Calcareous nannofossil biostratigraphy of the Cenomanian/Turonian boundary interval of ODP Leg 207 at the Demerara Rise

During Ocean Drilling Program Leg 207 at Demerara Rise (western tropical Atlantic) expanded sections of organic-rich laminated shales of late Cretaceous age, including the Cenomanian/Turonian boundary interval (CTBI) were recovered. These sediments yield rich assemblages of calcareous nannofossils, offering a unique possibility to calibrate the stable carbon isotope curve (δ¹³C_org). The calcareous nannofossils of Sites 1258, 1259, 1260 and 1261 have been investigated in order to gain a detailed biostratigraphic framework for the CTBI in the tropical latitudes. The bioevents observed have been correlated with the characteristic δ¹³C_org excursion of the CTBI. The events occurring during the excursion, and slightly below and above it, include the first occurrences of the calcareous nannofossil species Cylindralithus biarcus, Quadrum gartneri, Quadrum intermedium, Eprolithus octopetalus, Eprolithus eptapetalus, Eiffellithus eximius and the last occurrences of Corollithion kennedyi and Axopodorhabdus albianus. These bioevents have been correlated with those of other biostratigraphically and chemostratigraphically studied CTBI sections.     

Surface-water chemistry and fertility variations in the tropical Atlantic across the Paleocene/Eocene Thermal Maximum as evidenced by calcareous nannoplankton from ODP Leg 207, Hole 1259B

Calcareous nannoplankton assemblages at Ocean Drilling Program (ODP) Site 1259 on Demerara Rise (western equatorial Atlantic) underwent an abrupt and fundamental turnover across the Paleocene/Eocene Thermal Maximum (PETM) ~55.5 m.y. ago. The PETM is marked by a dissolution interval barren or nearly-barren of nannofossils due to the rapid acidification of the world oceans. Toweius, Fasciculithus, and Chiasmolithus sharply decrease at the onset, whereas Chiasmolithus, Markalius cf. M. apertus, and Neochiasmolithus thrive immediately after the event, which also signals the successive first appearances of Discoaster araneus, Rhomboaster, and Tribrachiatus. The environmental indications of these changes were further investigated by correspondence analysis on quantitative nannofossil counts. The PETM event has been attributed to CO₂-forced greenhouse effects. At Site 1259, the elevated pCO₂ and subsequent lowered surface-water pH values at the onset of the PETM caused intensive carbonate dissolution, producing the nannofossil-barren interval. The chemically stressed habitats may well have also induced the evolution of ephemeral nannofossil “excursion taxa”, such as Rhomboaster and malformed discoasters (D. araneus and Discoaster anartios). Based on its sudden increase, Markalius cf. M. apertus is considered to have been a local opportunistic species that took advantage of the surface-water changes. At the same time, a presumably higher runoff from continental areas fertilized the western equatorial Atlantic as indicated by an increase in the abundance of r-mode specialists preferring high-nutrient conditions, such as Chiasmolithus, Coccolithus pelagicus, and Hornibrookina arca. Contrasts between the results of this study and previous work at ODP Site 690 in the Southern Ocean, the New Jersey continental margin, and the central paleoequatorial Pacific further demonstrate that the response to the PETM can be influenced by local differences in geologic setting and oceanographic conditions.     

Earliest Zanclean age for the Colombacci and uppermost Di Tetto formations of the “latest Messinian” northern Apennines: New palaeoenvironmental data from the Maccarone section (Marche Province, Italy)

The occurrence of planktonic foraminifers in the latest Messinian deposits (uppermost Di Tetto Formation and Colombacci Formation) of the Marche Province (Apennine foredeep, Italy) has stimulated a debate since the 1970s. An earlier palynological study of the entire Maccarone section revealed a pronounced, and a sudden increasing frequency of saccate pollen grains which indicates more distal conditions, and thus a transgression. At first attributed to tectonic activity, this transgression is now interpreted as representing the Zanclean marine transgression after the discovery of Ceratolithusacutus, the calcareous nannofossil marker of the earliest Zanclean in the Mediterranean Sea. Evidence from marine dinoflagellate cysts and planktonic foraminifers supports this result. The Colombacci Formation and uppermost part of the Di Tetto Formation (i.e. the entire p-ev2 stratigraphic unit) belong to the earliest Zanclean. The so-called Lago Mare no longer has a regional chronostratigraphic sense, and should be understood as the invasion of Paratethyan organisms via surface waters owing to a connection at high sea-level between the Aegean Sea and the Eastern Paratethys (Dacic Basin). A new robust environmental reconstruction of the northern Apennine foredeep is proposed, which respectively considers the effects of tectonics and Mediterranean eustasy.     

