83 research outputs found
Lâinfluence de la glaciation du Gondwana Ă lâOrdovicien supĂ©rieur et au Silurien de la Baltique. Un test de la cyclicitĂ© de lâenvironnement Ă lâaide des isotopes du carbone
étudié et trois autres au Llandovérien et au commencement du Wenlock. Les nouvelles données de Hamoumi [1999] déplacent
le dĂ©but de lâĂ©poque glaciale au Caradocien infĂ©rieur quand la mer Baltique passe des moyennes aux basses latitudes
de lâhĂ©misphĂšre austral [Torsvik et al., 1996]. En mĂȘme temps le centre de la glaciation se dĂ©place du nord de
lâAfrique au sud de lâAmĂ©rique.
Malgré la distance considérable entre les régions polaires de Gondwana et les régions subtropicales de la mer Baltique,
tous les Ă©vĂ©nements glaciaires susnommĂ©s sont dâune certaine maniĂšre reflĂ©tĂ©s en mer Baltique de lâest. Le mĂ©canisme
de cette influence est discutable dans le détail mais, les processus climatiques et océaniques jouent un rÎle
essentiel. Il est généralement admis que les glaciations sont marquées par les excursions positives de valeurs de Ύ18
O et
ÎŽ13
C provoquĂ©es par lâaugmentation de la couche de glace aux rĂ©gions polaires, la bioproduction, le dĂ©placement du carbone
organique dans les sĂ©diments et le refroidissement de lâocĂ©an.
Les relations connues entre ces différents facteurs permettent de corréler les événements glaciaires de Gondwana
avec les changements simultanés de la courbe des isotopes fixée en mer Baltique. Par ailleurs, le modÚle du cycle des
isotopes du carbone océanique de Jeppsonn [1990] est mis en perspective avec les valeurs réelles mesurées.
Les excursions positives de ÎŽ13
C (les valeurs maximum entre parenthÚses) sont évaluées pour la Baltique : le Caradocien
moyen (2,2 â°), lâAshgill infĂ©rieur (2,5 â°), lâHirnantien (6 â°), lâAĂ©ronien infĂ©rieur (3,7 â°), le Telychien infĂ©rieur
(2,7 â°), le Wenlock infĂ©rieur (5,2 â°). Les changements pour la plupart sont en corrĂ©lation avec les baisses du
niveau de lâocĂ©an, ayant Ă©videmment un caractĂšre glacio-eustatique. La corrĂ©lation positive se trouve aussi entre la glaciation
et les changements de la biodiversitĂ© largement connus comme la crise Oandu (au Caradocien), lâextinction en
masse de Hirnantia et lâĂ©vĂ©nement Ireviken au Wenlock.
Les données analysées permettent de conclure que : (1) les quatre glaciations du Gondwana identifiées notamment
sur la base de tillites et dâargiles microconglomĂ©ratiques et biostratigraphiquement datĂ©es sont dans les profils baltiques clairement marquĂ©s par lâexcursion de la courbe des isotopes du carbone ; (2) trois anomalies positives mineures
Ă lâAshgill et au Caradoc ainsi que des donnĂ©es sur lâabondance spĂ©cifique des algues, indiquent la prĂ©sence dâune pĂ©riode
climatique plus froide Ă lâOrdovicien infĂ©rieur. Ces donnĂ©es sont en faveur dâun dĂ©but plus prĂ©coce de la glaciation
du Gondwana, mais des datations nouvelles des roches glaciogéniques considérées sont nécessaires pour le
confirmer ; (3) le test du modĂ©le ocĂ©anologique de Jeppsson Ă lâaide des isotopes du carbone a souvent montrĂ© des contradictions
entre le modÚle établi et les valeurs mesurées ; (4) on ne devrait pas représenter les épisodes climatiques-
ocĂ©aniques seulement sur la base de la distribution dâun petit nombre dâespĂšces de conodontes connus mais aussi Ă
lâaide de marqueurs mettant en Ă©vidence les changements plus gĂ©nĂ©raux du milieu marin fondĂ©s sur les critĂšres lithologiques,
géochimiques ou/et paléontologiques
Linking the progressive expansion of reducing conditions to a stepwise mass extinction event in the late Silurian oceans
The late Ludlow Lau Event was a severe biotic crisis in the Silurian, characterized by resurgent microbial facies and faunal turnover rates otherwise only documented during the "big five" mass extinctions. This asynchronous late Silurian marine extinction event preceded an associated positive carbon isotope excursion (CIE), the Lau CIE, although a mechanism for this temporal offset remains poorly constrained. Here, we report thallium isotope data from locally reducing late Ludlow strata within the Baltic Basin to document the earliest onset of global marine deoxygenation. The initial expansion of anoxia coincided with the onset of the extinction and therefore preceded the Lau CIE. Additionally, sulfur isotope data record a large positive excursion parallel to the Lau CIE, interpreted to indicate an increase in pyrite burial associated with the widely documented CIE. This suggests a possible global expansion of euxinia (anoxic and sulfidic water column) following deoxygenation. These data are the most direct proxy evidence of paleoredox conditions linking the known extinction to the Lau CIE through the progressive expansion of anoxia, and most likely euxinia, across portions of the late Silurian oceans
The Alvarez impact theory of mass extinction; limits to its applicability and the âgreat expectations syndromeâ
