Abstract
Lukeneder, A. and Grunert, P. 2013. Palaeoenvironmental evolution of the Southern Alps across the Faraoni Level equivalent: new data from the Trento Plateau (Upper Hauterivian, Dolomites, N. ...Italy). Acta Geologica Polonica,
63
(1), 89-104. Warszawa. New stratigraphic and palaeoenvironmental data are presented for the northeastern part of the Trento Plateau (Puez area, Southern Alps, Italy). The studied section corresponds to the upper Hauterivian Balearites balearis and “Pseudothurmannia ohmi” ammonite zones and normal palaeomagnetic chron upper M5. A c. 30-cm-thick bed is identified as the equivalent of the Faraoni Level, based on its position within the Pseudothurmannia mortilleti Subzone, the composition of its ammonite fauna and the peak of a minor positive trend in the δ 13 C bulk record. Microfacies and geochemical proxies compare well with those of the southeastern part of the Trento Plateau and indicate palaeoceanographic continuity along the eastern margin of the plateau. The abundances of radiolarians and nannoconids suggest a turnover in the trophic structure from eutrophic conditions around the Faraoni Level equivalent to oligotrophic conditions. Low organic matter and sulphur content and frequent bioturbation indicate a well-oxygenated environment. Ammonite diversity and life-habitat groups document the influence of sea level on the plateau: while epi- and mesopelagic ammonites occur commonly during sea-level highstands, all life-habitat groups become reduced during a sea-level lowstand in the Spathicrioceras seitzi and P. ohmi subzones. The Faraoni event is heralded by a faunal turnover expressed as the diversification of epipelagic ammonites. Palaeoenvironmental conditions along the eastern margin of the Trento Plateau during the Faraoni event contrast with those of the organic-rich black shales in the west. A re-evaluation of the depositional model based on the new results suggests a general water depth of 300-500 m for the plateau. The severe reduction of mesopelagic ammonites during the sea-level lowstand indicates a shallowing towards the epi-/mesopelagic boundary. During the Faraoni event, the eastern areas of the Trento Plateau were located at the upper limit of the oxygen-minimum layer and were thus only occasionally affected by oxygen depletion, whereas the western areas were located well within the upper part of the oxygen-depleted layer.
In order to improve our understanding of the relationships between the late Hauterivian oceanic anoxic Faraoni event, contemporaneous platform drowning along the northern Tethyan margin and global ...environmental change in general, we established high-resolution
δ
13C and
δ
18O curves for the late Hauterivian and the entire Barremian stage. These data were obtained from whole-rock carbonate samples from the Veveyse de Châtel-Saint-Denis section (Switzerland), the Fiume-Bosso section and the nearby Gorgo a Cerbara section (central Italy), and the Angles section (Barremian stratotype, France).
We observe an increase of 0.3‰ in mean
δ
13C values within sediments from the middle Hauterivian
Subsaynella sayni ammonite zone to the Hauterivian–Barremian boundary;
δ
13C values remain essentially stable during the early Barremian. During the latest early Barremian and most of the late Barremian,
δ
13C values increase slowly (until the
Imerites giraudi zone) and the latest Barremian is characterized by a negative trend in
δ
13C values, with minimal values at the Barremian–Aptian boundary. During the earliest Aptian,
δ
13C mean values start to rise again and attain
+
2.25‰.
We interpret the evolution of the
δ
13C record as resulting from the interaction between changes in the carbon cycle in the Tethyan basin and the adjacent platforms and continents. In particular, changes towards warmer and more humid conditions on the continent and coeval phases of platform drowning along the northern Tethyan margin may have contributed to enhance the oceanic dissolved inorganic carbon (DIC) reservoir which may have pushed the
δ
13C record towards more negative values and exerted a general attenuation on the
δ
13C record. From this may have come the general change from a heterozoan to a photozoan carbonate platform community, which influenced the evolution in
δ
13C values by increasing the export of aragonite and diminishing export of dissolved organic carbon into the basins.
