A Pb isotope tracer of ocean-ice sheet interaction: the record from the NE Atlantic during the Last Glacial/Interglacial cycle

Abstract

Ice sheet-ocean interactions are both a response to climate forcing and a source of climate feedback, releasing freshwater to the surface ocean and influencing climate and atmospheric CO2 through changes in ocean circulation. Documenting the outcomes of these interactions for recent glacial cycles is important given current and future scenarios of polar ice retreat. However, this is currently hampered by lack of accurate constraints on ice sheet development and demise. Marine sedimentary Pb isotope records have potential to investigate these aspects of ice sheet feedbacks at high temporal resolution because of the sensitivity of the Pb isotope composition to continental weathering intensity and solute flux. Here we present a Pb isotope record sourced from the FeMn oxyhydroxide fraction in marine sediments from ODP Site 980 on Feni Drift (2168 mbsl, Rockall Trough, NE Atlantic), spanning the last 43 ka. The location of Site 980 at the northern edge of the British–Irish Ice Sheet (BIIS) makes it well-placed to monitor changes in BIIS development as it responded to migration of the Polar Front during the Last Glacial/Interglacial cycle. The data reveal millennial-scale cyclicity in Pb isotope composition, reminiscent of Dansgaard–Oeschger cycles, from the start of the record until Heinrich event 2 (43–24 ka), characterised by extreme shifts to radiogenic compositions (i.e. variation in 206Pb/204Pb from ∼18.9 to 20.5). The period 24–17.5 ka is also characterised by exceptionally radiogenic and highly variable Pb isotope compositions, associated with the rapid and repeated expansion and collapse of the BIIS. The presence of such radiogenic Pb isotope compositions during periods of maximum ice sheet activity support interpretation of the subglacial environment as an active weathering environment, contributing to biogeochemical cycles through the transport vectors of meltwater release and debris-laden ice calving.