SSM perspective
Background
The future climate evolution and the impact it might have on a repository for radioactive waste is important when assessing the long-term safety. In a project funded by SSM (Holmlund et al. 2016), bathymetric data from the Southern Quark area between Sweden and Åland, provided by the Swedish Maritime Administration, were analysed, as well as terrestrial data from a glacial morphological mapping campaign. A broad range of landforms could be observed in these data and the overall glacial morphology pointed towards a powerful glacial impact involving an abundance of meltwater. Traces of mass wasting on the bottom of the deep trough through the Southern Quark were found. The genesis, timing and size of these deposits are important to resolve in order to assess their potential impact on nuclear facilities in the Forsmark area.
This study examined the bottom conditions in the Southern Quark, with a particular focus on indications from the previous study by Holmlund et al. (2016) on submarine mass wasting. Furthermore, in order to increase the understanding of the underlying processes that have shaped the glacial traces observed in the Southern Quark area, a study were performed on present glaciers in northern Sweden.
Results
The mass wasted rock debris affects an area >400 m wide of the seafloor in front of the steep wall that gave away. Rock slides on land occur regularly as a consequence of long-term rock disintegration from, for example, fluvial weathering on rather steep slopes (Abele, 1994). In the submarine environment, mass movements of sediments are common and may be triggered by mechanisms such as, for example, high sedimentation rates, glacial erosion, sea-level change and tectonic movements (Leynaud et al., 2009). The occurrence of submarine rock slides is, however, much less documented and therefore also less known. In general, the survey of the area shows strong bottom currents and a very dynamic oceanography in the investigated area today and it is likely that the area were equally dynamic during the deglaciation. Beneath the top sediment layers are buried channels that are probably derived from deglaciation and a plentiful supply of melt water. Thus, a plausible time when the rock slide could have occurred is during the deglaciation, but the genesis of the rock slide remains unknown.
The results from the present glaciers in northern Sweden show clearly the effect of the thermal conditions and they also show both that the boundary between basal frozen and basal melting conditions is sharp and it may move its position over time due to changes in ice thickness and climate change. In an analogue to basal conditions beneath the Weichselian ice sheet there may have been short distances between high erosion rates and hardly no erosion at all. The transition zone between these two different environments is not stable over time and is rather expected to have varied in its location.
Relevance
The project intends to increase the knowledge regarding the significance of the marine collapse structures outside Forsmark (Southern Quark). Knowledge of any post-glacial earthquakes (PGF) that have or may occur near a final repository for spent nuclear fuel is of high interest to SSM since large earthquakes can induce secondary fault movements in the repository volume which could damage deposited canisters. In addition, the identification of stronger PGFs would mean that SKB’s assumed earthquake frequency needs to be improved.
Future potential erosion during several glacial cycles are an important climate related process to consider for a repository of spent nuclear fuel since a lowering of the bedrock surface can affect the assessment of future human intrusion as well as the assessment of radionuclide transport. The study of Scandinavian alpine glaciers aims to improve our understanding of ice behaviour and sharp unconformities in erosion rates.
Need for further research
A geohazard from large mass movements on the seafloor, whether of rocks or sediments, is the generation of a tsunami (Ioualalen et al., 2010). A numerical simulation of a potential tsunami caused by the identified rock slide in the Southern Quark comprises one of several topics for follow-up studies that could be made based on the material acquired in this study, as it may have bearing on present nuclear facilities in the Forsmark area.