Spatial distribution patterns of calcareous nannofossils across the Callovian–Oxfordian transition in the French Subalpine Basin

Callovian–Oxfordian calcareous nannofossils are documented from four sections distributed along a proximal–distal (slope to central basin) transect in the French Subalpine Basin (south-eastern France). The sections were dated and correlated using integrated ammonite, dinoflagellate and nannofossil biostratigraphies. The nannofossil assemblages are constantly dominated by Watznaueria britannica. However, significant spatial changes in nannofossil assemblage composition are recorded. Nannofossil assemblages recovered in slope and slope-to-basin transition sections are characterized by high abundances, lowest diversities, highest percentages of smallest-sized morphotypes of W. britannica, subordinated W. britannica/manivitiae (a morphotype described in this study), and are indicative of turbulent, unstable and/or eutrophic conditions. Towards the central part of the basin, as the nutrient content and turbulence declined and paleoenvironmental stability increased, the nannofossil assemblages become more diverse and characterized by lower relative abundances of W. britannica but higher contribution of larger morphotypes and higher relative abundances of W. barnesiae/fossacincta, Biscutum dubium and Zeugrhabdotus erectus. These latter small coccoliths, markers of high surface-waters fertility for the Cretaceous, were probably not competitive with respect to smaller-sized W. britannica in unstable and eutrophic surface waters for the Jurassic. An increase in the surface water productivity is recorded both in proximal and distal sections at the Callovian–Oxfordian transition, and is demonstrated both by an increase in nannofossil total abundances and in the relative abundances of taxa adapted to high-trophic levels. Thermal minimum in surface waters, as indicated by an increase in the percentages of the cool-water nannofossil taxa, is associated to increased productivity. Further studies will be necessary to demonstrate whether surface-water productivity changes, already recorded in the eastern Paris Basin, were global.     

The record of mid Cretaceous oceanic anoxic events from the Ionian zone of southern Albania

Along the Sopoti section of Mali Gjere mountain are found two distinct mid-Cretaceous marly-shaly-siliceous intervals that occur near the top of the Vigla Limestone Formation (Aptian-Albian) of the Ionian zone in southern Albania. The lower interval is relatively rich in carbonate content (36% CaCO₃ on average) and radiolaria (10% on average). It contains black shale levels rich in organic matter (up to 8.5% total organic carbon (TOC)) of marine origin, which did not experience any strong thermal maturation as suggested by their very low T_max values. The age of the lower interval is latest Barremian to Early Aptian based on its calcareous nannofossil and radiolarian assemblages. It is therefore regarded as the equivalent of the Fourcade Level of Greece, reflecting the Oceanic Anoxic Event 1a (OAE1a). The upper interval is richer in both carbonate content (60% CaCO₃ on average) and radiolaria (20% on average) but it is practically devoid of any preserved organic matter. Its age straddles the Aptian-Albian boundary based on integrated biochronologic data of dinoflagellates, calcareous nannofossils and radiolaria. It is tentatively considered as the sedimentary expression of OAE1b (sensu Leckie et al., 2002) in the Ionian zone of Albania. The presence of large Assipetra nannoliths in both shaly-siliceous intervals and the relative abundance of radiolaria suggest that their accumulation took place during periods of higher productivity in the Ionian zone of Albania.     

Spatial distribution of Late Pliensbachian (Early Jurassic) calcareous nannofossils within the Lusitanian basin (Portugal)

Quantitative analyses of Pliensbachian calcareous nannofossils have been carried out on a proximal-distal transect in the Lusitanian Basin (Portugal). The studied sections Vale Venteiro near Tomar and Peniche represent proximal and distal environments with respect to the emerged land of the Iberian Meseta (to the East). The upper portion of the Vale das Fontes Fm (Davoei and Margaritatus ammonite Zones) is studied in both sections and correlated by means of ammonite and nannofossil integrated biostratigraphy. A careful analysis of the preservation state of nannofossils is performed. Preservation state is moderate to good in the two settings; changes in nannofossil assemblages are therefore considered as primary. Samples were analysed for nannofossil absolute and relative abundances, species diversity and wt%CaCO₃. The analysis of calcareous nannofossils (absolute abundance, percentage, average abundance) shows changes in the community structure in space (i.e., from proximal to distal), and vertically within the water column. This pattern suggests a partitioning of habitats within the photic zone, and with respect to emerged lands. Proximal environments within the Lusitanian Basin were probably more eutrophic, because of the proximity of emerged lands to the East (Iberian Meseta). This was the source area for nutrients delivered into the basin by river transport. Distal environments were likely characterized by a greater water depth and by a deeper light penetration leading to a relatively more expanded photic zone. The assemblages recorded in the proximal setting exhibit high mean relative abundance of placolith-bearing coccolithophorids (Lotharingius spp., Biscutum spp. and Similiscutum spp.) and of Schizosphaerella spp., while distal assemblages are dominated by Crepidolithus crassus and Schizospharella spp. Crepidolithus crassus is interpreted as a deep-dweller coccolithophorid, inhabiting preferentially distal and deeper settings in the Lusitanian Basin. Placolith-bearing coccolithophorids were more abundant in proximal settings with respect to emerged lands, under relatively elevated trophic conditions. The probable calcareous dinocyst Schizospharella spp. proliferated in surface waters of both proximal and distal environments.     

