BENTHIC FORAMINIFERA AND EARLY EOCENE HYPERTHERMAL EVENTS (SE ATLANTIC OCEAN)

Upper Paleocene - lower Eocene sediments recovered at 5 sites over a 2 km depth transect in the SE Atlantic (Walvis Ridge; 4700-2500 m) during ODP Leg 208 provide a unique opportunity to study the relation between the Paleocene-Eocene Thermal Maximum (PETM) and the Benthic Foraminiferal Extinction Event (BFE). The P/E boundary was marked by a prominent dissolution layer (clay layer), thickening from the shallowest to the deepest site. As a result, benthic assemblages at the shallowest site were poor or absent over about 10 cm (~7.5 kyr), whereas at the deepest site they were absent over about 20 cm (~110 kyr). Despite the difference in completeness of the records, the net extinction across this interval was similar at all sites: ~ 50% of species. At the deeper sites buliminid species (possibly indicating high food supply-low oxygen) showed a net decline over the dissolution interval, with peaks in abundance alternating with peaks in abundance of Nuttallides truempyi (possibly indicating low food) over that interval at the shallower sites. Abyssaminid species increased in abundance at all sites, more at the deeper sites. The Paleocene indicator species Stensioeina beccariiformis was more common at the shallower sites, and disappeared coevally at the BFE, exhibiting no depth gradient in extinction time. The cause of the extinction is difficult to deduce from the assemblage composition. Low oxygen conditions, high or low food supply, and carbonate dissolution have all been mentioned as prime causes of extinction, but extinction rates of ~50% occurred globally, at sites with little and with severe dissolution, at sites with and without sedimentary indications for low oxygen conditions. Indications for high/low food vary from site to site. Several early Eocene events resemble the PETM, but have a lesser intensity. These episodes, like the PETM, were marked by low diversity faunas with abundant abyssaminids. Abundant abyssaminids may not indicate specific environmental conditions, but may be opportunistic taxa colonizing a disturbed environment. The disturbing factor common to many deep ocean locations during the PETM and during later hyperthermal episodes was the oceanic warming itself, which may have been the main factor causing extinction, possibly by affecting metabolic rates of deep-sea faunas.