THE SURFACE OCEAN PRODUCTIVITY RESPONSE OF DEEPER WATER BENTHIC FORAMINIFERA IN THE ATLANTIC-OCEAN

We test the relationship of deep sea benthic foraminiferal assemblage composition to the surface ocean productivity gradient in the low lati tude Atlantic Ocean using 81 surface sediment samples from a water dep th range between 2800 and 3500 m. The samples are selected so that the surface ocean productivity gradient, controlling the flux of organic carbon to the seabed, will be the most important environmental variabl e. The first two principal components of the assemblage data account f or 73% of data variance and are clearly linked to the productivity gra dient across the Atlantic. These components, show that under higher pr oductivity the assemblages contain a higher abundance of Uvigerina per egrina, Melonis barleeanum, Globobulimina spp. and other taxa with pro bable infaunal microhabitats. Alabaminella weddellensis, a species lin ked to episodic phytoplankton debris falls, is also important in these assemblages. As productivity decreases there is a regular shift in as semblage composition so that low productivity assemblages are dominate d by Globocassidulina subglobosa and several Cassidulina species along with Epistominella exigua. We hypothesize that these taxa are epifaun al to very shallow infaunal since nearly all organic carbon oxidation occurs near the sediment-water interface in low productivity settings. Discriminant function analysis of the foraminiferal assemblages, with groups selected on the basis of surface ocean productivity, shows cle ar separation among five productivity levels we used. This analysis de monstrates that productivity variations have a strong influence on ass emblage composition. Finally, we used two groups of samples from the R io-Grande Rise representing water depths from 2007 to 2340 m and 2739 to 3454 m to test for effects produced by changing water depth. All th ese samples are from a low productivity region and represent nearly id entical environmental conditions. Although the low productivity nature of all the Rio-Grande Rise samples is obvious, there are assemblage d ifferences between our depth groups. We cannot account for the assembl age differences with changes in organic carbon flux, dissolution effec ts or other physical/chemical properties of the ocean. Thus there are as yet unidentified factors related to water depth which cause some as semblage variation in the low productivity setting we investigated.