1,721,006 research outputs found
Nannofossil carbonate fluxes during the Early Cretaceous : phytoplankton response to nutrification episodes, atmospheric CO2 and anoxia
No full textGreenhouse episodes during the Valanginian and Aptian correlate with major perturbations in the C cycle and in marine ecosystems, carbonate crises, and widespread deposition of Corg-rich black shales. Quantitative analyses of nannofossil micrite were conducted on continuous pelagic sections from the Southern Alps (northern Italy), where high-resolution integrated stratigraphy allows precise dating of Early Cretaceous geological events. Rock-forming calcareous nannofloras were quantified in smear slides and thin sections to obtain relative and absolute abundances and paleofluxes that are interpreted as the response of calcareous phytoplankton to global changes in the ocean-atmosphere system. Increased rates of volcanism during the formation of Ontong Java and Manihiki Plateaus and the Paranà-Etendeka large igneous province (LIP) are proposed to have caused the geological responses associated with early Aptian oceanic anoxic event (OAE) la and the Valanginian event, respectively. Calcareous nannofloras reacted to the new conditions of higher pCO2 and fertility by drastically reducing calcification. The Valanginian event is marked by a 65% reduction in nannofossil paleofluxes that would correspond to a 2-3 times increase in pCO2 during formation of the Paranà-Endenteka LIP. A 90% reduction in nannofossil paleofluxes, which occurred in a 1.5 myr-long interval leading into OAEla, is interpreted as the result of a 3-6 times increase in pCO2 produced by emplacement of the giant Ontong Java and Manihiki Plateaus. High pCO2 was balanced back by an accelerated biological pump during the Valanginian episode, but not during OAE1a, suggesting persisting high levels of pCO2 in the late Aptian and/or the inability of calcareous phytoplankton to absorb excess pCO2 above threshold values. Copyright 2004 by the American Geophysical Union
Late Barremian to early Aptian calcareous nannofossil paleoceanography and paleoecology from the Ocean Drilling Program Hole 641C (Galicia Margin)
No full textCalcareous nannofossil assemblages at Site 641C (Galicia Margin, North Atlantic) were investigated in order to determine changes in fertility and temperature of surface waters. Taxa such as Zeughrabdotus spp. < 3.5 mu m, Biscutum constans, Discorhabdus rotatorius and Diazomatolithus lehmanii, which thrived in higher fertility conditions, are particularly abundant across the CM0 interval as opposed to those with oligotrophic affinities such as Watznaueria spp. and Nannoconus spp., which are generally reduced in abundance. The abundances of nannoconids are much lower than those observed in Tethyan sections, indicating higher fertility conditions. Slumpings and low recovery prevent the identification of the onset of the "nannoconid crisis", but a sharp drop in nannoconid abundances, observed prior to the CM0 interval, correlates with the "nannoconid decline" observed in several Tethys sections. The normalized ratio between low and high fertility taxa (Fertility Index) was used to characterize the nannofossil assemblages in terms of productivity changes. The highest values of the Fertility Index were observed across magnetic chron CM0. The paucity of cold water taxa such as Seribiscutum spp. and Repagulum parvidentatum suggests warm water conditions throughout the deposition of upper Barremian-lower Aptian sediments on the Galicia Margin. (c) 2006 Elsevier Ltd. All rights reserved
Morphometric changes of the calcareous nannofossil taxon Discoaster multiradiatus across the Paleocene-Eocene Thermal Maximum: biotic and abiotic factors
No full textSize measurements of the calcareous nannofossil taxon Discoaster multiradiatus were carried out across the Paleocene-Eocene Thermal Maximum (PETM) in Ocean Drilling Program Holes 690B (Maud Rise, Weddell Sea) and 1209B (Shatsky Rise, Pacific Ocean). Morphometric investigations show that D. multiradiatus specimens are generally larger at ODP Site 1209 rather than at ODP Site 690. A small increase in size of this species is recorded at ODP Site 1209, whereas significant variations characterize ODP Site 690. A marked shift in diameter size was observed in the lower portion of the PETM at ODP Site 690, which coincides with the onset and the peak of the Carbon Isotope Excursion (CIE). The occurrence of the largest specimens of D. multiradiatus at the onset and the peak of the CIE coincides with an inferred massive introduction of carbon dioxide, increasing temperature, and a likely drop in primary productivity. This contradicts the size-reduction hypothesis under excessive pCO2 levels. Instead, the size increase of D. multiradiatus could be the result of a significant influx of larger-sized specimens from lower latitudes during the warmest and the most nutrient-depleted interval of the PETM. In addition, the shift in size recorded in the lower part of the PETM could result from the occurrence of a new echophenotype that occupied short-lived ecological niches triggered by changes in water stratification, temperature, and primary productivit
Size variations of the calcareous nannofossil taxon Discoaster multiradiatus (Incertae sedis) across the Paleocene–Eocene thermal maximum in ocean drilling program holes 690B and 1209B
No full textSize measurements of the calcareous nannofossil taxon Discoaster multiradiatus were carried out across the Paleocene–Eocene Thermal Maximum (PETM) in Ocean Drilling Program Holes 690B (Maud Rise, Weddell Sea) and 1209B (Shatsky Rise, Pacific Ocean). Morphometric investigations show that D. multiradiatus specimens are generally larger at ODP Site 1209 than at ODP Site 690. A limited increase in size of D. multiradiatus is recorded at ODP Site 1209, whereas significant enlargements characterize ODP Site 690. Preservation is comparable at both sites: nannofossils are moderately preserved with some evidence of etching/overgrowth in the PETM interval. Yet, D. multiradiatus variations do not correlate with preservation state and morphometric data most likely represent primary signals rather than diagenetic artifacts. There is a direct relationship between D. multiradiatus size and paleotemperatures: largest specimens are coeval with global warming associated with the PETM, inferred to result