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Featured Application: A challenging issue for humankind is the understanding of the future biota state within the context of global change (anthropogenic CO2 emissions, global warming, ocean acidification, eutrophication, anoxia). Studies on modern biomaterials reveal that the climate-induced environmental stress is affecting the stability of ecosystems. However, modern analyses are limited to decades/historical times whereas the paleontological record can reconstruct the impact of past global climate analogues on a thousand/hundreds/million-year perspective. This work fits the goal to deciphering how marine calcareous nannofossils, benthic and planktic foraminifera, unicellular algae and protozoan, reacted to the global warming known as Middle Eocene Climatic Optimum (MECO), centered at ~40 Ma ago and lasting ~500–600 kyr. Our detailed integrated reconstruction from the Neo-Tethyan geological setting (eastern Turkey) with thousand years' resolution (~100 m/My) reveals that the MECO warming peak induced marked water eutrophication, reduction in oxygen availability at the sea floor and carbonate dissolution. Part of the analyzed marine biota proved to be resilient to the MECO perturbation through marked modifications within assemblages whereas the most specialized planktic foraminifera declined permanently their abundance permanently. Biota and environment only partially recovered the pre-event conditions after the MECO warming peak. Our study, recording transient and permanent changes, allows to understand how paleoenvironment and marine biota reacted to the global MECO perturbation. The Middle Eocene Climatic Optimum (MECO; ~40 Ma), which interrupted for ~500–600 kyr the long-term cooling trend culminating at the Eocene/Oligocene boundary, still requires a comprehensive understanding of the biotic resilience. Here we present a high-resolution integrated foraminiferal and calcareous nannofossil study across the MECO from the expanded and continuous Tethyan Baskil section (eastern Turkey) that offers a complete magneto-biostratigraphic and geochemical framework. The five MECO phases identified reveal a transition from oligotrophic (pre-MECO) to eu-mesotrophic conditions, possibly related to accelerated hydrological cycle, during the initial MECO and MECO δ13C negative excursion phases. The MECO WARMING PEAK phase, marking the highest carbonate dissolution interval, records the most striking biotic changes, such as peak in warm and eutrophic nannofossils, virtual disappearance of the oligotrophic planktic foraminiferal large Acarinina and Morozovelloides, and peak in eutrophic deep dwellers Subbotina. Benthic foraminifera suggest in this phase an improvement in the quality of organic matter to the seafloor. The post-MECO phase shows only a partial recovery of the pre-event conditions. Large Acarinina and Morozovelloides did not recover their abundance, possibly due to cooler conditions in this phase. Our reconstruction reveals how paleoenvironment and marine biota from the studied Neo-Tethyan setting reacted to the MECO perturbations. [ABSTRACT FROM AUTHOR]

Here we present relative abundances of planktic foraminifera that span the Early Eocene Climatic Optimum (EECO) at Ocean Drilling Program (ODP) Site 1258 in the western equatorial Atlantic. The EECO (~53.3−49.1 Ma) represents peak Cenozoic warmth, probably related to high atmospheric CO2, and when planktic foraminifera, a dominant component of marine sediment, exhibit a major biotic response. Consistent with previous work, the relative abundance of the genus Morozovella, which dominated early Paleogene tropical-subtropical assemblages, markedly and permanently declined from a mean percentage of ~32% to less than ~7% at the beginning of the EECO. The distinct decrease in Morozovella abundance occurred at Site 1258 within ~20 kyr before a negative excursion in δ13C records known as the J event and which defines the beginning of EECO. Moreover, all morozovellid species except M. aragonensis dropped in abundance permanently at Site 1258, and this is related to a reduction in test-size. Comparing our data with that from other locations, the remarkable switch in planktonic foraminifera assemblages appears to have begun first with unfavourable environmental conditions near the Equator and then extended to higher latitudes. Several potential stressors may explain observations, including some combination of algal photosymbiont inhibition (bleaching), a sustained increase in temperature, or an extended decrease in pH. [ABSTRACT FROM AUTHOR]

