ä13C Isotope Analysis of Pennsylvanian Carbonates in Arrow Canyon, Nevada

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Julia Kahmann and Dr. Scott Ritter, Geology

Carbonate rocks, when sampled and analyzed for their carbon isotopes, carry an isotopic signature that may be used as a correlation tool. When other tools do not suffice, or the present correlation needs to be enhanced, isotope analysis is an appropriate alternative. Arrow Canyon Nevada has extensive, continuous, cyclical packages of pristine limestone making the location a prime candidate for isotopic analysis. As an isotopic signal for the limestone cycles is established, the resulting isotope carbon curve may be used as a correlative to91 for other basins along the Pennsylvanian paleo-margin.

This study has prompted a closer look into the analysis of limestone for a carbon excursion signal. The majority of isotope analysis has been completed in an effort to reconstruct ancient climates, specifically atmospheric carbon dioxide levels and relative atmospheric temperatures. With this as a goal, deriving a primary signal is essential. As such, the sampling is very precise and specific. Brachiopod skeletal grains are considered the best to sample, having an unaltered calcite. Unaltered calcite is the key to a primary signal. Here is where this study deviates from the majority of isotope analysis, as its purpose is correlation, rather than climate interpretation. Using isotopes in correlation does not require the precision of paleo-climate reconstruction. Merely a consistent isotope signal is sufficient for a particular basin to be correlated with other basins in the region. Therefore whole-rock sampling, as opposed to skeletal grain sampling, was considered adequate and would yield reproducible results (Ripperdan, 2001) for this study.

As this study progressed, modifications to the method of isotopic analysis were made. Carbon dioxide gas (CO2) must be evolved from the limestone in order for the sample to be ready for mass spectroscopy. Previously, samples were prepared using an “off -line” CO2 extraction apparatus where only 6 samples for every 8 hours of lab work could be prepared. One by one, each sample was analyzed for its carbon isotopic ratio (13C/2C). Duration of mass spectroscopy per sample was another 12 hour process after extraction. Upon consultation from Dr. Steve Nelson of BYU and Dr. Chris Romanek of the University of Georgia a more efficient method was devised: “off-line” extraction was replaced by an “on-line” extraction procedure. This new “on-line” extraction prepared and ran over 40 samples in one day for isotopic analysis using a Delta Plus ThermoQuest Finnigan Gas Bench. The Gas Bench prepared and ran samples for mass spectroscopy, whereas previously it was a laborious two-step procedure. Powdered whole rock samples (1g) were flushed with helium gas, digested in 100% phosphoric acid, placed directly in the gas bench, reacted for 6 hours, and then analyzed by the spectrometer. For every 5 samples, a standard of UCLA-Carrera, NBS-19, or LVSEC were run to determine standard deviations and accuracy of the technique. This new method proved to be accurate with a standard deviation of 0.03. Previously published results have reported a 0.05 deviation in analysis (Brand, 1982). This modification in method has proved to be more efficient for the isotope laboratory at BYU.

Results of the study have produced a preliminary isotope curve that closely matches trends in the stratigraphic architecture of Arrow Canyon outlined by Aaron Leavitt (2002). The excursion (ä13C/12C) tends to be more positive (increase in 13C) with increasing depth. Yet, a few high excursions are also seen at shallower depths. The positive excursion implies higher rates of carbon burial and bioproductivity (Kump, 1999; Bruckschen et at, 1999). As more organic carbon is buried, the resulting ocean water becomes progressively heavier. However, as calcite is precipitated (preferentially incorporates 13C), the ocean water will be light in carbon. The curve produced in this study shows a signal of enhanced burial, as well as precipitation of calcite. Enhanced burial will show a positive excursion, and high rates of calcite precipitation will present a more negative excursion. Further analysis must be done to determine if shallow waters versus waters at depth will present a consistently distinguishable excursion value. Methods of isotope correlation are still being refilled and have lead to many improvements. . This research and experience has benefited my academic pursuits greatly. The research and mentoring provided by the department has served as a springboard for the Master of Science degree I am currently pursing at Baylor University. Strides are also being made for my intent to pursue a PhD in this field of study. The greatest value of this research, however, was in the one on one consultation I had with my mentor and professors. Their attention inspired questions and drive. It is that drive, as well as the research experience, that continues to fuel my academic goals.

References

  • Brand, Uwe, 1982, The oxygen and carbon isotope composition of Carboniferous fossil components: sea water effects: Sedimentology, v. 29, p.139-147.
  • Bruckschen P., Oesmann, S., Veizer, J., 1999, Isotope stratigraphy of the European Carboniferous: proxy signals for ocean chemistry, climate and tectonics: Chemical Geology, v. 162, p.127-163.
  • Kump, Lee R, Arthur, M., 1999, Interpreting carbon-isotope excursions: carbonates and organic matter: Chemical Geology, v.161, p.181-198.
  • Leavitt, Aaron, 2002, Stratigraphic Architecture of Mid-Ramp, Icehouse Carbonates: A case study from Desmoinesian strata (Middle Pennsylvanian) of Arrow Canyon, Southern Nevada, Masters, BYU, 53 p.
  • Ripperdan, Robert L., 2001, Stratigraphic Variations in Marine Carbonate Carbon Isotope Ratios in Stable Isotope Geochemistry eds. John w. Valley , David R. Cole, Reviews in Mineralogy & Geochemistry, vol.43, p.637-662.