Millennial and sub-millennial-scale variability in sediment colour from the Barra Fan, NW Scotland: implications for British ice sheet dynamics

Sediment colour, together with other proxy data, provides a novel, rapid and non-destructive tool in the investigation of glacier-influenced sedimentation on the Barra Fan, NW Scotland. Lightness (L*) and reflectance (400-700 nm) measurements at this site provide a quantitative estimate of changes in calcium carbonate and clay content. Interstadials are carbonate-rich/clay-poor (higher L* and reflectivity), whereas stadials are carbonate-poor/clay-rich (lower L* and reflectivity). Detailed sedimentological investigations suggest that the last British Ice Sheet (BIS) extended to the outer continental shelf-break shortly after 30 ka Bp. This climatic response of the BIS to global cooling at the Marine Isotope Stage (MIS) 3 - 2 transition marks a significant increase in sediment delivery to the Barra Fan. Prior to 30 ka BP, strong Dansgaard/Oeschger (D/O) cyclicity dominates the record. After 30 ka BP, shorter periodicities prevailed as the BIS reached its maximum extent. Glacier dynamics plays a significant role in the delivery of ice-rafted debris (IRD) across this margin, highlighting the inherent difficulties of correlating millennial-scale IRD events when the IRD is derived from different ice sheets. An event stratigraphy based upon carbonate-rich interstadials provides a more robust means of amphi-Atlantic correlation during this interval.