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Project Progress

Diagnosing Ediacaran ocean chemistry:
Late Neoproterozoic (Ediacaran) strata from northwestern Canada provide a thick and rich sedimentological record, preserving intercalated carbonates and shale extending from the ~635 million year old Marinoan glacial deposits up through the ~541 million year old Precambrian–Cambrian boundary. This region also holds one of the classic localities for the study of early animal life, with the ensuing suggestion that this temporal interval captures a gross change in the O₂ content of Earth’s atmosphere. To test this hypothesis and bring records of northwestern Canada into line with other Ediacaran, fossil-bearing basins, we provide a detailed geochemical reconstruction from the Wernecke Mountains of the Yukon. Where possible, we also extend these records to the Ogilvie Mountains to the west and previously published data from the Mackenzie Mountains to the east.

Our work in the Wernecke Mountains is set against a composite d¹³C record for carbonate that preserves three distinct Ediacaran isotope excursions, the lowermost of which (preserved in the Gametrail Formation) is a putative Shuram excursion equivalent (Macdonald et al., 2013). What emerges from a multi-proxy (Fe speciation, sulfur isotopes, major and trace elements analyses) reconstruction is a picture of a persistently anoxic and ferruginous Ediacaran ocean. Notably absent is geochemical evidence for a prominent oxygenation event, an expectation given the appearance of animals and large swings in d¹³C. The new insight gained through these data challenge the idea of an Ediacaran jump in atmospheric oxygen, which in turn muddles the link between animal evolution and local geochemical environments.

The Underlying Cryogenian from NW Canada and Mongolia and extensions to ¹⁷O:
The stratigraphy beneath the Marinoan glaciation in both northwest Canada and Mongolia are amenable to the same geochemical tools described above. This work is actively ongoing and generating exciting results. For instance, Fe geochemistry from the interglacial of northwest Canada preserves a remarkably similar story to that of the Ediacaran in that region—this finding diagnoses the impact that the Marinoan Snowball Earth had on the Earth’s climate, specifically pO2. In perfect complement to this prediction (which is still in progress), through NAI funding a new ¹⁷O extraction fluorination line was completed in the Johnston lab and a Photon Machines CO₂ laser system arrived in early December. This analytical equipment will service the measurement of ¹⁷O in Neoproterozoic sulfate minerals, which has been shown to record hints of atmospheric O₂. This work is being conducted by Dr. Ben Cowie, a post-doc in the Johnston Lab hired with NAI funding and who is solely dedicated to this project. The initial project, which is targeting the Ravensthroat barite from the Marinoan cap carbonate in the Mackenzie Mountains of Canada, is nearing completion. More exciting than these results, which will be submitted to EPSL and were solely the result of NAI finding, is a new project carried out jointly with a student of Francis Macdonald (Uyanga Bold) whose interests lie in Cryogenian carbon cycling and the generation of the stratigraphically overlying Marinoan barite. It is becoming clear from field relationships that a dolomitization associated with the sub-Marinaon d¹³C anomaly is also quite sulfate rich, and may be feeding sulfate to the Marinoan cap barite. This is uniquely testable with ¹⁷O. More importantly, this project outlines a new direction for this isotopic tool-tightly coupling field geology and stratigraphic relationships with a geochemical fingerprints. We look forward to continuing this work in the coming year.