Bottom currents interpreted from iceberg ploughmarks revealed by multibeam data at Tromsøflaket, Barents Sea

Abstract

Multibeam data have been acquired on eastern Tromsøflaket, a bank in the southwestern Barents Sea comprising a plateau (< 200 m depth) bounded by glacially eroded troughs. These new data reveal the morphology of Tromsøflaket in spectacular detail and show that much of the area has been intensively scoured by iceberg ploughmarks. Multibeam bathymetry and backscatter data together with samples and video data have been used to interpret the distribution of surficial seabed sediments. The general pattern of this distribution reflects the morphology. Coarse sediments occur on ridges and shallow bank areas, while finer sediments occur in depressions, on the slopes of the bank and in the deeper areas. One exception to this pattern is observed in the Sørøydjupet trough (ca. 300m depth) where coarse sediments occur. Recent sedimentation is dominated by mud and fine sand. Detailed examination of the multibeam backscatter data however reveals a high variability of sediments, especially in and around iceberg ploughmarks. We observe coarser sediments on the levees either side of the ploughmarks than inside the furrows. Moreover we frequently note asymmetries in the distribution of fine sediments across these ploughmark features. Drawing inspiration from previous studies of bottom currents which use the distribution of sediments across bedforms (e.g. sand waves) we have interpreted the direction of bottom currents on Tromsøflaket by examining the local distribution of fine sediments across iceberg ploughmarks, which are the most prominent bedform in this area. Further integration of our results with existing grain-size - current-speed models have allowed estimates of the bottom current speeds to be obtained. Such interpretations are particularly valuable in this area where availability of bottom current measurements sparse. Our approach makes valuable and novel use of multibeam data and results show good agreement with the limited set of available current data and models. We can identify near bottom pathways for the Norwegian Atlantic Current and Norwegian Coastal Current, the two major current systems in this region, and the patterns of circulation help to explain the sedimentation regime and identify recent erosion and deposition areas.