L’influence de la glaciation du Gondwana à l’Ordovicien supérieur et au Silurien de la Baltique. Un test de la cyclicité de l’environnement à l’aide des isotopes du carbone

étudié et trois autres au Llandovérien et au commencement du Wenlock. Les nouvelles données de Hamoumi [1999] déplacent le début de l’époque glaciale au Caradocien inférieur quand la mer Baltique passe des moyennes aux basses latitudes de l’hémisphère austral [Torsvik et al., 1996]. En même temps le centre de la glaciation se déplace du nord de l’Afrique au sud de l’Amérique. Malgré la distance considérable entre les régions polaires de Gondwana et les régions subtropicales de la mer Baltique, tous les événements glaciaires susnommés sont d’une certaine manière reflétés en mer Baltique de l’est. Le mécanisme de cette influence est discutable dans le détail mais, les processus climatiques et océaniques jouent un rôle essentiel. Il est généralement admis que les glaciations sont marquées par les excursions positives de valeurs de δ18 O et δ13 C provoquées par l’augmentation de la couche de glace aux régions polaires, la bioproduction, le déplacement du carbone organique dans les sédiments et le refroidissement de l’océan. 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 modèle du cycle des isotopes du carbone océanique de Jeppsonn [1990] est mis en perspective avec les valeurs réelles mesurées. Les excursions positives de δ13 C (les valeurs maximum entre parenthèses) sont évaluées pour la Baltique : le Caradocien moyen (2,2 ‰), l’Ashgill inférieur (2,5 ‰), l’Hirnantien (6 ‰), l’Aéronien inférieur (3,7 ‰), le Telychien inférieur (2,7 ‰), le Wenlock inférieur (5,2 ‰). Les changements pour la plupart sont en corrélation avec les baisses du niveau de l’océan, ayant évidemment un caractère glacio-eustatique. La corrélation positive se trouve aussi entre la glaciation et les changements de la biodiversité largement connus comme la crise Oandu (au Caradocien), l’extinction en masse de Hirnantia et l’événement Ireviken au Wenlock. Les données analysées permettent de conclure que : (1) les quatre glaciations du Gondwana identifiées notamment sur la base de tillites et d’argiles microconglomératiques et biostratigraphiquement datées sont dans les profils baltiques clairement marqués par l’excursion de la courbe des isotopes du carbone ; (2) trois anomalies positives mineures à l’Ashgill et au Caradoc ainsi que des données sur l’abondance spécifique des algues, indiquent la présence d’une période climatique plus froide à l’Ordovicien inférieur. Ces données sont en faveur d’un début plus précoce de la glaciation du Gondwana, mais des datations nouvelles des roches glaciogéniques considérées sont nécessaires pour le confirmer ; (3) le test du modéle océanologique de Jeppsson à l’aide des isotopes du carbone a souvent montré des contradictions entre le modèle établi et les valeurs mesurées ; (4) on ne devrait pas représenter les épisodes climatiques- océaniques seulement sur la base de la distribution d’un petit nombre d’espèces de conodontes connus mais aussi à l’aide de marqueurs mettant en évidence les changements plus généraux du milieu marin fondés sur les critères 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 „great expectations syndrome”

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

Palaeoclimate inferred from δ18O and palaeobotanical indicators in freshwater tufa of Lake Äntu Sinijärv, Estonia

We investigated a 3.75-m-long lacustrine sediment record from Lake Äntu Sinijärv, northern Estonia, which has a modeled basal age >12,800 cal yr BP. Our multi-proxy approach focused on the stable oxygen isotope composition (δ18O) of freshwater tufa. Our new palaeoclimate information for the Eastern Baltic region, based on high-resolution δ18O 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 Äntu Sinijärv sediments by abrupt appearance of diagnostic pollen (Betula nana, Dryas octopetala), highest mineral matter content in sediments (up to 90 %) and low values of δ18O (less than −12 ‰). Globally recognized 9.3- and 8.2-ka cold events are weakly defined by negative shifts in δ18O values, to −11.3 and −11.7 ‰, 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 −11.5 to −10.5 ‰. Relatively stable environmental conditions, represented by only a small-scale increase in δ18O (up to 1 ‰) and high pollen concentrations between 5,000 and 3,000 cal yr BP, were followed by a decrease in δ18O, reaching the most negative value (−12.7 ‰) recorded in the freshwater tufa ca. 900 cal yr BP

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

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 (δ13 C values reach 3.2‰) in the lower Wenlock, (2) low shifts (1.3‰ and 1.6‰) at two levels in the upper most Wenlock Siesartis Formation corresponding to the Monograptus ludensis Biozone, (3) the most prominent excursion (δ13 C values reach 8.2‰) 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