UC San Diego

All chemical elements are dissolved in the ocean and the differences in their concentrations and relative amounts of their isotopes can be used to understand internal processes throughout the ocean. The isotopic composition of trace metals in the ocean varies as a function of changes in paleo circulation, source provenance, weathering on the continents, and marine biological processes. These proxies have broad spatiotemporal applications and provide a wealth of information about nutrients sources to the ocean, internal biogeochemical cycling, and the evolution of oceans. To gain a better understanding of the how various terrigenous sources contribute to the oceanic nutrient inventory and whether stable iron (Fe) isotopes can be used as a tracer of Fe source, Chapter I in this dissertation thesis applied the Fe isotopic composition together with measurements of chemical lability of glaciofluvial silt, loess, combustion byproducts from biomass burning, and volcanic ash sourced from Southcentral Alaska. This chapter has built a Fe isotope inventory of sources to the Gulf of Alaska and tracked the variations in solubility between different Fe sources. Despite the significant developments in establishing ocean circulation proxies, our current geochemical tools lack the specificity to track composition changes in the ocean without influence from continental processes. Therefore, in Chapter II, we developed a new analytical routine for analysis of zirconium (Zr) isotopes of water samples and applied this method to natural seawater samples collected from a water column profile off the coast of California. We find that the seawater samples are highly fractionated relative to solid-Earth values and display marked variability in δ94/90Zr as a function of depth. It suggested that Zr isotopic compositions may be sensitive to seawater chemical properties and source. In chapter III, we applied Zr isotope analyses to oceanic sediment samples including abyssal clays, carbonate sediments, and Fe-Mn nodules sampled from the global ocean to explore the potential spatial Zr isotopic heterogeneity and whether authigenic marine sediments faithfully preserve the Zr isotopic composition of seawater they originated from. We find that Zr isotopic compositions of authigenic oceanic sediments vary in different ocean basins, and Zr isotopic compositions in seawater, authigenic sediments, and Fe-Mn nodules/crusts are tightly coupled to the elemental Zr/Hf ratios which are fractionated by particle scavenging processes. Stable isotopes of Zr may be a useful proxy for tracing water mass movement and possibly particle scavenging rates which can in turn be used to gauge biological productivity without a significant impact from fluctuations in physical or chemical weathering rates or source rock provenance.