131,632 research outputs found
Calcareous nannofossils biostratigraphy (Upper Bajocian – Lower Bathonian) of the Ravin du Bès section (Bas Auran, Subalpine Basin, SE France), evolutionary trends of Watznaueria barnesiae and new enigmatic morphotypes of genus Rucinolithus
No full textA biostratigraphic and evolutionary study based of calcareous nannofossils was performed on the Ravin du Bès section (Bas Auran area, SE France), proposed as formal candidate of Global Boundary Stratotype Section and Point (GSSP) for the base of the Bathonian stage (Fernàndez-Lòpez et al., 2007). Semiquantitative estimates of total nannofloral abundance and single species abundance were carried out. The following biohorizonts were identified and calibrated to ammonite biostratigraphy: the first occurrence (FO) of Watznaueria aff. W. barnesiae, the FO of Pseudoconus enigma; the FO of Rucinolithus sp.; the last occurrence (LO) of Hexalithus magharensis; the FO of Stephanolithus speciosum octum and the FO of Watznaueria barnesiae. These results, consistent with biostratigraphic scheme previously proposed (Erba 1988, 1990; de Kaenel & Bergen 1993; de Kaenel et al. 1996; Bown & Cooper 1998; Mattioli & Erba 1999) confirm that the calcareous nannofossils are good biostratigraphic markers for the Bajocian/Bathonian boundary interval. Moreover, the finding of P. enigma within of the Sub-Mediterranean province allows a direct calibration between Tethyan and Boreal nannofossil events and biozones.
This study showed an evolutionary trend from Watznaueria communis to Watznaueria barnesiae that seems to support the theory of punctuated equilibria rather than a phyletic gradualism.
We also documented the occurrence of new morphotypes of uncertain polycycloliths. These enigmatic nannoliths are very similar to specimens of the Cretaceous taxon R. terebrodentarius, whose peculiar structure poses doubts on its origin. In fact, as previously speculated (Tremola & Erba 2002; Erba 2004), R. terebrodentarius nannoliths might be CaCO3 precipitates or biocalcification by bacteria under peculiar oceanographic conditions rather than products of coccolithophorid algae.
REFERENCES
Bown, P.R., Cooper, M.K.E, 1998. Jurassic. In: Bown, P.R. (EDS.), Calcareous Nannofossil Biostratigraphy. British Micropaleont. Soc. Publ. Series. Kluwer Academic Publishers, London: 34-85.
De Kaenel, E., Bergen, J.A., 1993. New early and Middle Jurassic coccolith taxa and biostratigraphy from the eastern proto-Atlantic (Morocco, Purtugal and DSDP Site 547B). Eclogae Geol. Helv., 86: 861-907.
De Kaenel, E., Bergen, J.A., von Salis Perch Nielsen, K., 1996. Jurassic calcareous nannofossils biostratigraphy of western Europe. Compilation of recent studies and calibration of bioevents. Bull. Soc. Geol. Fr., 167: 15-28.
Erba, E., 1988. Calcareous nannofossils from the Bas Auran section. In: M., Innocenti, C., Mangold, G., Pavia and H. Torrens, A proposal for the formal ratification of the basal boundary stratotype of the Bathonian stage based on a Bas Auran section (S.E. France). 2nd International Symposium on Jurassic Stratigraphy, 333-346.
Erba, E., 1990. Calcareous nannofossil biostratigraphy of some Bajocian sections from Digne area (SE france). Mem. Descr. Carta geol. Ital.,40: 237-356.
Erba, E. 2004. Calcareous nannofossils and Mesozoic Oceanic Anoxic Events. Marine Micropaleont. 52, 85-106.
Fernando-Lòpez, S.R., Pavia, G., Erba, E., Guiomar, M., Henriques, M.H., Lanza, R., Mangold, Morton, N., C., Olivero, D., Tiraboschi, D., 2007. Formal proposal for the Global Boundary Stratotype Section and Point (GSSP) of the Bathonian Stage, at the base of the Zigzag Zone in the Ravin du Bès Section (Bas-Auran, Sudalpine Basin, SE France). International Subcommission of Jurassic Stratigraphy. Bathonian Working Group Ballot: 1-43.
