Onset Of Carbon Isotope Excursion Research Articles

Palynofloral Change Through the Paleocene‐Eocene Thermal Maximum in the Bighorn Basin, Wyoming

AbstractTo better understand the effect of the Paleocene‐Eocene Thermal Maximum (PETM) on continental ecosystems, we studied 40 new palynological samples from the Bighorn Basin (BHB), northwestern Wyoming, USA. We see palm and fern abundances increase in the last 20–40 ka of the Paleocene, then dramatically with the onset of the carbon isotope excursion (CIE) defining the base of the PETM. Palynomorphs of plant groups with modern temperate climate distributions are absent from the CIE body, and this is when tropical plants are most diverse and abundant. During the CIE recovery, pollen of mesophytic/wetland plants become more common while tropical taxa persist. In the post‐CIE early Eocene tropical taxa are rare and temperate forms abundant, similar to the late but not latest Paleocene. Changes in the palynoflora are more easily detected if reworked palynomorphs are removed from analyses. We interpret palynofloral changes to indicate warming in the latest Paleocene, rapid warming and drying with the CIE onset, dry tropical climates through the CIE body, a return to wetter floodplains during a very warm CIE recovery, and cooler wet conditions in the post‐PETM early Eocene. These inferences are consistent with geochemical and paleobotanical proxies. Strikingly similar patterns in the palynoflora and megaflora suggest changes in vegetation were a basin‐wide phenomenon. These rapid, climatically forced changes in floral composition occurred without major extinction, perhaps indicating nearby refugia in which plants adapted to cooler and wetter climates persisted through the PETM.

Clay hydroxyl isotopes show an enhanced hydrologic cycle during the Paleocene-Eocene Thermal Maximum

The Paleocene-Eocene Thermal Maximum (PETM) was an abrupt global warming event associated with a large injection of carbon into the ocean-atmosphere system, as evidenced by a diagnostic carbon isotope excursion (CIE). Evidence also suggests substantial hydrologic perturbations, but details have been hampered by a lack of appropriate proxies. To address this shortcoming, here we isolate and measure the isotopic composition of hydroxyl groups (OH−) in clay minerals from a highly expanded PETM section in the North Sea Basin, together with their bulk oxygen isotope composition. At this location, we show that hydroxyl O- and H-isotopes are less influenced than bulk values by clay compositional changes due to mixing and/or inherited signals and thus better track hydrologic variability. We find that clay OH− hydrogen-isotope values (δ2HOH) decrease slowly prior to the PETM and then abruptly by ∼8‰ at the CIE onset. Coincident with an increase in relative kaolinite content, this indicates increased rainfall and weathering and implies an enhanced hydrologic cycle response to global warming, particularly during the early stages of the PETM. Subsequently, δ2HOH returns to pre-PETM values well before the end of the CIE, suggesting hydrologic changes in the North Sea were short-lived relative to carbon-cycle perturbations.

Calcium isotope composition ofMorozovellaover the late Paleocene–early Eocene

AbstractOcean acidification (OA) during the Paleocene-Eocene thermal maximum (PETM) likely caused a biocalcification crisis. The calcium isotope composition (δ44/40Ca) of primary carbonate producers may be sensitive to OA. To test this hypothesis, we constructed the first high-resolution, high-precision planktic foraminiferal δ44/40Ca records before and across the PETM. The records employ specimens of Morozovella spp. collected from Ocean Drilling Program Sites 1209 (Shatsky Rise, Pacific Ocean) and 1263 (Walvis Ridge, Atlantic Ocean). At Site 1209, δ44/40Ca values start at −1.33‰ during the Upper Paleocene and increase to a peak of −1.15‰ immediately before the negative carbon isotope excursion (CIE) that marks the PETM onset. Values remain elevated through the PETM interval and decrease into the earliest Eocene. A shorter-term record for Site 1263 shows a similar trend, although δ44/40Ca values are on average 0.22‰ lower and decrease shortly after the CIE onset. The trends support neither diagenetic overprinting, authigenic carbonate additions, nor changes in the δ44/40Ca value of seawater. Rather, they are consistent with a kinetic isotope effect, whereby calcite δ44/40Ca values inversely correlate with precipitation rate. Geologically rapid Ca isotope shifts appear to reflect the response of Morozovella to globally forced changes in the local carbonate geochemistry of seawater. All data combined suggest that the PETM-OA event occurred near the peak of a gradual reduction in seawater carbonate ion concentrations during a time of elevated atmospheric pCO2, potentially driven by North Atlantic igneous province emplacement.

