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

Millennial-scale climate variability has been detected globally throughout late Pleistocene paleoenvironmental records, yet the trigger and response mechanisms remain under debate. It has been suggested that deglacial variations in upper ocean stratification in high latitudes and associated changes in thermohaline overturning circulation might have played a key role in changing atmospheric CO2 concentrations (“polar stratification hypothesis”), but information to evaluate this relationship in the subarctic Pacific are scarce. Oceanic-atmospheric CO2 exchange also strongly depends on the efficiency of the biological pump, which is closely coupled to the nutrient cycle. This thesis examines the relationship between past changes in subarctic Pacific upper ocean stratification and nutrient (silicic acid) utilization, using oxygen (δOdiat) and silicon (δSidiat) stable isotopes of diatom silica, for the first time at millennialscale resolution. In combination with new information on subsurface temperature/salinity, paleoproductivity and iceberg discharge from the Cordilleran Ice Sheet as well as additional information e.g. on sea ice distribution and sea surface temperatures, this thesis aims to improve the understanding of subarctic Pacific climate development from the last glacial to interglacial period (~50-6 ka BP). The δOdiat and δSidiat measurements were performed using a new instrumentation set-up, which allows for the efficient combined silica δO and δSi analysis and requires a comparatively low amount of silica material (~1.5-2.0 mg). The results of this thesis are presented in three manuscripts. Apart from introducing the new instrumentation set-up, the first manuscript investigates potential isotope effects on δOdiat and δSidiat associated with (1) contamination with non-diatom silicates (e.g. clay minerals, radiolarians) and (2) species-related isotope effects (vital effects and environmental effects, e.g. related to seasonality). The results demonstrate the importance of high-purity diatom samples for isotope analysis and support previous findings indicating the absence of discernable species-related silicon isotope effects. Observed vital and/or environmental effects influencing diatom δOdiat highlight the necessity to separate diatom samples according to species or species groups, assigned to the same environmental boundary conditions, to obtain reliable isotopic results. The second manuscript concentrates on the relationship between upper ocean stratification, nutrient utilization and paleoproductivity from the last glacial to interglacial period (~25-6 ka BP). Consistent with the “polar stratification hypothesis”, glacial data indicate stratified, fresh and highly utilized surface waters, as well as low productivity. The deglacial period (~17.511.7 ka BP) is characterized by a sequence of paleoceanographic events occurring in surface waters. These can be linked to changes in sea-ice and water mass formation in the subarctic Pacific realm as well as to changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC), but do not mirror the deglacial pattern of the Heinrich Stadial (HS) 1 cooling (~17.5-14.6 ka BP) or Bølling/Allerød (B/A) warming (~14.6-12.8 ka BP), recorded in Greenland ice cores. An indicated increased availability of silicic acid during early HS1 is possibly associated with an increased formation of North Pacific Intermediate Water, characterized by a comparatively high silicic acid concentration. An alternative explanation might be an increased silicic acid supply to surface waters related to deep convective overturning. However, surface waters are still fresh and stratified during that time,