PhD Oral Exam - Yeganeh Mirzaei, Chemistry

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Coastal sediments are major long-term sinks of terrestrial and marine organic carbon (OC), operating as a key control in global biogeochemical cycle. The fate of OC in these hotspots is governed by competition between OC degradation and preservation, two sides of the same coin, yet operating through different timescales and mechanisms. Through elemental, isotopic, molecular, microbial and mineral assessments of the sediments across the largest semi-enclosed estuary and gulf of the world, this thesis offers mechanistic and multivariate insights into the short-term biotic degradation and long-term abiotic preservation of OC under varying regimes. Coastal sinks are found to be more vulnerable to perturbation than assumed; significant inputs of fresh biolabile OC can fuel microbial remineralization to mobilize > 10% of previously stable OC stocks, unexpectedly modifying the molecular composition of sediments. If left unprimed, biochemically recalcitrant compounds and minerally protected OC survive the degradation, becoming selectively preserved in sedimentary archives. OC composition, sediment mineralogy, and local environmental conditions (e.g. redox, ionic strength) show superimposed effects on the preservation mechanisms. Upon traversing the land–ocean transect, terrestrial OC undergoes progressive sorting, microbial reworking, erosion from mineral surfaces, and displacement from organomineral complexes, declining ~2% per 10 km seaward. Along this gradient, the incorporation of marine OC in sediments increases, not only because of greater primary production and fast assimilation of its labile compounds in microbial biomass, but also because of its preferential association to clays, and co-precipitation with amorphous iron phases in higher salinities and benthic redox interfaces. These findings hence refine our understanding of coastal carbon cycling, call for caution in sedimentary-based paleo-reconstructions, and shed light on the necessity of multivariate and source-specific frameworks in correctly tracing the dynamic and convoluted history of sedimentary OC across land-ocean systems.