Sediment transport by sea ice in the Chukchi and Beaufort Seas: Increasing importance due to changing ice conditions?

Sediment-laden sea ice is widespread over the shallow, wide Siberian Arctic shelves, with off-shelf export from the Laptev and East Siberian Seas contributing substantially to the Arctic Ocean's sediment budget. By contrast, the North American shelves, owing to their narrow width and greater water depths, have not been deemed as important for basin-wide sediment transport by sea ice. Observations over the Chukchi and Beaufort shelves in 2001/02 revealed the widespread occurrence of sediment-laden ice over an area of more than 100,000 km² between 68 and 74°N and 155 and 170°W. Ice stratigraphic studies indicate that sediment inclusions were associated with entrainment of frazil ice into deformed, multiple layers of rafted nilas, indicative of a flaw-lead environment adjacent to the landfast ice of the Chukchi and Beaufort Seas. This is corroborated by buoy trajectories and satellite imagery indicating entrainment in a coastal polynya in the eastern Chukchi Sea in February of 2002 as well as formation of sediment-laden ice along the Beaufort Sea coast as far eastward as the Mackenzie shelf. Moored upward-looking sonar on the Mackenzie shelf provides further insight into the ice growth and deformation regime governing sediment entrainment. Analysis of Radarsat Synthetic Aperture (SAR) imagery in conjunction with bathymetric data help constrain the water depth of sediment resuspension and subsequent ice entrainment (>20 m for the Chukchi Sea). Sediment loads averaged at 128 t km^–2, with sediment occurring in layers of roughly 0.5 m thickness, mostly in the lower ice layers. The total amount of sediment transported by sea ice (mostly out of the narrow zone between the landfast ice edge and waters too deep for resuspension and entrainment) is at minimum 4×10⁶ t in the sampling area and is estimated at 5–8×10⁶ t over the entire Chukchi and Beaufort shelves in 2001/02, representing a significant term in the sediment budget of the western Arctic Ocean. Recent changes in the Chukchi and Beaufort Sea ice regimes (reduced summer minimum ice extent, ice thinning, reduction in multi-year ice extent, altered drift paths and mid-winter landfast ice break-out events) have likely resulted in an increase of sediment-laden ice in the area. Apart from contributing substantially to along- and across-shelf particulate flow, an increase in the amount of dirty ice significantly impacts (sub-)ice algal production and may enhance the dispersal of pollutants.