Abstract. Knowledge of past variations in sea-ice extent is crucial
for understanding the relationship between climate change and changes in
sea ice. Diatom assemblages could be applied as a proxy for paleo-sea-ice
extent; this requires accurate information on the modern species that are
indicative of sea ice. Scanning electron microscope observations were
performed on modern diatom assemblages in sea ice, sinking particles, and
surface sediments in the Sea of Okhotsk. A sea-ice sample was collected in the
southwestern Sea of Okhotsk near Hokkaido island in February 2013.
Fragilariopsis cylindrus was the dominant diatom species in the sea-ice sample, accounting for 87 % of the total diatom assemblage. Time-series sediment traps were
deployed during 1998–2000 at two stations, M4 and M6, off Sakhalin island.
Total diatom fluxes ranged from 105 to 108 valves m−2 d−1 with noticeable seasonality. During the sea-ice covering period,
the total diatom flux decreased by 1 or 2 orders of magnitude. The highest
diatom fluxes were observed in spring and summer. The diatom species
composition in sinking particles also showed pronounced seasonal changes.
During summer and fall, the Shionodiscus trifultus group and Neodenticula seminae were the major diatom taxa. During the
sea-ice covering period, Fragilariopsis cylindrus and Bacterosira bathyomphala resting spores were abundant. Both the
sea-ice-related species showed similar flux patterns except for the spring
bloom after sea-ice retreat: F. cylindrus fluxes exhibited pronounced spring bloom peaks
of 108 valves m−2 d−1; in contrast, the fluxes of
Bacterosira bathyomphala resting spores during the spring bloom were 1 order of magnitude lower than
those of F. cylindrus. Surface-sediment core XP98-MC4 was obtained near station M6 sediment-trap site off Sakhalin island. The relative abundance of
Fragilariopsis cylindrus in the surface-sediment diatom assemblage was only 6.4 %, markedly lower
than that in the sediment-trap samples (43.4 %). In the surface sediment,
the relative abundances of diatom taxa with heavily silicified valves such
as B. bathyomphala resting spores, Shionodiscus variantius, and Thalassionema nitzschioides were greater than their relative abundances in
sinking particles.
Diatom fluxes in surface sediments of the Goban Spur continental margin, NE Atlantic Ocean
