The Eocene-Oligocene turnover of Deep-Water Agglutinated Foraminifera at ODPSite 647, Southern Labrador Sea (North Atlantic)

Abstract

We present a new record of deep-water agglutinated foraminifera (DWAF) across the Eocene-Oligocene Transition (EOT) in the southern Labrador Sea (ODP Site 647). We studied 82 samples from Cores 647A-37R to -27R, and recovered over 100 species and generic groups. The EOT represents an interval of rapid climatic change connected with global cooling, Antarctic glaciation, a substantial decrease in atmospheric CO2, and concurrent changes in the composition of deep waters in the world ocean. Our high-resolution quantitative study of the DWAF faunal succession in abyssal Hole 647A confirms earlier findings that the EOT was an interval of significant faunal turnover. The faunal succession in Hole 647A is subdivided into two assemblages based on the stratigraphic ranges of characteristic benthic foraminiferal species: late Eocene Spiroplectammina trinitatensis-Reticulophragmium amplectens Zone and early Oligocene Ammodiscus latus-Turrilina alsatica Zone. The boundary between these zones, i.e., the Eocene/Oligocene (E/O) boundary, is characterized by the disappearance of 11 DWAF taxa, most of them organically-cemented taxa. The boundary interval was also characterized by a striking sharp decrease in DWAF abundance and diversity. Organically-cemented DWAF taxa increased in abundance in the early Oligocene, but their diversity and abundance never recovered to Eocene values. These data suggest a deepening of the calcite compensation depth and associated changes, such as more vigorous ocean circulation in coincidence with the E/O boundary interval. The analysis of DWAF morphogroups reveals an acme in robust suspension-feeding tubular forms previous to the extinction, suggesting increased bottom water activity. The E/O boundary interval is characterized by an increase in DWAF infaunal taxa and Spiroplectammina species, probably related to increased productivity, as already suggested by the analysis of benthic elongate-cylindrical foraminifera at the same locality. The faunal turnover across the EOT at Hole 647A suggests more vigorous deep-ocean circulation in the latest Eocene and across the E/O boundary interval in the Labrador Sea. It seems reasonable to link the disappearance of DWAF at the E/O boundary at Site 647 to more than one mechanism, including inferred higher productivity, possible competition from calcareous benthic foraminifera, changes in the CO2 content of the atmosphere and ocean, the deepening of the calcite compensation depth, and concurrent changes in taphonomic conditions caused by changes in the water masses in the North Atlantic.