Weathering fluxes and sediment provenance on the SW Scottish shelf during the last deglaciation

Riccardo Arosio, Kirsty Crocket, Geoffrey Nowell, Louise Callard, John Howe, Sara Benetti, Derek Fabel, Steven Moreton, Chris W. Clark

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Abstract

The reconstruction of ice sheet dynamics can shed a light on long-term ice stream activity, and in turn provide constraints on the response of modern ice sheets to climate change. The Hebrides Ice Stream (HIS) occupied the entire western Scottish shelf during the last glacial cycle and drained a large portion of the northern sector of the British Irish ice Sheet. To investigate deglaciation dynamics of the HIS after the Last Glacial Maximum, Pb isotope records were extracted from the FeMn oxyhydroxide and detrital fractions of the recovered laminated glacimarine mud sequences to monitor the changing activity of HIS during its retreat. Lead isotope ratios in FeMn oxyhydroxides provide timing and some source information of glacially weathered inputs to the marine environment. The FeMn oxyhydroxide fraction in the samples is dominated by allochthonous particles (pre-formed) and shows a marked decrease from radiogenic (≤20.05 206Pb/204Pb) at ~21 cal ka BP to less radiogenic Pb isotope compositions (~19.48) towards the Windermere Interstadial (15.4-13 ka BP). This decrease represents a reduction in the flux of subglacially-derived radiogenic Pb to the continental shelf and is associated with the break-up of ice-streaming in western Scotland around that time. The Pb, Sr and Nd isotopic signatures of the detrital fraction indicate a preponderance of Moine-sourced fine sediments (originated from the NW Highlands) in the core locations from ~18 to 14 ka BP (Dimlington Sta - Windermere Intersta.), most likely dictated by the orientation of ice flow, tidal and oceanic current directions and sediment delivery. Ice rafted debris in a ~21 ka BP old basal diamicton contains instead volcanic-derived material, suggesting different provenance for different grain sizes. The FeMn oxyhydroxide 208Pb/204Pb ratio shows an unusual inversion relative to the other Pb isotope ratios, and is attributed to the introduction of secondary weathering phases from a source with contrasting 208Pb/204Pb but similar 206Pb/204Pb and 207Pb/204Pb. In the detrital fraction, the inversions are constrained to periodic spikes, which may indicate an increased contribution from a high Th/U source, potentially the neighbouring Archaean amphibolitic Lewisian basement in the Outer Hebrides. These results indicate that geochemical investigation on continental shelves may be applied to reconstruct regional ice stream dynamics on sub-millennial time scales.
Original languageEnglish
Pages (from-to)81-98
Number of pages17
JournalMarine Geology
Volume402
Early online date31 Aug 2017
DOIs
Publication statusPublished - 1 Aug 2018

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Keywords

  • Glacial sediments
  • Pb geochemistry
  • FeMn oxyhydroxides
  • Provenance
  • LAST GLACIAL MAXIMUM
  • Western Scotland

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