An integrated study of multiple geochemical proxies (TOC, δ13Ccarb, trace metallic elements, iron speciation) on the late Hauterivian Faraoni Oceanic Anoxic Event has been performed along the ...hemi-pelagic section of Río Argos (south-eastern Spain) in order to better constrain the environmental perturbations linked to this event in the Subbetic domain.
In the studied section, the TOC is relatively low (<0.5wt.%) except for one sample at the base of the Faraoni horizon, where it reaches 1.5wt.%. The δ13Ccarb signal is characterized by a minor long-term increase, primarily attributed to an enhanced organic matter burial. However, both organic and isotopic signals are probably affected by dilution effect of carbonates and a possible diagenetic overprint to primary organic and isotopic signal, due to a possible alteration of organic matter during early diagenesis, cannot be ruled out.
For the inorganic signal, the trace metallic elements show contrasted record during the late Hauterivian. While detrital proxies (Th, Ti, Zr) record rather constant signal, the productivity (Cu, Ni, P) and redox (U, V, Mn, Fe) proxies show noticeable variations. The organic-rich interval at the base of the Faraoni horizon records higher values of Cu, Ni and P as well as of U and V, linked to bottom-water oxygen depletion and enhanced primary productivity. The rest of the Faraoni horizon records punctual peaks of productivity and redox proxies but rather constant TOC concentration. As for organic and isotopic signals, parts of inorganic signal, linked to organic matter (as those of productivity proxies), have been probably reworked and partly influenced by carbonates. However, inorganic geochemical proxies are consistent suggesting rather oxygenated water-masses associated with moderate productivity during the late Hauterivian. Based on redox ratios (U/Th vs. V/(V+Ni), FeHR/FeT, Fepy/FeHR), oxygen concentrations were determined and it clearly appears that only the organic-rich interval at the base of the Faraoni horizon might record ferruginous anoxic conditions.
Compared to other hemi-pelagic Mediterranean regions (the Ultrahelvetic Basin from Switzerland and the Vocontian Trough from France), the Río Argos section shows good similarities concerning TOC and δ13Ccarb signals, suggesting moderate organic matter burial during the Faraoni Event in proximal environments. The main difference concerns the redox variations. Contrary to Italy and France sections where anoxic bottom-water conditions persist up to the end of the Faraoni Event, the Río Argos section shows no truly anoxia onset except at the base of the Faraoni horizon, which we rather interpret as local intra-basinal chemical variations.
► Integrated study of multiple geochemical proxies of the Faraoni Event in Spain ► Low TOC enrichment and slightly δ13Ccarb increase within the Faraoni horizon. ► Possible diagenetic overprint to TOC and δ13Ccarb signals ► Higher values of productivity and redox proxies within the Faraoni horizon ► Conditions of Oceanic Anoxic Event only recorded at the base of the Faraoni horizon.
A detailed geochemical analysis was performed on the upper part of the Maiolica Formation in the Breggia (southern Switzerland) and Capriolo sections (northern Italy). The analysed sediments consist ...of well-bedded, partly siliceous, pelagic carbonate, which lodges numerous thin, dark and organic-rich layers. Stable-isotope, phosphorus, organic-carbon and a suite of redox-sensitive trace-element contents (RSTE: Mo, U, Co, V and As) were measured. The RSTE pattern and Corg:Ptot ratios indicate that most organic-rich layers were deposited under dysaerobic rather than anaerobic conditions and that latter conditions were likely restricted to short intervals in the latest Hauterivian, the early Barremian and the pre-Selli early Aptian. Correlations are both possible with organic-rich intervals in central Italy (the Gorgo a Cerbara section) and the Boreal Lower Saxony Basin, as well as with the facies and drowning pattern in the Helvetic segment of the northern Tethyan carbonate platform. Our data and correlations suggest that the latest Hauterivian witnessed the progressive installation of dysaerobic conditions in the Tethys, which went along with the onset in sediment condensation, phosphogenesis and platform drowning on the northern Tethyan margin, and which culminated in the Faraoni anoxic episode. This episode is followed by further episodes of dysaerobic conditions in the Tethys and the Lower Saxony Basin, which became more frequent and progressively stronger in the late early Barremian. Platform drowning persisted and did not halt before the latest early Barremian. The late Barremian witnessed diminishing frequencies and intensities in dysaerobic conditions, which went along with the progressive installation of the Urgonian carbonate platform. Near the Barremian-Aptian boundary, the increasing density in dysaerobic episodes in the Tethyan and Lower Saxony Basins is paralleled by a change towards heterozoan carbonate production on the northern Tethyan shelf. The following return to more oxygenated conditions is correlated with the second phase of Urgonian platform growth and the period immediately preceding and corresponding to the Selli anoxic episode is characterised by renewed platform drowning and the change to heterozoan carbonate production. Changes towards more humid climate conditions were the likely cause for the repetitive installation of dys- to anaerobic conditions in the Tethyan and Boreal basins and the accompanying changes in the evolution of the carbonate platform towards heterozoan carbonate-producing ecosystems and platform drowning.
