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The biotic response of calcareousnannoplankton to environmental and climatic changes during theEocene–Oligocene transition was investigated at a high resolution at OceanDrilling Program (ODP) Site 1263 (Walvis Ridge, southeast Atlantic Ocean) andcompared with a lower-resolution benthic foraminiferal record. During thistime interval, global climate, which had been warm under high levels ofatmospheric CO2 (pCO2) during the Eocene, transitioned into thecooler climate of the Oligocene, at overall lower pCO2. At Site 1263,the absolute nannofossil abundance (coccoliths per gram of sediment;N g−1) and the mean coccolith size decreased distinctly after the E–Oboundary (EOB; 33.89 Ma), mainly due to a sharp decline in abundance oflarge-sized Reticulofenestra and Dictyococcites, occurringwithin a time span of ~ 47 kyr. Carbonate dissolution did not varymuch across the EOB; thus, the decrease in abundance and size of nannofossilsmay reflect an overall decrease in their export production, which could haveled to variations in the food availability for benthic foraminifers. The benthic foraminiferal assemblage data are consistent with a globaldecline in abundance of rectilinear species with complex apertures in thelatest Eocene (~ 34.5 Ma), potentially reflecting changes in the foodsource, i.e., phytoplankton. This was followed by a transient increasedabundance of species indicative of seasonal delivery of food to the sea floor(Epistominella spp.; ~ 33.9–33.4 Ma), with a short peak inoverall food delivery at the EOB (buliminid taxa; ~ 33.8 Ma).Increased abundance of Nuttallides umbonifera (at ~ 33.3 Ma)indicates the presence of more corrosive bottom waters and possibly thecombined arrival of less food at the sea floor after the second step ofcooling (Step 2). The most important changes in the calcareous nannofossil and benthiccommunities occurred ~ 120 kyr after the EOB. There was no majorchange in nannofossil abundance or assemblage composition at Site 1263 afterStep 2 although benthic foraminifera indicate more corrosive bottom watersduring this time. During the onset of latest-Eocene–earliest-Oligoceneclimate change, marine phytoplankton thus showed high sensitivity tofast-changing conditions as well as to a possibly enhanced, pulsed nutrientsupply and to the crossing of a climatic threshold (e.g., pCO2decline, high-latitude cooling and changes in ocean circulation)

Microfossil evidence for trophic changes during the Eocene–Oligocene transition in the South Atlantic (ODP Site 1263, Walvis Ridge)