A diverse and abundant assemblage of deep-water agglutinated foraminifera (DWAF) has been recovered from an uppermost Hauterivian ammonite-rich horizon (Pseudothurmannia ohmi Biozone) from the ...Central Western Carpathians. The section records the last occurrence of the calcareous nannofossil species Litraphidites bollii (Thierstein) preceding the Faraoni anoxic level. Various morphogroups of agglutinated foraminifera show changes in response to trophic regime shift and decreasing oxygenation of the bottom and pore waters in a relatively short time-series. The DWAF data were compared with calcareous nannofossil assemblages and foraminifera analysed from thin sections (calcareous benthic and planktonic foraminifera). Four ecozones were identified, each with a specific taxonomic composition of foraminiferal assemblages. The zone-to-zone changes include fluctuations in the richness and relative abundance of morphogroups and planktonic foraminifera, and dwarfism in certain groups of foraminifera.
The Geological Time Scale shows large uncertainties on durations and ages of Berriasian to Albian stages (Early Cretaceous), which impact climate and paleoceanographic reconstructions. Here, we ...provide a new astrochronology of the Hauterivian Stage anchored on (1) recent biostratigraphically well-constrained published radio-isotopic dates, and (2) a previously published astrochronology of the Valanginian Stage. A new duration of the Hauterivian Stage is assessed here at 5.93±0.41myr. The retained age model, anchored on a latest CA-ID-TIMS U–Pb age from a tuff level in the Hauterivian of the Neuquén Basin (Argentina), dates the base of the Valanginian Stage at −137.05±1.0Ma, the base of the Hauterivian Stage at −131.96±1.0Ma, and the top of the Hauterivian Stage at −126.02±1.0Ma. In addition, the onset of the mid-Valanginian Weissert Event is dated at −135.22±1.0Ma and the onset of the Faraoni Event at −126.73±1.0Ma. The duration of the mid-Valanginian carbon-isotope excursion, associated to the Weissert Event, is assessed at 5.85myr, with a rapid phase of increasing δ13C values (0.60myr), a phase of stable δ13C values (1.48myr), and smooth decrease in δ13C values (3.77myr). The calibration provided here highlights that the onset of the activity of the Paraná–Etendeka province and the start of the Weissert Event coincided, suggesting that the Paraná–Etendeka province may have played a major role on the climatic and oceanographic changes during the mid-Valanginian.
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•New Geological Time Scale for the Valanginian–Hauterivian times•Onset of the Paraná–Etendeka volcanism concomitant to start of the Weissert Event•CO2 releases may have been not enough to turn the Weissert Event into an OAE.
The response of clay mineral assemblages to potential orbital forcing is tested in Mesozoic hemipelagic marl–limestone rhythmites of the Río Argos section (Betic Cordillera, Southeastern Spain). ...Along the section, marls are pervasively enriched in kaolinite and illite, whereas limestones are enriched in smectite-rich illite/smectite mixed-layers, suggesting that marl–limestone alternations are produced by cyclic high-frequency fluctuations of continental runoff. Spectral analyses show that clay mineral assemblages evolve accordingly to precession, obliquity and eccentricity cycles. Durations of ammonite zones are assessed at 535kyr for the Late Hauterivian Pseudothurmannia ohmi Zone and at 645kyr and for the Early Barremian Taveraidiscus hugii Zone. These durations are in agreement with other cyclostratigraphic estimates but significantly differ from the Geologic Time Scale 2004 and 2008. Clay minerals display enhanced amplitude of the eccentricity cycles during the Faraoni Oceanic Anoxic Event due to enhanced continental weathering conditions prevailing at that time. Sedimentary expression of the 405-kyr eccentricity is disturbed by palaeoclimate changes during the Faraoni OAE, challenging the hypothesis of Cretaceous OAE triggered by eccentricity cycles. Although palaeoceanographic events (e.g. Faraoni OAE) may induce disturbances in the clay mineral record, this study demonstrates the potential of these minerals to be used as a proxy for orbital calibration in Mesozoic times.
► Clay mineral analyses from high-resolution sampled bulk rocks ► Spectral analyses to detect orbital forcing on clay mineralogy ► Clay minerals prove they can be used for orbital calibrations.
