Bathymetric profiles, bottom-camera photographs, surface-sediment samples, and biological samples were obtained through a 44 m long hydrohole in the Canadian Ice Island as it drifted over the Canadian polar margin from western Ellesmere Island to Meighen Island. Continuous bathymetric profiles show a 300 km wide shelf with a narrow, dissected, shallow (< 200 m) inner shelf and a deep (300–800 m), sloping outer shelf. Extensive areas of siliceous sponge communities are present on the seafloor beneath the polar pack ice on the central shelf. A clear biological zonation is evident: above 130 m, large living sponges support a diverse benthic community and form reef mounds up to 10 m high; from 130 to 150 m, there are few living sponges, but extensive mats of sponge spicules are present; from 150 to 300 m, there are few, small living sponges, and radiocarbon dating of mud-covered spicule mats indicates that formerly extensive sponge communities have died during the past 500–2000 years. This zonation may reflect a relative-sea-level depth increase of ~40–60 m during the past 1000 years. Alternatively, the zonation may indicate a 20–30 m upward shift in the warm Atlantic water layer during the past 500–1000 years, perhaps reflecting global climatic warming. The study of factors that determine the distribution and growth of the modern polar sponge communities has important implications for understanding the paleoecology of Paleozoic demosponge–bryozoan bioherms in the eastern Canadian Arctic Islands.

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