Offshore murtoos indicate warm-based Fennoscandian ice-sheet conditions during the B?lling warming in the northern Gulf of Riga, Baltic Sea

Glacial landforms provide invaluable information on the dynamics and extent of past ice sheets. In planform view, murtoos are triangular-shaped subglacial landforms that have recently been described from areas previously covered by the Fennoscandian Ice Sheet and whose formation has been linked to meltwater activity. We document the first occurrence of offshore murtoos and associated eskers in a high-resolution multibeam and subbottom profile dataset from the northern Gulf of Riga, Baltic Sea. In this study area, the murtoos appear to be longer (range 43-748 m), narrower (range 12-394 m) and less high (range 0.5-7.8 m), on average, compared to those previously reported on land, although this may be partly due to post-glacial sediments that smooth the present-day seafloor topography and may have buried smaller murtoos. The direction of murtoo longitudinal axes suggests a local NNE-SSW paleo-ice stream direction. In sub-bottom boomer profiles, the murtoos are characterized by moderate amplitude, sub-parallel discontinuous internal reflectors, indicating a varying sandy diamicton and sand composition, which is compatible with the previously interpreted mode of deposition by pulsed subglacial meltwater flow. Eskers that are found in close association with the murtoos are shorter (mean length 538 m) and more curved than their counterparts on nearby land areas, and they lack a general alignment. In boomer profiles, the eskers are characterized by low-amplitude, sub-parallel continuous reflectors, suggesting a better-sorted sandy composition. The curved and poorly aligned external form of the eskers suggests deposition under restricted subglacial drainage conditions. We conclude that murtoos and related landforms in the northern Gulf of Riga were formed during the Bolling warming at ca. 14.5-14.1 cal kyr BP, when large amounts of meltwater were delivered to the ice-sheet bed.