PALEOCENE TO EOCENE BENTHIC FORAMINIFERAL ISOTOPES AND ASSEMBLAGES: IMPLICATIONS FOR DEEPWATER CIRCULATION

Early Paleogene warm climates may have been linked to different modes and sources of deepwater formation. Warm polar temperature of the Paleocene and Eocene may have resulted from either increased atmospheric trace gases or increased heat transport through deep and intermediate waters. The possibility of increasing ocean heat transport through the production of warm salined waters (WSDW) in the Tethyan region has generated considerable interest. In addition, General Circulation Model results indicate that deepwater source regions may be highly sensitive to changing basin configurations To decipher deepwater changes we examined detailed benthic foraminiferal faunal and isotopic records of the late Paleocene through the early Eocene (similar to 60 to 50 Ma) from two critical regions: the North Atlantic (Bay of Biscay Site 401) and the Pacific (Shatsky Rise Site 577). These records are compared with published data from the Southern Ocean (Maud Rise Site 690, Islas Orcadas Rise Site 702). During the late Paleocene, similar benthic foramaniferal delta O-18 values were recorded at all four sites. This indicates uniform deepwater temperatures consistent with a single source of deep water. The highest delta C-13 value were recorded in the Southern Ocean and were 0.5 parts per thousand more positive than those of the Pacific. We infer that the Southern Ocean was proximal to a source of nutrient-depleted deep water during the late Paleocene. Upper Paleocene Reflector A(b) was cut on the western Bermuda Rise by cyclonically circulating bottom water, also suggesting a vigorous source of bottom water in the Southern Ocean. A dramatic negative excursion in both carbon and oxygen isotopes occurred in the latest Paleocene in the Southern Ocean. This is a short-term (< 100 kyr), globally synchronous event which also is apparent in both the Atlantic and Pacific records as a carbon isotopic excursion of approximately 1 parts per thousand. Faunal analyses from the North Atlantic and Pacific sites indicate that the largest benthic foraminiferal faunal turnover of the Cenozoic was synchronous with the isotopic excursion, lending support to the hypothesis that the extinctions were caused by a change in deepwater circulation. We speculate that the Southern Ocean deepwater source was reduced or eliminated at the time of the excursion. During the early Eocene, Southern Ocean delta C-13 values remained enriched relative to the North Atlantic and Pacific. However, the Southern Ocean was also enriched in delta O-18 relative these basins. We interpret that these patterns indicate that although the Southern Ocean was proximal to a source of cool, nutrient-depleted water, the intermediate to upper deepwater sites of the North Atlantic and Pacific were ventilated by a different source that probably originated in low latitudes, i.e., WSDW.