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

Abstract

The reconstruction of past ice sheet dynamics can inform 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) flowed across part of the western Scottish shelf to the shelf-break during the last glacial cycle. To investigate the deglacial dynamics of the HIS following the Last Glacial Maximum (LGM), lead (Pb) isotope records were extracted from the FeMn oxyhydroxide and detrital fractions of recovered laminated glacimarine mud sequences to monitor the changing activity of HIS during its retreat. These provide, respectively, relative timing of glacially weathered inputs to the marine environment and some source information on the eroded sediments. The FeMn oxyhydroxide fraction is dominated by pre-formed particles and shows a marked decrease from radiogenic at ~21cal ka to less radiogenic Pb isotope compositions towards 15.4–13ka. This decrease represents a reduction in the flux of subglacially-derived radiogenic Pb to the continental shelf, and it is interpreted as the result of the break-up of the ice-stream in western Scotland around that time. The Pb, Sr and Nd isotopic signatures of the detrital fraction indicate a preponderance of fine sediments originated from the NW Highlands throughout the period studied (~21 to 15cal ka BP), most likely dictated by the orientation of tidal and oceanic current directions and sediment delivery. Both fractions show inversion of the 208Pb/204Pb ratio relative to the other Pb isotope ratios. This is observed only in one core site in the detrital fraction, and extended to all cores in the FeMn oxyhydroxide fraction. This behaviour highlights the influence of ocean currents in restricting the detrital but encouraging dispersal of the FeMn oxyhydroxide signal. Periodic increased contributions from a high Th/U source, potentially the neighbouring Archaean amphibolitic Lewisian basement in the Outer Hebrides, are proposed as the source of these 208Pb/204Pb inversions. This study demonstrates how geochemical investigation on continental shelves can be used to constrain the activity and flow sources of palaeo-ice streams, and the utility of combining detrital and FeMn oxyhydroxides to determine the combined influence of the continental sources of material and their dispersal in the marine environment.