The middle Eocene climatic optimum (MECO, ~40 Ma) was a transient period of global warming that interrupted the secular Cenozoic cooling trend. We investigated the paleoceanographic, ...paleoenvironmental, and paleoecological repercussions of the MECO in the southeast Atlantic subtropical gyre (Ocean Drilling Program Site 1263). TEX86 and δ18O records support an ~4°C increase in surface and deepwater temperatures during the MECO. There is no long‐term negative carbon isotope excursion (CIE) associated with the early warming, consistent with other sites, and there is no short‐term negative CIE (~50 kyr) during the peak of the MECO, in contrast to what has been observed at some sites. This lack of a CIE during the peak of the MECO at Site 1263 could be due to poor sediment recovery or geographic heterogeneity of the δ13C signal. Benthic and planktic foraminiferal mass accumulation rates markedly declined during MECO, indicating a reduction of planktic foraminiferal production and export productivity. Vertical δ13C gradients do not indicate major changes in water column stratification, and there is no biomarker or micropaleontological evidence that hypoxia developed. We suggest that temperature dependency of metabolic rates could explain the observed decrease in foraminiferal productivity during warming. The kinetics of biochemical reactions increase with temperature, more so for heterotrophs than for autotrophs. Steady warming during MECO may have enhanced heterotroph (i.e., foraminiferal) metabolic rates, so that they required more nutrients. These additional nutrients were not available because of the oligotrophic conditions in the region and the lesser response of primary producers to warming. The combination of warming and heterotroph starvation altered pelagic food webs, increased water column recycling of organic carbon, and decreased the amount of organic carbon available to the benthos.
Key Points
Uniform surface to bottom temperature increase during MECO
No water column stratification or primary productivity changes during MECO
Warming may have directly affected pelagic and benthic ecosystems
The Middle Eocene Climatic Optimum (MECO) was one of the most severe, short-term global climate perturbations of the Cenozoic that occurred at ca. 40Ma and was characterized by a gradual 4–6°C ...temperature increase of intermediate and deep-waters. We investigated the response to the MECO of the deep-sea ecosystem in the central-western Tethys, through a quantitative study of bathyal benthic foraminiferal assemblages in the expanded and continuous Alano section (northeastern Italy), for which data on stratigraphy, lithology, isotope and trace element geochemistry, and calcareous microplankton were available. During the gradual warming of MECO (lasting between 350 and 650kyr) marine export productivity increased, causing a significant but transient restructuring of benthic foraminiferal faunas, which changed gradually from assemblages typical for oligo-mesotrophic sea floor conditions to assemblages indicative of more eutrophic conditions. Just after the peak MECO conditions, which lasted less than 100kyr, a prolonged phase of environmental instability (~500kyr) occurred, marked by even more highly increased export productivity leading to bottom-water oxygen depletion, as reflected in deposition of organic-rich sediments and multiple peaks of bi-triserial opportunistic benthic foraminiferal taxa, including buliminids, bolivinids and uvigerinids. The high productivity may have been caused by a strong influx of nutrient-bearing fresh water into the basin, due to the increased vigour of the hydrological cycle during the warm period, and this increased fresh-water influx might have been a factor in enhancing water column stratification, thus exacerbating the hypoxic conditions, which persisted about 400–500kyr. After deposition of the organic-rich layers the environmental perturbation ended, and benthic foraminiferal assemblages recovered while conditions became very similar to what they were before the MECO. The environmental disturbance during and directly after the MECO thus strongly but transiently affected benthic foraminiferal assemblages in the central western Tethys.
•We studied benthic foraminifera from a Tethyan section spanning the MECO.•Benthic foraminifera reflect paleoenvironmental changes across the MECO.•Foraminiferal changes suggest variations in productivity and sea-floor oxygenation.•Sea-floor eutrophication and oxygen depletion follow the MECO peak warming.•No permanent effects on benthic foraminifera have been detected.
We present trace metal geochemistry and stable isotope records for the middle Eocene Alano di Piave section, NE Italy, deposited during magnetochron C18n in the marginal Tethys Ocean. We identify a ...∼500 kyr long carbon isotope perturbation event we infer to be the middle Eocene climatic optimum (MECO) confirming the Northern Hemisphere expression and global occurrence of MECO. Interpreted peak climatic conditions are followed by the rapid deposition of two organic rich intervals (≤3% TOC) and contemporaneous positive δ13C excursions. These two intervals are associated with increases in the concentration of sulphur and redox‐sensitive trace metals and low concentrations of Mn, as well as coupled with the occurrence of pyrite. Together these changes imply low, possibly dysoxic, bottom water O2 conditions promoting increased organic carbon burial. We hypothesize that this rapid burial of organic carbon lowered global pCO2 following the peak warming and returned the climate system to the general Eocene cooling trend.
