ABSTRACT The Popigai (100 km in diameter) and the Chesapeake Bay (40–85 km diameter) impact structures formed within ~10–20 k.y. in the late Eocene during a 2 m.y. period with enhanced flux of 3 He-rich interplanetary dust to Earth. Ejecta from the Siberian Popigai impact structure have been found in late Eocene marine sediments at numerous deep-sea drilling sites around the globe and also in a few marine sections outcropped on land, like the Massignano section near Ancona in Italy. In the Massignano section, the Popigai layer is associated with an iridium anomaly, shocked quartz, and abundant clinopyroxene-bearing (cpx) spherules, altered to smectite and flattened to “pancake spherules.” The ejecta are also associated with a significant enrichment of H-chondritic chromite grains (>63 μm), likely representing unmelted fragments of the impactor. The Massignano section also contains abundant terrestrial chrome-spinel grains, making reconstructions of the micrometeorite flux very difficult. We therefore searched for an alternative section that would be more useful for these types of studies. Here, we report the discovery of such a section, and also the first discovery of the Popigai ejecta in another locality in Italy, the Monte Vaccaro section, 90 km west of Ancona. The Monte Vaccaro section biostratigraphy was established based on calcareous nannoplankton, which allowed the identification of a sequence of distinct bioevents showing a good correlation with the Massignano section. In both the Monte Vaccaro and Massignano sections, the Popigai ejecta layer occurs in calcareous nannofossil zone CNE 19. The ejecta layer in the Monte Vaccaro section contains shocked quartz, abundant pancake spherules, and an iridium anomaly of 700 ppt, which is three times higher than the peak Ir measured in the ejecta layer at Massignano. In a 105-kg-size sample from just above the ejecta layer at Monte Vaccaro, we also found an enrichment of H-chondritic chromite grains. Because of its condensed nature and low content of terrestrial spinel grains, the Monte Vaccaro section holds great potential for reconstructions of the micrometeorite flux to Earth during the late Eocene using spinels.

We present the results of integrated biostratigraphic (planktonic foraminifera, calcareous nannofossils, and dinoflagellates), magnetostratigraphic, and cyclostratigraphic analyses of the lower part of Monte Cagnero section (Umbria-Marche Apennines of Italy), a continuous and complete succession of pelagic limestone and marls that provides the means for an accurate and precise astrochronologic calibration of the Eocene-Oligocene transition. This 38.5-m-thick section overlaps the Oligocene section, which, at meter level 188, contains the Rupelian-Chattian boundary corresponding to the O4-O5 planktonic foraminiferal zonal boundary within the upper half of magnetochron C10n. The Eocene-Oligocene boundary at Monte Cagnero, as defined by the last occurrence of hantkeninid planktonic foraminifers (E14-E15 zonal boundary), is found at meter level 114.1, in the upper part of calcareous nannofossil zone CP16a, and very near the Aal-Gse dinocyst zonal boundary. Paleomagnetic analysis has identified all the magnetic reversals from the lower C13r to the lower C12n, precisely overlapping the base of the Oligocene magnetostratigraphic succession and placing the Eocene-Oligocene boundary in the upper part of C13r, in full agreement with the global stratotype section and point (GSSP) at Massignano. Spectral analysis of calcium carbonate data from bulk samples, collected at 5 cm intervals, indicates that orbital forcing of depositional cycles (i.e., limestone versus marl alternations) is dominant at frequencies corresponding to the theoretical astronomical curves of eccentricity, obliquity, and precessional cycles throughout the studied Eocene-Oligocene transition. Correlation with the astrochronologic time scale allows an age assignment of 33.95 Ma for the Eocene-Oligocene boundary, which is in close agreement with the astrochronologic age for the boundary in the GSSP of Massignano obtained in a similar study by R.E. Brown and colleagues in this volume. Thus, the Monte Cagnero section represents a candidate parastratotype for the Eocene-Oligocene GSSP of Massignano in the eventuality that the oxygen and carbon stable isotope shifts defining the oxygen isotope Oi-1 glaciation will be preferred over the last occurrence of hantkeninids as marker for the boundary, since, at Massignano, the beginning of this isotope shift is barely represented in the uppermost part of the exposed section. The excellent integrated stratigraphic framework of Monte Cagnero presented here will significantly facilitate further high-resolution isotope and paleoecological studies across the time of transition from a hothouse to icehouse Earth.