The Early Cretaceous (latest Hauterivian) Faraoni Oceanic Anoxic Event (F-OAE) in the Río Argos section (Caravaca region, southern Spain), the candidate for the Global Boundary Stratotype Section and ...Point (GSSP) of the Hauterivian–Barremian transition, has been studied. Its ichnological bed-by-bed analysis allows for interpretation of oxygenation changes through the Faraoni Level interval and improvement upon previous characterization of oxygen conditions prior, during and after the F-OAE. The trace fossil assemblage belongs to the Zoophycos ichnofacies and it includes Chondrites intricatus, Chondrites targionii, Halimedides isp., Palaeophycus isp., ?Patagonichnus isp., Planolites isp., Rhizocorallium isp., Taenidium isp., Thalassinoides isp., Trichichnus linearis, Zavitokichnus fusiformis, and Zoophycos isp. Their diversity in particular beds fluctuates. Beds with four to six ichnotaxa reflect a multi-tiered macrobenthic trace maker assemblage living in good oxic and trophic conditions. In one bed below and one bed above the Faraoni Level (both marls without primary lamination), there are only two or three, mostly opportunistic ichnotaxa (Trichichnus, Chondrites, Planolites). They record dysoxic conditions. At the base of the Faraoni Level, one thicker and two thinner beds of marly mudstones (21.2 and 3.5 cm thick, respectively) are characterized by primary lamination. At the top and in the basal part of the thicker bed and in the thinner beds some trace fossils are present. These beds were deposited under anoxic conditions and later colonized by trace makers either from overlying beds deposited under oxic conditions or from the level of a greenish lamina in the lower part of the thicker bed, recording short episodes of dysoxic conditions. The thinner anoxic beds are separated by marls deposited under dysoxic conditions.
•Ichnological analysis of the Faraoni Oceanic Anoxic Event is conducted.•Trace fossil composition is relatively uniform in the studied interval.•Prevalent good environmental conditions (oxic conditions and food) are interpreted.•Local decreasing in diversity below and above the Faraoni Level reveals dysoxia.•Absence of traces in laminated beds at the base of the Faraoni Level indicates anoxia.
The 405‐kyr eccentricity cycle is a consistent orbital parameter throughout the Phanerozoic that is associated with long‐term variations in global continental weathering. However, a lack of reliable ...geological evidence has hampered the understanding of the relation between the 405‐kyr eccentricity cycle and continental weathering during the Cretaceous. Os isotopic ratios (187Os/188Os) of the sedimentary record reflect the balance between radiogenic Os derived from continental weathering and Os derived from unradiogenic sources (e.g., hydrothermal activity, weathering of mafic rocks, and extraterrestrial sources). This ratio is therefore considered as a good proxy for the evaluation of short‐term changes in continental weathering patterns. To trace orbital‐paced continental weathering, this study reconstructs the marine Os isotopic records in upper Hauterivian to lower Barremian (Lower Cretaceous) carbonate rocks in central Italy, where previous studies have reported that variations in clay mineral composition are paced by the 405‐kyr cycle. Our new Os isotopic record documents periodic oscillations of 187Os/188Os between 0.7 and 0.9 that correspond to the 405‐kyr Earth's eccentricity cycle. Because the sedimentary interval with radiogenic 187Os/188Os values (∼0.9) corresponds to a time interval characterized by a humid climate in areas surrounding the Tethys, variations in the 187Os/188O values likely reflect cyclic changes in continental weathering caused by eccentricity‐paced intensification of monsoonal activity at low latitudes. This variation could have been further amplified by increased input of radiogenic Os from Paleozoic shale and Precambrian crust at higher latitudes that resulted from a latitudinal shift of the intertropical convergence zone.
Key Points
Hauterivian marine 187Os/188Os cyclically varied following the 405‐kyr eccentricity
405‐kyr eccentricity enhanced monsoon that intensified the continental weathering
187Os/188Os record does not support the massive volcanic event at the Faraoni Level
In the uppermost Hauterivian sediments of the western Tethys, a short-lived anoxic event (Faraoni event) is documented both in the form of an interval enriched in organic matter (pelagic realm) and ...in a condensed interval enriched in glauconite and phosphate (shelf realm). This latter interval represents the onset of a drowning episode on the Helvetic carbonate platform along the northern tethyan margin that lasted throughout the early Barremian. This drowning episode marks a turning point in the way the platform carbonate factory functioned: during the Hauterivian carbonate production was dominated by heterozoans, whereas during the late Barremian a photozoan assemblage developed that is preserved in the so-called Urgonian limestone. The late Hauterivian Faraoni oceanic anoxic event is of particular interest because it is not accompanied by a major positive shift in δ
13C unlike other oceanic anoxic events during the Cretaceous (Valanginian, early Aptian, Cenomanian–Turonian boundary).