Mattioli, E., Erba, E., 1999. Synthesis of calcareous nannofossil events in Tethyan Lower and Middle Jurassic successions. Riv. Ital. Paleontol. Stratigr., 105: 373- 376.
Tremolada, F., Erba, E., 2002. Morphometric analyses of Aptian Assipetra infracretacea and Rucinolithus terebrodentarius nannoliths: Implications for taxonomy, biostratigraphy and paleoceanography. Marine Micropaleont., 44: 77-92
Calcareous nannofossil paleofluxes: proxies for pCO2 fluctuations through the Aptian
No full textThe Cretaceous has been punctuated by episodes of widespread organic matter burial in oxygen-depleted oceans known as Oceanic Anoxic Events (OAEs) (Schlanger & Jenkyns, 1976). In particular, the Aptian (~121 to ~113 Ma) has been characterized by a super-greenhouse climate and profound environmental perturbations, including the early Aptian OAE 1a, thought to be related to the emplacement of the Ontong Java Plateau.
The aim of this work is to study variations in biogenic carbonate production of calcareous nannofossils through the Aptian in order to detect if and how it has been affected by high pCO2. Calcareous nannoplankton is an excellent proxy for reconstructing present and paste surface water conditions, being extremely sensitive to changes in physical and chemical conditions of the oceans (pH). Studies on living form have attested a direct response of these organisms to changes in CO2 concentrations (e.g. Krug et al. 2010). Production of dwarf/malformed coccoliths has also been documented in the geological record through OAE 1a (Erba et al. 2010).
Here we present quantitative analyses of nannofossil micrite in thin sections, reconstructing nannofossil absolute abundances and calcite paleofluxes in three drill sites: the Cismon core (Northern Italy), Piobbico core (Central Italy) and DSDP Site 463 in the mid-Pacific Mountains. In each of the three sites, nannofossil absolute abundances and paleofluxes show a progressive drastic reduction starting in the latest Barremian. This decrease is due to a worldwide decrease in the rock-forming nannoconids, “nannoconids decline”, that culminates with the “nannoconids crisis” just prior to OAE 1a (Erba, 1994; Erba & Tremolada, 2004). At the end of OAE 1a, nannofossil biocalcification increases again, but it never reaches pre-anoxia values. In the late Aptian, nannofossil paleofluxes reach high values during the Nannoconus truittii acme, followed by a final decrease through the Aptian/Albian boundary interval.
Calcite paleofluxes fluctuations reveal a drastic reduction in nannoplankton calcification interpreted as the adaptive response of these organisms to perturbed surface-water conditions that favoured small and less calcified forms and caused false extinction among heavily calcified nannoconids (Lazarus effect). We conclude that, despite metabolic processes, CO2 concentrations influence the ocean chemistry and the carbonate system. In particular, the correlation between reduced biocalcification rates and intervals of intense volcanism, suggest that mid-Cretaceous nannoplankton biocalcification and nannofossil paleofluxes were strongly controlled by excess volcanogenic CO2.
REFERENCES
Erba, E. (1994). Nannofossils and superplumes: The early Aptian “nannoconid crisis”. Paleoceanography, 9: 483-501.
Erba, E. & Tremolada, F. (2004). Nannofossil carbonate fluxes during the Early Cretaceous: phytoplankton response to nutrification episodes, atmospheric CO2 and anoxia. Paleoceanography, 19: 1008.
Erba, E., Bottini, C., Weissert, H.J., Keller, C.E. (2010). Calcareous Nannoplankton response to surface-water acidification around Oceanic Anoxic Event 1a. Science 329: 428–432.