Deep-sea benthic foraminifera live in the largest habitat on Earth, constitute an important part of its benthic biomass, and form diverse assemblages with common cosmopolitan species. Modern deep-sea benthic foraminiferal assemblages are strongly influenced by events affecting their main food source, phytoplankton (a relationship known as bentho-pelagic coupling). Surprisingly, benthic foraminifera did not suffer significant extinction at the end of the Cretaceous, when phytoplankton communities underwent severe extinction. Possibly, bentho-pelagic coupling was less strong than today in the warm oceans of the Cretaceous–Paleogene, because of differences in the process of food transfer from surface to bottom, or because more food was produced below the photic zone by litho-autotrophs. Alternatively, after the end-Cretaceous extinction the food supply from the photic zone recovered in less time than previously thought. In contrast, deep-sea benthic foraminifera did undergo severe extinction (30%–50% of species) at the end of the Paleocene, when planktic organisms show rapid evolutionary turnover, but no major extinction. Causes of this benthic extinction are not clear: net extinction rates were similar globally, but there is no independent evidence for global anoxia or dysoxia, nor of globally consistent increase or decrease in productivity or carbonate dissolution. The extinction might be linked to a global feature of the end-Paleocene environmental change, i.e., rapid global warming. Cenozoic deep-sea benthic faunas show gradual faunal turnover during periods of pronounced cooling and increase in polar ice volume: the late Eocene–early Oligocene, the middle Miocene, and the middle Pleistocene. During the latter turnover, taxa that decreased in abundance during the earlier two turnovers became extinct, possibly because of increased oxygenation of the oceans, or because of increased seasonality in food delivery. The Eocene-Oligocene was the most extensive of these turnovers, and bentho-pelagic coupling may have become established at that time.

The coccolithophores (or calcareous nannoplankton) have proven to be remarkably sensitive to changes in the earth system. However, their history is often expressed in terms of changes in species richness, a methodology that became suspect with the discovery of cryptic species through molecular techniques. To avoid this problem, I describe the extant coccolithophores in terms of morphostructural characters, tracing their changes through the Neogene. I conclude that these are regulated by a morphological strategy that favors small size of cells and coccoliths. I show that this strategy developed as a result of morphologic convergences in different lineages and in taxa inhabiting different strata of the photic zone. A long-term trend through the Neogene resulted in similar innovations in different lineages, and, also, in the loss of large and complex coccoliths. Superimposed on this trend, a short (∼2 m.y.) Pliocene turnover involved both the loss of morphostructural groups that were successful through the Paleogene and Miocene, and a critical, permanent shift to smaller size in the dominant Family Noelaerhabdaceae. The life strategy exhibited by the extant calcareous nannoplankton is rooted in this turnover, so that the same morphological strategy (Pleistocene Morphological Strategy; PLMS) has regulated the coccolithophores since the latest Pliocene–earliest Pleistocene. It is possible that biologic pressure from the microplankton induced the shift to smaller cells; it is equally possible that the progressive change in size and fine structure of coccoliths through the Neogene is linked to an increase in the Mg/Ca ratio and decrease in Ca 2+ concentration in what Stanley and Hardie have called Aragonite III sea. The Pliocene turnover was likely induced by glacial intensification in the middle Pliocene, and sustained by the progressive cooling from the warm early Pliocene to the cold Pleistocene. The PLMS thus results from the combined forcing of ocean chemistry and climatic change. The physiognomy of the extant coccolithophores, far from indicating their failure as a result of unfavorable seawater chemistry, demonstrates the remarkable adaptability of a group that evolved and first radiated in Aragonite II sea, thrived in Calcite II sea, and has “reinvented” itself in Aragonite III sea by adopting a unique morphological strategy.

In the present study, we investigate upper Paleocene to lower Eocene planktic forami niferal assemblages in Egypt and Jordan across a middle neritic to upper bathyal transect of the Tethyan continental margin. In particular, we evaluate the planktic foraminiferal turnover across the Paleocene-Eocene Thermal Maximum (PETM). Dissolution affects the planktic assemblages more intensively than previously considered, especially in the marls below the PETM. High numbers of Subbotina , fluctuating planktic/benthic (P/B) ratios, and low numbers of planktic foraminifera per gram (PFN) are indicative of dissolution, probably as a consequence of deep weathering. Hence, high numbers of Subbotina in this area do not indicate cooling. Despite this taphonomic overprint, we observe that well-diversified planktic foraminiferal assemblages of Subzone P5a abruptly changed into oligotaxic assemblages dominated by Acarinina during the PETM. Because various biotic and geochemical proxies indicate increased nutrient supply to the basin, we argue that the blooming of Acarinina is not indicative of oligotrophic conditions. Instead, we postulate that (low-trochospiral) Acarinina may have been better adapted to thrive under stressed environmental conditions, possibly because they hosted symbionts different from those in Morozovella .