For the past three decades, the Alvarez impact theory of mass extinction, causally related to catastrophic meteorite impacts, has been recurrently applied to multiple extinction boundaries. However, these multidisciplinary research efforts across the globe have been largely unsuccessful to date, with one outstanding exception: the Cretaceous-Paleogene boundary. The unicausal impact scenario as a leading explanation, when applied to the complex fossil record, has resulted in force-fitting of data and interpretations ("great expectations syndrome". The misunderstandings can be grouped at three successive levels of the testing process, and involve the unreflective application of the impact paradigm: (i) factual misidentification, i.e., an erroneous or indefinite recognition of the extraterrestrial record in sedimentological, physical and geochemical contexts, (ii) correlative misinterpretation of the adequately documented impact signals due to their incorrect dating, and (iii) causal overestimation when the proved impact characteristics are doubtful as a sufficient trigger of a contemporaneous global cosmic catastrophe. Examples of uncritical belief in the simple cause-effect scenario for the Frasnian-Famennian, Permian-Triassic, and Triassic-Jurassic (and the Eifelian-Givetian and Paleocene-Eocene as well) global events include mostly item-1 pitfalls (factual misidentification), with Ir enrichments and shocked minerals frequently misidentified. Therefore, these mass extinctions are still at the first test level, and only the F-F extinction is potentially seen in the context of item-2, the interpretative step, because of the possible causative link with the Siljan Ring crater (53 km in diameter). The erratically recognized cratering signature is often marked by large timing and size uncertainties, and item-3, the advanced causal inference, is in fact limited to clustered impacts that clearly predate major mass extinctions. The multi-impact lag-time pattern is particularly clear in the Late Triassic, when the largest (100 km diameter) Manicouagan crater was possibly concurrent with the end-Carnian extinction (or with the late Norian tetrapod turnover on an alternative time scale). The relatively small crater sizes and cratonic (crystalline rock basement) setting of these two craters further suggest the strongly insufficient extraterrestrial trigger of worldwide environmental traumas. However, to discuss the kill potential of impact events in a more robust fashion, their location and timing, vulnerability factors, especially target geology and palaeogeography in the context of associated climate-active volatile fluxes, should to be rigorously assessed. The current lack of conclusive impact evidence synchronous with most mass extinctions may still be somewhat misleading due to the predicted large set of undiscovered craters, particularly in light of the obscured record of oceanic impact events
Facies development and sequence stratigraphy of the Ludfordian (Upper Silurian) deposits in the Zbruch River Valley, Podolia, western Ukraine: local facies overprint on the ÎŽ13Ccarb record of a global stable carbon isotope excursion
Palaeoclimate inferred from ÎŽ18O and palaeobotanical indicators in freshwater tufa of Lake Ăntu SinijĂ€rv, Estonia
We investigated a 3.75-m-long lacustrine sediment record from Lake Ăntu SinijĂ€rv, northern Estonia, which has a modeled basal age >12,800 cal yr BP. Our multi-proxy approach focused on the stable oxygen isotope composition (ÎŽ18O) of freshwater tufa. Our new palaeoclimate information for the Eastern Baltic region, based on high-resolution ÎŽ18O data (219 samples), is supported by pollen and plant macrofossil data. Radiocarbon dates were used to develop a core chronology and estimate sedimentation rates. Freshwater tufa precipitation started ca. 10,700 cal yr BP, ca. 2,000 years later than suggested by previous studies on the same lake. Younger Dryas cooling is documented clearly in Lake Ăntu SinijĂ€rv sediments by abrupt appearance of diagnostic pollen (Betula nana, Dryas octopetala), highest mineral matter content in sediments (up to 90 %) and low values of ÎŽ18O (less than â12 â°). Globally recognized 9.3- and 8.2-ka cold events are weakly defined by negative shifts in ÎŽ18O values, to â11.3 and â11.7 â°, respectively, and low concentrations of herb pollen and charcoal particles. The Holocene thermal maximum (HTM) is palaeobotanically well documented by the first appearance and establishment of nemoral thermophilous taxa and presence of water lilies requiring warm conditions. Isotope values show an increasing trend during the HTM, from â11.5 to â10.5 â°. Relatively stable environmental conditions, represented by only a small-scale increase in ÎŽ18O (up to 1 â°) and high pollen concentrations between 5,000 and 3,000 cal yr BP, were followed by a decrease in ÎŽ18O, reaching the most negative value (â12.7 â°) recorded in the freshwater tufa ca. 900 cal yr BP
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The climatic significance of Late Ordovician-early Silurian black
