Abstract
Abstract. 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 14 m 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 0.4 m a−1 following inundation to around 0.1 m a−1 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.
Highlights
Submarine permafrost refers to solid earth material below modern sea level that has perennial temperatures below 0 ◦C
Our results indicate that permafrost degradation rates at Muostakh Island are consistent with those inferred for nearshore sites elsewhere on the Siberian shelf (Overduin et al, 2007, 2015)
Geoelectric surveys along the borehole profile 28 years later show that land has been inundated as a result of coastal erosion and suggest that the submarine permafrost created has degraded at mean rates of between 0.6 and 0.1 m a−1 over decades to centuries
Summary
Submarine permafrost refers to solid earth material below modern sea level that has perennial temperatures below 0 ◦C. Deglaciation after the Last Glacial Maximum led to sea-level rise in the Laptev Sea region more than 10 times as rapid as modern rates (Bauch et al, 2001), inundating most of the more than 1.5 million km East Siberian shelf region within several thousand years. Both processes (coastal erosion and sea-level rise) covered cold (less than −2 ◦C at the seasonal damping depth) and thick (hundreds of metres) permafrost with seawater and seasonal sea ice, separating it from the cold air with mean annual temperatures of less than −10 ◦C. Sediment bulk electrical resistivity increases by orders of magnitude between icebonded, freshwater sediment and ice-free, saline sediment (Scott et al, 1990), making it suitable for the detection of the ice-bonded permafrost table
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