GMAT 考满分题库

Despite an abundance of major nutrients in the surface waters of parts of the ocean, extremely low concentrations of dissolved iron are believed to play a crucial role in limiting the biological productivity of these remote regions. Phytoplankton, the basis of freshwater food chains and all aerobic life as well as the source of most of Earth's atmospheric oxygen, require iron for various biochemical processes. Thus, a lack of iron in surface waters has detrimental effects.

In temperate and tropical oceans, iron reaches surface waters via the dissolution of eolian- transported continental dust. Previously, little was known about iron distribution in the surface waters of non-temperate oceans such as the Arctic Ocean. Recent advances, however, have resulted in an analytical methodology capable of determining iron concentrations in ambient surface waters. Studies indicate that concentrations across the Arctic Basin are relatively high and quite variable, ranging from 3.2 nM in the western Arctic to 0.75 nM in the Nansen Basin.

The highest values of iron concentration occur in regions with ice floes containing significant quantities of surface sediment. The hypothesis that ice-rafted sediment is the source of high iron values is bolstered by the presence of large amounts of aluminum in the same regions. The entrainment of sediments from the edge of the basin into floes during the winter freezing process along with the subsequent advection and partial melting of the ice at the center of the basin provides a means of transporting reactive trace metals, such as iron, to the center of the basin. The partial melting of floes during the summer appears sufficient to transport high concentrations of iron to both surface and stratified waters. It seems, however, that any change resulting in the diminution of ice- edge freezing in winter might lead to significant changes in the nature and magnitude of primary productivity in the central Arctic.