2013 Annual Science Report
Massachusetts Institute of Technology Reporting | SEP 2012 – AUG 2013
Life and Environments: Geochemistry of Late Precambrian Oxygenation
The first year of work marked a successful transition from the goals and projects defining our last NAI node and the initiation of new, exciting research lines. Recently, our work on the Ediacaran transition in the Earth system culminated in an integrated geochemical study that both covers the state of the late Precambrian world, but also serves as a critical tie point for our upcoming work on Cryogenian ocean and atmospheric chemistry. This entails the extension of similar tools to those we applied in the Ediacaran, as well as the development of a new 17O system in the Johnston Lab that will serve as a central measurement for the upcoming projects.
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.
PROJECT MEMBERS:David Johnston
RELATED OBJECTIVES:Objective 4.1
Earth's early biosphere.
Production of complex life.
Co-evolution of microbial communities
Effects of environmental changes on microbial ecosystems