The Ordovician-Silurian transition (455-430 Ma) is characterized by repeated climatic perturbations, concomitant with major changes in the global oceanic redox state best exemplified by the periodic deposition of black shales. The relationship between the climatic evolution and the oceanic redox cycles, however, remains largely debated. Here, using an ocean-atmosphere general circulation model accounting for ocean biogeochemistry (MITgcm), we investigate the mechanisms responsible for the burial of organic carbon immediately before, during and right after the latest Ordovician Hirnantian (445-444 Ma) glacial peak. Our results are compared with recent sedimentological and geochemical data. We show that the late Katian time slice (445 Ma), typified by the deposition of black shales at tropical latitudes, represents an unperturbed oceanic state, with regional organic carbon burial driven by the surface primary productivity. During the Hirnantian, our experiments predict a global oxygenation event, in agreement with the disappearance of the black shales in the sedimentary record. This suggests that deep-water burial of organic matter may not be a tenable triggering factor for the positive carbon excursion reported at that time. Our simulations indicate that the perturbation of the ocean circulation induced by the release of freshwater, in the context of the post-Hirnantian deglaciation, does not sustain over sufficiently long geological periods to cause the Rhuddanian (444 Ma) oceanic anoxic event. Input of nutrients to the ocean, through increased continental weathering and the leaching of newly-exposed glaciogenic sediments, may instead constitute the dominant control on the spread of anoxia in the early Silurian
PĆĂdolĂ carbon isotope trend and upper Silurian to lowermost Devonian chemostratigraphy based on sections in Podolia (Ukraine) and the East Baltic area
Insufficient knowledge of carbon isotope cycling in the latest Silurian initiated the study of two regions at the western and southwestern margins of Baltica in order to obtain a more complete picture about the carbon isotope trend through the PĆĂdolĂ. Shallow and open shelf carbonate rocks of the Dniester River outcrops and Kotuzhiny core in Podolia and deep shelf rocks of the East Baltic area, especially the Lithuanian cores, were studied for bulk-rock isotope analysis. The data sets of both regions begin with the mid-Ludfordian excursion and include also some part of the lowermost Devonian. The data show a new minor twin positive ÎŽ13C excursion (peak values 0.8â1.7â°) in the upper Ludfordian. The PĆĂdolĂ carbon isotope trend begins with a low of negative ÎŽ13C values, succeeded by the lower to middle PĆĂdolĂ âstabilityâ interval (variable values below or close to 0â° with a slight rising trend). The upper PĆĂdolĂ begins with a medium to major excursion (peak values 2.3â4.5â°), which reflects the pattern of the carbon isotope trend on the west of the Baltica palaeocontinent. Its wider significance awaits confirmation from observations elsewhere. The carbon isotope excursion at the SilurianâDevonian boundary, named here the SIDE excursion (its ÎŽ13C values range from 1.6â° in deep shelf settings to 3.8â° in shallower ones and 4.5â° in brachiopod shells), has been traced on several continents, and now also in Baltica. This excursion can serve as a well-dated global chemostratigraphic correlation tool. The shape of the excursion indicates the completeness of the studied section. We conclude that carbon isotope chemostratigraphy may contribute to subdividing the PĆĂdolĂ Series into stages and that Baltica sensu lato seems to be the right place for such a development
The Wenlock-Ludlow carbon isotope trend in the Vidukle core, Lithuania, and its relations with oceanic events
A Wenlock to Ludlow terrigenous-carbonate succession in the Vidukle core in Central Lithuania represents a deep shelf environment with a general upwards-shallowing trend, interrupted by brief deepening episodes. The carbon isotope trend, based on 115 whole-rock analyses, shows three main excursions: (1) a major excursion (ÎŽ13 C values reach 3.2â°) in the lower Wenlock, (2) low shifts (1.3â° and 1.6â°) at two levels in the upper most Wenlock Siesartis Formation corresponding to the Monograptus ludensis Biozone, (3) the most prominent excursion (ÎŽ13 C values reach 8.2â°) occurs in the upper Ludlow Mituva Fm. The upper Ludlow excursion is dated by the last occurrences of Polygnathoides siluricus below the main shift and the appearance of Ozarkodina wimani and O. crispa above the excursion. The excursion stratigraphically coincides with the Lau oceanic Event and is correlated with the mid-Ludfordian Neocucullograptus kozlowskii-Bohemograptus bohemicus tenuis Biozone. Changes in the carbon isotope trend are in general harmony with some aspects of the rock (CaO, terrigenous component) and fossil content of the section. The data presented are consistent with an arid climate model for the Ludfordian isotope event
Three global carbon isotope shifts in the Silurian of Podolia (Ukraine): stratigraphical implications
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