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A new and more precise bathymetric zonation is proposed based on the distribution of benthic foraminifers primarily from the northern continental slope and abyssal plain of the Gulf of Mexico. Ninety-nine bathymetric indicator species selected from the 328 foraminiferal species identified in this study form the basis for the bathymetric subdivisions. Fourteen benthic species have upper depth limits within the neritic zone. Thirty-two species, including four rare auxiliary species, have upper depth limits within the upper bathyal zone. Twenty-eight species, including five rare auxiliary species, have upper depth limits within the middle bathyal zone, while 19 species are characteristic of the lower bathyal zone. Six species have upper depth limits within the abyssal zone, supplemented by abundance values from 4 additional species.

Twenty-six of the 99 bathymetric indicator species are considered to be isobathyal forms and form the framework for the bathymetric zonation. Fifty-seven species show varying upper depth limits associated with the Mississippi River deltaic area. Of these, 43 species show depressed upper depth limits, whereas 14 have elevated upper depth limits.

The bathymetric distribution of foraminifers from more than 20 genera representing either successions of valid taxonomic species or morphologic gradations of single species (clines) are used as auxiliary bathymetric indicators.

Six general faunal trends provide supplemental ecologic information. A dramatic increase occurs in foraminiferal/ostracode ratios with distance from shore and with increase in water depth. Radiolarians are of greatest abundance in bottom sediments from the lower bathyal and abyssal zones. Numbers of benthic species increase with increasing depth from shore into the bathyal zone; beyond this water depth the numbers decrease somewhat. Agglutinated foraminifers become more abundant with depth, increasing from about 5 percent of the benthic population in the upper bathyal zone to values of about 15 percent or more in many samples from the lower bathyal and abyssal zones. Planktonic foraminifers show a general increase in abundance to values of 50 percent of the foraminiferal assemblages in the lower neritic zone and to more than 90 percent in the lower bathyal and abyssal zones. Several planktonic species develop tests with a thick crystalline crust in upper bathyal and deeper water depths.

Benthic foraminiferal distribution in either clastic or carbonate environments also provides supplemental ecologic information. At least 20 species are more characteristic of clastic facies in the western Gulf of Mexico than in eastern carbonate facies, whereas four species are more characteristic of carbonate facies.

Several faunal-geochemical boundary associations were noted. Bathymetric faunal changes occur at a temperature boundary at a water depth of 3,000 feet in the Gulf of Mexico, a prominent oxygen-minimum zone within the upper bathyal zone, and an Eh gradient off the Mississippi delta. It is clear that any one geochemical factor does not control the bathymetric zonation observed in the Gulf of Mexico. Hydrostatic pressure is suggested to represent a primary limiting factor controlling benthic foraminifer bathymetric distribution in view of the similar depth zonations of benthic foraminifers in many different oceanic water masses.

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