Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1997
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.
This research was supported by the National Science Foundation Graduate
Fellowship Program, the JOI!USSAC Ocean Drilling Graduate Fellowship Program, and
by NSF grant OCE-9201286 to D.C. McCorkle.
Atmospheric carbon dioxide
Polarstern (Ship) Cruise
Woods Hole Open Access Server