No abstract

Carbonate‐forming organisms play an integral role in the marine inorganic carbon cycle, yet the links between carbonate production and the environment are insufficiently understood. Carbonate production is driven by the abundance of calcifiers, and the amount of calcite produced by each individual (their size and weight). Here we investigate how foraminiferal carbonate production changes in the Atlantic, Pacific and Southern Ocean in response to a 4‐5°C warming and a 0.3 surface ocean pH reduction during the Palaeocene‐Eocene Thermal Maximum (PETM). To put these local data into a global context, we apply a trait‐based plankton model (ForamEcoGEnIE) to the geologic record for the first time. Our data illustrates negligible change in the assemblage test size and abundance of foraminifers. ForamEcoGEnIE resolves small reductions in size and biomass, but these are short‐lived. The response of foraminifers shows spatial variability linked to a warming‐induced poleward migration and suggested differences in nutrient availability between open‐ocean and shelf locations. Despite low calcite saturation at high latitudes, we reconstruct stable foraminiferal size‐normalised weight. Based on these findings, we postulate that sea surface warming had a greater impact on foraminiferal carbonate production during the PETM than ocean acidification. Changes in the composition of bulk carbonate suggest a higher sensitivity of coccolithophores to environmental change during the PETM than foraminifers.This article is protected by copyright. All rights reserved.

<p>During the Pliocene the planktic ecosystem, for the first time in its evolutionary history, experienced a separation of the tropical seaways. At the same time, low latitude planktic foraminifers reached sizes not seen for millions of years. We set out to examine whether the closure of the Central American Seaway (CAS) led to the reorganisation of the planktic food web and enabled this growth. As many plankton are not well preserved in the fossil record, we applied a trait-based ecosystem model for plankton – ForamEcoGEnIE – to an open and closed CAS Pliocene environment. ForamEcoGEnIE is an extension of the size-structured 3-D plankton ecosystem model, “EcoGEnIE” that includes non-spinose planktic foraminifers as a new functional group based on the costs and benefits of key traits (e.g. growth, grazing, calcification). We test whether the planktic food web and planktic foraminiferal physiology responded to this change in paleogeography by quantifying changes in plankton biomass and size. In large regions of the ocean, we observe no change in phytoplankton, zooplankton, or foraminiferal biomass in response to a closed CAS. However, we note an increase in biomass at the eastern equatorial Pacific and a decrease in the North Atlantic in response to the closure of the CAS. ForamEcoGEnIE predicts an increase in non-spinose foraminiferal body size at the eastern equatorial Pacific and a small decrease in the North Atlantic. We attribute the Pacific response to increased upwelling due to the closure of the CAS and in the North Atlantic we suggest the reduction in biomass and size is linked with the reorganisation of surface ocean currents. As much of the ocean shows no response, we tentatively conclude that the closure of the CAS did not induce a major reorganisation of the planktic ecosystem. </p>

<p>Size is fundamentally important in individual planktic foraminifers as it determines the number of offspring and hence reproductive success. Over evolutionary timescales, individuals tend to increase in size, altering the average size of assemblages; in the last million years, individuals were larger than at any other time in the geological record. This pattern is specifically driven by size increases in tropical and subtropical taxa in the Plio-Pleistocene. Here we use a taxon-free approach to assess what facilitated this novelty and to quantify the response of planktic foraminifers to both long and short time-scale environmental changes. We focus on the Pliocene, a time interval characterized by the closure of the Central American Seaway (CAS) and the short glaciation at marine isotope stage (MIS) M2. We measured size in foraminiferal assemblages using automated microscopy across 24 globally distributed PRISM locations, from subpolar to tropical environments. The 95<sup>th</sup> percentile was calculated on the maximum diameter measurements of 1.28 million specimens. Although there is a slight decrease in the average size of the assemblage in the high latitudes from the Early to Late Pliocene, with minimal changes in the tropics, results indicate little to no effect on foraminiferal size across MIS M2. The results show an unexpected stability and resilience against the reorganization of the tropical oceans associated with the closure of the CAS.</p>

<p>The scientific community is increasingly focused on the study of past climate analogues to better comprehend future implications of global warming on marine ecosystem and biogeochemical cycles. Through an integrated calcareous plankton and geochemical approach, we examined the Terche and Madeago (NE Italy) Tethyan sections, that encompass the hyperthermal Eocene Thermal Maximum 2 (ETM2, ~54 Ma). The ETM2 shares similarities with the current climate context such as global warming, carbon cycle perturbations, high <em>p</em>CO<sub>2</sub> and ocean acidification, thus representing a key event in which investigate links between climate and biotic changes.</p><p>Our planktic foraminiferal and calcareous nannofossil records show significant, though transient, changes in both the sections across this event. Our record of multiple dissolution proxies from both the sections ensures that dissolution did not affect calcareous plankton assemblages. Planktic foraminifera exhibit a marked increase in warm index surface-dweller <em>Acarinina</em> paralleled by a decline in abundance of the deeper-dweller chiloguembelinids and subbotinids. This implies that the ETM2 warming impacted the entire upper water column. Both chiloguembelinids and subbotinids, recognized as eutrophic and colder taxa, may have suffered, beside warming, of reduced food supply at the thermocline due to the increased surface-water remineralization of organic matter as induced by the significant warmth. Increase of Cretaceous calcareous nannofossils testifies reworking related to enhanced hydrological cycle that also generated input of nutrients from land. Surface water eutrophication during the ETM2 was inferred by rise in calcareous nannofossil eutrophic indices. This group proved to be more sensitive to the nutrient supply rather than warming. The ETM2 consequence on marine calcifiers test-size were not previously explored. We provide here new evidence of a striking test size reduction in planktic foraminiferal assemblages (up to 40%) during the ETM2 that involved both the surface and deeper-dweller taxa that is particularly marked at Madeago. Although loss of symbionts (bleaching) is known to affect test calcification, it cannot represent a likely cause in the studied case as both symbiotic and asymbiotic planktic foraminifera were affected at the same scale across the ETM2. The increased abundance of small placoliths <em>Toweius</em> and <em>Ericsonia</em> indicates that size reduction also impacted nannofossils, though to a lesser degree.</p><p>We collected Hg (ppb), TOC (wt%) and Hg/TOC (ppb/wt%) data throughout our sections to test whether the reduction in size was related to environmental stressors not commonly linked to the ETM2. An increase in Hg was indeed detected in both sections at the base of the ETM2 and not corresponding to the intervals of reworking. Coeval submarine igneous events in this area might have introduced biolimiting metals thus involving the calcareous plankton productivity and possibly affecting test sizes. We hypothesize that the striking plankton dwarfism here recorded across the ETM2 is the result of the combined effect of paleoenvironmental perturbations induced by this event and increase in biolimiting metals. Our study alerts on possible consequences related to intense warming as associated to igneous events. Further data from different locations are needed to evaluate the geographic extension of impact on test-size variations in calcareous plankton assemblages.</p>