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

Abstract

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