Krug, S.A., Schulz, K.G., Riebesell, U. (2010). Effects of CO2-induced changes in seawater carbonate chemistry speciation on Coccolithus braarudii: a conceptual model of coccolithophorid sensitivities. Biogeosciences Discuss., 7: 8763–8778.
Schlanger, S.O. & Jenkyns, H.C. (1976). Cretaceous oceanic anoxic events: causes and consequences. Geol. Mijnb., 55: 179–184
Calcareous nannofossil paleofluxes as proxy of pCO2 fluctuations during the Early Cretaceous
No full textCalcareous nannoplankton is an excellent proxy for detecting present and paste surface water conditions, being extremely sensitive to changes in temperature, fertility, salinity and chemical conditions of the oceans. We investigated a time interval of the geological history that includes the early Aptian Oceanic Anoxic Event 1a (OAE1a) (Schlanger and Jenkyns, 1976; Arthur et al., 1990), characterized by pronounced global changes in the composition and structure of the ocean-atmosphere system due to increased volcanogenic CO2 emissions associated with the emplacement of the Ontong Java and Manihiki Plateau.
In order to detect the response of calcareous phytoplankton to global changes and C-cycle perturbation, quantitative analyses of nannofossil micrite were conducted in thin sections, reconstructing nannofossil absolute abundances and calcite paleofluxes in two drill sites: the Cismon core (Northern Italy) and the DSDP 463 Site in the mid-Pacific Mountains.
At both sites, in the Barremian –Aptian interval, nannofossil absolute abundances and paleofluxes show a progressive drastic reduction, marked by a worldwide decrease in the rock-forming nannoconids “nannoconids decline”, that culminates with the “nannoconids crisis” just prior to the Selli event (Erba, 1994; Erba and Tremolada, 2004). At the end of the OAE1a, nannofossil biocalcification increases again, but it never reaches pre-anoxia values. In the late Aptian, nannofossil paleofluxes reach maximum values during the Nannoconus truittii acme, followed by a final decrease through the Aptian/Albian boundary interval.
Calcite paleofluxes fluctuations reveal a drastic reduction in nannoplankton calcification interpreted as the adaptive response of these organisms to perturbed surface-water conditions that favoured small and less calcified forms and caused false extinction among heavily calcified nannoconids (Lazarus effect). We speculate that mid-Cretaceous nannoplankton biocalcification and nannofossil paleofluxes were strongly controlled by excess volcanogenic CO2. Despite metabolic processes, CO2 concentrations influence the ocean chemistry and the carbonate system.
Excess CO2 conditions were perhaps enhanced by introduction of biolimiting metals at hydrothermal fields during construction of submarine Large Igneous Provinces
Warming and cooling episodes across the early Aptian Oceanic Anoxic Event 1a: the role of pCO2 on climate changes
No full textThe early Aptian Oceanic Anoxic Event 1a (OAE 1a: ~120 Ma) is a global phenomenon of organic matter burial in oxygen-depleted oceans. Volcanism, associated with the emplacement of the Ontong Java Plateau (OJP), is thought to be the main triggering mechanism for global anoxia, ocean acidification and greenhouse conditions (e.g. ERBA, 1994; LARSON & ERBA, 1999; JONES & JENKYNS, 2001; MÉHAY et alii, 2009; TEJADA et alii, 2009; BOTTINI et alii., in press). Independent studies on TEX86, sporomorphs and oxygen isotopes (e.g. MENEGATTI et alii., 1998; DUMITRESCU et alii., 2006; ANDO et alii, 2008; KUHNT et alii., 2011; MILLAN et alii., 2011; KELLER et alii., 2011) provided evidence for climate instability during OAE 1a. Nevertheless, the direct/indirect role of OJP volcanism, weathering rates and pCO2 on climate changes has not been established.
We have performed a high-resolution integrated nannofossil-geochemical investigation of OAE 1a in three sites: Cismon core (Italian Southern Alps), Piobbico core (Umbria-Marche basin, Italy) and DSDP Site 463 Mid-Pacific Mountains. The existing stratigraphic framework for the three sites and available cyclochronology for the Cismon core (MALINVERNO et alii., 2010) allows high-resolution dating of biotic and environmental fluctuations.
