GREENHOUSE WORLD DEEP-SEA BENTHIC FORAMINIFERA IN OPEN OCEANS: LIMITS OF UNIFORMITARIANISM?

Studies of deep-sea foraminifera in open-ocean regions from the Arctic Ocean to the Equatorial Pacific and Indian Oceans have documented that the food-supply from primary producers in surface waters is of prime importance for the benthic foraminifera on the sea floor. In such open-ocean regions far from continental margins, growth and reproduction of biota including foraminifera is severely food-limited. Not only the averaged, total annual amount of organic carbon deposited to the sea floor, but also the seasonal variability in productivity influence the total abundance of benthic foraminifera, as well as the species composition of the assemblages: there is strong bentho-pelagic coupling in the present day oceans. It is an open question, however, whether this present situation is analog to deep-sea benthic foraminifera in the oceans of the Paleogene and Late Cretaceous, when deep-water temperatures averaged about 8-12^oC. We would expect metabolic rates to have been much higher, and if oceanic productivity had been similar to today this would have resulted in faunas appearing to be oligotrophic. There is evidence that surface productivity was, in fact, lower in these warm oceans, but Late Cretaceous-Paleogene open ocean deep-sea benthic foraminifera are comparable to Recent higher productivity faunas. Additional evidence for decoupling of the bentho-pelagic linkage is seen in records of the Cretaceous-Tertiary boundary. At this time surface water productivity declined severely, but open ocean deep-sea benthic foraminifera were much less affected than shallower faunas, even though their present counterparts are strongly dependent upon surface productivity. The lack of bentho-pelagic coupling in the oceans of the Greenhouse World might be explained by different processes of carbon transfer from surface to deep oceans than in an Icehouse World, or by a greater importance of primary productivity of food particles on the ocean floor itself, by chemosynthetic bacteria.