LAUSR

Abstract In 2016, approximately 1800 previously unobserved seabed domes were found in high-resolution acoustic soundings of the Lumparn Bay in the Aland Islands, northern Baltic Sea. As similar geological features have only rarely been reported in earlier literature globally, the aim of this study was to characterize the domes, to investigate their geochemical composition, as well as to determine if they had any effects on the composition of the benthic fauna. The domes were generally about 1 m high and had a diameter of 20–30 m, while the largest ones were up 60 m in diameter. The domes resembled the initial processes in pockmark formation where gas is trapped under the sediment surface, doming the sediment upwards. However, no pockmarks were found in the area, indicating that doming does not result in pockmark formation. High sulphur concentrations indicate hydrogen sulphide formation within the dome field area while large zones with acoustic blanketing (acoustic interference) also indicate the occurrence of gas, most likely formed in biogenic processes. Concentrations of chromium, copper, cadmium, zinc, lead, mercury and arsenic showed some differences in character between the domes and reference sites. In general concentrations varied less in the domes and were somewhat higher in the reference sites. Comparison with sediment data from the Gulf of Finland revealed that heavy metal concentrations were on low levels in the Lumparn Bay. The non-metric multidimensional scaling (NMDS) showed some, although not significant, differences between the benthic invertebrate communities of the domes and the reference sites.