Diatoms, a group of unicellular algae, are limited to the upper 100 m of the water column, due to their light requirement. Benthic diatoms, consequently, are restricted to the continental shelf, and primarily the inner shelf, coasts, and estuaries. Planktonic diatoms occur on the shelf (neritic) and in deeper waters (pelagic)—certain genera are restricted to one region while others occur in both. Salinity appears to exercise an important control on these distributions, so that a salinity front frequently produces a sharp boundary between populations. The shelf-slope break is usually associated with a sharp salinity gradient, where low-salinity shelf waters encounter a high-salinity oceanic current. The result is that benthic and low-salinity planktonic diatoms characterize shelf sediments, while higher-salinity planktonics and an absence of benthics characterize the slope and ocean basin sediments.
Two techniques for identifying the shelf-slope break are proposed, and examples given from the Bering Sea. The first involves use of the ratio of % benthic specimens to % pelagic-planktonic specimens in a series of sediment samples. The correlation of this ratio with depth is r = .85. A second technique uses quantitative population counts along a shelf-to-basin transect. In this case the shelf-slope break and its associated transition zone are seen to exert an ecologic control on species abundance. Other work in the literature, while not directly addressed to this question, suggests that similar results would be obtained in the Miocene diatomaceous sections of the U.S. East Coast and Japan.
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The Shelfbreak: Critical Interface on Continental Margins
The shelfbreak is that point where the first major change in gradient occurs on the outermost edge of the continental shelf. Although this environment delimits the boundary between two principal and well-defined provinces, the continental shelf and slope - and thus is of the first order of importance on continental margins - it has received surprisingly little specific attention in either modern oceans or in the rock record. This volume, the first compendium dedicated specifically to the shelfbreak, was derived from an SEPM Research Symposium convened at the joint Annual Meeting of SEPM and AAPG on June 2, 1981. The material is organized in a manner to illustrate examples of the shelfbreak in both modern oceans and the rock record.