The nannofossil Temperature Index correlated with the oxygen-isotope record has revealed systematic and synchronous changes in the three sites. A warming pulse is registered prior to the onset of OAE 1a and corresponds to a prominent volcanic phase documented in the Os-isotope records (TEJADA et alii., 2009; BOTTINI et alii., in press). During OAE 1a, in correspondence of the core of the negative carbon-isotope excursion, temperature shows a maximum. This interval coincides with the most intense volcanic phase, as suggested by biomarkers, calcareous nannofossils and Os-isotope (MÉHAY et alii, 2009; TEJADA et alii, 2009; ERBA et alii., 2010; BOTTINI et alii., in press). This correspondence is suggestive of a (super)greenhouse climate triggered by excess volcanogenic CO2. Our data indicate that the beginning of the prolonged volcanic phase during OAE 1a coincides with warmest temperatures.
The end of global anoxia is paralleled by a cooling episode which slightly postpone the end of the major OJP volcanism.
High resolution analyses allow the identification of rapid cooling and warming during OAE 1a. Warm conditions in the early part of OAE 1a are interrupted by a brief (~35 ky) cooling interlude traced either by oxygen isotopes and calcareous nannofossils. This temperature change follows a ~100 kyrs-long interval characterized by accelerated continental weathering rates (BOTTINI et alii., in press). Arguably, warming at OAE 1a onset promoted methane hydrate dissociation (also suggested by carbon isotopes and biomarkers analyses; MEHAY et alii., 2009), which was perhaps instrumental in triggering continental weathering. Subsequent CO2 draw down, possibly during OJP quiescence, might explain the brief cooling interlude annihilated by warmest temperatures coeval with the onset of OJP paroxysmal phase.
In the second part of OAE 1a two more cooling events (around segments C4 and C6 of the carbon-isotope curve) sandwich an interval of intermediate and fluctuating temperatures. Major and minor cooling episodes correlate with high TOC content, suggesting that burial of organic matter acted as storage of excess CO2, thus temporarily mitigating greenhouse conditions, although under persisting OJP activity.
The data collected combined with an improved chronology show that volcanism of OJP caused general global warming. The excess burial of organic matter acted as an additional and alternative process influencing climate change.
The micropaleontological data presented confirm and provide evidence for even more temperature fluctuations during OAE1a, although under persisting OJP volcanism. This observation may imply that volcanism was not the only factor influencing climate. The ocean/atmosphere system during OAE 1a was extremely dynamic and affected by a series of complex processes. In fact, positive and negative feedbacks alternated over 1.2 My and controlled climate variability
Morphological changes of calcareous nannofossils during Oceanic Anoxic Event 2
No full textThe Cenomanian - Turonian Oceanic Anoxic Event 2 (OAE2) is one of the most pronounced perturbations of the Cretaceous that induced major changes in the marine environment and severe disturbances of the biosphere. It is thought to be linked to extensive volcanism and particularly to the emplacement of a Large Igneous Province (LIP) that acted as a natural source of excess CO2. Particularly, OAE2 seems to be strictly connected with the formation of the Caribbean Plateau, which triggered a chain of events leading to enhance ocean fertility, accelerated organic carbon burial, widespread ocean anoxia, and acidification.
Calcareous nannoplankton is sensitive to chemical - physical - trophic changes and must have reacted to the OAE2 extreme environmental conditions. Indeed, nannofossil data show that they experienced a turnover, a general decrease in species richness and locally a decline in abundance of the fertility indicator species.
In this study we present new results of a detailed morphometric analysis of 4 nannofossil species during OAE2 from two different areas: the Mediterranean area (Novara di Sicilia section, North-eastern Sicily) and the Sussex area (Eastbourne section, United Kingdom). These sections have been chosen based on integrated high-resolution stratigraphy: the C isotopic anomaly is taken as primary tool of dating and correlation of OAE2 events. Furthermore the Novara di Sicilia section represents only the lower portion of OAE2, whereas the Eatsbourne section, covering the entire OAE2 interval, provides the opportunity to characterize changes before, during and after the paleoenvironmental perturbation.
