Ecological and evolutionary response of Tethyan planktonic foraminifera to the middle Eocene climatic optimum (MECO) from the Alano section (NE Italy)

Abstract

The enigmatic middle Eocene climatic optimum (MECO) is a transient (~500kyr) warming event that significantly interrupted at ~40 Ma the long-term cooling through the middle and late Eocene, eventually resulting in establishment of permanent Antarctic ice-sheet. This event is still poorly known and data on the biotic response are so far scarce. Here we present a detailed planktonic foraminiferal analysis of the MECO interval from a marginal basin of the central-western Tethys (Alano section, northeastern Italy). The expanded and continuous Alano section provides an excellent record of this event and offers an appealing opportunity to better understand the role of climate upon calcareous plankton evolution. A sapropel-like interval, characterized by excursions in both the carbon and oxygen bulk-carbonate isotope records, represents the lithological expression of the post-MECO event in the study area and follows the δ¹⁸O negative shift, interpreted as representing the MECO warming.
High-resolution quantitative analysis performed on both >38 μm and >63 μm fractions reveals pronounced and complex changes in planktonic foraminiferal assemblages indicating a strong environmental perturbation that parallels the variations of the stable isotope curves corresponding to the MECO and post-MECO intervals. These changes consist primarily in a marked increase in abundance of the relatively eutrophic subbotinids and of the small, low-oxygen tolerant Streptochilus, Chiloguembelina and Pseudohas-tigerina. At the same time, the arrival of the abundant opportunist eutrophic Jenkinsina and Pseudoglobigerinella bolivariana, typical species of very high-productivity areas, also occurs. The pronounced shift from oligotrophic to more eutrophic, opportunist, low-oxygen tolerant planktonic foraminiferal assemblages suggests increased nutrient input and surface ocean productivity in response to the environmental perturbation associated with the MECO. Particularly critical environmental conditions have been reached during the deposition of the sapropel-like beds as testified by the presence of common giant and/or odd morphotypes. This is interpreted as evidence of transient alteration in the ocean chemistry.
The enhanced surface water productivity inferred by planktonic foraminiferal assemblages at the onset of the event should have resulted in heavier δ¹³C values. The recorded lightening of the carbon stable isotope preceding the maximum warmth therefore represents a robust indication that it derives principally by a conspicuous increase of pCO2. The increased productivity of surface waters, also supported by geochemical data, may have acted as mechanism for pCO2 reduction and returned the climate system to the general Eocene cooling trend. The oxygen-depleted deep waters and the organic carbon burial following the peak of the MECO event represent the local response to the MECO warming and suggest that high sequestration of organic matter, if representing a widespread response to this event, might have contributed to the decrease of pCO2 as well. Though the true mechanisms are still obscure, several lines of evidence indicate a potential pressure on planktonic foraminiferal evolution during the MECO event including permanent changes besides transient and ecologically controlled variations.

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