EXAMINATION OF THE DIATOM-BOUND NITROGEN ISOTOPE PROXY: NITRATE CONSUMPTION IN THE POLAR OCEAN

• Main Author: Horn, Matthew G. S.
• Format: Text
• Language: unknown
• Published: DigitalCommons@URI 2011
• Subjects: Southern Ocean
• Online Access: https://digitalcommons.uri.edu/oa_diss/74; https://doi.org/10.23860/diss-horn-matthew-2011; https://digitalcommons.uri.edu/context/oa_diss/article/1092/viewcontent/Horn_uri_0186A_10088.pdf

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 ...