A new varve sequence from Windermere, UK, records rapid ice retreat prior to the Lateglacial Interstadial (GI-1). (1st December 2019) - Avery, Rachael S.
Abstract: Annually laminated sediments (varves) provide excellent temporal resolution to study rapid environmental change, but are rare in the early part of the Last Termination (∼19–∼11.7 ka BP). We present a new >400 varve year (vyr) varve sequence in two floating parts from Windermere, a lake at the southern margin of the mountains of northwest England. This sequence records the final retreat of the Windermere glacier at the southern edge of the Lake District Ice Cap during the transition from Heinrich Stadial 1 (∼18–∼14.7 ka BP) into the Lateglacial Interstadial (∼14.7–∼12.9 ka BP). Laminated sediments from four lake cores from Windermere's northern and southern basins were investigated and shown to be varved. These sequences are integrated with seismic reflection evidence to reconstruct south-to-north deglaciation. Seismic and sedimentological evidence is consistent with gradual stepped ice retreat along the entire southern basin and into the northern basin between 255 and 700 vyr prior to the appearance of significant biota in the sediment that heralded the Lateglacial Interstadial, and had retreated past a recessional moraine (RM8) in the northern basin by 121 vyr prior to the interstadial. The Lateglacial Interstadial age of this biota-bearing unit was confirmed by 14 C-dating, including one date from the northernmost core of ∼13.5 cal ka BP. A change in mineralogy in all four cores as the glacier retreated north of the Dent Group (the northernmost source of calcareous bedrock) and a decrease in coarse grains in the varves shows that the ice had retreated along the entire North Basin at ∼70 vyr prior to the Lateglacial Interstadial. The estimated retreat rate is 70–114 m yr −1 although buried De Geer moraines, if annual, may indicate retreat of 120 m yr −1 with a ≥3 year stillstand at a recessional moraine halfway along the basin. The glacier then retreated north of the lake basin, becoming land-terminating and retreating at 92.5–49 m yr −1 . The northernmost core has a varve sequence ending at least 111 vyr after the other core chronologies, due to the increased proximity to remnant ice in the catchment uplands into the early Lateglacial Interstadial. We show that almost all of the glacier retreat in the Windermere catchment occurred before the abrupt warming at the onset of the Lateglacial Interstadial, in keeping with similar findings from around the Irish Sea Basin, and suggesting a similar retreat timescale for other radial valley glaciers of the Lake District Ice Cap. The seismic and core evidence also show the potential for a much longer varve chronology extending at least 400 and potentially over 1000 vyr further back into Heinrich Stadial 1 (18–14.7 ka BP), suggesting that glacier retreat in the Windermere valley initiated at least before 15.5 ka BP and perhaps 16 ka BP. Highlights: New pre-Lateglacial Interstadial varve sequence from lake cores, Windermere, UK. Varves record the deglaciation of the catchment into the Lateglacial Interstadial. Glacier retreat rate along lake basin up to 120 m yr −1 ; retreat into valleys ∼49 m yr −1 . Varves from the northernmost core persist ≥150 yr into the Lateglacial Interstadial. Seismic and core evidence for a much longer varve record from Heinrich Stadial 1.