Polygonal patterns, with dimensions and shapes similar to modern polygons covering extensive parts of the permafrost-dominated land areas of the Arctic and subarctic, exist on the seafloor of the eastern Laptev Sea. The mapped patterns in the Laptev Sea resemble macro-scale frost polygons associated with thermal-contraction cracking of the ground (Lachenbruch 1962). The formation of these permafrost-related landforms probably occurred during the Late Pleistocene when most of the Laptev Sea shelf was sub-aerially exposed and free from ice-sheet cover (Jakobsson et al. 2014). Postglacial sea-level rise transformed these areas from a periglacial permafrost landscape into a modern shallow shelf (Gavrilov et al. 2006).
Description
Large numbers of closed, multi-sided, roughly equidimensional seabed polygons are revealed by side-scan sonar imagery and sub-bottom profiles from the SE Laptev Sea (Fig. 1). These landforms are mapped in water depths between about 10 and 17 m over a maximum stretch of around 700 m. The total number of seafloor areas with similar patterns exceeded two dozen.
Geophysical images and profiles showing evidence of polygonal patterns in the southeastern part of the Laptev Sea. (a, c, e, f) Side-scan sonar images of polygonal patterns. Acquisition system Hydra-250/500. Frequency 500 kHz. (b) Sub-bottom profile across the polygonal patterns. VE×8. Acquisition system Innomar SES-2000 Standard. Frequency 8 kHz. (d) Sub-bottom profile across the polygonal patterns. VE×6. Acquisition system Innomar SES-2000 Standard. Frequency 6 kHz. (g) Bathymetric image of the study area. Data taken from digitized navigation charts (Nikiforov et al. 2004). (h) Location of study area (red box map from IBCAO v. 3.0).
The mapped polygons vary widely in shape and size. Their horizontal dimensions range from a few metres to some tens of metres and their bathymetric relief reaches 1 m (Fig. 1b, d). The geometrical shapes vary in planform from tetragonal (Fig. 1a), through pentagonal or hexagonal (Fig. 1e), to subrounded seabed elements (Fig. 1c). However, some of the mapped landforms have been broken down by modern erosional processes (Fig. 1e). The observed polygonal patterns can be divided into two types: high-centred polygons with trenches surrounding the polygon centre (Fig. 1a, c) and low-centred polygons consisting of elevated ridges that enclose central depressions (Fig. 1e).
The polygonal seabed landforms mapped closest to Dmitry Laptev Strait are the most reworked (Fig. 1f). As a result, these are characterized by small vertical dimensions with generally not more than 30 cm bathymetric relief.
The horizontal dimensions of coastal polygonal nets commonly vary from approximately 5 m up to 40 m (French 2007), and the major known types of multi-sided frost polygons are either low-centred or high-centred (Minke et al. 2007). This general characterization is consistent with the landforms mapped in the southeastern part of the Laptev Sea.
Sub-bottom profiles reveal that the mapped seafloor areas with polygonal features in the eastern Laptev Sea are often accompanied by zones below the seafloor in which it is not possible to trace continuous sub-bottom reflections (Fig. 1b, d). This is probably due to a plurality of vertical cracks, genetically linked to the polygonal patterns. These cracks, at least close to the seafloor, are now ice-free and are infilled by subsequent sedimentation.
Interpretation
The mapped polygonal seafloor patterns in the Laptev Sea are interpreted to comprise fragments of a relict sub-aerial polygonal permafrost relief. Most of the East Siberian Arctic shelf appears to have escaped glaciation during the Last Glacial Maximum (Jakobsson et al. 2014). Instead, the exposed shelf experienced cold and dry atmospheric conditions, ideal for the formation and further development of permafrost-related landforms.
The most widespread permafrost-related landforms are the polygons that cover c. 250 000 km2 of the modern circum-Arctic region (Minke et al. 2007). The origin of polygonal patterns is closely connected with frost cracking (Lachenbruch 1962), low temperatures and low precipitation (Romanovskii 1977).
The environment of the eastern Laptev Sea changed 7–8 ka BP, when the transgressing sea level reached areas with modern water depths of 10–17 m (Gavrilov et al. 2006). At about 4–5 ka BP, sea level stabilized and environmental conditions became similar to those of today.
At present, the southeastern part of the Laptev Sea is strongly influenced by heat and freshwater influx from the Lena and Yana rivers. The water column is therefore stratified and has a high resistance to vertical thermohaline- and wave-mixing during the relatively short sea-ice-free period during summer. Similar conditions probably persisted during most of postglacial time; it is therefore likely that dynamic bottom erosion during the Holocene transgression only played a minor role (Dudarev et al. 2014). For this reason, the flooded seafloor patterns have been preserved relatively intact. This is exemplified by the polygonal elements mapped on the seafloor of the southeastern part of the Laptev Sea, which are of the same kind as those found along the Arctic coasts today. However, the heat brought by the Lena and Yana river flows (Dudarev et al. 2008) has resulted in some seafloor polygon destruction and ‘smearing’ of the mapped permafrost landforms in the easternmost area of Dmitry Laptev Strait (Fig. 1f).
- © 2016 The Author(s). Published by The Geological Society of London. All rights reserved
