Internal dynamics condition centennial-scale oscillations in marine-based ice-stream retreat

Abstract

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.

Ice margin oscillations during deglaciation of the northern Irish Sea Basin

“…This phase corresponded to a reduction in net margin retreat rate in the interval from 20.3 ± 0.7 ka (BL3) to 19.3 ± 0.8 ka (BL5), with evidence for at least 10 ice‐marginal oscillations (Thomas et al ., ) in relatively quick succession (within 1 ka) as sectors on and to the east of the Isle of Man developed a terrestrially terminating ice margin. These still‐stands and readvances (BL5–6) are similar in character to the centennial‐scale oscillations recorded on the Llŷn Peninsula (Smedley et al ., ). This period of ice stability coincided with both stable climatic and oceanic conditions (Fig.…”

“…Net ice retreat was faster from the coast of North Wales to the central Isle of Man, constrained here at rates of 80–180 m a −1 , and then slowed to 20 m a −1 across the northern Isle of Man (BL3–4) before stabilizing at net retreat rates around 15–20 m a −1 towards the terrestrial hinterlands of southern Scotland. The faster net retreat rates (80–180 m a −1 ) were similar to the faster rates on the northern Llŷn Peninsula where the ice front was less constrained by trough geometry (84–139 m a −1 ) (Smedley et al ., ). The slower net retreat rates (13–20 m a −1 ) experienced in the NISB were similar to those reconstructed for the southern Llŷn Peninsula (8–41 m a −1 ) (Smedley et al ., ) and southern Irish coast (26 m a −1 ) (Small et al ., ).…”

“…The faster net retreat rates (80–180 m a −1 ) were similar to the faster rates on the northern Llŷn Peninsula where the ice front was less constrained by trough geometry (84–139 m a −1 ) (Smedley et al ., ). The slower net retreat rates (13–20 m a −1 ) experienced in the NISB were similar to those reconstructed for the southern Llŷn Peninsula (8–41 m a −1 ) (Smedley et al ., ) and southern Irish coast (26 m a −1 ) (Small et al ., ). Slowing of net ice‐margin retreat appears to have occurred when the ice was pinned against the bedrock obstruction of the Isle of Man, and was characterized by a series of ice‐marginal oscillations within the interval 19.9 ± 0.7 and 18.3 ± 1.1 ka ; these are recorded in stratigraphical evidence for a series of standstill events and limited (<1 km) readvances of the ice margin (Thomas et al ., ).…”

“…) shows a broad match between retreat and increasing summer insolation (Berger and Loutre, ). Following the short‐lived advance of the ISIS into the Celtic Sea, overall ice‐marginal retreat in the southern ISB was punctuated by still‐stands and oscillations, recorded from south to north along the coastlines of Ireland and Wales (Smedley et al ., ; Small et al ., ). Faster (e.g.…”

“…Smedley et al . () have shown that retreat of the ISIS margin from the southern Irish Sea Basin was complete by 20.3 ± 0.6 ka; it follows that deglaciation of the NISB coincided with summer insolation‐related warming at ∼45°N (Bintanja et al ., ) and overlapped the timing of peaks in ice‐rafted debris flux to the southern Celtic Sea associated with Heinrich Event 1 (H1) (Haapaniemi et al ., ).…”