The global geochemical cycles of iron and calcium: Using novel isotope systems to understand weathering, global mass budgets, natural reaction rates, and paleoclimate

The study of global geochemical cycles is critical to many aspects of earth science. By understanding how and on what timescales elements move between geochemical reservoirs, we can interpret geochemical variations in the geological record and examine the past. Traditional geochemical proxies utilize variations in the oxygen, carbon, and boron isotopic composition of marine carbonates, for example, to determine paleo-sea surface temperature, sea level regressions and transgressions, and paleo-pH in the surface ocean. Studies of these types have successfully probed the geological past and offered insight into the mechanisms which control the movement of elements within geochemical cycles.; The current study attempts to extend the tools available for paleoenvironmental analysis by examining the utility of two novel stable isotope systems, iron and calcium, in the context of elemental cycling. Chapter 2 presents an initial look at the Fe isotope composition of various components of the modern Fe cycle and constructs an iron budget for the modern ocean. Chapter 3 takes a detailed look at calcium isotope records derived from marine carbonates and uses numerical modeling to deduce relative calcium fluxes, marine calcium concentrations, and atmospheric carbon dioxide levels over the past 20 million years. Chapters 4 and 5 present Sr and Ca isotope measurements in marine carbonates and associated pore fluids and detail numerical modeling methods that are used to constrain the equilibrium fractionation factor between calcite and solution, calcium carbonate recrystallization rates in the sedimentary column, and diagenetic alteration of Ca isotope signals over geologic time scales. The overall goal of the current study is to examine the usefulness and fidelity of Fe and Ca isotopes as tools to probe the geologic past.