Master of Science Adele Westgård will Wednesday August 13th, 2025, at 12:15 hold her Thesis Defense for the PhD degree in Science. The title of the thesis is:
« Constraining the environmental and biological controls on the geochemical composition of Neogloboquadrina pachyderma: Towards developing robust proxies for reconstructing polar surface ocean hydrography »
Rapid ongoing climate change includes warmer sea surface temperatures, salinity changes, reduced sea ice cover, and melting ice sheets in the polar regions. This has consequences for climate and society regionally and globally. However, there are large knowledge gaps regarding the sensitivity of these systems. Records of past changes in polar climate and ocean-cryosphere interactions can provide invaluable insight to the impacts of ongoing warming. The planktic foraminifer, Neogloboquadrina pachyderma, is a key species in high latitude marine ecosystems and integral for palaeoceanographic reconstructions of the subpolar and polar oceans. The composition of their calcium carbonate shells broadly reflects the conditions they grew in, e.g., shell Mg/Ca increases with temperature. The species exhibits a thick outer crust with significantly different element/Ca (e.g., lower Mg/Ca) compared to the inner lamellar calcite. This large intrashell variability combined with a lack of culture-based low temperature calibrations result in large uncertainties in Mg/Ca-based temperature reconstructions in the polar regions. In addition, there are currently no robust trace element proxies for reconstructing salinity or carbonate chemistry.
Results presented in this thesis are based on culture experiments of N. pachyderma in controlled conditions including temperature, salinity, and carbonate chemistry, and high-resolution shell element composition was obtained using Laser Ablation Inductively Coupled Plasma Mass Spectrometry. The main outcomes of this study are: 1) N. pachyderma is a resilient species which utilise strategies such as dormancy and asexual reproduction to outlast unfavourable conditions; 2) A method was developed to geochemically distinguish the crust and lamellar calcite which revealed distinct differences in element/Ca between the two components; and 3) A cold water-tailored culture-based Mg/Ca-temperature calibration with separate regressions for the crust and lamellae, and refined understanding of environmental influences on shell composition. Overall, the methods presented in this thesis reduce uncertainties in high latitude palaeoenvironmental reconstructions towards better understanding of ocean-climate interactions.
Supervisory Committee:
1st Opponent: Professor Anna Nele Meckler, University in Bergen
2nd Opponent: Professor Jeroen Groeneveld, Institute of Oceanography, National Taiwan University
Internal member and leader of the committee: Professor Anders Schomacker, IG, UiT
The defence and trial lecture will be streamed from these following links at Panopto:
Defence (12:15 - 16:00)
Trial Lecture (10:15 - 11:15)
The thesis is available at Munin Here.