Calcareous nannofossil biostratigraphy and turnover dynamics in the late Campanian–Maastrichtian of the tropical South Atlantic

The complete late Campanian–Maastrichtian succession of the South Atlantic reference DSDP Site 525A has been investigated to establish a detailed record of calcareous nannofossil biohorizons. A complete holostratigraphy of the core is presented for this interval and allows for global correlations to other Tethyan and Boreal reference sections for that interval. The new study allows for proposing a new time scale with a tie of Tethyan and Boreal nannofossil zonations. A new subzonation is proposed for the late Campanian UC16 Zone in the Tethyan Realm and the last occurrences of Uniplanarius trifidus and Zeugrhabdotus praesigmoides are demonstrated as markers of the Campanian–Maastrichtian boundary. The difference between cumulative first and last occurrences reflects the turnover dynamics of calcareous nanoplankton in the South Atlantic and highlights 6 major events. Five of them appear well related to major changes in sea-surface temperatures whereas the sixth event is likely the expression of a global decrease in primary productivity in the late Maastrichtian. Surprisingly, the turnover dynamics in calcareous nannoplankton appears completely unrelated to that observed in planktic foraminifera which shows only 4 major events with different timings. The two groups thus seem to have responded either very differently to similar environmental constraints or to different direct environmental constraints in the photic zone, although caused by the same global changes in climate.     

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 CO₂ 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.     

Eocene–Oligocene calcareous nannofossils from Maud Rise and Kerguelen Plateau (Antarctica): paleoecological and paleoceanographic implications

The evolution of the Southern Ocean climate during the late Eocene–late Oligocene interval is examined through high-resolution, quantitative calcareous nannofossil analyses on samples from the Southern Ocean sections on Maud Rise and Kerguelen Plateau. We determined the abundance patterns of the counted species to clarify the biostratigraphy, which we correlated with high-resolution magnetostratigraphy [Roberts, A.P., Bicknell, S.J., Byatt, J., Bohaty, S.M., Florindo, F., Harwood, D.M., 2003a. Magnetostratigraphic calibration of Southern Ocean diatom datums from the Eocene–Oligocene of Kerguelen Plateau (Ocean Drilling Program Sites 744 and 748). In: Florindo, F., Cooper, A.K., O'Brien, P.A. (Eds.), Antarctic Cenozoic Palaeoenvironments: Geologic Record and Models. Palaeogeogr., Palaeoclimatol., Palaeoecol. 198 145–168; Florindo, F., Roberts, A.P., in press. Eocene–Oligocene magnetobiochronology of ODP Sites 689 and 690, Maud Rise, Weddell Sea, Antarctica. Geol. Soc. Am. Bull.], and used this data to interpret paleoceanographic changes through the late Eocene to late Oligocene. Percentage plots of the individual species, compared with R-mode principal component and cluster analysis results, allowed us to divide the assemblages into three groups: temperate-water taxa, cool-water taxa, and no temperature-affinity taxa. We attempt correlations between these paleoecological groups and the major sea-surface temperature (SST) variations with tectonic and paleoceanographic changes in the Southern Ocean. During the late Eocene, the nannofossil assemblage data reveal that there were several minor SST decreases (coolings) from 36 to 34 Ma, before the Eocene/Oligocene (E/O) boundary. A sharp cooling event, dated at 33.54 Ma (earliest Oligocene), occurred about 160 kyr after the E/O boundary, which is dated at 33.7 Ma. Relatively stable, cool conditions are interpreted to persist until the latest Oligocene, when an increase in abundance of temperate-water taxa, which corresponds to an antithetical decrease in abundance of cool-water indicators, is recorded.On the basis of our dating, the opening of the Drake Passage, allowing shallow-water circulation, began by 33.54 Ma at the latest, while the establishment of deep-water connections through the Tasmanian Gateway occurred at 33 Ma, as suggested by Exon et al. [Proc. ODP, Init. Rep. 189 (2001) 1].     

The response of calcareous nannofossil assemblages to the Paleocene Eocene Thermal Maximum at the Walvis Ridge in the South Atlantic

Compositions and abundances of calcareous nannofossil taxa have been determined in a ca 170 kyrs long time interval across the Paleocene/Eocene boundary at 1-cm to 10-cm resolution from two ODP Sites (1262, 1263) drilled along the flank of the Walvis Ridge in the South Atlantic. The results are compared to published data from ODP Site 690 in the Weddell Sea. The assemblages underwent rapid evolution over a 74 kyrs period, indicating stressed, unstable and/or extreme photic zone environments during the PETM hyperthermal. This rapid evolution, which created 5 distinct stratigraphic horizons, is consistent with the restricted brief occurrences of malformed and/or weakly calcified morphotypes. The production of these aberrant morphotypes is possibly caused by major global scale changes in carbon cycling in the ocean–atmosphere system, affecting also photic zone environments. No marked paleoecologically induced changes are observed in abundances of the genera Discoaster, Fasciculithus and Sphenolithus at the Walvis Ridge sites. Surprisingly, there is no significant correlation in abundance between these three genera, presumed to have had a similar paleoecological preference for warm and oligotrophic conditions.