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

Abstract

a b s t r a c t a r t i c l e i n f oThe enigmatic middle Eocene climatic optimum (MECO) is a transient (∼ 500 kyr) 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 δ 18 O negative shift, interpreted as representing the MECO warming. High-resolution quantitative analysis performed on both N38 μm and N 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 Pseudohastigerina. 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 δ 13 C 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 pCO 2 . The increased productivity of surface waters, also supported by geochemical data, may have acted as mechanism for pCO 2 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, m...

Enhanced primary productivity and magnetotactic bacterial production in response to middle Eocene warming in the Neo-Tethys Ocean

“…It also has the lowest concentrations of the coarsest fraction, which is used as a proxy for carbonate dissolution (e.g. Hancock and Dickens, 2005;Colosimo et al, 2006;Leon-Rodriguez and Dickens, 2010;Luciani et al, 2010). Dissolution during this interval is also supported by increased values of the planktonic foraminiferal FI, greater relative abundances of agglutinated foraminifera that are less prone to dissolution, and a clear decrease of the P/P + B ratio (Fig.…”

“…This index has been largely used as an indicator of carbonate dissolution (e.g. Hancock and Dickens, 2005;Colosimo et al, 2006;Leon-Rodriguez and Dickens, 2010;Luciani et al, 2010).…”

“…More recently, the MECO event has also been recognized at the Alano di Piave section, northeast Italy ( Fig. 1a; Luciani et al, 2010). In the Alano section the MECO event is followed by deposition of two organic-rich intervals (ORG1 and ORG2; Spofforth et al, 2010), which are thought to represent rapid organic carbon burial contemporaneous with the global pCO 2 drawdown during the post-MECO recovery interval.…”

Ecology of foraminifera during the middle Eocene climatic optimum in Kutch, India

“…The planktic foraminifer O. beckmanni is reported to be an excursion taxon coinciding with the MECO event (Edgar et al, 2010). Acarinina is a symbiont-bearing planktic foraminifer indicative of warm tropical seawater and oligotrophic conditions (Luciani et al, 2010). Acarinina species show good abundance at certain levels in this interval but further diversify and become abundant in the overlying carbonates.…”

“…A turnover in eberdian, silicoflagellates and dinoflagellates has been observed during the MECO (Witkowski, Bohaty, McCartney, & Harwood, 2012). The Alano section in NE Italy is among the few on land sections to have been well studied for the response of benthic foraminifera (Galazzo, Giusberti, Luciani, & Thomas, 2013), planktic foraminifera (Luciani et al, 2010) and nannoplankton (Toffanin et al, 2011).…”