Stratigraphic and sedimentological aspects of the worldwide distribution of Apectodinium in Paleocene/Eocene Thermal Maximum deposits

Abstract The Paleocene/Eocene Thermal Maximum (PETM) is characterized by pronounced global warming and associated environmental changes. In the more-or-less two decades since prior regional syntheses of Apectodinium distribution at the PETM, extensive biological and geochemical datasets have elucidated the effect of rising world temperatures on climate and the biome. A Carbon Isotope Excursion (CIE) that marks the Paleocene/Eocene Boundary is associated with an acme of marine dinocysts of the genus Apectodinium in many locations. Distinctive foraminiferal and calcareous nannofossil populations may also be present. For this updated, dinocyst-oriented view of the PETM, data from worldwide locations have been evaluated with an emphasis on stratigraphic and sedimentological context. What has emerged is that a change in lithology is common, often to a distinctive siltstone or claystone unit, which contrasts with underlying and overlying lithotypes. This change, present in shallow marine/coastal settings and in deep-water turbidite deposits, is attributed to radical modifications of precipitation and erosional processes. An abrupt boundary carries the implication that some time (of unknowable duration) is potentially missing, which then requires caution in the interpretation of the pacing of events in relation to that boundary. In most instances an ‘abrupt’ or ‘rapid’ CIE onset can be attributed to a data gap at a hiatus, particularly in shallow shelf settings where transgression resulted from sea-level rise associated with the PETM. Truly gradational lower boundaries of the PETM interval are quite unusual and, if present, are poorly known so far. Gradational upper boundaries are more common, but erosional upper boundaries have been reported. Taxonomic changes have been made to clarify identification issues that have adversely impacted some biostratigraphic interpretations. Apectodinium hyperacanthum has been retained in Wetzeliella , its original genus. The majority of specimens previously assigned to Apectodinium hyperacanthum or Wetzeliella ( Apectodinium ) hyperacanthum have been reassigned to an informal species, Apectodinium sp. 1. Dracodinium astra has been retained in its original genus as Wetzeliella astra and is emended.

Unraveling short- and long-term carbon cycle variations during the Oceanic Anoxic Event 2 from the Paris Basin Chalk

Abstract The Oceanic Anoxic Event 2 (OAE2, ca. 94.6 Ma) is one of the major perturbations in the global carbon cycle during the Phanerozoic. Stable carbon isotopes (δ13C) from marine and continental sedimentary environments document this carbon cycle perturbation with a pronounced (> 2‰) positive carbon isotope excursion (CIE). Although the OAE2 stratigraphic interval has been intensively studied in terms of paleoceanography and paleoclimatology, several climatic and carbon cycle aspects are not yet well-understood. In particular, cyclic short-term Milankovitch-scale δ13C variations within the OAE2 and their potential implications for the global carbon cycle have been rarely addressed. Here, we present high-resolution (5 cm, ~2 kyr) δ13C data spanning the OAE2 from the Paris Basin Chalk (Poigny Craie-701 drill-core) to show high amplitude short-term δ13C oscillations, superimposed on the major CIE. Time-series analysis indicates that short-term oscillations are astronomically paced, with eccentricity cycles being the most prominent. Orbital forcing of δ13C variations is further supported by time-series analysis of the English Chalk (Eastbourne section). We suggest that orbitally paced carbon cycle oscillations were amplified by considerable emission of greenhouse gases from volcanism that caused the overall CIE. Astronomical calibration of the whole OAE2 (the perturbation and recovery phases) from the Poigny record provides a duration equivalent to eight to eight and a half short eccentricity cycles. Cyclostratigraphic correlations among several OAE2 key records indicate the same duration of the whole CIE. However, duration of the interval from the onset of CIE till the Cenomanian-Turonian boundary (CTB) is significantly different from one basin to another. In particular, a difference of almost two short eccentricity cycles is highlighted between the Anglo-Paris and Western Interior basins. According to cyclostratigraphic approach and correlations, the entry of W. devonense was at least 200 kyr later in the Western Interior Basin (WIB, USA) than in Europe. Key calcareous nannofossil biohorizons (e.g., Quadrum gartneri) are also stratigraphically upshifted in the WIB with respect to the European sections, hence concurring with the hypothesis of a younger CTB in the WIB. We ascribe such significant temporal offset to diachroneity of the CTB, which is likely the result of different, regional biotic responses to the global OAE2 paleoenvironmental perturbation.

