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Project Progress

The Mesoproterozoic Era is widely thought to be quite “boring,” whereas the subsequent Neoproterozoic is characterized by large perturbations to geochemical cycles, climate instability, a putative rise of oxygen, and eukaryotic radiation. We seek to understand the cause and timing of the onset of volatile geochemical cycling that is characteristic of the Neoproterozoic. Large isotopic variability during the Neoproterozoic has previously been proposed to be the product of alteration (Knauth and Kennedy, 2009) or dynamic behavior of a large dissolved organic carbon pool (Rothman et al., 2003). Through our stratigraphic and geochemical studies in Mongolia we identified a large Cryogenian carbonate carbon isotope anomaly that covaries in organic carbon isotopes (Johnston et al., 2012). Although these data exclude these previous hypotheses for large carbon isotopic variability during the Neoproterozoic, we still do not understand what is driving the apparent temporal variability in the carbon cycle. In Mongolia we have begun to layer trace element, clay minerology, and biostratigraphy onto these isotopic records. All of these data suggest a major change across the Tayshir anomaly from marine, weakly oxidized conditions, to anoxic, potentially non-marine conditions. Late Neoproterozoic (Ediacaran) strata from around the globe carry evidence for a biospheric revolution that is long thought to carry a genetic link to atmospheric oxygen. Fortunately, these classic models can now be tested with an expanding toolbox of geochemical techniques and ever improving stratigraphic frameworks. Here we focus on new storylines emerging from northwest Canada and place these data in the context of published chemostratigraphy from other localities.

Northwest Canada holds one of the classic localities for the study of early animal life, and as such, provides an opportunity to directly test hypotheses discussed above for Mongolia and more broadly about evolving environments (biological and chemical). More specifically, recently submitted work on a detailed geochemical reconstruction of the Wernecke Mountains, Yukon (Johnston et al., submitted). Given recent stratigraphic studies (Macdonald et al., submitted), we are able to extend these records to the Ogilvie Mountains to the west and Mackenzie Mountains to the east. What emerges from a multiproxy (Fe speciation, sulfur isotopes, major and trace elements analyses) reconstruction is a picture of a persistently anoxic and ferruginous Ediacaran ocean, despite a major change in sediment protolith up-section and an increasing prevalence of Fe-bearing illite. Our work in the Wernecke Mountains is set against a composite δ¹³C record for carbonate that preserves three distinct Ediacaran isotope excursions, the lowermost of which (preserved in the Gametrail Formation) is a putative Shuram anomaly equivalent. Notably absent is geochemical evidence for a prominent oxygenation event, an expectation given the appearance of animals and large swings in δ¹³C. The new insight gained through the integration of these data further challenge the idea of a dramatic Ediacaran jump in atmospheric oxygen, which in turn muddles the link between animal evolution and local geochemical environments. In many ways, these data are consistent with that which we find in Mongolia.

Older stratigraphic units exposed in northwest Canada allow for similar tests of global coherence with Mongolia and other continents. The Proterozoic record of the northwestern Canadian Cordillera is an attractive place to explore the Mesoproterozoic to Neoproterozoic transition because there are several well-exposed, low-grade, mixed carbonate siliclastic successions that were deposited between 1.5 and 0.7 Ga. Correlations of Proterozoic strata have been recently revisited and refined with new mapping, geochemistry, and stratigraphic analysis, and geochronology and have identified an additional late Mesoproterozoic to early Neoproterozoic succession between Sequence A and Sequence B of Young (1979, 1981). These rocks of the Pinguicula, Hematite Creek, and lower Fifteenmile groups are situated between the geochemically well-characterized 1.5 to 1.27 Ga Dismal Lake Group and the 0.81 Ga upper Fifteenmile Group (and coeval Little Dal Group). This season we mapped the geology and collected samples through measured sections in the early Neoproterozoic Hematite Creek Group and in the Cryogenian Keele Formation. These data will allow us to construct higher resolution paleontological and geochemical records across the Mesoproterozoic-Neoproterozoic transition and the Cryogenian glacial interlude, and assess the relationship between oxidation state, changes in the carbon cycle, glaciation, and the diversification of eukaryotes.