Oxygen and Silicon Stable Isotopes of Diatom Silica: Reconstructing Changes in Surface Water Hydrography and Silicic Acid Utilization in the Late Pleistocene Subarctic Pacific

Deglacial variations in upper ocean nutrient dynamics and stratification in high latitudes, as well as associated changes in thermohaline overturning circulation, are thought to have played a key role in changing atmospheric CO2 concentrations. This thesis examines the relationship between past changes in subarctic Pacific upper ocean stratification and nutrient (silicic acid) utilization, using oxygen and silicon stable isotopes of diatom silica, for the first time at millennial-scale resolution and analyzed with a new and efficient instrumentation set-up. The isotopic data, presented in three manuscripts, show a consistent picture of millennial-scale variability in upper ocean stratification and silicic acid utilization during the last ~50 ka BP, e.g. indicating that the subarctic Pacific was a source region for atmospheric CO2 during the last deglaciation (late Heinrich Stadial 1 and the Bølling/Allerød). The presented results demonstrate the high potential of combined diatom oxygen and silicon stable isotope analysis especially for, but not restricted to, marine regions characterized by a low biogenic carbonate content like the subarctic Pacific and the Southern Ocean.