Effects of size-dependent sediment mixing on deep-sea records of the Paleocene-Eocene Thermal Maximum

Abstract Stratigraphic features of the carbon isotope excursion (CIE) marking the Paleocene-Eocene Thermal Maximum (PETM; ca. 55.8 Ma) are used to study ocean-climate change and carbon cycling during this ancient global warming event. Yet discrepancies in its timing and amplitude exist between bulk-carbonate and planktic-foraminifera δ13C records. Here we examine these disparities through the lens of δ13C compositions of size-segregated planktic shells across the pre-CIE to CIE transition in the iconic PETM section of Ocean Drilling Program Site 690 in the Weddell Sea. Our results show that the stratigraphic position of the CIE onset is dependent upon shell size, which we attribute to preferential mixing of smaller shells with pre-CIE δ13C values up into the overlying CIE interval. Hence, the transitory loss of size-dependent δ13C signatures in photosymbiotic planktic foraminifera is a taphonomic artifact, not a geochemical signal of symbiont “bleaching” during the PETM. Our results also indicate that many salient features of the Site 690 bulk-carbonate δ13C record are aberrations caused by size-dependent sediment mixing, and as such, should not be viewed as primary signals of ocean-climate change during what is arguably one of the best ancient analogs for future ocean-climate change.

The extraterrestrial impact evidence at the Palaeocene–Eocene boundary and sequence of environmental change on the continental shelf

We have identified clear evidence of an extraterrestrial impact within the onset of the carbon isotope excursion (CIE) that defines the Palaeocene-Eocene (P-E) boundary hyperthermal event (approx. 56 Ma) from several sites on the eastern Atlantic Coastal Plain and offshore. We review and update the state of the evidence for an impact at the P-E boundary, including a K-Ar cooling age of the ejecta that is indistinguishable from the depositional age at the P-E, which establishes the ejecta horizon as an isochronous stratigraphic indicator at the P-E. Immediately above the ejecta peak at the base of the coastal plain Marlboro Clay unit, we identify a sharp increase in charcoal abundance coincident with the previously observed dramatic increase in magnetic nanoparticles of soil pyrogenic origin. We therefore revisit the observed sequence of events through the P-E boundary on the western Atlantic Coastal Plain, showing that an extraterrestrial impact led to wildfires, landscape denudation and deposition of the thick Marlboro Clay, whose base coincides with the spherule horizon and CIE onset. The Sr/Ca ratio of the spherules indicates that the carbon responsible for the onset may be vaporized CaCO3 target rock mixed with isotopically light carbon from the impactor or elsewhere. Crucially, we do not argue that the impact was responsible for the full manifestation of the CIE observed globally (onset to recovery approx. 170 kyr), rather that a rapid onset was triggered by the impact and followed by additional carbon from other processes such as the eruption of the North Atlantic Igneous Province. Such a scenario agrees well with recent modelling work, though it should be revisited more explicitly.This article is part of a discussion meeting issue 'Hyperthermals: rapid and extreme global warming in our geological past'.