The Faraoni Level is a short-lived oxygen-deficient event that took place during the latest Hauterivian. In order to improve our understanding of the palaeoenvironmental conditions that occurred ...during this event, we have analysed the contents of several redox-sensitive trace elements (U, V, Mo, As, Co, Cd, Cu, Zn, Ni, Pb, Cr) from bulk limestone samples of late Hauterivian-early Barremian age from three reference sections. U, V, Mo and As show consistent and significant enrichments during the Faraoni event whereas the other redox-sensitive trace elements analysed here are not systematically enriched. In order to explain this discrepant behaviour, we propose that the Faraoni Level was deposited during a period of anoxic conditions near the sediment-water interface. The distinctive peaks in U, V, Mo and As contents are traceable throughout the three studied sections and represent a good correlation tool which helps to identify the Faraoni Level and its equivalents in the western Tethyan realm and outside of the Tethys. For example, a peak in U contents in upper Hauterivian sediments of the northwestern Pacific realm (ODP leg 185, site 1149) may well be an expression of the Faraoni event in this particular basin.PUBLICATION ABSTRACT
A conspicuous renewal in the ammonite faunas of the Mediterranean Tethys occurred in the latest Hauterivian. This faunal turnover took place following a stepwise pattern. The first step occurred at ...the boundary between the
Pseudothurmannia ohmi Subzone and the
Pseudothurmannia mortilleti Subzone, coinciding with the base of the so-called Faraoni Level. This is a C
org-rich interval that has been recognised in several basins of the Mediterranean Tethys and seems to be the expression of a short-lived oxygen-deficient event. It correlates with a well-documented second-order peak transgression. The oxygen depletion preferentially affected the deep nektic ammonites, which would explain the extinctions within this group around the Faraoni Level. On the contrary, an increase in the trophic resources in the photic zone favoured the diversification of epipelagic ammonites. Concurrently, an abrupt change took place at this level in the nannoconid assemblage composition.
A minor second event, located at the base of the
Pseudothurmannia picteti Subzone, was marked by the replacement of a few planktic ammonite species by closely related forms, and the structure of the ammonite assemblage was not substantially altered. The coincidence of this event with a further restructuring of the calcareous nannofossil assemblage suggests that some changes had to occur in the planktic ecosystem during the sea-level highstand subsequent to the peak transgression.
The third and last stage of the renewal process started in the upper part of the
P. picteti Subzone, coinciding with a drastic sea-level fall. It is characterised by the extinction of many of the species that had appeared in the two previous events, resulting in an extensive modification of the assemblage structure. The regression would probably cause a drop in the primary productivity and, consequently, an improvement in the oxygenation level of the sea bottom. This would explain the extinction of several planktic ammonite species and the appearance of new nektic and nektobenthic lineages.
