59th Annual Report on Research 2014

The proposal, funded in 2013, was designed to understand the influences on the clumped isotope signature during well constrained diagenesis. To this end a graduate student was recruited to start in the Fall of 2013. In addition two other graduate students are working on the general area of research, although these are funded by other mechanisms. The following aspects of the early diagenesis have been examined.

1. The clumped isotopic signature in unaltered non-biogenic sediments
2. The influence of solid state conversion of aragonite to calcite on the clumped isotope signature
3. The role of dolomitization in controlling the clumped isotope signature

1. Clumped isotopic composition of unaltered non-biogenic sediments

At the time of writing the clumped isotope method has been applied to many geological studies, but there have as yet been no investigations of unaltered non-biogenic carbonates, common in the geological record. In order to remedy this we have analyzed five representative samples of different facies selected from a set of ~300 samples previously collected from Great Bahama Bank [1]. Analyses of these samples show that while the calculated temperature (21-35^oC) agrees reasonably well with that expected, there is a strong facies dependence. The muddier facies, found mainly in the interior of GBB have higher temperatures while the coarser facies which presented closer to the margin show cooler temperatures.

This facies dependence of the temperatures measured using the clumped isotope method actually agrees well with the expected distributions of temperatures on GBB, which are higher in the interior and lower on the margin. Using these temperatures, the original δ¹⁸O values of the sediments [2], and the relationship between temperature, the δ¹⁸O_w, and the δ¹⁸O of aragonite [3] it is possible to estimate the δ¹⁸O_w of the fluids (Figure 2) which formed the sediments and using relationships between salinity and δ¹⁸O for GBB [2], the salinity. Preliminary results were presented at the Fall AGU meeting [4].

2 .The influence of solid state conversion of aragonite to calcite on the clumped isotope signature

Although aragonite rapidly is changed to calcite in the geological record, there are instances of original aragonite being preserved for millions of years. Such fossils have been assumed to been unaltered and their chemical composition has been used to make inferences regarding the evolution of corals and changes in the chemistry of sweater through time [5, 6]. We have been conducting experiments by heating aragonite and examining the relationship between the inversion of the aragonite to calcite and the rate at which the clumped isotopic signature is reset. These results indicate that the clump isotopic signature is altered at a faster rate than the inversion to calcite [7].

3. The role of dolomitization in controlling the clumped isotope signature.

The understanding of dolomitization using clumped isotopes is possibly one of the more exciting avenues for clumped isotopes. As of yet studies which have explored this have not been mature and there remain many unknown phenomenon. These include aspects such as whether there is a different relationship between temperature and the clumped isotopic signature in dolomites and whether the fractionation of the clumped isotopic signature during the reaction with phosphoric acid is different. We have explored many of the aspects and presented data at different meetings [8-10]. We have applied our results to the geologically young dolomites from the Bahamas and obtained values which are consistent with the previous geological models.

References

1. Reijmer, J.J.G., et al., A reevaluation of Facies on Great Bahama Bank I: New Facies Maps of Western Great Bahama Bank in Perspectives in Carbonate Geology: A Tribute to the Career of Robert Nathan Ginsburg, IAS Special Publication, P.K. Swart, G.P. Eberli, and J.A. McKenzie, Editors. 2009, Wiley-Blackwell: Oxford. p. 29-46.
2. Swart, P.K., J.J. Reijmer, and R. Otto, A reevaluation of facies on Great Bahama Bank II: Variations in the d¹³C, d¹⁸O and mineralogy of surface sediments, in Perspectives in Carbonate Geology: A Tribute to the Career of Robert Nathan Ginsburg, IAS Special Publication, P.K. Swart, G.P. Eberli, and J.A. McKenzie, Editors. 2009, Wiley-Blackwell: Oxford. p. 47-60.
3. Grossmann, E.L. and T.-L. Ku, Oxygen and carbon isotope fractionation in biogenic aragonite: temperature effects. Chemical Geology, 1986. 59: p. 59-74.
4. Atasoy, D. and P.K. Swart, Clumped Signatures of Shallow Water Corals in Tobago. AGU Fall Meeting, 2014: p. PP51A-1102.
5. Stanley, G.D., Jr. and P.K. Swart, Evolution of the coral-zooxanthellae symbiosis during the Triassic: a geochemical approach. Paleobiology, 1995. 21(2): p. 179-199.
6. Gothmann, A.M., et al., Fossil corals as an archive of secular varitions in seawater chemistry. In Review.
7. Staudigel, P. and P.K. Swart, Clumped isotopic effects during the inversion of aragonite to calcite. 4th International Clumped Isotope Workshop, Zurich, 2014.
8. Murray, S., M. Arienzo, and P.K. Swart, Clumped Isotopes in Bahamian Dolomites: A Rosetta Stone? . AGU Fall Meeting, PP54A-02, 2014.
9. Murray, S., M. Arienzo, and P.K. Swart, A calibration for clumped isotopes in low-temperature dolomites. Geochimica et Cosmochimica Acta, 2013. In Prep.
10. Swart, P.K., et al., Dolomitization in the Ghawar Field: An Update Based on the Clumped Isotope Technique. Geochimica et Cosmochimica Acta, 2013. In Preparation.