Abstract
The Middle Eocene Climatic Optimum (MECO; ~40 Ma), which interrupted for ~500–600 kyr the long-term cooling trend culminating at the Eocene/Oligocene boundary, still requires a comprehensive understanding of the biotic resilience. Here we present a high-resolution integrated foraminiferal and calcareous nannofossil study across the MECO from the expanded and continuous Tethyan Baskil section (eastern Turkey) that offers a complete magneto-biostratigraphic and geochemical framework. The five MECO phases identified reveal a transition from oligotrophic (pre-MECO) to eu-mesotrophic conditions, possibly related to accelerated hydrological cycle, during the initial MECO and MECO δ13C negative excursion phases. The MECO WARMING PEAK phase, marking the highest carbonate dissolution interval, records the most striking biotic changes, such as peak in warm and eutrophic nannofossils, virtual disappearance of the oligotrophic planktic foraminiferal large Acarinina and Morozovelloides, and peak in eutrophic deep dwellers Subbotina. Benthic foraminifera suggest in this phase an improvement in the quality of organic matter to the seafloor. The post-MECO phase shows only a partial recovery of the pre-event conditions. Large Acarinina and Morozovelloides did not recover their abundance, possibly due to cooler conditions in this phase. Our reconstruction reveals how paleoenvironment and marine biota from the studied Neo-Tethyan setting reacted to the MECO perturbations.
Highlights
The middle Eocene climatic optimum (MECO) is still an enigmatic global warming event, which occurred at about 40 Ma and lasted ~500–600 kyrs, that temporarily interrupted the long-term cooling trend initiated at the end of the early Eocene climate optimum (EECO, ~49 Ma) (e.g., [1,2,3,4])
Lower planktonic to benthic (P/B) values were found in the interval between 168 and 173 m (Figure 3, Table S1)
Such changes can be ascribed to enhanced organic matter availability to the seafloor both in terms of quantity and quality and a possible lowering of oxygen availability at the seafloor, as indicated by lowest values of Nuttalides spp. together with two peaks of Lenticulina spp. (Figure 4). This evidence agrees with a more stratified water column and consequent more efficient biological pump, as suggested by lower δ13 C and higher difference in δ18 O values between deep and surface dwelling planktic foraminifera [3]. This interval shows a further decrease of C/A values that would reflect enhanced carbonate dissolution at the sea floor [31] (Figure 3)
Summary
The middle Eocene climatic optimum (MECO) is still an enigmatic global warming event, which occurred at about 40 Ma and lasted ~500–600 kyrs, that temporarily interrupted the long-term cooling trend initiated at the end of the early Eocene climate optimum (EECO, ~49 Ma) (e.g., [1,2,3,4]). Bulk carbonate oxygen isotope records indicate ~4–6 ◦ C warming (assuming ice-free conditions), with a gradual onset and brief peak temperatures, followed by a rapid return to pre-event conditions [1]. The MECO has been described as a global carbon cycle perturbation [11], but the lack of a carbon isotope excursion (CIE) contemporaneous with warming and the duration of the climate anomaly suggest that the event was more complex than the earlier Paleogene hyperthermals [12]. Marine carbonate δ13 C records show considerable geographic and bathymetric variability but are commonly characterized by rising rather than declining δ13 C values during the initial gradual warming [1,5,9,13]. An early stage of Ethiopian flood basalt activity might have occurred between 45 and 35 Ma [14], and the “Mid-Tertiary Ignimbrite Flare-Up” in Mexico started around 40–38 Ma [15]
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