The Middle Eocene Climatic Optimum (MECO) impact on Southern Atlantic planktic foraminifera (Site 702)

Understanding the resilience of the ecosystems within the context of the ongoing global climate change is a
pressing challenge for humankind. The combination of the huge archive available in the geological record
with studies on modern biota is essential to formulate realistic predictions, and the Paleogene is one of the
most climatically dynamic periods in Earth´s history, offering this crucial opportunity. Here we focus on the
Middle Eocene Climatic Optimum (MECO), a global warming event during which marine bulk and benthic
carbonate δ 18 O values steadily declined by roughly 1‰ in over ~400 kyr, usually interpreted as a 3–6 °C
increase in global temperature followed by a rapid return to pre‐event conditions. This event record
temperatures and pCO2 that Earth will reach whether anthropogenic emissions will not stop (RCP8.5). A
number of characteristics, including greater‐than‐expected deep‐sea carbonate dissolution, a lack of globally
coherent negative δ 13 C excursion in marine carbonates, a duration longer than the characteristic timescale of
carbon cycle recovery, and the absence of a clear trigger mechanism, make the MECO one of the most
enigmatic events in the Cenozoic, dubbed a middle Eocene “carbon cycle conundrum”.
The paleoenvironmental and biotic consequences of the MECO are still poorly constrained, however, and
here we focus on the response of planktic foraminifera, which are extremely sensitive to the physical and
chemical state of the oceans. Quantitative studies of planktic foraminiferal assemblages from South Atlantic
ODP Site 702 allowed us to characterize the MECO at this key southern high-latitude setting. The magneto
and stable isotope stratigraphy are well constrained at this site, together with the calcareous nannofossils and
benthic foraminiferal response (Rivero-Cuesta et al., 2019, Paleoceanography and Paleoclimatology).
Our results indicate a pronounced southern migration of the warm index Acarinina coupled by a marked
decline in the abundance of the cold index Subbotina. Additionally, the low-latitude species Orbulinoides
beckmanni occurs only at the MECO peak. The post-MECO assemblages show a recovery of the pre-event
abundances, with the exception of the genus Chiloguembelina, which shows a striking increase in abundance
and suggests an intensification of the Oxygen Minimum Zone. A further result of our study is the greater
sensitivity of planktic foraminifera to the MECO with respect to calcareous nannofossils, as changes in
planktic foraminiferal assemblages started ~2 kyr before the calcareous nannofossil turnover.