The biotic response of calcareous nannoplanktonto environmental and climatic changes during the Eocene–Oligocene transition was investigated at a high resolution atOcean Drilling Program (ODP) Site 1263 (Walvis Ridge,southeast Atlantic Ocean) and compared with a lowerresolution benthic foraminiferal record. During this time interval, global climate, which had been warm under high levels of atmospheric CO2 (pCO2) during the Eocene, transitioned into the cooler climate of the Oligocene, at overalllower pCO2. At Site 1263, the absolute nannofossil abundance (coccoliths per gram of sediment; N g−1) and the meancoccolith size decreased distinctly after the E–O boundary(EOB; 33.89 Ma), mainly due to a sharp decline in abundance of large-sized Reticulofenestra and Dictyococcites, occurring within a time span of ∼ 47 kyr. Carbonate dissolution did not vary much across the EOB; thus, the decreasein abundance and size of nannofossils may reflect an overalldecrease in their export production, which could have led tovariations in the food availability for benthic foraminifers.The benthic foraminiferal assemblage data are consistentwith a global decline in abundance of rectilinear specieswith complex apertures in the latest Eocene (∼ 34.5 Ma), potentially reflecting changes in the food source, i.e., phytoplankton. This was followed by a transient increased abundance of species indicative of seasonal delivery of food to thesea floor (Epistominella spp.; ∼ 33.9–33.4 Ma), with a shortpeak in overall 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 the combined arrival of less food atthe sea floor after the second step of cooling (Step 2).The most important changes in the calcareous nannofossil and benthic communities occurred ∼ 120 kyr after theEOB. There was no major change in nannofossil abundanceor assemblage composition at Site 1263 after Step 2 although benthic foraminifera indicate more corrosive bottom waters during this time. During the onset of latestEocene–earliest-Oligocene climate change, marine phytoplankton thus showed high sensitivity to fast-changing conditions as well as to a possibly enhanced, pulsed nutrient supply and to the crossing of a climatic threshold (e.g., pCO2 decline, 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)

Bordiga M
;
2015-01-01

Abstract

The biotic response of calcareous nannoplanktonto environmental and climatic changes during the Eocene–Oligocene transition was investigated at a high resolution atOcean Drilling Program (ODP) Site 1263 (Walvis Ridge,southeast Atlantic Ocean) and compared with a lowerresolution benthic foraminiferal record. During this time interval, global climate, which had been warm under high levels of atmospheric CO2 (pCO2) during the Eocene, transitioned into the cooler climate of the Oligocene, at overalllower pCO2. At Site 1263, the absolute nannofossil abundance (coccoliths per gram of sediment; N g−1) and the meancoccolith size decreased distinctly after the E–O boundary(EOB; 33.89 Ma), mainly due to a sharp decline in abundance of large-sized Reticulofenestra and Dictyococcites, occurring within a time span of ∼ 47 kyr. Carbonate dissolution did not vary much across the EOB; thus, the decreasein abundance and size of nannofossils may reflect an overalldecrease in their export production, which could have led tovariations in the food availability for benthic foraminifers.The benthic foraminiferal assemblage data are consistentwith a global decline in abundance of rectilinear specieswith complex apertures in the latest Eocene (∼ 34.5 Ma), potentially reflecting changes in the food source, i.e., phytoplankton. This was followed by a transient increased abundance of species indicative of seasonal delivery of food to thesea floor (Epistominella spp.; ∼ 33.9–33.4 Ma), with a shortpeak in overall 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 the combined arrival of less food atthe sea floor after the second step of cooling (Step 2).The most important changes in the calcareous nannofossil and benthic communities occurred ∼ 120 kyr after theEOB. There was no major change in nannofossil abundanceor assemblage composition at Site 1263 after Step 2 although benthic foraminifera indicate more corrosive bottom waters during this time. During the onset of latestEocene–earliest-Oligocene climate change, marine phytoplankton thus showed high sensitivity to fast-changing conditions as well as to a possibly enhanced, pulsed nutrient supply and to the crossing of a climatic threshold (e.g., pCO2 decline, high-latitude cooling and changes in ocean circulation).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/20.500.14083/15271
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