Resilience of planktic and benthic foraminifera across the Middle Eocene Climatic Optimum (MECO) along a shallow water mixed siliciclastic and carbonate succession in NW Italy

Abstract

The early Paleogene is characterized by several warming episodes that are evaluated as analogues to the ongoing climate change. Specifically, we focalize on the Middle Eocene Climatic Optimum (MECO, centered at ~40 Ma) which is one of the major Eocene global warming events, characterized by ~4–6°C warming, shifts in the global carbon cycle and rise in atmospheric pCO2. Even though the MECO is a still enigmatic event, studies on paleobiotic effects across this interval are yet rather limited. We present here new quantitative analysis on planktic and benthic foraminifera to evaluate the impact of the MECO on the investigated biotic groups along the Sealza section (Liguria, NW Italy). This succession is interpreted as the result of a drowning ramp that was affected by continuous tectonic activity and offers the exceptional opportunity to compare the biotic variations across the MECO in shallow-water assemblages with the deep-water communities. The MECO interval at Sealza is constrained by the stable isotope oxygen data and by the occurrence of the species Orbulinoides beckmanni that has a range mostly coincident with the MECO event. Planktic foraminiferal abundance is generally scarce, as expected from a shallow-water succession. The most abundant genus is Subbotina, which however records its lowest abundance within MECO interval. We interpret this record as a response to the MECO warming because this genus is a cold-water index. The genera Acarinina and Morozovelloides show low abundance in the lower part of the section, but they record an increase across the MECO interval, as expected by these warm indices. The most abundant genus among benthic foraminifera that are well preserved and easy recognizable, is the epifaunal? Cibicidoides, adapted to oxygenated conditions. This genus records a marked decrease across the MECO. Similarly, the opportunistic genus Heterolepa shows an increase in abundance across the MECO interval. Another abundant genus is Anomalinoides that is very abundant in the lower part of the section but also displays a decrease across the interval corresponding to the MECO but differently from Cibicidoides, it did not recover in the post-MECO and is absent at the top of the section. This suggests that Cibicidoides was more resilient than Anomalinoides which proved to be less flexible to the paleoenvironmental changes induced by the MECO warming. The decrease of Cibicidoides coupled to the increase in abundance across the MECO by the genera Uvigerina and Bolivina, infaunal forms tolerating low-oxygen conditions, possibly indicate less oxygenation at the bottom and a change in the quality of organic matter (i.e., more labile) reaching the seafloor.