Timing of glacial retreat in the Wicklow Mountains, Ireland, conditioned by glacier size and topography
Journal of Quaternary Science · November 2017
Reconstructing the deglacial history of palaeo‐glaciers provides vital information on retreat processes, information which can inform predictions of the future behaviour of many of the world's glaciers. On this basis, this paper presents 170 Schmidt Hammer exposure ages from moraine boulders and glacially sculpted bedrock to reveal the post‐Last Glacial Maximum (LGM) history of the Wicklow Mountains, Ireland. These data suggest that large ice masses survived for 4–7 ka after retreat of the Irish Sea Ice Stream and were sustained by summit ice fields until ∼16.6 ka. Post‐LGM retreat was driven by climate and involved numerous short‐term ice front oscillations (≤1 ka), with widespread moraine deposition during Heinrich Stadial 1. In contrast, marked asynchroneity in the timing of Younger Dryas deglaciation is closely linked to snow redistribution which demonstrates the sensitivity of small cirque glaciers (≤1 km2) to local topography. This result has important implications for palaeoclimate reconstructions as cirque glacier dynamics may be (at least partly) decoupled from climate. This is further complicated by post‐depositional processes which can result in moraine ages (e.g. 10Be) which post‐date retreat. Future palaeoclimate studies should prioritize cirques where snow‐contributing areas are small and where post‐depositional disturbance of moraines is limited.
This study provides the first comprehensive glacial retreat history for the Wicklow Mountains, Ireland. 170 Schmidt Hammer exposure ages from cirque and valley moraines and from a summit overridden by ice at the LGM demonstrate that significant ice masses persisted for 4-7 ka after retreat of the Irish Sea Ice Steam and were sustained by summit ice-fields until ~16.6 ka. Post-LGM
retreat involved numerous oscillations of glacier termini during the retreat phase, with widespread
moraine deposition related to stabilisation or re-advance of valley glaciers during the Oldest Dryas,
potentially in response to cooling during Heinrich Stadial 1 (HS1). However, these moraines reflect
short-term oscillations (≤ 1 ka) of the ice front during the long-term retreat phase (~8 ka), which
was driven by reduced moisture availability and winter aridity. These data match numerical ages
(10Be, 36Cl, 14C) from comparable mountains caps at the margins of the Irish Sea basin and in SW
Ireland which support a model of widespread and persistent alpine glaciation during the post-LGM
period. Significant ice masses (Length: ≤ 5 km) were present until the onset of Greenland Interstadial
1 and in some mountain massifs, for up to ~4 ka after initial summit emergence following the LGM.
Valley glacier retreat in the Wicklow Mountains was driven by climate, with time-progressive
deglaciation from low to high elevation (R2 = 0.9116). In contrast, marked asynchroneity in the
timing of Younger Dryas (YD) deglaciation (11.4 - 12.3 ka), unrelated to site elevation, latitude or
equilibrium line altitude (ELA), is accounted for by macro-topography and the redistribution of snow
and avalanche material, sufficient to locally supress the ‘local’ (non-climatic) ELA and promote glacier survival. Contrasting synchroneity in the timing of glacial retreat during these periods is conditioned by glacier size, with small YD glaciers (< 1 km2) highly sensitive to local topographic controls. This result has important implications for palaeoclimate reconstructions based on dating of cirque moraines (e.g. 10Be, SHED), as cirque glacier dynamics may be (at least partly) decoupled from climate. This is further complicated by post-depositional processes which can result in ages which post-date retreat. As a result, future palaeoclimate reconstructions should prioritise cirques where snow and avalanche contributing areas (Ac) are small and where the potential for post-depositional disturbance is limited (matrix-poor, boulder rich moraines).