The role of biological production in pleistocene atmospheric carbon dioxide variations and the nitrogen isotope dynamics of the southern ocean

Abstract

This dissertation contributes to the search for a cause of glacial/interglacial variations in atmospheric carbon dioxide. The hypotheses addressed involve changes in low and high-latitude biological export production. A modelling exercise demonstrates that the paleoceanographic record of calcite preservation places constraints on hypothesized changes in low latitude biological production. The model results indicate that large, production-driven changes in the depth of the calcite saturation horizon during the last ice age would have caused a similar deepening of the calcite lysocline, even when the effect of sediment respiration-driven dissolution is considered. Such a large glacial lysocline deepening is not evident on an ocean-average basis. The results indicate very few mechanisms by which low latitude production could have driven Pleisotocene carbon dioxide variations, generally arguing against a low latitude cause for these variations. The use of N isotopes as a paleoceanographic proxy for nitrate utilization in Southern Ocean was investigated. In order to examine the generation of the link between nitrate utilization and N isotopes in the surface ocean, the isotopic composition of nitrate was studied. The first step in this work was the development of a new method to measure the isotopic composition of nitrate which is amenable to the generation of large, precise data sets. Results from the Southern Ocean demonstrate that the Antarctic and Subantarctic represent distinct regimes of N isotope dynamics. The findings support the use of N isotopes as a proxy for nitrate utilization in the Antarctic. A study of diatom microfossil-bound N in sediments suggests that this N is native to the diatoms, that it is invulnerable to early diagenesis, and that its isotopic compositon varies with that of the sinking flux. Paleoceanographic records of diatom-bound N isotopic composition corroborate the conclusion, previously based on bulk sediment isotopic data, that nitrate utilization was elevated in the glacial Antarctic, representing a major cause of lower glacial atmospheric carbon dioxide levels.