Stable carbon, oxygen, and strontium isotope records were obtained from uppermost Hauterivian to lowermost Aptian belemnite rostra, which were collected in well-dated sections from the Vocontian ...Trough (southeastern France). This data set complements previously published belemnite-isotope records from the uppermost Berriasian-Hauterivian interval from the same basin. The belemnite carbon and oxygen isotope record is compared to the carbonate bulk-rock isotope record from the same sections, and from additional Italian sections. With regards to their long-term trends, both belemnite and whole-rock δ
18O records are well correlated, except for the uppermost Hauterivian-lower Barremian interval, within which they deviate. This discrepancy is interpreted to be linked to the latest Hauterivian Faraoni oceanic anoxic event and its early Barremian aftermath. The Faraoni level is characterized by enhanced sea-water stratification, probably induced by the onset of a warmer and more humid climate along the northern Tethyan margin. The early Barremian was characterized by stronger vertical sea-water mixing reflected by a decrease in density contrast between sea-surface and deeper waters. The belemnite oxygen isotope record shows a more stable evolution with smaller fluctuations than its bulk-rock counterpart, which indicates that deeper water masses were not as much subjected to density fluctuations as sea-surface water. The comparison of belemnite and bulk-rock carbon isotope records allows observing the impact of regional influence exerted by platform carbonate ooze shedding on the carbon cycle. Discrepancies in the two records are observed during time of photozoan carbonate platform growth. The strontium isotopic record shows a gradual increase from the uppermost Berriasian to the uppermost lower Barremian followed by a rapid decrease until the uppermost Barremian and a renewed small increase within the lowermost Aptian. The major inflection point in the uppermost lower Barremian appears to predate the onset in the formation of the Ontong-Java volcanic plateau.
A Late Hauterivian interval (i<127.5 Ma), called the Faraoni Event, which is characterised by the deposition of deep-marine black shales in the Mediterranean Tethys, is demonstrably of sufficient ...geological brevity to be qualified as an anoxic event. This event lies within the Pseudothurmannia catulloi ammonite subzone, coincides with the extinction of the calcareous nannofossil species Lithraphidites bollii, and records an increase in a globular planktonic foraminifer. High quantities of marine organic matter were preserved in pelagic successions from northern and central Italy, Switzerland, southeastern France, southern Spain and probably elsewhere in the Mediterranean Tethys and Atlantic Ocean. Carbon-isotope stratigraphy from Tethyan and Atlantic sections shows a minor positive excursion in the uppermost part of the Hauterivian and Lowermost Barremian, suggesting accelerated extraction of organic carbon from the ocean reservoir just after the Faraoni Event. The duration of this short event is less than 100 ka according to cyclostratigraphy and coincides with a third-order sea-level rise. It is likely that similar forcing mechanisms responsible for global OAEs operated during this short time interval.Original Abstract: Un intervalle fini-Hauterivien (i<127,5 Ma), caracterise par le depot de black shales dans les bassins pelagiques de la Tethys mediterraneenne, possede beaucoup des specificites des evenements anoxiques pour etre homologue sous le nom d'evenement Faraoni. Cet evenement est date de la sous-zone d'ammonites a Pseudothurmannia catulloi, coincide avec l'extinction du nannofossile calcaire Lithraphidites bollii, et enregistre une augmentation de l'abondance d'un foraminifere planctonique globuleux (Gorbachikella). De grandes quantites de matiere organique marine ont ete preservees dans les series pelagiques des bassins d'Italie centrale et septentrionale, de Suisse, du Sud-Est de la France et du Sud de l'Espagne, et probablement ailleurs, en Tethys mediterraneenne et atlantique. La stratigraphie isotopique du carbone de series tethysiennes et atlantiques montre une legere excursion positive dans la partie terminale de l'Hauterivien et la base du Barremien, suggerant une acceleration du stockage du carbone organique dans le reservoir sedimentaire juste apres l'evenement Faraoni. La duree de cet evenement est de l'ordre de 100 ka, d'apres la cyclostratigraphie, et coincide avec une phase transgressive de troisieme ordre. Il est fort probable que les memes facteurs forcants, a l'origine des evenements anoxiques globaux, soient la cause de ce court evenement.
Calcareous 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.
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