PhD defence: Anne-Sofie Crüger Ahm – Københavns Universitet

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PhD defence: Anne-Sofie Crüger Ahm

Anne-Sofie Crüger Ahm defends her thesis,

Refining the Neoproterozoic and Early Paleozoic record of carbon cycling and seawater chemistry using quantitative geochemical models of redox dynamics and carbonate diagenesis

Associate Professor Christian J. Bjerrum, IGN

Assessment Committee:
Dr. Alexandra Turchyn, University of Cambridge - UK
Dr. David S. Jones, Department of Geology, Amherst college - US
Assistant Professor Emily C. Pope (chair), SNM

This thesis explores the ancient sedimentary archive of Earth history and examines enigmatic climatic transitions that may have shaped the evolution of animal life. Much of our understanding of the climatic history of Earth is based on chemical and isotopic measurements of ancient sediments. One of the main limitations of this archive, however, is the susceptibility of sediments to diagenesis that can alter the primary chemical signals. For example, the concentration of iron (Fe) in specific mineral phases is used to infer oxygen concentrations in the ancient water column. Chapter 2 of this thesis demonstrates that modern weathering processes remobilize reactive Fe in outcrop samples which leads to a loss of specific minerals phases (pyrite) and a gain of others (Fe-oxides). These results highlight the need for caution when using Fe-speciation to catalog past environment redox change from purely outcrop based records.
Despite the prevalence of diagenesis in sedimentary rocks there are currently few robust geochemical tools capable of providing quantitative information on the extent of alteration from the primary signal. In order to fill this gap, Chapter 3 presents a numerical model of marine carbonate diagenesis that tracks dissolution of primary carbonates and re-precipitation of secondary minerals. The model is ground-truthed using measurements of calcium, magnesium, carbon, and oxygen isotopes in carbonate sediments from the modern Bahamas platform. This model can be used as a `looking glass' to see through diagenesis and provide more robust estimates for past seawater chemistry.
Ancient carbonate rocks with extreme negative carbon isotopes are found worldwide bracketing the Marinoan glaciation (635 Ma). There is no scientific consensus as to whether these excursions originate from a primary perturbation in the carbon cycle or from diagenetic alterations. Chapter 4 merges new measurements of calcium, magnesium, and strontium isotopes in these sediments with the diagenetic model developed in Chapter 3 to offer new insights into the potential origin of these extreme isotope anomalies.

The thesis is available for inspection at the PhD administration office, 04.1.417