Evidence for Arctic paleothermokarst and melt features controlled by sea-level rise in regional sub-bottom profile data

Permafrost extends offshore in the Arctic as submarine permafrost. Near the edge of stable, continuous permafrost offshore, fresh groundwater flux contributes to regions of actively deforming thermokarst. Large-magnitude (up to ~30 m) sinkholes have been observed to form over less than a decade, posing a significant threat to offshore infrastructure. Additionally, having recently been discovered in sub-Arctic environments, thermokarst formation may be possible in a wider range of conditions than previously thought. It is critical to understand the evolution of degrading submarine permafrost, thermokarst, and the critical parameters necessary to predict the locations and magnitudes of seabed impacts.

We present acoustic sub-bottom profile (3.5 kHz Chirp) data collected in the Canadian Beaufort Sea, highlighting observations just offshore of a well-studied, actively deforming thermokarst field near the shelf edge. We map a seismic horizon that marks the top of ice-bearing sediment in the active thermokarst region; seaward, this horizon becomes a choppy unconformity punctuated by low-amplitude reflections that we interpret as fluid escape pathways. Beneath this horizon, an acoustically transparent layer persists seaward until characteristically and abruptly gaining coherent layering. We map the seaward edge of the acoustically transparent layer and interpret it as paleothermokarst. Laminated shallow seismic stratigraphy and a deep continuous acoustic basement surface are also characteristic of the interpreted paleothermokarst zone and absent from active thermokarst regions. The seaward edge of interpreted paleothermokarst has remarkably consistent seafloor depths at our mapped crossings (-372 m ± 34 m), suggesting an influence of depth-related processes controlling this edge. Lowstand sea level (-120 m) and potentially colder bottom-water temperatures may have allowed lowstand thermokarst to form ~187 m deeper than the thermokarst along the Canadian Beaufort margin today (which has seaward edge depths of -185 m ± 24 m). We suggest that the locus of active thermokarst (de)formation has moved landward over time in response to rising sea level, and expect the most active deformation at the landward edge of the thermokarst field.