Academic dissertation for the Degree of Doctor of Philosophy in Marine Geology at Stockholm University to be publicly defended on Friday November 7, 2025 in William-Olsson lecture hall, Geoscience building, Stockholm University.

Zoom link:https://stockholmuniversity.zoom.us/j/65514136155

Supervisor

• Helen K. Coxall, Professor, Department of Geological Sciences, Stockholm university.
• Martin Jakobsson, Professor, Department of Geological Sciences, Stockholm university.

Opponent

• Michael Henehan, School of Earth Sciences,University of Bristol.

Abstract

Planktonic foraminifera, marine heterotrophic protists with calcite tests, are widely used in palaeoceanographic and palaeoclimatic reconstructions. Recent advances in marine biology and molecular genetics continue to refine our understanding of their diversity and ecology, enhancing their utility as palaeoenvironmental proxies. However, reliable application still depends on robust taxonomy and clear insights into species evolution, classification, and the (palaeo)ecology of both extant and extinct species. Considerable progress has been made, but important gaps remain, particularly for Neogene–Quaternary planktonic foraminifera.

This PhD explores evolutionary, ecological, and biostratigraphic aspects of the genus Neogloboquadrina in the high latitude North Atlantic and Arctic regions during the Neogene to Quaternary, with a particular focus on N. pachyderma. Although a dominant component of modern and fossil high latitude planktonic foraminifera assemblages, the evolutionary trajectory of N. pachyderma towards polar specialisation remains poorly understood. Through integrated taxonomic, morphological, and isotopic analyses, this PhD provides new insights into its long-term ecological strategies and palaeoceanographic significance. 

Paper 1 revises the taxonomy and biostratigraphy of planktonic foraminifera from DSDP Site 407, west of the Reykjanes Ridge. This core spans 25 million years and provides a near-continuous North Atlantic record of Neogloboquadrina evolution. Revisiting material first studied over 45 years ago, the work applies modern taxonomic concepts to all taxa, refines species-level identifications, and documents the rise of N. pachyderma as a key high-latitude taxon.

Paper 2 examines living N. pachyderma collected from the water column in the central Arctic. It investigates the reproductive strategy and records unusually abundant aberrant dextral forms, providing the first field-based evidence of a schizont reproductive stage, previously suggested only in laboratory cultures. These findings highlight the species’ reproductive plasticity and adaptive strategies under perennial sea ice, offering insight into how it maintains ecological dominance in polar environments.

Paper 3 presents stable carbon and oxygen isotope analyses from three high-latitude North Atlantic sites. The data track the ecological and biogeographic evolution of N. pachyderma and related species from the late Miocene to Pleistocene, casting light on how N. pachyderma expanded poleward in response to global cooling. The study documents the species’ persistence through pronounced late Pliocene to Pleistocene climate shifts and its eventual rise to dominance in northern high-latitude environments.

Paper 4 integrates these results within the Neogene–Quaternary Planktonic Foraminifera Work Group, producing an updated taxonomy and a stratophenetic phylogenetic tree for all Neogloboquadrina species and their ancestor.

This thesis offers in improved understanding of Neogloboquadrina species in polar ecosystems, integrating taxonomic revisions, pelagic reproductive strategies, and stable isotope data to provide deeper insights into past climate and oceanographic conditions and ecosystem resilience. The findings improve our ability to use these organisms as palaeoceanographic tracers, with important implications for reconstructing past high-latitude climate dynamics.

Link to DiV