We have analyzed four (hemi-)pelagic sections with regards to their phosphorus content to better understand the palaeoceanographic conditions related to this anoxic event and the associated changes in the shallow-water carbonate factory. The sections are located in Angles (SE France), Fiume-Bosso and Gorgo a Cerbara (central Italy), and Veveyse de Châtel-St. Denis (west Switzerland). We calculated phosphorus mass accumulation rates by using a cyclostratigraphic approach in order to obtain an adequate age model. We observe a comparable and correlatable long-term trend for the four sections, which suggests that the phosphorus mass accumulation rates and temporal changes therein are representative for the western tethyan pelagic realm. The Faraoni event is marked by a minimum in phosphorus accumulation and a positive shift in the C
org/P
tot ratios, which is interpreted as a reflection of the decreased capacity of storing and preserving phosphorus in oxygen-depleted sediments. Moreover, the onset in the decrease in phosphorus accumulation coincides with a sea level rise, while the Faraoni level itself corresponds to a maximum flooding interval. This phase of sea-level rise may have been important in the establishment of marine connections between the boreal and tethyan realms and, as such, in the exchange of nutrient-enriched waters. The model for the origin of the Faraoni oceanic anoxic event proposed here incorporates these aspects together with a positive feedback loop generated by phosphorus regeneration and a negative feedback loop related to changes in the ocean oxygen cycle.
The subsequent long-term changes in phosphorus burial rates during the Barremian suggest that the Faraoni event marks the onset of a long period of environmental instability with regards to platform growth, leading to periodic phases of eutrophication and drowning of the northern tethyan carbonate platform. This environmental crisis ended during the late Barremian with the onset of the deposition of the Urgonian limestone under oligotrophic conditions.
An orbital floating time scale of the Hauterivian–Barremian transition (Early Cretaceous) is proposed using high-resolution magnetic susceptibility measurements. Orbital tuning was performed on the ...Río Argos section (southeast Spain), the candidate for a Global boundary Stratotype Section and Point (GSSP) for the Hauterivian–Barremian transition. Spectral analyses of MS variations, coupled with the frequency ratio method, allow the recognition of precession, obliquity and eccentricity frequency bands. Orbitally-tuned magnetic susceptibility provides minimum durations for ammonite biozones. The durations of well-constrained ammonite zones are assessed at 0.78myr for Pseudothurmannia ohmi (Late Hauterivian) and 0.57myr for Taveraidiscus hugii (Early Barremian). These results are consistent with previous estimates from the other reference section (Angles, southeast France) and tend to show that the Río Argos section displays a complete succession for this time interval. They differ significantly from those proposed in the Geologic Time Scale 2008 and may help to improve the next compilation. The Faraoni Oceanic Anoxic Event, a key Early Cretaceous oceanographic perturbation occurring at the P. ohmi/P. catulloi subzone boundary has a duration estimated at 0.10–0.15myr, which is similar to previous assessments.
► We provide an orbital tuning for the Hauterivian–Barremian transition. ► New durations are proposed for biozones, significantly different from the GTS. ► Our data indicate the Río Argos section is a valid candidate for GSSP. ► A new temporal framework is proposed for the Hauterivian–Barremian transition.
To better understand the sedimentary environment of the rift basin under the influence of a warm global climate, Lingshan Island in eastern Shandong Province (China) was studied. Inorganic ...geochemical indexes (B, equivalent B, B/Ga, Ga, V, Couch’s palaeosalinity, Adams’s palaeosalinity, Sr/Ba, Sr/Cu, Rb/Sr, Al
2
O
3
/MgO, and CaO/MgO·Al
2
O
3
) were analyzed and a quantitative calculation of palaeosalinity was carried out based on the Adams and Couch methods. The sedimentary environment of the rift basin at Lingshan Island was determined according to the morphology and distribution of its palaeobiota as well as inorganic geochemical indexes. We demonstrate that eastern China had high-temperature drought-like conditions during the Early Cretaceous, which may have been influenced by the Faraoni event. The dry-hot climate transformed the rift basin at Lingshan Island into saline lacustrine basin. Therefore, the salinization of water was not influenced by seawater but was the response of terrestrial strata to the warm, dry climate. This study helps to understand the sedimentary background of Cretaceous rift basins in eastern China and the influence of a warm climate on China’s terrestrial strata.
