Between a Rock and a Precipitate: How Dolomite Geochemistry Encodes Environmental Parameters in a Modern Environment

Abstract

Carbonate rocks record information about the environmental conditions under which they formed, offering a window into Earth history. Among carbonate rocks, dolomite, CaMg(CO3)2, is abundant in the geologic record while elusive in modern environments due to kinetic barriers in formation. Geochemical signals measured in dolomite, however, are frequently used as evidence of changes to the global carbon cycle. To refine interpretations of ancient dolomite, this thesis investigates dolomite precipitation in the Coorong region of southern Australia, examining how dolomite geochemistry encodes environmental information. To that end, I create mineralogical and geochemical fingerprints of 49 samples collected from the carbonate-precipitating lakes. I develop a method for quantifying the relative abundances of carbonates from XRD spectra. I also measure carbon and oxygen isotope ratios and elemental composition (Mg, Ca, Sr, U), and, on a subset of samples, clumped carbonate isotopes to constrain formation temperatures. I use these fingerprints to evaluate whether dolomite precipitates record signals from a global carbon reservoir or are instead influenced by non-global controls like carbonate mixing processes and local environmental conditions on geochemistry. I identify two groups of isotopically and elementally distinct dolomite precipitates—one in equilibrium with atmospheric CO2, and one that is not. Finally, I use the fingerprints to test hypotheses about dolomite formation and show that varying levels of seawater-groundwater mixing can explain the observed chemical differences in dolomite precipitates.