The distribution and characterization of gas domes in Lumparn Bay, Åland Islands, northern Baltic Sea

In 2016, approximately 1800 previously unobserved seabed domes were found in high-resolution acousticsoundings of the Lumparn Bay in the Åland Islands, northern Baltic Sea. As similar geological features have onlyrarely been reported in earlier literature globally, the aim of this study was to characterize the domes, to in-vestigate their geochemical composition, as well as to determine if they had any effects on the composition of thebenthic fauna.The domes were generally about 1 m high and had a diameter of 20–30 m, while the largest ones were up60 m in diameter. The domes resembled the initial processes in pockmark formation where gas is trapped underthe sediment surface, doming the sediment upwards. However, no pockmarks were found in the area, indicatingthat doming does not result in pockmark formation. High sulphur concentrations indicate hydrogen sulphideformation within the domefield area while large zones with acoustic blanketing (acoustic interference) alsoindicate 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 re-ference 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 lowlevels in the Lumparn Bay. The non-metric multidimensional scaling (NMDS) showed some, although not sig-nificant, differences between the benthic invertebrate communities of the domes and the reference sites.