Observations of frazil ice formation and upward sediment transport in the Sea of Okhotsk: A possible mechanism of iron supply to sea ice

In the Sea of Okhotsk, sediment incorporation, transport and release by sea ice potentially plays important roles in the bio-related material (such as iron) cycle and ecosystem. The backscatter strength data of bottom-mounted Acoustic Doppler Current Profilers have suggested signals of frazil ice down to 30 m depth, and signals of upward sediment transport throughout the water column simultaneously in the region northeast of Sakhalin, with a water depth of ∼100 m. Such events occurred under turbulent conditions with strong winds of 10–20 m s−1. During such events, newly formed ice was present near the observational sites, shown by satellite microwave imagery. Sediment dispersion from the bottom occurred in association with strong currents of 1.0–1.5 m s−1. During these events, the mixed layer reaches near the bottom due to wind-induced stirring, inferred from the high frequency component of vertical velocity. Thus the winter time turbulent mixing brings re-suspended sediment up to near the ocean surface. This study provides the first observational evidence of a series of processes on the incorporation of sedimentary materials into sea ice: sedimentary particles are dispersed by the strong bottom current, subsequently brought up to near the surface by winter time mixing, and finally incorporated into sea ice through underwater interaction with frazil ice and/or flooding of sea ice floes. This wintertime incorporation of bottom sediment into sea ice is a possible mechanism of iron supply to sea ice which melts in spring, and releases bio-reactive iron into the ocean.