A major change in calcareous nannofossil assemblages has been reported at the Paleocene-Eocene Thermal Maximum (PETM) on a global scale. To document the response of the nannoplankton communities below, within, and above the PETM, we studied in detail six successions, representing a wide range of environments and latitudes. Calcareous nannofossil response was different in discrete paleogeographic areas. Several classical Tethyan sections (Alamedilla, Caravaca, Zumaia [Spain], Contessa [Central Italy], and Wadi Nukhl [Egypt]), plus the high-latitude Ocean Drilling Program reference Site 690 (Weddell Sea) were re-investigated using high resolution calcareous nannofossil quantitative analyses. Five assemblage zones were identified: two before the onset of the Carbon Isotope Excursion (CIE) and three after it. Before the PETM, several changes were observed in both high and low latitudes that are characterized by well-defined increases of r-selected taxa ( Biscutum and Prinsius ). These changes probably were in response to an upwelling pulse that increased nutrients in surface waters. These events, which predate the geochemical and oceanic changes at the PETM, indicate that there were global events occurring before the actual CIE onset. At Site 690, the principal calcareous nannofossil change coincides with the onset of the CIE and is characterized by the rapid replacement of cold-water taxa by warm-water taxa. This change resulted from a sudden expansion of warm-water low-latitude assemblages into higher latitudes, probably due to an abrupt increase of surface-water temperatures. An increase in species richness here is due to the migration of several genera (i.e., Discoaster and Fasciculithus ) south from warmer areas and to decreased dissolution. Moreover, an increase in abundance of Thoracosphaera spp. (calcareous dinoflagellate) below and within the CIE also indicates a stressed surface-water environment. In the Tethyan sections, the response of the calcareous nannofossil assemblages to the PETM is more complex. As at the Southern Ocean Site 690, calcareous nanno-fossil fluctuations begin below the onset of the CIE and increase in frequency and amplitude at the benthic foraminifera extinction (BFE). At this level, calcareous nannofossil diversity and abundance abruptly decrease, and the Rhomboaster spp.– Discoaster araneus (R-D) association appears. The occurrence of the R-D association together with Thoracosphaera suggests that during the PETM there was a change to stressed ocean-surface conditions. Calcareous nannofossil recovery occurred later in the Tethys than at the southern high latitudes, where it occurred before the CIE recovery. Furthermore, the nanno-floral assemblages after the δ 13 C recovery still indicate stressed conditions, suggesting that the plankton communities did not completely recover until later.

The comparison between calcareous nannofossils during the early Aptian Oceanic Anoxic Event 1a (OAE1a) and the Paleocene-Eocene Thermal Maximum (PETM) suggests different nannofloral reactions to extreme greenhouse conditions. Both events were likely characterized by major changes in nutrient concentrations, temperature, and p CO 2 levels. OAE1a corresponds to an increase in opportunistic taxa associated with eutrophic surface-water conditions. Eutrophy also resulted in the demise of an oligotrophic group, the nannoconids. Nannofloral assemblages of the PETM interval suggest nutrient-depleted surface waters at open-ocean sites including those at high and low latitudes. However, the upper part of the PETM shows a return to mesotrophic conditions documented by the increase in abundance of mesotrophic taxa. PETM records from shelf sites are characterized by an increase in nannofossil taxa indicative of mesotrophic conditions, suggesting an increase in productivity. Fluctuations in primary productivity affected composition and abundance of calcareous nannofossil assemblages during both events. Whereas fertility increased in the global ocean during OAE1a, mesotrophic conditions mostly characterized proximal settings during the PETM. Nannofloral changes could have been partially triggered by the warming, but the influence of high p CO 2 levels is not evident. Reductions in nannofossil calcification and paleofluxes are associated with the OAE1a, but the role of p CO 2 variations in nannofloral calcification during the PETM is not obvious. In both events, variations in lysocline/CCD depth and enhanced dissolution and/or diagenesis strongly affected nannofossil assemblages in some locations, but the overall nannofloral changes reveal a primary paleoecological and paleoceanographic signal.