Response of larger benthic foraminifera to the Paleocene-Eocene thermal maximum and the position of the Paleocene/Eocene boundary in the Tethyan shallow benthic zones: Evidence from south Tibet

Research Article| August 01, 2018 Response of larger benthic foraminifera to the Paleocene-Eocene thermal maximum and the position of the Paleocene/Eocene boundary in the Tethyan shallow benthic zones: Evidence from south Tibet Qinghai Zhang; Qinghai Zhang † 1Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, and CAS Centre for Excellence in Tibetan Plateau Earth Sciences, Lincui Road 16-3, 100101 Beijing, China2Department of Geosciences, University of Bremen, Klagenfurter Straße 4, 28359 Bremen, Germany †zhang@itpcas.ac.cn Search for other works by this author on: GSW Google Scholar Helmut Willems; Helmut Willems 2Department of Geosciences, University of Bremen, Klagenfurter Straße 4, 28359 Bremen, Germany3Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, East Beijing Road 39, 210008 Nanjing, China Search for other works by this author on: GSW Google Scholar Lin Ding; Lin Ding 1Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, and CAS Centre for Excellence in Tibetan Plateau Earth Sciences, Lincui Road 16-3, 100101 Beijing, China Search for other works by this author on: GSW Google Scholar Xiaoxia Xu Xiaoxia Xu 2Department of Geosciences, University of Bremen, Klagenfurter Straße 4, 28359 Bremen, Germany Search for other works by this author on: GSW Google Scholar GSA Bulletin (2019) 131 (1-2): 84–98. https://doi.org/10.1130/B31813.1 Article history received: 24 Mar 2017 rev-recd: 21 May 2018 accepted: 19 Jun 2018 first online: 01 Aug 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation Qinghai Zhang, Helmut Willems, Lin Ding, Xiaoxia Xu; Response of larger benthic foraminifera to the Paleocene-Eocene thermal maximum and the position of the Paleocene/Eocene boundary in the Tethyan shallow benthic zones: Evidence from south Tibet. GSA Bulletin 2018;; 131 (1-2): 84–98. doi: https://doi.org/10.1130/B31813.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Paleocene-Eocene thermal maximum (PETM) is one of the most pronounced global warming events in the Cenozoic. This event was associated with a large negative carbon isotope excursion (CIE) and with major changes in the atmosphere, hydrosphere, and biosphere. However, how the larger benthic foraminifera (LBFs) in the shallow Tethyan Ocean responded to the PETM remains controversial. In this study, we investigate two shallow-marine, LBF-rich carbonate sections from south Tibet, aiming to locate the position of the Paleocene/Eocene (P/E) boundary in the Tethyan shallow benthic zones (SBZs) and to examine the response of the LBFs to the PETM. Carbon isotope compositions of bulk carbonate were measured to constrain the stratigraphic position of the CIE onset marking the P/E boundary in the sections, and the LBFs were studied in rock thin sections in order to assess their biostratigraphy and to construct the SBZs. The combination of the carbon isotope data and constructed SBZs shows that the P/E boundary is located within SBZ5, not at the SBZ4/SBZ5 transition as proposed in the Western Tethyan domain. At the P/E boundary, no evident compositional change in LBF assemblages can be observed. However, a major compositional change in LBF assemblages occurs in the CIE recovery, characterized by the sudden disappearance of Miscellanea, Ranikothalia, Setia, Orbitosiphon, and the initial dominance of porcellaneous-walled Alveolina and Orbitolites together with small miliolids and rotaliids. We tentatively speculate that this compositional change in LBF assemblages may be related to a eutrophication event, likely resulting from intensified continental weathering during the CIE recovery of the PETM. You do not currently have access to this article.

An unexpected record of the PETM in terrestrial and organic sediments of Avesnois, between the Paris and Belgian Basins, NW Europe

An unexpected record of the PETM in terrestrial and organic sediments of Avesnois, between the Paris and Belgian Basins, NW Europe

Onset of carbon isotope excursion at the Paleocene-Eocene thermal maximum took millennia, not 13 years

The Paleocene-Eocene thermal maximum (PETM) may represent the best paleo-analog for rapid and massive carbon release to the ocean and atmosphere. Thus, constraining the carbon release rate at its onset is critical. Wright and Schaller (1) use records from apparently rhythmically layered shelf sediments to argue that the layering is annual and that the onset of the carbon isotope excursion (CIE, fingerprint for carbon release) in the surface ocean was complete in 13 y. Using basic carbon cycle and climate considerations, we show this is not feasible. In fact, Wright and Schaller’s isotope records indicate that the CIE onset took at least several millennia. This finding rules out a cometary origin of the carbon release.