In analogy to the work of (ERBA et alii, 2010) on the Early Aptian OAE1a, biometric analyses were performed at high resolution on B. constans, D. rotatorius, Z. erectus and W. barnesiae in order to evaluate the influence of paleoenvironment on coccolith size and morphology. Size measurements of calcareous nannofossils were carried out on smear slides with a light microscope: in each sample 30 specimens of B. constans, D. rotatorius, Z. erectus and 50 specimens of W. barnesiae have been measured by using the Qcapture Pro suite.
Morphometric analyses highlight that:
1. In both sections dwarf coccoliths are present even before the OAE2 onset.
2. There is a species - specific response in both section: B. constans displays the most pronounced reduction in size, D. rotatorius records a well express reduction in size too as opposed to Z. erectus which diminishes in size to a lesser extend.
3. In both sections, the mean size of W. barnesiae is within the holotype and normal range size
4. In the Novara di Sicilia section dwarf coccoliths record the strongest reduction in size relative to Eastbourne samples.
5. Pronounced dwarfism is observed in discrete intervals before and within the carbon isotope excursion.
6. During OAE2 coccoliths return to normal sizes around the carbon isotopic peak “A”.
Comparison of morphometric analyses through OAE1a (ERBA et alii, 2010) and OAE2 shows analogies and differences. During OAE1a dwarfism and malformation are restricted to the onset of the carbon isotopic anomaly, in the core of the negative shift coeval with most profound paleoenvironmental perturbations. In the OAE2 interval dwarfism is most pronounced in the last part of the C isotopic anomaly, and coccolith deformation is negligible. During the two major Cretaceous OAEs, the ocean experienced extreme CO2 concentrations, acidification and an increase in fertility. Coccolith dwarfism suggests a link between nannoplankton calcification and high pCO2, although enhanced fertility and/or presence of toxic metals in seawater might have been instrumental as well. We speculate that during OAE1a and OAE2 excess CO2 played a fundamental role in ruling nannoplankton calcification efficiency, and that coccolith dwarfism might be a proxy of ocean acidification. Different patterns and degree of dwarfism and malformation during OAE1a and OAE2 suggest unequal volcanic CO2 emissions (rates, pulses, amount). We conclude that analogous causes (LIPs emplacement) have induced partially similar response at different times.
REFERENCE
ERBA E., BOTTINI C., WEISSERT H.J. & KELLER C.E., L., (2010) – Calcareous nannoplankton response to surface- water acidification around Oceanic Anoxic Evenet 1. Science, 329, 428-432
Las múltiples y extrañas despedidas de José Liboy Erba
No full textEl presente trabajo intenta hacer un balance de la obra del autor puertorriqueño José “Pepe” Liboy Erba. En nuestro recorrido ubicaremos al escritor dentro del sistema literario de la isla, proponiendo a Liboy como uno de los atípicos de la literatura reciente producida en el territorio. Luego revisaremos algunas de las características más sobresalientes de su poética, para sugerir que la escritura de Liboy es afín, más que a alguna tradición literaria en particular, a la estética cinematográfica de David Lynch. Hablamos entonces de una poética de lo raro, una cotidianidad alucinatoria que deforma y se asemeja a una lógica del ensueño o la pesadilla. Trazaremos algunos ejemplos que apuntan hacia el ocultamiento del sentido en la obra, y otra serie de técnicas, como la elipsis narrativa, la oralidad, las permutaciones de los cuentos y el inacabamiento, que continuamente refuerzan la idea de una escritura inestable, con ejes deformantes y grotescos. En otras palabras, intentaremos presentar su escritura como un modelo que establece una relación problemática con la tradición literaria y los temas recurrentes en el plano intelectual puertorriqueño
Origin of the early Albian oceanic anoxic event 1b in the western tethys (Piobbico core, central Italy) and north Atlantic (ODP Leg 171b site 1049c): calcareous nannofossil evidence.
