ALBERT

Description: Submarine permafrost is usually created by the inundation of terrestrial permafrost by seawater. The inundation of permafrost follows coastal erosion or relative sea level rise. Low modern sea level rise rates in Siberia mean that coastal erosion is the main mechanism of formation of submarine permafrost. Coastal sections composed of fine-grained sediments that have high ground-ice contents, such as the long Yedoma coastline of eastern Siberia, are especially vulnerable to mechanical and thermal erosion processes [Romanovskii et al., 2004]. When such frozen soft sediments thaw and/or erode, then the state of the permafrost is determined by the transition from sub-aerial to submarine conditions, and the processes that accompany this transition. These include removal of the upper horizons of material, sediment dynamics along the beach and shore face profile, saltwater diffusion, changing thermal regime and sea ice dynamics. For example, driven by the influence of warm and salty seawater, permafrost begins to thaw once inundated due to thermal and chemical degradation. As a result, submarine permafrost degradation may be rapid near the coast, and shows a spatially variable dependence on this set of processes [Overduin et al., 2007]. Our objectives are to investigate changes in coastal geomorphology in combination with geophysical investigations of submarine permafrost distribution in the near-shore zone (〈 10 m water depth), in order to infer which processes dominate permafrost degradation in this highly dynamic coastal setting.

Description: This study focuses on a coupled understanding of coastal erosion and submarine permafrost dynamics at 3 arctic sites, where long-term observations of coastal change, historical subsea permafrost core data from the 1980s and recently obtained geophysical data of subsea permafrost are available. In this context, observations of changes in coastal geomorphology are coupled with geophysical observations along a historic submarine sub-bottom profiles. The rate of permafrost degradation in the littoral zone is controlled by a number of factors. The sea bottom water temperature and salinity control warming and salt penetration into the sediment column. Sediment deposition, re-suspension and transport by wave action and entrainment in ice are important in determining initial rates of degradation in the water depths where wave cycles reach the sea bed. Where the water depth is less than the sea ice thickness, bottom-fast ice (BFI) forms and affects the sea bed. The duration of inundation controls the length of time over which these factors influence the sea bed and increases with seaward distance from the coastline. Thus, the rate of coastal retreat affects the inclination of the IBPF table within the sediment. If erosion is rapid, and permafrost degradation rates in the littoral zone are negligible, the permafrost table lies close to the sea bed (upper figure A). If other factors are similar, a low coastal retreat rate leads to a steeper inclination of the IBPF table (middle figure B). Observations show inclinations between 0.14 and 2° below horizontal [Overduin et al., 2007] that vary between and within sites, suggesting that the relative importance of factors affecting IBPF table inclination varies spatially between sites and over time at each site (e.g. lower figure C). Here we present observations of the geoelectrical resistivity of the seabed, and compare interpreted IBPF positions from this observations with observations from the literature. We include near shore (〈 10 m water depth) IBPF table inclinations determined by probing, drilling and temperature and their dependence on coastline position change rates.

Description: Coastal erosion and flooding transform terrestrial landscapes into marine environments. In the Arctic, these processes inundate terrestrial permafrost with seawater and create submarine permafrost. Permafrost begins to warm under marine conditions, which can destabilize the sea floor and may release greenhouse gases. We report on the transition of terrestrial to submarine permafrost at a site where the timing of inundation can be inferred from the rate of coastline retreat. On Muostakh Island in the central Laptev Sea, East Siberia, changes in annual coastline position have been measured for decades and vary highly spatially.We hypothesize that these rates are inversely related to the inclination of the upper surface of submarine ice-bonded permafrost (IBP) based on the consequent duration of inundation with increasing distance from the shoreline. We compared rapidly eroding and stable coastal sections of Muostakh Island and find permafrost-table inclinations, determined using direct current resistivity, of 1 and 5 %, respectively. Determinations of submarine IBP depth from a drilling transect in the early 1980s were compared to resistivity profiles from 2011. Based on borehole observations, the thickness of unfrozen sediment overlying the IBP increased from 0 to 14m below sea level with increasing distance from the shoreline. The geoelectrical profiles showed thickening of the unfrozen sediment overlying ice-bonded permafrost over the 28 years since drilling took place. We use geoelectrical estimates of IBP depth to estimate permafrost degradation rates since inundation. Degradation rates decreased from over 0.4m/a following inundation to around 0.1m/a at the latest after 60 to 110 years and remained constant at this level as the duration of inundation increased to 250 years. We suggest that long-term rates are lower than these values, as the depth to the IBP increases and thermal and porewater solute concentration gradients over depth decrease. For the study region, recent increases in coastal erosion rate and changes in benthic temperature and salinity regimes are expected to affect the depth to submarine permafrost, leading to coastal regions with shallower IBP.