Lower Cretaceous pedogenic carbonates exposed in SE China have been dated by U–Pb isotope measurements on single zircons taken from intercalated volcanic rocks, and the ages integrated with existing ...stratigraphy. δ
13C values of calcretes range from –7.0‰ to –3.0‰ and can be grouped into five episodes of increasing–decreasing values. The carbon isotope proxy derived from these palaeosol carbonates suggests pCO2 mostly in the range 1000–2000 parts per million by volume (ppmV) at S(z) (CO2 contributed by soil respiration) = 2500 ppmV and 25°C during the Hauterivian–Albian interval (c. 30 Ma duration). Such atmospheric CO2 levels are 4–8 times pre-industrial values, almost double those estimated by geochemical modelling and much higher than those established from stomatal indices in fossil plants. Rapid rises in pCO2 are identified for early Hauterivian, middle Barremian, late Aptian, early Albian and middle Albian time, and rapid falls for intervening periods. These episodic cyclic changes in pCO2 are not attributed to local tectonism and volcanism but rather to global changes. The relationship between reconstructed pCO2 and the development of large igneous provinces (LIPs) remains unclear, although large-scale extrusion of basalt may well be responsible for relatively high atmospheric levels of this greenhouse gas. Suggested levels of relatively low pCO2 correspond in timing to intervals of regional to global enrichment of marine carbon in sediments and negative carbon isotope (δ
13C) excursions characteristic of the oceanic anoxic events OAE1a (Selli Event), Kilian and Paquier events (constituting part of the OAE 1b cluster) and OAE1d. Short-term episodes of high pCO2 coincide with negligible carbon isotope excursions associated with the Faraoni Event and the Jacob Event. Given that episodes of regional organic carbon burial would draw down CO2 and negative δ
13C excursions indicate the addition of isotopically light carbon to the ocean–atmosphere system, controls on the carbon cycle in controlling pCO2 during Early Cretaceous time were clearly complex and made more so by atmospheric composition also being affected by changes in silicate weathering intensity.
Within the Valanginian–Hauterivian series of the northern Tethyan margin, two organic-rich laminated black shale intervals corresponding to the Valanginian Weissert and the late Hauterivian Faraoni ...oceanic anoxic events (W-OAE and F-OAE, respectively) are well documented. However, in Tunisia as part of the southern Tethyan margin, despite several biostratigraphic and structural studies awarded to this period, geochemical characterization based mainly on wireline logs data was undertaken. Here, we aimed to confirm the extension of these Valanginian–Hauterivian organic-rich deposits throughout a bulk and biomolecular geochemical (Rock–Eval pyrolysis and gas chromatography–mass spectrometry analysis) study carried out on selected surface outcrops in northern Tunisia. To that end, three hemi-pelagic sections belonging to the Tunisian backbone (Jebel Zaghouan), the Tunisian trough (Jebel Oust) and the diapiric zone (Jebel Boulahouajeb) have been chosen and analysed with highly interest to their organic matter (OM) contents and biomolecular composition. Rock–Eval pyrolysis shows that Jebel Oust section presents total organic carbon (
TOC
) contents ranging from 0.18 to 0.57 wt% with an average of 0.38 wt% and low hydrogen index (
HI
) values ranging from 15 to 74 mg HC/g TOC indicating a type IV OM. Its hydrocarbon generation potential (
HGP
) is very low where the average value is around 0.19 mg HC/g rock, while Jebel Zaghouan section reveals
TOC
values reaching up to 0.56 wt% with a low
HGP
up to 1.01 mg HC/g rock and high maturity levels attested by
T
max
values ranging from 449 to 460 °C. Jebel Boulahouajeb section shows low
TOC
values ranging from 0.22 to 0.69 wt% with an average of 0.35 wt%. Its
HGP
is very low with a maximum value of 1.67 mg HC/g rock and presents marginally mature to mature stage where
T
max
values ranging from 438 to 450 °C with an average of 443 °C. This maturity distribution was guided by the basin architecture marked by the presence of subsiding zones in local faulted blocks and palaeohighs occupied by Triassic salt domes and horsts. Molecular biomarker analyses from the Valanginian–Hauterivian Boulahouajeb and Jebel Zaghouan facies indicated a mainly marine origin of the OM and a suboxic depositional environment in a normal marine water column. Throughout this study, we confirm the installation, in the southern Tethyan margin (northern Tunisia), of dysoxic depositional conditions during the early Valanginian and late Hauterivian times corresponding to the Weissert and Faraoni events, respectively. However, these depositional environments were not favourable for preservation of high quantities of OM. This was mainly controlled by the presence of an oxygen minimum zone generally disturbed by detrital discharges. Additionally, the results highlight the important interplay of the basin architecture, the various faults developed with an extensional tectonic regime during the Valanginian–Hauterivian period, the warm climate conditions and the eustatic sea-level variations where transgressive systems tracts coincide with black shales deposited majorly under dysoxic conditions.
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