No full textThe mid-Cretaceous greenhouse climate represents the most extreme warmth of the past 300 My, characterized by intense volcanism and increased ocean crust production. Moreover, the mid-Cretaceous is associated with oceanic anoxic events (OAEs), periods of elevated carbon buriaI in marine sediments.
Calcareous nannoplankton were sensitive to changes in temperature, fertility, and chemistry of surface waters. Consequently, abundance and composition of nannofossil assemblages are used to reconstruct paleoclimatic fluctuations and the functioning of the ocean/atmosphere system. Specifically, calcareous nannofloras are used for characterization of the earliest Albian OAE1b in order to assess the role of paleoclimatic changes leading to possible perturbations of surface water masses as triggers of anoxia.
Oceanic Drilling Program (ODP) Site 1049C (Blake Nose) recovered Aptian to Eocene sediments. The upper Aptian – lower Albian sequence consists of marls and calcareous marls interrupted by a prominent black shale interval (46 cm thick), that has been identified as the local expression of OAE 1b. Calcareous nannofossils were investigated every 20 cm in the upper Albian – lower Albian sediments and the early Albian OAE1b interval was sampled at high-resolution (1 cm). A similar sampling was applied to the coeval interval in the Piobbico core, drilled in the Umbria-Marche Basin (central Italy).
In both sections nannofossils are well preserved and the dominant species are W.barnesiae, R. asper, B. costans, Z. erectus, D.rotatorius, L. carniolensis. The warm-water taxa (R.asper and Z.diplogrammus) increase in abundance and temperature index shows warmer conditions within OAE 1b. This result is consistent with the oxygen stable isotope record, suggesting a significant warming at the onset of OAE1b.The meso-eutrophic taxa (Z.erectus, D.rotatorius and B.costans) increase in abundance 50 cm before OAE1b in the Piobbico core. and 2 meters before OAE 1b at Site 1049C. Within black shales the fertility taxa and nutrient index are generally high although fluctuating, and a systematic relationship with the d13C excursion is not observed.
Similar patterns have been documented for the Vocontian Trough (Herrle, 2003b). The nannofossil record suggests that productivity increased well before OAE1b and, consequently, cannot be the trigger of anoxia. In the western Tethys - North Atlantic area, OAE 1b is associated with a strong increase in surface-water temperatures as recorded by calcareous nannofloras. As suggested by Erbacher et al (2002), OAE1b might be regarded as a “super-sapropel”: extremely warm conditions and increased precipitation and continental runoff induced abnormal stratification of the water column with development of dysaerobic-anoxic conditions and accumulation/preservation of organic matter on the sea floor.
References:
Erbacher, J., Huber, B.T., Norris, R.D., Markey, M., 2001. Increased thermohaline stratification as a possible cause for an oceanic anoxic event in the Cretaceous periode. Nature 409, 325-327.
Herrle, J.O., Pross, J., Friedrich, O., Kössler, P., Hemleben, C., 2003b. Forcing mechanisms for mid-Cretaceous black shale formation: evidence from the upper Aptian and lower Albian of the Vocontian basin (SE France). Palaeogeogr. Palaeoclimatolog. Palaeoecol., 190, 399-426
Synthesis of 4,5-Dihydro-1H-Imidazole-4-Carboxylates from alfa-Amino Acid Amidines
No full textStarting from serine methyl esters, aldehydes or ketons and tosylazide various beta-hydroxy-substituted amidines were obtained. Via Mitsunobu intramolecular cyclisation, optically pure methyl-2-alkyl-1-tosyl-4,5-dihydro-1H-imidazole-4-carboxylates 5a-d were synthetise
Nannofossil paleofluxes during the Aptian: proxies for paleo-CO2, climate, fertility and chemistry of the oceans
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