Rates of ice-stream retreat over decades can be determined from repeated satellite surveys
and over millennia by paleoenvironmental reconstructions. Centennial time scales are
an important temporal gap in geological observations of value in process understanding
and evaluation of numerical models. We address this temporal gap by developing a 3 ka and
123 km retreat time series for the Irish Sea ice stream (ISIS), a major outlet draining the
last British-Irish ice sheet. The Llŷn Peninsula (northwest Wales, UK) contains numerous
ice-marginal indicators from which we reconstructed a robust chronological framework of
margin oscillations. The landscape documents the retreat of a former marine-terminating
ice stream through a topographic constriction, across a reverse bed slope, and across variations
in calving margin width. New dating constraints for this sequence were integrated in
a Bayesian sequence model to develop a high-resolution ice-retreat chronology. Our results
show that retreat of the ISIS during the period 24–20 ka displayed centennial-scale oscillatory
behavior of the margin despite relatively stable climatic, oceanic, and relative sea-level
forcing mechanisms. Faster retreat rates coincided with greater axial trough depths as the
ice passed over a reverse bed slope and the calving margin widened (from 65 to 139 km). The
geological observations presented here over a 123-km-long ice-retreat sequence are consistent
with theory that marine-based ice can be inherently unstable when passing over a reverse
bed slope, but also that wider calving margins lead to much faster ice retreat.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)