from excess atmospheric CO2 due to (partial) oxidation of massive quantities of methane. Size increases and largest specimens of D. multiradiatus occur at different stratigraphic levels within PETM at ODP Sites 690 and 1209. A marked shift in diameter size was observed at the onset and peak of the Carbon Isotopic Excursion (CIE) at ODP Site 690, but only at the end of CIE and initial recovery interval at ODP Site 1209. This diachroneity is puzzling, but indeed correlates well with reconstructed changes in surface and thermocline water masses temperature and salinity in the PETM interval at low and high latitudes. The presumed high concentrations of carbon dioxide seem to have not influenced the morphometry of D. multiradiatus. The major size increase of D. multiradiatus in the CIE of ODP Site 690 could represent the migration of larger-sized allochtonus specimens that moved from peri-equatorial/subtropical areas to higher latitudes during the warmest interval of the PETM, although no direct evidence of distinct populations/subpopulations has been obtained from the frequency diagrams. As a result, we infer that D. multiradiatus is a proxy of water masses stratification and might be used for deriving temperature–salinity–nutrient conditions in the mixed layer and thermocline and their dynamics
Calcareous nannofossil fluctuations during the late Hauterivian in the Cismon core (Venetian Alps, northeastern Italy) and in selected sections of the Umbria-Marche Basin (central Italy) : paleoceanographic implications of the Faraoni Level
No full textCalcareous nannofossil quantitative analyses were carried out on upper Hauterivian sediments from the Cismon core and results from the uppermost Hauterivian "Faraoni Level" were compared to nannofloral fluctuations in abundance and composition recorded in the S.S. Arceviese road and Palazzo D'Arcevia sections, Umbria-Marche Basin (central Italy). Fragile taxa are generally absent since the preservation is poor in all lithologies analyzed, but dissolution-resistant taxa show remarkable fluctuations. Smear-slide investigations reveal that limestones are characterized by high abundances of nannoconids (principally narrow-canal nannoconids) suggesting oligotrophic conditions throughout the interval investigated. Conversely, nannoconids are rare or absent in black shales, whereas Assipetra infracretacea and Rucinolithus terebrodentarius are prevailing. In addition, high abundances of pentaliths are recorded in limestones through the interval preceding the deposition of the Faraoni Level. This finding possibly suggests a decrease in salinity of surface waters. Different paleoceanographic regimes seem to have triggered repetitive changes in abundance and composition of nannofloral assemblages. Limestones were plausibly deposited under oligotrophic conditions, whilst black shales may suggest increasing primary productivity. Thin section investigations show a drop in abundance of pentaliths and a marked decrease of narrow-canal nannoconids starting definitively before and reaching a minimum in the Faraoni Level. In the black shales bounding the ammonite-rich Guide Bed, A. infracretacea and R. terebrodentarius become dominant. Such nannofloral changes may suggest enhanced primary productivity and more arid conditions associated with dysoxic bottom waters. We stress the fact that nannofloral variations recorded in the Faraoni Level are not exclusive, but match the repetitive nannofloral-lithological fluctuations preceding and following it. Perhaps, the paleoenvironmental changes triggering the Faraoni event were not affecting calcareous phytoplankton or represent the over-reaching of threshold conditions after gradual and progressive modifications in nutrient availability
Ipertensione arteriosa con esaltata attività mineraloattiva da abuso di spray nasale contenente fluoridrocortisone.
No full textEarly Jurassic schizosphaerellid crisis in Cantabria, Spain: Implications for calcification rates and phytoplankton evolution across the Toarcian oceanic anoxic event
No full text[1] The Toarcian oceanic anoxic event ( similar to 183 Myr ago) represents a global perturbation marked by increasing organic carbon burial and a general decrease in calcium carbonate production likely triggered by elevated carbon dioxide levels in the atmosphere. Here we present quantitative analyses of calcareous nannofossil diversity and abundance from the Castillo de Pedroso section in Cantabria, northern Spain. We compare these data with geochemical data ( C and O isotopes) obtained from biogenic and bulk carbonate records in order to highlight the response of calcareous phytoplankton to major climatic and paleoceanographic changes. The Pliensbachian/ Toarcian boundary is characterized by an abrupt decrease in abundance of Schizosphaerella punctulata, the most important lithogenic contributor to ( hemi) pelagic carbonates in the Early Jurassic. The early Toarcian nannofloral assemblages show an increase in abundance of Mitrolithus jansae and small-sized r-selected taxa and a progressive decrease in S. punctulata percentages. The deep dwellers M. jansae and S. punctulata experienced a major crisis slightly prior to the deposition of the Toarcian black shales that are characterized by high abundances of eutrophic taxa such as Lotharingius spp. and Biscutum spp. The return of S. punctulata associated with lower percentages of eutrophic taxa was observed just above the Toarcian black shales. The Toarcian episode reveals that high CO2 levels and increasing primary productivity probably triggered a shift in abundance from highly calcified nannoliths such as S. punctulata and M. jansae to small-sized r-selected coccoliths that overall record a biocalcification crisis at the onset and during the Toarcian episode
A review of calcareous nannofossil changes during the early Aptian Oceanic Anoxic Event 1a and the Paleocene-Eocene Thermal Maximum: The influence of fertility, temperature, and pCO2
No full textThe 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 pCO2 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 pCO2 levels is not evident. Reductions in nannofossil calcification and paleofluxes are associated with the OAEla, but the role of pCO2 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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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