The Eocene-Oligocene transition (EOT) at ∼34 Ma constitutes one of the major episodes of climate change that occurred in the last 50 myr, associated with the establishment of permanent Antarctic ice sheets and the initiation of meridional overturning circulation. We present a high-resolution quantitative study (>125-μm fraction) of a group of calcareous, deep-sea, benthic foraminifera with elongate-cylindrical morphologies (Elongate Gp.) across the EOT at high latitudes in the North Atlantic Ocean (ODP Hole 647A). This group experienced significant declines in abundance and diversity during periods of global cooling, disappearing almost completely during the mid-Pleistocene Climate Transition.

The Elongate Gp. was a common component of deep-sea benthic foraminiferal assemblages at Hole 647A (average 20%). The family Stilostomellidae dominated during the late Eocene and at the EOT; the Nodosariidae slightly dominated during the early Oligocene. During the EOT at Hole 647A, an abrupt decline in the abundance and diversity of agglutinated foraminifera occurred, but not a distinct taxonomic turnover of the Elongate Gp. Siphonodosaria jacksonensis was the most abundant species and showed the most significant changes in relative abundance across the EOT, from almost no record to a peak of >50% of total benthic foraminifera coincident with the lowest diversity and highest absolute abundance of the Elongate Gp. We attribute these changes to the influence of more than one mechanism, including increased productivity, deepening of the calcite compensation depth, and more vigorous deep-ocean circulation.

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