O depletion gradually decreases (i.e. approaching normal) when going up towards the overlying Ranford sandstone. A positive correlation between the δ34S and the Δ17O exists, as does the case in South China and Svalbard. The CAS in the MLV cap dolostones in the Palm Spring section, however, does not bear distinct 17O depletion. Neither does the CAS in the Egan cap dolostones. Thin-section examination has revealed little or no pyrite grains in the MLV and Egan cap dolostones, ensuring trustable CAS data. The MLV cap dolostones are the first case where 17O anomaly is found in CAS in cap dolostones, representing a new host of the anomalously depleted 17O signal. This finding not only expands the geographic distribution of the 17O-anomalous signal, but also the type of host rocks or minerals that the sulfate resides. The presence and absence of distinct 17O anomalies in Texas/Mabel Downs (type section) and in Palm Spring, respectively, in the two time-equivalent yet spatially different MLV cap dolostones are consistent with sedimentological-petrographic features and paleogeographic reconstruction that indicates Texas/Mabel Downs was close to continental runoffs (a lagoon?) while Palm Spring was in an open ocean. The finding supports the proposed origin of sulfate 17O depletion and also an assignment of the MLV glaciation to Marinoan.