Date of Award
Doctor of Philosophy
Geological and Environmental Sciences
Stephen E. Kaczmarek, Ph.D.
Michelle Kominz, Ph.D.
Johnson Haas, Ph.D.
Franciszek Hasiuk, Ph.D.
Aragonite, calcite, carbon cycle, carbonate sediments and rocks, limestone microporosity, paleoclimate, paleoenvironment, paleoceanography
A significant proportion of modern marine calcium carbonate sediments is dominated by metastable aragonite and high Mg calcite that either dissolves or stabilizes to low-Mg-calcite (calcite) or dolomite during diagenesis. Sediment dissolution and stabilization have implications for the CaCO3 budget in the ocean and carbon burial rates. Yet, the diagenetic conditions that promote each process and their relative importance are poorly understood. Further, stabilization most commonly produces calcite microcrystals that exhibit various textures and host micropores. Despite their ubiquity in the rock record, the controls on microcrystal textures remain unclear. Here, laboratory experiments were used to investigate aragonite-to-calcite stabilization as well as calcite dissolution under various conditions. Results from stabilization experiments show that the well-documented Mg inhibitory effect of calcite precipitation during stabilization is valid only at high fluid:solid ratios (F:S). at low F:S, stabilization readily occurs despite the presence of Mg at near seawater concentrations. These observations suggest that the stabilization of aragonite-rich sediments to calcite-rich limestones is likely promoted by a decrease in F:S and the development of a closed system during burial. The results also show that calcite microcrystal texture is controlled by various depositional and diagenetic parameters including fluid chemistry, temperature, as well as aragonite reactant type and size. These findings challenge existing hypotheses by showing that certain calcite textures previously interpreted to reflect later stage dissolution and cementation, can form during early diagenesis. The results also show that rhombic calcite microcrystals form only under conditions that prevail in meteoric settings, implying that this crystal morphology can be used as a textural proxy for meteoric diagenesis. Results from our dissolution experiments show that the degree of fluid undersaturation with respect to calcite correlates with district textural features at the surface of dissolving crystals, suggesting that these distinguishing features can be used to discern dissolution conditions and mechanisms in natural settings.
Hashim, Mohammed, "Experimental Insights Into The Origin Of Microcrystalline Calcites" (2022). Dissertations. 3845.