Pronounced warming negatively impacts ecosystem resilience in modern oceans. To offer a long‐term geological perspective of the calcareous plankton response to global warming, we present an ...integrated record, from two Tethyan sections (northeastern Italy), of the planktic foraminiferal and calcareous nannofossil response to the Eocene Thermal Maximum 2 hyperthermal (ETM2, ∼54 Ma). Our study reveals pronounced changes in assemblage composition and a striking dwarfing of planktic foraminiferal tests of up to 40% during the event, impacting both surface and deeper dwellers. The increased abundance of small placoliths among calcareous nannofossils is interpreted as community size reduction. Literature and our foraminiferal size data from Sites 1263 and 1209 (Atlantic and Pacific Oceans) highlights that the pronounced dwarfism is restricted to the Tethyan area. The ETM2 is characterized by warm sea surface temperatures as indicated by our δ18O data, but this warming is of global extent and cannot explain the unique dwarfism. Excluding evolutionary modifications, other potential drivers of dwarfism (eutrophication, deoxygenation, metabolic adaptation) cannot explain the exceptional dwarfism by themselves. The smallest sizes are in close temporal association with peaks in volcanic derived Hg/Th‐Hg/Rb recorded just before and at the ETM2 which could not have been brought into our sections through weathering. In contrast, size reductions are absent below and above the ETM2 at Hg peaks where δ18O data do not show warm conditions. We speculate that the local input of toxic metals from submarine volcanic emissions could have acted synergistically to warming, causing the unique dwarfism.
Key Points
Calcareous plankton size in the Tethys during the Eocene Thermal Maximum 2 (ETM2) reveals marked dwarfism
Pronounced dwarfism was restricted to the Tethyan area, highlighting the importance of local signals in interpreting hyperthermals
Calcareous plankton were highly unstable across the ETM2 but ultimately resilient
Calcareous nannofossil, mineralogical and geochemical data are presented from the expanded Paleocene–Lower Eocene Forada section in the Venetian Pre-Alps. The short-lived Paleocene Eocene Thermal ...Maximum (PETM) is characterized by profound and temporally constrained changes among calcareous nannoplankton assemblages, triggered by global modifications of the ocean–atmosphere system. The Calcareous Nannofossil Excursion Taxa (CNET) represent transient adaptations to changes in the chemical (nutrients, CO
2, pH) and temperature structure of surface waters typifying the PETM. Oligotrophic taxa, such as
Sphenolithus, Zygrhablithus, Octolithus and
Fasciculithus, exbihit a sharp decrease in abundance, whereas reworked forms show a sharp increase. These changes reflect a response to the modified environmental conditions, characterized by sharp increase in nutrient availability. This increase likely resulted from a huge terrigenous input, suggesting that the siliciclastic pump was intensified by enhanced weathering and runoff. The high correlations between mineralogical/geochemical data and calcareous nannofossil fluctuations is consistent with this scenario. Highly-resolved age estimations are established for several calcareous nannofossil biohorizons based on cyclostratigraphy.
