Experimental and analytical investigations of non-skeletal carbonate production

Thesis

The overriding aim of this research is concerned with understanding the chemical controls on carbonate sedimentation in naturally occurring highly alkaline systems that produce enigmatic mineral assemblages. The main approach taken here is to characterize the kinetic pathways of Ca-Mg-carbonate precipitation and mineralization in systems lacking a biological carbonate factory and to understand the relationship between fluid chemistry and mineralogy in non-classical nucleation regimes. The first line of research explores the kinetics of Ca-Mg-carbonate nucleation and recrystallization in the presence of phosphate (PO₄) as an inhibitor in high-alkalinity experiments. We identify and constrain new crystallization pathways to Mg-rich carbonates magnesite and disordered dolomite from amorphous precursors that overcome energy barriers under Earth surface conditions. The second line of research explores CaCO₃ nucleation under Neoproterozoic seawater conditions. We identify and constrain saturation state thresholds required for nucleation in these settings that are influenced by micromolar concentrations of PO₄. The third line of research explores geological carbonates to test our hypotheses that PO₄ may have influenced the mid-Neoproterozoic CaCO₃ factory. We reveal new P records that identify abundant and widespread P distribution at significantly elevated PO₄ concentrations in Neoproterozoic Tonian-aged carbonate rocks. Together, this research indicates enhanced P-cycling in the Tonian that influenced the CaCO₃ factory and may have resulted in enhanced concentrations of bioavailable P that was crucial for biological evolution.

Authors: Roest-Ellis, S

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Limestone; Mg-calcite; Experimental; Geochemistry; Precamabrian; Carbonate; Magnesite; Neoproterozoic; Tonian; Phosphate
• Subjects: Carbonate rocks; Geochemistry; Carbonate minerals; Sedimentology