Glaciomarine sediment archives – Integrating traditional approaches and novel techniques to constrain ice-stream collapse in The Minch, NW Scotland

Abstract

This PhD thesis covers a number of topics relating to glaciomarine sediment archives. It presents a novel method for quantifying ice-rafted debris (IRD) and characterising glaciomarine sediments. It identifies strengths and weaknesses of existing IRD techniques and combines traditional and innovative approaches to constrain Minch Ice Stream behaviour before and after its demise. A new IRD method which automatically characterises glaciomarine sediments, based on X-ray imaging properties, affords significant advantages over existing IRD techniques – reducing analysis times, minimising human bias, and operating at the highest resolution of any technique. A participatory study of different researchers undertaking X-ray gravel counting (> 2 mm) on X-ray images to quantify IRD yielded significant inaccuracies between different users, which indicates that there is a strong need for automation in sediment characterisation. A multiproxy investigation of laminated glaciomarine sediments in the Minch (NW Scotland) indicates that colour-banded glaciomarine sediments formed via alternating sediment-laden meltwater plume locations, rather than simple suspension settling or seasonally deposited terrigenous v biogenic sedimentation. Finally, the timing and behaviour of the Minch Ice Stream is constrained before, during and after collapse, via a multiproxy approach. Two main stable phases are identified while the ice stream was grounded in the central Minch: the first remains poorly constrained, but occurred before 17.2 ka cal BP; the second was a ~ 600-year stable phase from 17.2 to 16.6 ka cal BP with a short 5-10 km ice shelf. At 16.6 ka cal BP, the Minch Ice Stream underwent a step-change in behaviour, entering a “runaway retreat” or “collapse phase”. These results indicate that sea-level-rise projections must consider marine ice sheet dynamics if they are to accurately predict the contributions of marine-terminating glaciers and ice streams such as those in West Antarctica and Greenland.