Atlantic Eocene planktonic foraminiferal historical biogeography and paleohydrographic indices

Abstract

Analysis of a quantitative and synoptic data base of Eocene Atlantic planktonic foraminifera has illuminated many facets of foraminiferal faunal structure and composition reflective of bioprovince and useful for empirical establishment of ecologic and paleohydrographic indices. Tropical indigenes, such as the morozovellids, turborotaliids, subquadrate acarininids, and some globigerinathekids dissolve most rapidly along with juveniles of most species. Tropical faunas are high in diversity and dominance, include oxygen minimum indices such as the biserial heterohelicids only in high productivity times such as the middle Eocene, are dominated by genera with high numbers of species and species consistently typical of warm water regions until the late Eocene when faunas include a higher number of middle than lower latitude types. Truly tropical genera are replaced by subtropical, eutrophic indicators, such as the globigerinathekids, and contract to the Florida Current region prior to their extinctions at the end of the middle Eocene. Atlantic late Paleogene tropical forms first appear in the Gulf of Mexico at the end of the middle Eocene. Middle latitude faunas include high numbers of chiloguembelinids, pseudohastigerinids, small and biconvex morozovellids, round and globigerinid-form acarininids, low-spired subbotinids in the late middle Eocene, high-spired subbotinids, large and typical globigerinids in the late Eocene. These forms have intermediate dissolution susceptibilities. Middle latitude faunas are often high in diversity in midddle latitude frontal regions. Dominance is low even in warm intervals when tropical indigenes increase in number in middle latitude faunas, especially in boundary currents. Low latitude species penetrate temperate areas also during cool, eutrophic periods of vigorous circulation such as the late middle Eocene. Typical of temperate slope faunas are high abundances of high-spired subbotinids and small-sized catapsydracids. High latitude faunas are usually low in diversity except in the middle to late Eocene when frontal regions were located in the southern Labrador Sea and over Rockall Bank. Typical forms are the low-spired subbotinids until the late middle Eocene, globorotaloids, catapsydracids, tenuitellids, and in the late Eocene, the biserial heterohelicids. Indigenous species are the most solution resistant and many are mesopelagic. During warmings rare low latitude indices penetrate high latitudes; subpolar faunas have greatest affinities with middle latitude faunas at these times.