Mapping bottomfast sea ice in Arctic lagoons using Sentinel-1 interferometery

Abstract

Sea ice is an important component of Arctic coastal ecosystems. Where the water is shallow enough, it can extend all the way to the seafloor and become bottomfast sea ice (BSI), the lateral extent of which depends upon ice thickness and the regional nearshore slope. Sea ice thickness is a well-known indicator of climate change in the Arctic and in areas with gently sloping seafloors, we expect the extent of BSI to be a sensitive indicator of changes in ice thickness. Contact with the seafloor can help cool and aggregate subsea permafrost and restrict under-ice habitats. It also prevents or reduces motion experienced by floating landfast ice in response to wind, ocean, and ice forcing. Bottomfast ice is in turn more stable than floating ice with implications for human activities on ice. BSI cannot easily be distinguished from floating landfast ice using optical imagery and synthetic aperture radar (SAR) is not typically able to penetrate to the bottom of saline ice. As a result, large-scale mapping of BSI has previously been limited to brackish waters near Arctic deltas, where (SAR) can detect the ice-water interface. However, recent work has demonstrated that SAR interferometry (InSAR) can be used to delineate BSI based on an absence of small-scale surface motion over time. Here, we utilize the Alaska Satellite Facility's Hybrid Pluggable Processing Pipeline (HyP3): A cloud-based infrastructure to process interferograms from the entire Sentinel-1 record over three lagoon systems across the Beaufort Sea coast of Alaska near Utqiagvik, Prudhoe Bay, and Kaktovik. We develop and test a mapping approach that discriminates bottomfast ice based on a near-zero gradient in interferometric phase change, which on floating lagoon ice is primarily caused by surface motion from tides and thermal stress. This enables the comparison of the date of onset, maximum extent, and seasonal evolution of BSI between the lagoons from 2016-2020. We also evaluate the use of electromagnetic sounding in tandem with in-situ drilling to verify BSI extent with greater detail. Based on this work, we argue that mapping BSI could significantly improve our understanding of Arctic lagoons in terms of detailed bathymetry, winter habitats, and saline stress on benthic communities, and the thermal regime of the underlying permafrost.