Detailed analyses of the benthic foraminiferal assemblages extracted with the cold acetolyse method together with high resolution geochemical and mineralogical investigations across the ...Paleocene/Eocene (P/E) boundary of the classical succession at Contessa Road (western Tethys), allowed to recognize and document the Paleocene–Eocene Thermal Maximum (PETM) interval, the position of the Benthic Extinction Event (BEE) and the early recovery of benthic faunas in the aftermath of benthic foraminiferal extinction. The stratigraphical interval spanning the P/E boundary consists of dominantly pelagic limestones and two prominent marly beds. Benthic foraminifera indicate that these sediments were deposited at lower bathyal depth, not deeper than 1000–1500 m. The Carbon Isotope Excursion (CIE) interval is characterized by high barite abundance with a peak at the base of the same stratigraphic interval, indicating a complete, although condensed record of the early CIE. A succession of events and changes in the taxonomic structure of benthic foraminifera has been recognized that may be of use for supra-regional stratigraphic correlation across the P/E boundary interval. The composition of the benthic foraminiferal assemblages, dominated by infaunal taxa, indicates mesotrophic and changing conditions on the sea floor during the last ∼
45 kyr of the Paleocene. The BEE occurs at the base of the CIE within the lower marly bed and it is recorded by the extinction of several deep-water cosmopolitan taxa. Then, the lysocline/CCD rose and severe carbonate dissolution occurred. Preservation deteriorated, the faunal density and simple diversity dropped to minimum values and a peak of
Glomospira spp. has been observed. Stress-tolerant and opportunistic groups, represented mainly by bi-and triserial taxa, dominate the low-diversity post-extinction assemblages, indicating a benthic foraminiferal recovery under environmental unstable conditions, probably within a context of sustained food transfer to the bottom. A three-phase pattern of faunal recovery is recognizable. At first the lysocline/CCD started to descend and then recovered. Small-sized “
Bulimina”,
Oridorsalis umbonatus, and
Tappanina selmensis rapidly repopulated the severely stressed environment. Later on,
Siphogenerinoides brevispinosa massively returns, dominating the assemblage together with other buliminids,
Nuttallides truempyi, and
Anomalinoides sp.1. Finally, a marked drop in abundance of
S. brevispinosa is followed by a bloom of the opportunistic and recolonizer agglutinated
Pseudobolivina that, for the first time, is recorded within the main CIE. A second interval of dissolution, but less severe than the previous one, has been recognized within the upper marly bed (uppermost part of the main CIE interval) and it is interpreted as a renewed, less pronounced shoaling of the lysocline/CCD that interrupted the recovery of benthic faunas. This further rise likely represents a response to persistent instability of ocean geochemistry in this sector of the Tethys before the end of the CIE. In the CIE recovery and post CIE intervals, the composition of the benthic foraminiferal assemblages suggests mesotrophic and unstable conditions at the sea floor. According to the geochemical proxy for redox conditions, the deposition of the PETM sediments at Contessa Road occurred in well-oxygenated waters, leading out a widespread oxygen depletion as major cause of the BEE. Changing oceanic productivity, carbonate corrosivity and global warming appear to have played a much more important role in the major benthic foraminiferal extinction at the P/E boundary.
The Forada section in the Venetian Pre-Alps of northern Italy represents an expanded record of the Paleocene–Eocene Thermal Maximum (PETM) at a depositional paleodepth of about 1 km
±
0.5 km. ...High-resolution planktonic foraminiferal analysis of this section, in a time interval of approximately 1.3 Myr across the Paleocene/Eocene boundary, reveals striking faunal changes that allow the identification of eight phases (a–h). The late Paleocene was represented by stable, warm and oligotrophic surface water conditions (phase a). Unstable environmental conditions start well before the onset of PETM (ca. 150 kyr, phase b) and involved a change towards eutrophy, as marked by the increase of
Subbotina and the concomitant decrease of
Morozovella. This step is also characterized by enhanced fragmentation and dissolution.
The interval corresponding to the main body of the carbon isotope excursion (CIE) is characterized by a marked increase of
Acarinina, though with some differences in the species composition and relative abundance, both in high-and low-latitudes, particularly in the Tethyan area. Forada is no exception to this pattern. However, at Forada, two prominent peaks in abundance of acarininids are recorded ca. 30 kyr prior to the onset of the CIE, thus suggesting an increase in temperature heralding the onset of the PETM (phase c). Interestingly, the lower peak in abundance of
Acarinina just precedes the 1‰ carbon isotope negative shift occurring below the onset of the main CIE. The basalmost Eocene, corresponding to the lower part of CIE curve, is represented by intense planktonic foraminiferal dissolution, implying an extraordinary rise of the CCD. This interval has an estimated duration of about 16 kyr (phase d).
The dominance of acarininids in the lower part of the CIE (phase e, f; ca. 14 and 22.5 kyr) is interpreted as a consequence of the extreme warmth coupled with eutrophic conditions characterizing the Forada depositional environment at that time. These acarininids include at Forada also the temporally constrained
Acarinina sibaiyaensis and
A. africana. The morphological similarity between these peculiar species with the radially elongated chambered forms characterizing the Cretaceous anoxic events, suggests the hypothesis that depletion of oxygen in the upper water column might have been one of the factors causing their conspicuous occurrence at the PETM.
