Abstract. Deglaciation in Svalbard was followed-up by seawater ingression and the deposition of marine (deltaic) sediments in fjord valleys, while elastic rebound resulted in fast land uplift and the exposure of these sediment to the atmosphere, therefore the formation of epigenetic permafrost. This was then followed by the accumulation of aeolian sediments, which froze syngenetically. The permafrost was drilled in the east Adventdalen valley, Svalbard, 3–4 km from the maximum up-valley reach of post-deglaciation seawater ingression, and its ground ice was measured for chemistry. While ground ice in the syngenetic part is basically fresh the epigenetic part reveals a frozen fresh-saline water interface (FSI), with chloride concentrations increasing from the top of the epigenetic part (depth of 5.5 m) to about 15 % that of seawater at 11 m. We applied a one-dimensional freezing model in order to examine the rate of top-down permafrost aggradation, which could accommodate with the observed frozen FSI. The model examined permafrost development under different scenarios of mean average air temperature, water-freezing temperature and the degree of pore-water freezing. We found that even at the relatively high temperatures of the Early to mid-Holocene, permafrost could aggrade quite fast, e.g. down to 15 to 33 m in 200 years, therefore allowing freezing of the fresh-saline water interface despite of the relatively fast rebound rate and the resultant increase in topographic gradients toward the sea. This could be aided by non-complete pore water freezing, which possibly lead to slightly faster aggradation, resulting in the freezing of the entire marine section at that location (23 m) within less than 200 years. We conclude that freezing should have occurred immediately after the exposure of the marine sediment to atmospheric conditions.
Permafrost in circum‐polar regions has been recently undergoing thawing, with severe environmental consequences, including the release of greenhouse gases and amplification of global warming. Although highly important, direct methods of tracking thawing hardly exist. In a research study conducted at Adventdalen, Svalbard, we identified a permafrost radioisotope fingerprint, and show that it can be used to track thawing. Ratios of long‐ to the shorter‐lived radium isotopes are higher in ground ice than in active layer water, which we attribute to the permafrost closed system and possibly to the long residence time of ground ice in the permafrost. Also, daughter–parent 224Ra/228Ra ratios are lower in permafrost than in the active layer. These fingerprints were also identified in a local stream, confirming the applicability of this tool to tracing thawed permafrost in periglacial watersheds. A combination of radium isotope ratios and 3H allowed the identification of recent intra‐permafrost segregation processes. The permafrost radium fingerprint should be applicable to other permafrost areas, which could assist in regional quantification of the extent of permafrost thawing and carbon emissions to the atmosphere.
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