<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&#180;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 &#948; 18 O values steadily declined by roughly 1&#8240; in over ~400 kyr, usually interpreted as a 3&#8211;6 &#176;C<br />increase in global temperature followed by a rapid return to pre&#8208;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&#8208;than&#8208;expected deep&#8208;sea carbonate dissolution, a lack of globally<br />coherent negative &#948; 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 &#8220;carbon cycle conundrum&#8221;.<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>
<p>Modern studies on marine ecosystems are limited in time so the evaluation of their stability under the ongoing CO<sub>2</sub> emissions and global warming remains uncertain and necessarily requires a long-term perspective. The dynamic early Paleogene climate offers the crucial opportunity to detect relationships among calcareous plankton, past carbon cycle perturbations and climate. Specifically, the EECO (~53-49 Ma) represents a key interval to investigate the planktic foraminiferal resilience on a long-term perspective as it records the peak temperature and <em>p</em>CO<sub>2</sub> of the entire Cenozoic. We investigated the Pacific Sites 1209-1210 and eastern Indian Ocean Site 762 following the evidence that the EECO marked impacted planktic foraminiferal assemblages at the Atlantic Oceans. Abrupt and permanent abundance decline (more than one-third) of the symbiont-bearing genus <em>Morozovella </em>occurred<em> </em>at the EECO beginning (J event, ~53 Ma) at sites 1209-1210, whereas <em>Acarinina</em> concomitantly increased, as from the Atlantic sites. Site 762 recorded the southern high-latitude migration of the warm <em>Acarinina</em> species coupled with the decline of the &#8216;cold&#8217; subbotininds. Another major change documented at the Pacific and Indian Oceans revealed to be similar to the Atlantic record and involved the coiling direction (ability to add chambers clock- or counter-clockwise) of the genus <em>Morozovella.</em> Indeed, the morozovellid coiling direction is dominantly dextral below the EECO but became sinistral within the EECO, although this change is registered ~ 200 kyr later at the Pacific Ocean and ~ 200 kyr before at Site 762 where it occurred at the K/X event (~52.8 Ma). Therefore, the morozovellids crisis observed in the Atlantic and Pacific Oceans can be mainly read as the dextral forms decline. Searching for the driving causes of the observed modifications, we performed stable isotope analysis on sinistral and dextral morozovellids morphotypes (possibly cryptic species) from sites 1209-1210 and 762. Results show that the sinistral forms generally record lower d<sup>13</sup>C values, once again as recorded for the Atlantic Ocean. This evidence suggests a reduced symbiosis relationship and/or a slightly deeper habitat, probably a strategy to sustain the stressors induced by the EECO. Our record advises on a causal relationship to chemical-physical modifications in the surface waters, such as the temperature increase. The increased temperature of at least 1&#176;C [Mg/Ca (LA)-ICP-MS] recorded by sinistral morozovellids within the EECO may have acted in the reduced photosymbiotic activity. Conversely, acarininids do not show preferential coiling nor below neither within the EECO and they reveal d<sup>13</sup>C values that imply major ecological flexibility, possibly enabling them to proliferate. The EECO also induced the virtual disappearance of the genus <em>Chiloguembelina</em> after the K/X event at all the Atlantic, Pacific and Indian sites investigated. This disappearance appears to be related to thermocline warming and Oxygen Minimum Zone enhanced oxygenation. Our records demonstrate the wide geographic and possibly global character of the striking modifications occurred in the planktic foraminiferal assemblages during the first ~800 kyr of the EECO. Our derived paleobiology gives new insights into planktic foraminiferal strategies adopted under long-term global warming.</p>
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