Investigating the impact of meteoric diagenesis on the geochemistry of carbonate eolianites, Eleuthera and Water Cay, the Bahamas
Semester of Graduation
Master of Science (MS)
Geology and Geophysics
The geochemical signatures imparted in major, minor, and trace elements, combined with light isotopes, suggest promising applications regarding the stabilization of meteorically altered limestone eolianites. Previous high-resolution studies have indicated that elements associated with carbonate diagenesis such as Mg and Sr can be valuable proxies for salinity and aragonite dissolution, respectively. In addition to testing these proxies, the analyses of several temperature-, diagenetic-, bioactive-, and redox-sensitive elements were evaluated using laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) to identify additional indicators during carbonate diagenesis. Two geochemical drivers of U were identified; (1) aragonite dissolution similar to Sr and (2) oxidation and mobilization associated with a protosol. Ba was interpreted to reflect its saturation with respect to porewaters from dissolved bioclasts, potentially regarding the element as a paleo-productivity proxy for the eolianite dune. Mg in calcite reflected different concentrations between the lower eolianite (> 2000 ppm) and upper eolianite (< 2000 ppm), suggesting differential cementation when paired with δ18O. We suggest that the permeability of the soil horizon restricts most percolation into the lower eolianite, thus distinguishing two separate pore water chemistries for the upper and lower units of the dune. We further suggest that the fluid chemistry of the upper unit is predominantly newly-recharged meteoric waters, and the lower unit is mainly brought up through the groundwaters, resulting in separately evolved pore systems and therefore different stabilities and recrystallization rates for low magnesian calcite. The elemental concentrations measured in ooid cores and rims were indistinguishable, indicating a similar mineralogy between the two spot types. The detrital contaminants, Al and Th, were measured in very low concentrations within the eolianites but were elevated within the protosol, confirming terrestrial input to the soil horizon. We suggest Fe, Zn, and Cu concentrations were elevated within the protosol as a result of oxide precipitation onset by the degradation of organic matter. Covariation of δ13C and δ18O occurred near the protosol, which we interpreted as an indicator of differences in the pore space evolution of the lower and upper eolianite.
Bonnot, Graham S., "Investigating the impact of meteoric diagenesis on the geochemistry of carbonate eolianites, Eleuthera and Water Cay, the Bahamas" (2021). LSU Master's Theses. 5289.
Herrmann, Achim D.
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