<p>Improved knowledge of the connection between striking variations in the abundance and coiling direction of the trochospiral planktic foraminiferal genus&#160;<em>Morozovella</em> and early Eocene carbon-cycle changes, is presented in this study as deriving from new data recorded from the Pacific Ocean (Shatsky Rise, Ocean Drilling Program Sites 1209, 1210). This location spans the Early Eocene Climatic Optimum (EECO; ~53-49 Ma), the interval when Earth surface temperatures and atmospheric&#160;<em>p</em>CO<sub>2</sub>&#160;reached the maximum peak of entire Cenozoic (Zachos <em>et al.,</em> 2001, Sciences; Anagnostou <em>et al.</em> 2016, Nature; Inglis <em>et al.,</em> 2020 Clim. Past). A significative impact of the EECO on planktic foraminiferal assemblages has recently been recorded in previous works from the Atlantic Ocean, where a definitive marked decline in abundance, diversity, test-size and change in coiling direction of the mixed-layer symbiont-bearing genus <em>Morozovella, </em>took place within the first ~600 kyr of this interval (Luciani <em>et al.,</em> 2016 Clim. Past; Luciani <em>et al.</em> 2017 Paleoceanogr.; Luciani <em>et al.,</em> 2017 GloPlaCha; D&#8217;Onofrio <em>et al.</em>, 2020 Geosciences; Luciani <em>et al.,</em> 2021 GloPlaCha). As registered in Atlantic sites, in the tropical Pacific Ocean Sites 1209 and 1210, the morozovellids drop permanently their relative abundance at the carbon isotope excursion (CIE) known as J event (~53 Ma), which marks the EECO beginning. A second major change affected all the morphologically defined species of&#160;<em>Morozovella</em>&#160;(possibly criptic species) at the Atlantic Ocean, resulting in a switch from dominant dextral to sinistrally coiling preference, within ~200 kyr after the K/X event (~52.8 Ma). Although the coiling direction preference of <em>Morozovella</em> at Shatsky Rise changed from dominant dextral to dominant sinistral after the K/X event as well as in the Atlantic sites, here the switch occurred with a delay of ~200 kyr. The recorded modifications clearly reflect important changes in evolution or environment. These changes may include temperature increase and pH decrease that could have reduced the symbiotic relationship and induced calcification crisis. Searching for the driving causes of the observed variations, our data clearly demonstrate their wide geographic and possibly global character and the evident relationship between the environmental perturbations occurred in the mixed-layer at the EECO and the striking changes on planktic foraminiferal assemblages during the first ~800 kyr of this intriguing interval.</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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