Exploring Geochemical Proxies for Identifying Cryptic Sequence Boundaries in the Geologic Past Using Modern Bahamian Analogues
Semester of Graduation
Master of Science (MS)
Geology and Geophysics
Previous studies of carbonate diagenesis have suggested that bulk Sr concentrations and inorganic δ13C values may be a useful proxy for identifying cryptic sequence boundaries in meteorically altered limestones due to geochemical processes associated with carbonate mineral stabilization in cement precipitation in the meteoric diagenetic environment. However, it is possible that natural geochemical variation in sediments may produce geochemical shift which could be misinterpreted as a cryptic sequence boundary. To test this, the Two Pines roadcut containing a previously identified protosol on Eleuthera, Bahamas, which has been affected by vadose meteoric diagenesis was sampled for bulk geochemical analysis. As well, modern sediment from around the island were collected from a variety of environments under the main assumption that these modern sediments represent the sedimentary environments which would have contributed to the formation of the limestone at the roadcut during the Pleistocene and display similar geochemical characteristics to those sediments. If the geochemical values in the meteorically altered limestone were significantly different than modern sediments, then the geochemical parameter was determined to have potential as a proxy for meteoric diagenesis. Viability of proxies for determining cryptic sequence boundaries were also determined from geochemical trends in the roadcut samples. The parameters measured included mineralogy by x-ray diffraction, inorganic δ13C and δ18O, rare earth elements and yttrium (REY), and trace elemental analysis (Ca, Mg, Sr, S, Al, Fe, Mn, Sc, V, Cr, Co, Ni, Cu, Zn, Ba, Ti, Th, U, Cd, Rb, and Mo). δ34S, organic δ13C, and organic δ15N and are reported and discussed for sediment samples only. Results show Sr does not appear to significantly record meteoric diagenesis in the roadcut samples, nor does it appear useful for identification of cryptic sequence boundaries. This follows for U concentrations as well. Inorganic δ13C and δ18O and mineralogy do indicate meteoric diagenesis but do not appear useful for identification of cryptic sequence boundaries. Sr, U, δ13C, and δ18O are potentially heavily influenced by the original mineralogy. Other potential proxies which show significant differences between the meteorically altered roadcut and modern unaltered carbonate sediments include REY, Mg, S, Cr, Co, Cu, and Zn.
Magette, Evan B., "Exploring Geochemical Proxies for Identifying Cryptic Sequence Boundaries in the Geologic Past Using Modern Bahamian Analogues" (2019). LSU Master's Theses. 4887.