Thin-walled planktic foraminifera bearing a biserial disposition of the chambers arecharacterized by relatively long stratigraphic ranges as originating in the Late Albian from thebiserial Heterohelix, diversified in the Late Cretaceous and survive to the present day, eventhough their abundance is highly variable through time and space. These forms attracted thescientific interest because they thrived in correspondence with extreme global climatic andpaleoceanographic events. However, paleobiology of Cenozoic biserial planktic foraminiferais not fully established. Largely accepted ecological interpretation ascribes themas low-oxygen tolerant, meso- to eutrophic thermocline dwellers inhabiting theOxygen Minimum Zone (OMZ) and thriving in stressed environments. However, thisgeneral ecological interpretation does not agree with all available records. Stableisotope data from Chiloguembelina wilcoxensis and Zeauvigerina aegyptiaca atDeep Sea Drilling Project (DSDP) Site 401 (eastern North Atlantic) analyzed in theinterval following the PETM, overlap with those of the symbiont-bearing Morozovellasubbotinae thus implying surface mixed layer habitat (D’Haenens et al. 2012, AustrianJournal of Earth Sciences 105/1). Similar habitat was derived for middle Eocene-LateOligocene Chiloguembelna cubensis (Premec Fucek et al. 2018 and referencestherein, Cushman Foundation Special Publication 46, chapter 17). Conversely, Sextonet al. (2006, Marine Micropaleontology 60) report Chiloguembelina ototara asinhabiting a thermocline habitat on the basis of stable isotope data from EoceneZone E14 at northwest Atlantic Ocean (ODP Site 1052B). These evidences indicatethat planktic biserial taxa may have changed their ecological niches through timeand over their geographic distribution. We present here new oxygen and carbonstable isotope data from early Eocene chiloguembelinids with the aim to assessingtheir habitat during this interval for which stable isotope record is lacking. Theearly Eocene is a crucial interval of time marked by significant climatic changeswhen temperatures across Earth’s surface rose toward an extended interval of peakCenozoic global warmth and high pCO2, the Early Eocene Climatic Optimum (EECO,∼54-48 Ma). Paleoceanographic reconstructions are essential to understand climaticchanges occurred during the EECO interval therefore we need a good knowledgeof ecological characters of planktic foraminifera, one of the most widely utilisedgroup for interpreting paleoenvironmental changes. To perform our analysis weselect the Ocean Drilling Program (ODP) south Atlantic Site 1263 that has goodsedimentary recovery and preserves relatively abundant chiloguembelinids at theEECO beginning. Our analysis was performed at the beginning of the EECO onChiloguembelina wilcoxensis and C. trinitatensis and on Morozovella, Acarinina, Subbotinaspecies and benthic Oridorsalis umbonatus belonging to Zone E5 (Wade et al. 2011,Earth Science Review 104). Even though species are slightly recrystalised, resultsclearly demonstrate that these species lived in sub-surface habitat as they showstable isotope data close to those of the thermocline dweller Subbotina. These datareveal therefore a different habitat with respect to north Atlantic earliest Eocene andmiddle Eocene-Late Oligocene records that designate mixed-layer habitat. Ourfindings provide critical implications for the paleoceanographic reconstruction ofthe upper water column and its changes occurred during the EECO at Site 1263. [ABSTRACT FROM AUTHOR]

The Early Eocene Climatic Optimum (EECO) expresses the multi-million year interval ofpeak Cenozoic global warmth and high atmospheric CO2 concentrations that occurredsometime between 54 and 48 million years ago. An outstanding issue is how life evolvedduring this crucial interval, and planktic foraminifera represent an excellent class in which toexamine such links. Here we present new data on planktic foraminferal response to the EECOfrom the equatorial Atlantic ODP Site 1258 (Demerara Rise). Our main result reveals that therelative abundance of the surface-dweller symbiont-bearing genus Morozovella, thatdominated early Paleogene planktic foraminiferal assemblages from tropical-subtropicalregions, markedly and permanently declined at the beginning of the EECO moving frommean percentage of ∼34% to less than ∼10%. This distinct decrease in abundanceoccurred within ∼20 kyr before a negative δ13C excursion known as the "J" event(∼ 53.3 Ma), which marks the onset of the EECO. The permanent decrease ofmorozovellids is associated with reduction in species diversity, but an increasein the abundance and diversity of another symbiont-bearing foraminifera genus,Acarinina. The remarkable turnover from Morozovella to Acarinina was geographicallywidespread, as it is now recorded at the start of the EECO in the subtropical Pacific (Site577), and elsewhere in the subtropical Atlantic (Sites 1051 and 1263). Interestingly,the timing of the drop in abundance is close but different at each site. Our newdata from Demerara Rise further a notion that the morozovellid decline began firstwith unfavourable environmental conditions near the equatorial Atlantic Oceanand then extended to higher latitudes. The triggering mechanism for the strikingplanktic foraminiferal turnover remains elusive, because both Morozovella andAcarinina existed in the mixed-layer. Most of the morozovellid species that becameextinct within the EECO reduced their maximum size. Recent culturing and openocean observations indicate that omega-calcite saturation state can affect variablysized foraminifera differently, such that larger planktic foraminifera preferentiallyreduce their calcification. The two dominant genera, Morozovella and Acarinina,may have had different tolerances to temperature and ocean chemistry. This wouldexplain why anti-phase variations in their abundances occurred during several earlyPaleogene hyperthermals that happened before the EECO. The EECO interval mayrepresent the time when optimal conditions for morozovellids diminished for asufficiently long time, such that acarininids dominated surface water habitats afterward. [ABSTRACT FROM AUTHOR]