The Wetumpka impact event, ca. 83.5 Ma in shallow waters of the northern Gulf of Mexico, caused minor ecosystem perturbations because of the physical effects of a 2.6-gigaton-equivalent impact detonation. The impact event and its consequences had relatively minor, but notable, paleobiologic effects (i.e., preservational effects, biostratigraphic effects, and impact-succession effects). The impact structure served as a local reservoir for an impact-entombed fossil record in two main ways. First, coarse to fine fragments of terrestrial vegetation, probably derived from the adjacent tropical forest, were swept up and incorporated into Wetumpka washback- and surgeback-deposited breccias and sands. Second, intact blocks of target sedimentary units, which contain an internal fossil component of their own, are part of the slump and fallback debris that partially fills the Wetumpka impact structure. Some of these target sedimentary rock blocks include updip sedimentary facies of these formations that no longer exist in outcrops anywhere in the region. In yet another paleoecologic effect, the Wetumpka impact crater apparently functioned as a minor terrestrial (island) ecosystem embedded within the shelfal marine realm for an unknown length of time. As one might expect from the relatively small size of this structure, there is no apparent regional or global biotic extinction event associated with this local catastrophe.

Various scenarios have been proposed to explain the Late Devonian mass extinction, foremost among which are bolide impact and sea-level fall. We hereby propose a gas hydrate-induced model based on detailed geochemical and sedimentological data. The period of enhanced organic carbon burial in Iran, in south China, and in subpolar Urals corresponds to a brief negative δ 13 C excursion of 3.5‰ at the Frasnian-Famennian (F-F) transition. Prior to this event, oceanic δ 13 C increased for a period of several million years. However, major perturbations of the carbon geochemical cycle, and corresponding sharp and strong negative spikes of δ 13 C, which require a large input of isotopic light carbon into the ocean, also characterize the boundary horizons. Oxygen isotope ratios show negative excursions of 1.7‰ in south China and 4.1‰ in subpolar Urals that parallel the negative excursions in δ 13 C values. Synchronous negative spikes of δ 18 O are likely to imply a rapid increase of ocean temperature. We propose that the F-F boundary event was ultimately caused by voluminous and abrupt release of methane from marine gas hydrate into the ocean and atmosphere to trigger rapid global warming. Assuming that the total amount of inorganic carbon of the Devonian ocean was 40,000 gigatons (Gt) and δ 13 C of gas hydrate methane was −80‰, only 2600 Gt carbon from the total amount of 10,000 Gt gas hydrate carbon could have changed the oceanic δ 13 C values from +1‰ to −3‰, the observed magnitude of the F-F boundary excursion. Therefore only ∼26% of the gas hydrate could have triggered the boundary events. Widespread rift-related, basaltic volcanism along eastern Laurussia and northern Gondwana during the middle Late Devonian is believed to have contributed greatly to the global warming surrounding the F-F boundary, which in turn would have triggered massive dissociation of methane hydrate, especially if paired with intensive igneous and tectonic activity and rapid sea-level fall.

Abstract Quantitative analyses of the calcareous nannofossil Reticulofenestra asanoi and related species have been performed on the Early-Middle Pleistocene transition in the Mediterranean Sea (ODP Sites 976 and 963) and Atlantic Ocean (DSDP Hole 610A) in order to improve the understanding of their stratigraphic distributions. Abundance patterns have allowed the identification of the lowest common occurrence (LCO) and highest common occurrence (HCO) of R. asanoi in a short interval below and above the lowest occurrence of Gephyrocapsa sp. 3. Correlation with oxygen isotope stratigraphy at Site 976 places the LCO of R. asanoi at the Marine Isotope Stage (MIS) 34-33 transition and its HCO at the MIS 23-22 transition. At Site 963, the HCO of R. asanoi (estimated age of 0.96 Ma correlated to MIS 25) is regarded as ‘artificially’ low, and its highest occurrence (estimated age 0.90 Ma correlated to MIS 23) has therefore been used for bio-chronostratigraphic correlation. The LCO of R. asanoi is estimated at 1.05 Ma at Site 963 and 1.17 Ma at Hole 610A, which suggests correlation to MIS 30 and MIS 35, respectively. These data suggest a possible diachrony for the LCO of R. asanoi .