Mar 2023
The clay-sized fraction of modern carbonate facies is mainly composed of fine aragonite needles (≤1µm by ~5 µm). In ancient limestones, however, the fine fraction has altered to micrite to microspar, both equant calcite crystals. Exactly how and when this alteration occurs is still hotly debated. Speleothems, mineral constructs in caves and mines, often have micritic layers, but it is not clear if these layers are primary or recrystallized; and if recrystallized, how to tell? I will present data from Pleistocene and Recent speleothems that will document primary minimicrite (<1 µm) layers progressively recrystallizing to microspar (10-20 µm) in months to years without any change in mineralogy (calcite-to-calcite recrystallization) and without any change in ambient conditions. I will argue that this recrystallization is an example of Ostwald ripening, where small crystals dissolve simultaneously with growth of larger crystals so as to minimize the surface free energy. This process may also contribute to the recrystallization of lime mud in near-surface sediments. A number of geochemical studies of modern sediment suggest early lime mud diagenesis driven by organic matter and/or mineralogy, but proof of recrystallization has been hard to find in the complex mixture of modern sediments. The results from the simpler cave system suggest that very rapid recrystallization of near-surface lime muds is not only possible, but likely.
Leslie Melim discovered geology rather late in her undergraduate career and so has a B.A. in History from Whitman College. Shifting time scales, she then completed her M.S. in geology at Western Washington University and a Ph.D. at Southern Methodist University, studying the dolomitization of the Permian Reef of Texas and New Mexico. Post-doctoral studies at the University of Miami, FL, expanded her study of carbonate diagenesis to the Neogene of Great Bahama Bank and the Devonian of the Canadian Rockies. Since 1996, she has taught at Western Illinois University, where she is Professor of Geology.
Her early research on the alteration of aragonite-rich sediments buried in seawater defined marine-burial diagenesis, which rather surprisingly mimics meteoric diagenesis. Her current research continues to revolve around carbonate diagenesis, but in a different setting: caves. She studies the interaction of geomicrobiology, water chemistry, and carbonate diagenesis in the origin and alteration of cave speleothems, with a focus on cave microbialites.