Unsettled puzzle of the Marlboro clays

Global warming during the Paleocene-Eocene thermal maximum (PETM) was associated with the release of large amounts of 12C-enriched carbon, as reflected in the carbon isotope excursion (CIE) observed globally in sedimentary records. The New Jersey shelf PETM sequences are expanded relative to deep-sea settings, offering high temporal resolution to unravel the pathway of carbon release, although the exact amount of time represented is unknown because of unconformities (1, 2). Wright and Schaller (1) propose an almost instantaneous atmospheric carbon release, based on the assumption that rhythmic couplets in the New Jersey sediments represent annual layers, and that the CIE onset occurred over 13 couplets. Understanding these sequences remains challenging because of complex and extreme environmental changes during the PETM, including eustatic sea-level changes (3) and the development of a river-influenced shelf with high accumulation rates of fine-grained sediment (2). We argue that: (i) there is no compelling evidence for annual rhythmic layers; (ii) the accumulation rates of 2 cm/y are highly improbable in view of the microfossil content; (iii) the cross-shelf δ13C gradient in bulk carbonate does not reflect overall dissolved inorganic carbon; and (iv) the paleodepth estimates are significantly less than published estimates (2, 3).

Magnitude and profile of organic carbon isotope records from the Paleocene–Eocene Thermal Maximum: Evidence from northern Spain

The Paleocene–Eocene Thermal Maximum (PETM), a hyperthermal event that occurred ca. 56 Ma, has been attributed to the release of substantial amounts of carbon, affecting the atmosphere, biosphere and the oceans. Current issues with respect to our understanding of the PETM include the amount of carbon released, the duration of carbon release, and the mechanism(s) of release, all of which are related to the magnitude and profile of the associated Carbon Isotope Excursion (CIE). High-resolution organic carbon profiles (δ13C) of six PETM sections in northern Spain are presented that span a transect from continental to marine environments. These data represent the highest-resolution isotope records for these sections and allow a comparison of the magnitude of the excursion, the shape of the vertical δ13C profile during the PETM episode, and the relative timing of the onset of the excursion across a linked sediment routing system. Previous studies using carbonate δ13C data have suggested that the continental Claret Conglomerate, found in this region, formed synchronously with a marine clay-rich siliciclastic unit, with these key lithological changes interpreted to be driven by increased seasonal rainfall-runoff in the warmer PETM climate. Our data suggest that deposition of these units did not immediately follow the CIE onset, indicating that there may be a temporal lag between the onset of the PETM warming and the response of the depositional systems in northern Spain. No systematic variation in the magnitude of the CIE between different depositional environments was found; the marine CIE magnitudes are at the higher end of those previously described (3.7±1.4‰), and the continental ranges are lower (3.1±1.3‰).

Marine and terrestrial environmental changes in NW Europe preceding carbon release at the Paleocene–Eocene transition

