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Project Progress

Timescales and Neoproterozoic history

Most geological hypotheses hinge on the ability to resolve time. In the Phanerozoic, geologists rely on a time scale constructed from ranges of fossil assemblages and calibrated with radioisotopic dates. In the best cases we can tell time at the ± 0.1-1.0% level. However, in the Precambrian, without index fossils, geologists have relied on the use of carbon and strontium isotope variations in carbonate rocks to make detailed correlations between widely disparate sections assuming synchroneity of large variations and some calibration with radioisotopic dates on volcanic ash beds. This method has been applied for two decades with apparent success and underpins much of our proposed work. However, recently, Derry (2010) proposed that extremely negative carbon isotopic values in Neoproterozoic rocks, and in particular the ‘Shuram anomaly’, are the product of alteration from migrating petroliferous brines and not global signatures of variations in the organic and inorganic carbon pools. If correct, this would complicate our understanding of Neoproterozoic earth history. However, the brine alteration hypothesis carries with it testable stratigraphic and geochemical predictions. In particular, the brines should also drive alteration of other isotopic systems, particularly strontium, depending on their composition, fluxes, and the composition of detrital components within the carbonate rocks. We consider evaluation of this an extremely high-priority endeavor.

To test the effects of brine migration on Neoproterozoic rocks, we have begun high-resolution, coupled, Corg, and Sr isotopic, and elemental studies through the pre-Sturtian Islay anomaly in the Coates Lake Group of NW Canada and the pre-Marinoan Tayshir anomaly in Mongolia. Both successions are of an exceptionally low metamorphic grade and consist of organic-rich limestone, allowing for Sr and Corg isotopic studies. This work reflects a close collaboration between the MIT radiogenic isotope lab (Bowring), the MIT organic geochemistry lab (Summons) and Francis MacDonald and David Johnson (Harvard).

The Coates Lake Group is a particularly attractive target because extensive studies on the sulfide mineralization have constrained the burial temperatures to be less than 100 degrees, and have constrained the composition, flux, and stratigraphic extent of brine migration. This framework will allow us to construct a three-component mixing model for Sr isotopes between the brine, detrital components, and carbonate. We can then attempt to map constraints from this model onto the carbon isotope profile. Preliminary results suggest that the brine front, Sr isotope alteration, and negative carbon isotopes correlate strongly with one another. Key to this effort will be estimations of sulfate concentrations in the brine, as sulfate can act as an oxidant of organic carbon in brines through thermochemical sulfate reduction. We then hope to take what we have learned from the three-component modeling in the Coates Lake Group to the Tayshir anomaly where carbonate carbon and organic carbon covary. Through this work we will:
1) Utilitze the Sr isotopic system to assess the possibility of brine alteration in large carbon isotope anomalies, and
2) After distinguishing the effects of alteration, we will refine the Neoproterozoic Sr isotopic curve, which will strongly complement the carbon isotopic curve, particularly where correlations of large carbon isotope anomalies are questioned.

To date we have analyzed more than 64 samples from the Coates Lake for ⁸⁷Sr/⁸⁶Sr and more than 50 from Mongolia. The data are very well behaved and show smooth variations with stratigraphy.