Using De Geer moraines to reconstruct ice sheet retreat in high resolution

This thesis explores the spatiotemporal properties of De Geer moraines (DGMs) in southwest Finland and assesses their utility as high-resolution ice margin indicators. This research provides new insights into DGM formation mechanisms, ice margin geochronometric potential, and discusses implications relative to ice sheet reconstructions. Key findings from this study include the development of a new Python-based ArcGIS toolbox for automated 3D morphometric analyses, quantified morphometric and internal architectural insights, and temporal associations with local clay-varved chronologies, producing an annual-resolution ice margin reconstruction of the southwest Finnish sector of the Fennoscandian Ice Sheet (FIS). A total of 3 966 DGMs were mapped across southwest Finland and were subdivided into prominent (2 581) and intermediate (1 385) moraines. Morphometric analyses find DGMs to be slightly sinuous and asymmetric in profile. Internal architecture of DGMs located across 4 sites in southwest Finland present proximal to distal transitions whereby proximal sides are characterised by compact laminae and thrust plane structures, and distal sides are characterised by poorly compact diamicton reworked by proglacial water currents. These findings support DGM formation at the grounding line of water-terminating ice margins and provide evidence to accurately position them within a wider ice sheet context. In addition, spatial analyses reveal DGM-subtypes, namely regularly spaced prominent DGMs and irregularly spaced intermediate DGMs, that depict interseasonal variations during formation. Specifically, regularly spaced prominent DGMs are suggested to form via push during winter readvances, whereas irregularly spaced DGMs are suggested to form during periods of summer retreat. Connections with local clay-varved chronologies show close alignment with annual rates of retreat and demonstrate that an annual signal can be established within De Geer terrain (DGT) enabling annual DGM isochrones to be constructed. The DGM-derived reconstruction in this study presents an annual 714 year long ice margin retreat pattern across southwest Finland between 11 615 – 10 901 cal. years BP, characterised by retreat rates ranging between ~500 – 2 000 m/yr-1. This is a significant refinement to that of the current highest resolution (e.g. annual compared to 100-year resolution) and provides detailed insights into grounding line processes whereby interrelationships between temperature, water depth and topography drive ice margin retreat. Specifically, the dynamic fluctuations of ice margin retreat observed at annual timescales infer hinge-driven calving processes as a result of thinning ice, increasing water depths and smooth, shallow-gradient bed topography. The findings from this study demonstrate that DGMs may be used as valuable ice margin proxies and can capture retreat rates at very high resolution. Furthermore, this work demonstrates how DGMs may offer valuable insight relative to grounding line forcing mechanisms at annual to centennial timescales. Insights produced from this study are particularly valuable for understanding ice sheet retreat dynamics at societally relevant timescales and may assist with projecting changes in contemporary ice sheets. Moreover, this study provides a methodological framework and foundation for future studies that aim to utilise DGMs as ice margin indicators and reveal detailed insights into complex grounding line processes.