Environmental changes associated with the Paleocene–Eocene thermal maximum (PETM, ∼56Ma) have not yet been documented in detail from the North Sea Basin. Located within proximity to the North Atlantic igneous province (NAIP), the Kilda Basin, and the northern rain belt (paleolatitude 54°N) during the PETM, this is a critical region for testing proposed triggers of atmospheric carbon release that may have caused the global negative carbon isotope excursion (CIE) in marine and terrestrial environments. The CIE onset is identified from organic matter δ13C in exceptional detail within a highly expanded sedimentary sequence. Pollen and spore assemblages analysed in the same samples for the first time allow a reconstruction of possible changes to vegetation on the surrounding landmass. Multiproxy palynological, geochemical, and sedimentologic records demonstrate enhanced halocline stratification and terrigenous deposition well before (103yrs) the CIE, interpreted as due to either tectonic uplift possibly from a nearby magmatic intrusion, or increased precipitation and fluvial runoff possibly from an enhanced hydrologic cycle. Stratification and terrigenous deposition increased further at the onset and within the earliest CIE which, coupled with evidence for sea level rise, may be interpreted as resulting from an increase in precipitation over NW Europe consistent with an enhanced hydrologic cycle in response to global warming during the PETM. Palynological evidence indicates a flora dominated by pollen from coastal swamp conifers before the CIE was abruptly replaced with a more diverse assemblage of generalist species including pollen similar to modern alder, fern, and fungal spores. This may have resulted from flooding of coastal areas due to relative sea level rise, and/or ecologic changes forced by climate. A shift towards more diverse angiosperm and pteridophyte vegetation within the early CIE, including pollen similar to modern hickory, documents a long term change to regional vegetation.

What happens when the ocean is overheated? The foraminiferal response across the Paleocene-Eocene Thermal Maximum at the Alamedilla section (Spain)

The global warming and major perturbation of the global carbon cycle that occurred during the Paleocene–Eocene Thermal Maximum (PETM) have been investigated in the lower bathyal–upper abyssal Alamedilla section (Spain). Geochemical anomalies and dramatic faunal changes (including the globally recognized extinction event of deep-sea benthic foraminifera and the rapid evolutionary turnover of planktic foraminifera and calcareous nannofossils) are associated with the PETM at Alamedilla. Biotic changes in the plankton and benthos indicate environmental instability ∼11–14 k.y. before the onset of carbon isotope excursion that marks the Paleocene/Eocene boundary. The reorganization of the planktic ecosystem points to warm and oligotrophic conditions in surface waters during the earliest Eocene, whereas faunal and geochemical data indicate that the extinctions of benthic foraminifera occurred over an interval with a high CaCO3 content and oxic conditions at the seafloor. The proliferation of disaster taxa ( Glomospira spp.) after the extinctions has been related to a potential source of isotopically light carbon in the western Tethys and North Atlantic. Significant changes in foraminiferal test size are documented across the PETM. We suggest that increased temperatures played an important role in benthic foraminiferal test size, increasing their metabolic rates and, consequently, their food requirements. Decreased planktic foraminiferal test size may be related to decreased nutrient availability or surface-water density. However, the differences in test size evolution among different species of both benthic and planktic foraminifera may be related to interspecific competition and ecological adaptations to direct or indirect consequences of the carbon addition during the PETM.

The Paleocene–Eocene Thermal Maximum record in the organic matter of the Claret and Tendruy continental sections (South-central Pyrenees, Lleida, Spain)

The Carbon Isotope Excursion (CIE) associated with the Paleocene–Eocene Thermal Maximum (PETM) has been detected for the first time in the total organic carbon (TOC) contained within the continental sediments of the Tremp Formation (South Central Pyrenees, Lleida, Spain). The δ 13C TOC magnitude of the CIE is ~ 4.3‰ in the Claret section and ~ 3.0‰ in the Tendruy section with minimum values of − 27.6‰ and − 26.7‰, respectively. Previous studies have detected the CIE in the South Central Pyrenees in both marine carbonates and within continental soil carbonate nodules. These studies have located the onset of the CIE either above or within the Claret Conglomerate, interpreted as a megafan produced by a profound change in the precipitation regimen at the beginning of the PETM. Our higher resolution δ 13C TOC study in the Claret and Tendruy sections places the onset of the CIE below the base of the Claret Conglomerate and therefore suggests a 4 to 9 kyr time lag between the onset of the CIE and an increase in intense seasonal precipitation rates. Furthermore, this study suggests that the CIE took place ~ 30–55 kyr after the deposition of important late Cernaysian Tremp mammalian sites (Claret-4, Tendruy-V, Tendruy and Palau) which contain the youngest known occurrence of endemic Paleocene mammalian taxa in Europe before the Mammalian Dispersal Event (MDE, migration of new mammal groups, such as perissodactyls, artiodactyls, primates, marsupials, carnivores, creodonts and rodents). The first immigrant mammals in Europe are recorded at Dormaal (Belgium) above a hiatus, the base of which has been dated to around the start of the CIE. Our data indicate that the MDE might have in fact occurred in Europe within a time interval of about 67 kyr around the CIE onset.

