Date of Award
Doctor of Philosophy in Oceanography
Rebecca S. Robinson
The ratio of stable nitrogen isotopes (δ15N) is used to infer nitrogen utilization in nutrient replete surface waters through time. Typically, the isotopic composition of bulk sedimentary nitrogen (δ15Nbulk) is measured. However the isotopic composition of this fraction is known to shift due to alteration during sinking and early burial. More recently, the nitrogen bound within silica microfossils (δ15NDB) has been targeted. This fraction is hypothesized to be protected from alteration during sinking and early burial by the biominerals encapsulating it, thus preserving the surface nitrate utilization signal through time. The goal of this project was two-fold, first determining the relationships and reliability of both the bulk and diatom-bound nitrogen isotope proxies over varying nutrient utilization and second, to investigate nitrate consumption in the Southern Ocean to better understand Earth’s history since the Last Glacial Maximum (LGM). Several polar diatoms species were cultured to determine the relationship between the isotopic composition of the seawater’s nitrate substrate, the bulk organic matter, and the diatom-bound fraction over varying degrees of nutrient utilization. A Rayleigh-type relationship was found to exist for all species, whereby δ15NO3 - > δ15Nbulk > δ15NDB. The isotopic enrichment factor (ε) and the offset between the bulk and diatom-bound fraction (δ15NDBoffset) were found to be relatively consistent within replicates, but varied greatly between species. This implies that species assemblage shifts through time may contribute to variations in the sedimentary δ15N signal. These relationships were then applied to downcore sedimentary records from the Southern Ocean at core TN057-13PC4, along with stable silicon isotopes (δ30Si) to investigate large-scale hydrographic changes between the LGM and the Holocene, today. Coupled N and Si measurements alongside opal accumulation rates provided a quasi-quantitative estimate of nutrient supply, or upwelling. Results suggest that large changes in water column stratification took place during the deglaciation, a highly variable period, which likely resulted in rising atmospheric pCO2 and caused a transient peak in nutrient supply to the low latitudes. Finally, bulk and diatom-bound δ15N values were investigated in five cores (TN057-13PC4, ODP 1094, PA9802-9PC, E17-9 and E23-14) from the Southern Ocean to determine the viability and variability within and between each proxy. Several factors including changes in species assemblage, ε, opal flux, nutrient dynamics, and alteration of the bulk and diatombound fraction may all contribute to the variability in measured δ15N downcore. Species assemblage composition clearly impacts downcore records of sedimentary δ15N. Experimental results from the first manuscript show that given assemblage changes, our culture data, for relevant species, do not suggest that the observed glacial/interglacial δ15N change in Antarctic sediment cores can be explained by bulkto-diatom-bound isotopic differences. In fact, large changes in species assemblage at TN057-13PC4 greatly affected the δ15N of sedimentary material and resulted in the removal of all δ15N data prior to 20 kya from the analysis of the deglaciation in the second manuscript. Discrepancies between bulk and diatom-bound δ15N were investigated in the third manuscript to determine the reliability of each proxy. This suggests that the diatom-bound material is not as protected as it was once thought to be and is likely subject to alteration.
Horn, Matthew G. S., "EXAMINATION OF THE DIATOM-BOUND NITROGEN ISOTOPE PROXY: NITRATE CONSUMPTION IN THE POLAR OCEAN" (2011). Open Access Dissertations. Paper 74.