Conclusions

Our work on this project sought to answer the following questions.

1. What was the redox state of shallow marine oceanic environments during precipitation of carbonates that formed  following Neoproterozoic glacial episodes?
2. Do carbonates previously identified as following the Pre-Sturtian, Sturtian, or Marinoan glacial episodes chemically correlate with other cap carbonates from the same age classification?
3. How well did samples preserve their original chemistry, i.e. are the compositions we measured reflective of the conditions of the original depositional environment? Or have the chemical compositions been significantly altered since deposition?
4. How well did our analytical methods work?

Our answers to these questions are as follows…

1. Our data are consistent with oxidation of Neoproterozoic oceans by the end of the Marinoan glacial episode.  Pre-Sturtian carbonates have REE patterns consistent with anoxic conditions (no Ce anomaly, positive Eu anomaly). Post and syn-Sturtian cap carbonates have REE patterns that in most cases lack Ce or Eu anomalies; however, some sample have patterns consistent with anoxia (basal section of Tindelpina Shale). Interestingly, the dropstone from the Rasthof formation has a pronounced negative Ce anomaly, which requires the existence of an oxygenated Pre-Sturtian ocean (at least locally), even though our Pre-Cryogenian samples have no evidence of oxic conditions. Finally, by Post-Marinoan time significant negative Ce-anomalies occur in the Noonday Dolomite in the absence of Eu anomalies. This is consistent is oxygenated oceans by ~650 Ma. Collectively, our results are consistent with oceans becoming progressively more oxidized from Pre-Sturtian through Marinoan glacial episodes, perhaps with minor oxygen excursions in-between.
2. REE patterns from our Pre-Sturtian samples generally do not correlate well in terms of concentration across localities; however, overall shape is similar. The post and syn-Sturtian cap carbonates (Tindelpina Shale and Scout Mountain Member) have a stronger correlation with respect to both correlation and shape. For our Post-Marinoan samples the Noonday Dolomite corresponds well with one of the Keilberg samples, but not the other. Where patterns and concentrations match well, we infer similar depositional conditions. This conclusion is consistent with glaciations of a global extent; however, more detailed trace element and isotopic work would be required to eliminate other explanations (similar latitude, tectonic position) for these correlations.
3. Samples are generally not well preserved. Most samples are dolomitized and show enrichments in Al and Si. Likewise, the REE concentrations are likely influenced to some extent by post-depositional chemical modification.  The samples that show the best preservation are the Beck Spring Dolomite(BSD 1.2d-a), mid-section Assem Limestone T3-15-12, the dropstone in the basal Rasthof Formation  (S-5c), and the upper Rasthof Formation (S-2).
4. All of our solution mode and laser ablation methods measured our target analytes with acceptable signal/noise ratios. Mo, probably due to its very low abundance, was not measurable. The solution mode Y+REE method was much more precise than the laser ablation methods used to measure Y and the REEs, in part because the integration times for each mass were nearly three times longer for the solution mode method. For laser ablation, the MREEs and HREEs had the lowest signal/noise ratios, which is likely why some of the sawtooth REE patterns occurred for our spot data averages.