Weichselian history of the Fennoscandian Ice Sheet centre, Kvarken archipelago, western Finland—flow shifts, thermal regime and deglaciation

Rapid thaw of the Earth's cryosphere in response to anthropogenic warming highlights the need to identify and understand the contrasting signatures of past ice‐sheet stability and collapse. The Kvarken archipelago, western Finland, at the centre of the former Fennoscandian Ice Sheet (FIS), has been designated a UNESCO World Heritage site in recognition of the exceptional preservation of glacial landforms exposed by rapid (10 mm a −1 ) postglacial isostatic uplift. Detailed mapping of hard and soft glacial bedforms allows reconstruction of ice sheet dynamics over the last glacial cycle. Striation orientation measurements ( n ~ 1100) were made at 700 sites on Precambrian gneiss and granite bedrock surfaces around the present shorelines. The striation data are classified into relative age categories and broader striation arrays to identify ice‐flow paths and then integrated with other directional indicators from overlying glacial sediments and landforms. The full data set reveals a sequence of six ice‐flow stages, which are then placed in a revised event and chronostratigraphic model for the wider Ostrobothnia region. After a major glacial erosion phase in Kvarken in late MIS 6, ice sheets returned in MIS 4 and MIS 3. During brief sliding phases, till sheets were deformed and eroded, but cross‐striations indicate that bedrock erosion was locally weak (<0.5 m) until after the last glacial maximum. At 11.6 ka, summer temperatures in Fennoscandia rose abruptly by 3–6 °C, and large volumes of meltwater reached the glacier bed. At 10.7–10.5 ka, a fast ice‐flow phase, the Gävle Oscillation, developed in the Bothnian basin. A <400‐year pulse of intense subglacial activity began in Kvarken, with abrupt and often pronounced switching of flow, production of new striation arrays, widespread erosion of till and bedrock and the formation of new soft bedforms. Existing till sheets were extensively glaciotectonically deformed into a young set of ribbed moraines. Local ice‐flow events led to the development of low‐relief megaflutings, drumlins and flutings. Bedrock was hydraulically damaged by large discharges of pressurized meltwater flowing in subglacial meltwater corridors, generating dense concentrations of large boulders. The ice margin retreated at 200–700, even 1000 m a −1 , with deposition of De Geer moraines in water depths of 220 m, before final deglaciation at 10.4–10.3 ka. Our Kvarken case study indicates that after slow erosion beneath stable ice sheet centres in MIS 4 and MIS 3/2, abrupt Early Holocene warming triggered ice sheet collapse and brought profound changes at the ice sheet bed over centennial time scales.