ATLANTIC EOCENE PLANKTONIC FORAMINIFERAL PALEOHYDROGRAPHIC INDICATORS AND STABLE ISOTOPE PALEOCEANOGRAPHY

Reconstruction of Atlantic Ocean vertical and meridional water characteristics is based on combined synoptic fossil census data, stable isotope records, and measurements of individual planktonic foraminiferal species. Correlations between planktonic foraminiferal indices and specific water types and/or chemical conditions are used to trace seven stages in the history of Atlantic circulation during Eocene climatic changes: (1) warm, equable, probably saline, productive ocean of the early Eocene characterized by lowest meridional and vertical thermal gradients and the maximal extent of tropical surface water, (2) early middle Eocene expansion of subtropical waters and faunas accompanying the first climatic cooling and eutrophication of equatorial surface waters, (3) maximal partitioning of water types, faunas, and bioprovinces in middle Eocene Zone Pll accompanying maximum planktonic foraminiferal diversity, accelerated near-surface and deepwater cooling, and intensified upwelling, (4) latest middle Eocene (Zone P14) maximal planktonic foraminiferal evolutionary overturn during accelerated cooling, deep and surface water ventilation, and transformation of tropical, surface water habitat with the expansion of temperate water types and faunas; this accompanied the reduction of meridional heat transport in boundary currents, (5) late Eocene bottom water and middle-latitude cooling, broadening of meridional temperature gradients, and expanded oxygen-minimum, but faunal homogenization following the decreased import of Mediterranean water ourflow, (6) weakening and/or shoaling of the nutricline, an oxygen minimum, and the thermocline preceding a diminution of primary productivity at the Eocene-Oligocene boundary, and (7) earliest Oligocene low-productivty ocean marked by diminished vertical thermal contrasts and homogeneous foraminiferal faunas dominated by small-sized individuals. The increased areal extent of upwelling with the outcropping of cooler waters over large geographic regions provides an important climatic-forcing mechanism in the latest middle and late Eocene.