Response of calcareous nannofossils to the Paleocene–Eocene Thermal Maximum: Observations on composition, preservation and calcification in sediments from ODP Site 1263 (Walvis Ridge — SW Atlantic)

In this study we present the results of a detailed analysis on calcareous nannofossil assemblages from sediment cores of ODP Site 1263 (Southern East Atlantic, Walvis Ridge). This section represents one of the few complete deep-sea sections that document the Paleocene–Eocene Thermal Maximum (PETM) in the pelagic realm. The PETM transient event was characterized by a brief, but intense interval of global warming, a global negative carbon isotope excursion (CIE), and widespread dissolution of seafloor carbonate sediments. Paired analysis at polarizing light microscope (LM) and Scanning Electron Microscope (SEM) documents the different “behavior” of nannofossils through the different phases of the PETM, at the onset of CIE, within the CIE, and during the recovery interval. The presence of anomalous specimens and morphotypes within some nannofossil taxa, recorded during previous LM high resolution analyses, has been further investigated in selected samples at the SEM. Besides the known representatives of the CIE-PETM “excursion nanno-flora”, as Rhomboaster calcitrapa group and Discoaster anartios, the analysis revealed the presence of peculiar morphotypes of Fasciculithus and deformed specimens of Discoaster nobilis group, Discoaster mediosus and Discoaster multiradiatus that are considered related to the anomalous amount of CO 2 in the ocean-atmosphere system during the early phase of PETM. Comparative analyses were performed in few selected samples from other PETM sections located at different latitudes in the Atlantic and Pacific oceans. Although the anomalous geochemical conditions during the PETM-CIE interval seem to have had some influence on the nannofossil production, calcification and assemblage composition, it results that local productivity together with post depositional (diagenetic) conditions were additional important controlling factors on nannofossil assemblages. Preliminary data from Eocene Thermal Maximum 2 (ETM2 or Elmo) suggest that nannofossil malformations are not exclusive of the PETM, and are associated to other episodes of perturbation of the C cycle.

Enhanced terrestrial weathering/runoff and surface ocean carbonate production during the recovery stages of the Paleocene‐Eocene thermal maximum

The carbonate saturation profile of the oceans shoaled markedly during a transient global warming event known as the Paleocene‐Eocene thermal maximum (PETM) (circa 55 Ma). The rapid release of large quantities of carbon into the ocean‐atmosphere system is believed to have triggered this intense episode of dissolution along with a negative carbon isotope excursion (CIE). The brevity (120–220 kyr) of the PETM reflects the rapid enhancement of negative feedback mechanisms within Earth's exogenic carbon cycle that served the dual function of buffering ocean pH and reducing atmospheric greenhouse gas levels. Detailed study of the PETM stratigraphy from Ocean Drilling Program Site 690 (Weddell Sea) reveals that the CIE recovery period, which postdates the CIE onset by ∼80 kyr, is represented by an expanded (∼2.5 m thick) interval containing a unique planktic foraminiferal assemblage strongly diluted by coccolithophore carbonate. Collectively, the micropaleontological and sedimentological changes preserved within the CIE recovery interval reflect a transient state when ocean‐atmosphere chemistry fostered prolific coccolithophore blooms that suppressed the local lysocline to relatively deeper depths. A prominent peak in the abundance of the clay mineral kaolinite is associated with the CIE recovery interval, indicating that continental weathering/runoff intensified at this time as well (Robert and Kennett, 1994). Such parallel stratigraphic changes are generally consonant with the hypothesis that enhanced continental weathering/runoff and carbonate precipitation helped sequester carbon during the PETM recovery period (e.g., Dickens et al., 1997; Zachos et al., 2005).