The Bothnian Sea ice stream: early Holocene retreat dynamics of the south‐central Fennoscandian Ice Sheet

Abstract

The Gulf of Bothnia hosted a variety of palaeo‐glaciodynamic environments throughout the growth and decay of the last Fennoscandian Ice Sheet, from the main ice‐sheet divide to a major corridor of marine‐ and lacustrine‐based deglaciation. Ice streaming through the Bothnian and Baltic basins has been widely assumed, and the damming and drainage of the huge proglacial Baltic Ice Lake has been implicated in major regional and hemispheric climate changes. However, the dynamics of palaeo‐ice flow and retreat in this large marine sector have until now been inferred only indirectly, from terrestrial, peripheral evidence. Recent acquisition of high‐resolution multibeam bathymetry opens these basins up, for the first time, to direct investigation of their glacial footprint and palaeo‐ice sheet behaviour. Here we report on a rich glacial landform record: in particular, a palaeo‐ice stream pathway, abundant traces of high subglacial meltwater volumes, and widespread basal crevasse squeeze ridges. The Bothnian Sea ice stream is a narrow flow corridor that was directed southward through the basin to a terminal zone in the south‐central Bothnian Sea. It was activated after initial margin retreat across the Åland sill and into the Bothnian basin, and the exclusive association of the ice‐stream pathway with crevasse squeeze ridges leads us to interpret a short‐lived stream event, under high extension, followed by rapid crevasse‐triggered break‐up. We link this event with a c. 150‐year ice‐rafted debris signal in peripheral varved records, at c. 10.67 cal. ka BP. Furthermore, the extensive glacifluvial system throughout the Bothnian Sea calls for considerable input of surface meltwater. We interpret strongly atmospherically driven retreat of this marine‐based ice‐sheet sector.

Controls on the early Holocene collapse of the Bothnian Sea Ice Stream

“…A large negative surface mass balance (end‐of‐run values of −3.1 to −4.1 m a −1 m −2 ) is independently capable of driving complete Bothnian Sea retreat within a time frame matching the terrestrial clay varve chronology for deglaciation [ Stroeven et al ., ]. Such surface mass balance rates are in fact rather moderate compared to an estimated −5 to −9 m a −1 m −2 for the southern Laurentide Ice Sheet during early Holocene [ Carlson et al ., ], and the geomorphological record exhibits abundant evidence of high‐discharge, well‐connected subglacial meltwater drainage [ Greenwood et al ., ]. Implementation of stepwise reductions in surface mass balance, however, did not drive instantaneous response or catastrophic retreat, possibly due to the time‐integrated effect of surface mass balance (Figure d).…”

“…That there is only an MSGL record of ice streaming up to 620 km along our flow line, beyond which lies a separate group of small (often bedrock) drumlins and crag and tails with a flow direction offset from the MSGLs, suggests that ice streaming occurred as a distinct event once retreat into the Bothnian Sea had already begun. Furthermore, this ice stream event did not extend the full length of the Bothnian Sea [ Greenwood et al ., ]. We would therefore expect to see initially low flow velocities, with an abrupt shift to high velocities once the margin reached approximately 620 km.…”

“…The island of Åland itself has, in the past, been proposed as a “sticky spot” [ Holmlund and Fastook , ], acting to inhibit ice flux through the south‐central sector of the FIS, which has led to overly rapid retreat in previous modeling studies [ Clason et al ., ]. Slowdown of retreat approximately 450 km along flow line, just south of Härnösand Deep, emerges consistently from our model runs and is also indicated by the appearance of transverse wedge‐like ridges superimposed on MSGLs [ Greenwood et al ., ] and which record a zone of reduced flow and margin stability after a period of ice streaming. Since modeled grounding line retreat only slows across this zone when surface crevasse propagation is a component of the forcing, we implicate enhanced crevassing as a control on stability in this system.…”

Past water flow beneath Pine Island and Thwaites glaciers, West Antarctica

“…This cross-sectional shape has been associated with high-magnitude discharges of water (e.g. Bretz, 1923Bretz, , 1969Gupta et al, 2007;Larsen and Lamb, 2016), although the shape of the channels in Pine Island Bay, and accordingly their b values, may be unrepresentative where signif- Geomorphologically mapped channel network within the highresolution swath bathymetric data and modelled subglacial drainage network. The major flow routing pathways and subglacial lake locations calculated in (c) are displayed as red arrows and blue polygons in (d), respectively.…”

Bathymetric Properties of the Baltic Sea

“…Furthermore, the macro-scale bedrock topography of the Baltic is overlain by Quaternary glacial, post-glacial and modern sediments and landforms, whose local-scale morphology is superimposed on the underlying relief. High TRI values with a locally very patchy distribution in Figure 4c reflect a drumlin field (Greenwood et al, 2017) superimposed on an otherwise rather low relief surface. Here the boundaries of the multibeam dataset that reveals these drumlins is also clearly seen in the 10 TRI-pattern highlighting the need to consider the underlying source data when interpreting seafloor morphology using DBMs.…”