The recovery in abundance of the specialized morozovellids and of other planktonic foraminiferal groups (e.g., biserials, globanomalinids, igorinids, planorotalids and pseudohastigerinids), occurring in the middle part of the CIE (ca. 30 kyr after the onset of the PETM), indicates an initial environmental recovery (phase g). A new stable state is definitely reached in the upper part of the Forada section where the relative proportions of the main component of planktonic foraminiferal assemblages move towards values similar to those of the late Paleocene conditions (phase h). However, the perturbation during the PETM produced significant changes in the ocean geochemistry that endured after the PETM event, as testified by the prominent high carbonate dissolution characterizing the marly levels, and the large variability in relative abundance among different components of the planktonic foraminiferal assemblages. These striking oscillations were not present in the latest Paleocene.
An integrated micropalaeontological (planktonic foraminifera and calcareous nannofossils), mineralogical and stable isotope investigation was carried out across the Cretaceous/Tertiary (K/T) boundary ...in a previously undescribed hemipelagic section (Forada section) cropping out in the Piave River Valley (Southern Alps, Belluno Province, NE Italy). Our results suggest that an apparently complete K/T transition with an expanded basal Danian is preserved in the Forada section. The K/T boundary is marked by a ca. 1.5-cm-thick clay interval intercalated in the pelagic carbonates of the
Scaglia Rossa Formation. The boundary clay, characterised by an iridium anomaly, is constituted by a basal 1–2 mm-thick “green layer” and an overlying “red layer”. Below the boundary clay, the calcareous nannofossil
Micula murus Zone and the planktonic foraminiferal
Plummerita hankteninoides Zone, that characterise the terminal Maastrichtian in complete sections, have been identified. Virtually all the calcareous plankton biohorizons normally utilised in the early Danian have been recognised. Interestingly, the ranking of the various biohorizons is the same observed elsewhere if considering the single plankton groups separately, while several inconsistencies emerge if considering the relative succession of the two calcareous plankton groups simultaneously. We caution to infer hiatuses on a section based only on the spacing of the biohorizons of a single group because of the risk of diachroneities in the stressed early Danian environment. By comparing the “distance” of the key biohorizons (i.e. the First Occurrence (FO) of
Cruciplacolithus primus and the Last Occurrence (LO) of
Parvularugoglobigerina eugubina) from the boundary clay at Forada and in other complete classical sections, we have inferred that the early Danian is expanded in the Forada section. The section seems ideally suited for studying the recovery pattern of the pelagic ecosystem and calcareous plankton communities in the aftermath of the K/T catastrophe in a pelagic setting of the western central Tethys. The adaptive radiation of planktonic foraminifera was very rapid in contrast with the delayed diversification of coccolithophores. The latter show a ca. 21-cm-thick “dead zone” that is followed by a long-lasting survival interval (ca. 180-cm-thick, that might represent some 200 kyr) dominated by persistent Cretaceous coccolithophores and calcareous dinoflagellate
Thoracosphaera spp. The temporal pattern in recovery of the coccolithophore and planktonic foraminiferal communities is decoupled as well pointing to their varying sensitivities to the K/T perturbation. It appears that the ecospace occupied by coccolithophores in the terminal Maastrichtian was occupied in the initial Danian by the calcareous dinoflagellates (
Thoracosphaera spp.), that remained a significant component of the assemblage up to the base of the
Prinsius dimorphosus Zone of Romein Romein, A.J.T., 1979. Lineages in Early Paleogene calcareous nannoplankton. Utrecht Micropaleontological Bulletins, 22. 231 pp., probably in the early Chron C29n. On the contrary, planktonic foraminifera seem to reach stable conditions earlier, in the late
Cruciplacolithus primus Zone. The early Danian interval considered is also characterised by low Sr/Ca ratio of the biogenic carbonates that would suggest dramatically low coccolithophore productivity conditions, in agreement with previous flux studies. We speculate that the altered ecological structure of phytoplankton community and the low productivity of coccolithophores severely affected the efficiency of the transfer of organic carbon from the photic zone to the deep ocean, thus supporting the early Danian “living ocean” model of D'Hondt et al. D'Hondt, S., Donaghay, P., Zachos, J.C., Luttenberg, D., Lindinger, M., 1998. Organic carbon fluxes and ecological recovery from the Cretaceous–Tertiary mass extinction. Science 282, 276–279.
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