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

Farmers’ Trust in Extension Staff and Productivity: An Economic Experiment in Rural Areas of Iran

Publication history: Accepted - 25 June 2022; Published - 4th May 2023.Farmers’ trust in extension staff may improve the performance of agricultural extension services and productivity through transferred knowledge and new farming practices. Using the trust game and trust questionnaire, this study measured farmers’ trust in extension staff. Measures obtained from the two methods were statistically different. We examined the relationship between the measured trust and agricultural productivity to control socio-economic factors. The findings revealed an insignificant relationship between trust and productivity that might be due to inappropriate attributes of extension programs. This emphasizes the need for more participation of farmers in researching and structuring training programs. While age had a negative impact on trust, traditional farmers with high experience showed a high level of trust. This indicates that young farmers who mostly inherited their lands from their parents and have occupations other than farming or practice modern farming, do not trust the extension staff. Farm size positively influences productivity by reducing the number of laborers per hectare. This emphasizes that the traditional way of farming is the cause of low productivity in Iranian agriculture

Lower and Middle Ordovician conodonts from the subsurface of SE Estonia and adjacent Russia

The distribution of conodonts in the Lower–Middle Ordovician beds was studied in five drill core sections of south­eastern Estonia. Ten conodont zones and six subzones, from the Paroistodus proteus Zone to the Eoplacognathus lindstroemi Subzone, were established. The peculiarity of the studied sequence is that the Volkhov Stage is notably poor in conodonts, in particular Baltoniodus species. Large specimens of Drepanodus arcuatus and Protopanderodus rectus, however, are found in great numbers in the uppermost Volkhov and lower Kunda stages. The deeper shelf conodont faunas recognized in SE Estonia (Central Baltoscandian confacies belt) are compared with shallow shelf faunas of northern Estonia (North Estonian confacies belt)

Ordovician conodont diversity in the northern Baltic

The diversity data presented in this paper are based on recently revised collections of conodonts from outcrops (northern Estonia) and core sections of Estonia and western and northern Latvia. Based on variations in the abundance of taxa, four intervals with different general trends in diversity changes can be recognized in the Ordovician part of the succession: (1) the proavus–crassus zones and (2) crassus–anserinus zones, both intervals forming quite distinct diversity cycles; (3) the variabilis–ventilatus zones, characterized in general by stable diversity values, and (4) the ventilatus Zone–the topmost Ordovician, with a steady increase in diversity up to the ordovicicus Zone, followed by a decline during the End-Ordovician Extinction Event. Diversity was lowest in the earliest and latest Ordovician but reached maximum values at the end of the Early Ordovician radiation of conodonts, in the middle and upper Darriwilian, and just before the start of the End-Ordovician Extinction Event. Four supersequences (transgressive–regressive cycles) are proposed here for the northern Baltic Ordovician succession. The diversity changes recognized in the conodont succession demonstrate general correlation with these supersequences: boundaries between supersequences are characterized by low diversity values; diversity increases more or less rapidly in the lower, transgressive parts of the supersequences and decreases in their upper parts

Darriwilian (Middle Ordovician) conodont biostratigraphy in NW Estonia

Darriwilian conodonts have been studied in numerous sections of Baltoscandia, but few data are available from the classical outcrop area in northern Estonia. In this paper we report the succession of Darriwilian conodonts and chitinozoans from the bed-by-bed sampled Uuga and Osmussaar cliffs, NW Estonia. Standard Baltoscandian conodont and chitinozoan zones and subzones were identified from the topmost Volkhov to Uhaku regional stages. The global Dapingian–Darriwilian boundary coincides with the Volkhov–Kunda boundary in NW Estonia. The Kunda Stage is represented by its middle or upper part only, and the Aseri Stage is probably missing in both sections studied. The lowermost part of the Lasnamägi Stage with the conodont Yangtzeplacognathus foliaceus is also very condensed. The rest of the Lasnamägi and Uhaku stages, corresponding to the main part of the Pygodus serra conodont Zone, is well represented and can be correlated across Estonia and Sweden. Subtle regional variations in this interval indicate very uniform depositional conditions over wide areas of the Baltoscandian palaeobasin. Our results suggest that the Y. foliaceus Subzone in Estonia needs further assessment. Stratigraphically well-constrained Y. protoramosus appears to be more common than previously thought, and is thus a valuable regional subzonal index. The integrated conodont and chitinozoan scale provides more than 20 biostratigraphically important levels for local and regional correlations, probably approaching temporal resolution in the order of 0.1 Ma for the late Darriwilian

A bio-chemostratigraphical test of the synchroneity of biozones in the upper Silurian of Estonia and Latvia with some implications for practical stratigraphy

The paper discusses the reliability of different biozones in terms of their synchroneity when crossing facies boundaries within a sedimentary basin. Graptolite biozones are the most trusted ones, but also biozones based on conodonts, chitinozoans and less ‘authoritative’ groups like ostracodes and vertebrates are used. The integration of bio- and chemostratigraphy aids the understanding of the pattern and timing of fossil distribution. Despite different environments most of the analysed biozones are in general synchronous units and in the majority of cases their time signals do not contradict each other. The much discussed Ozarkodina crispa (and its two morphs) has a stratigraphical range in the East Baltic that is longer than commonly recognized elsewhere. Its first occurrences in Estonia have been reported from shallow-water facies below the Mid-Ludfordian Carbon Isotope Excursion. The geographical distribution of Oz. crispa is much wider in the upper Ludfordian open shelf rocks and a few occurrences are also known in the lowest Přídolí in the East Baltic. Analogous ecostratigraphical trends are characteristic of some other fossil species. The analysis demonstrates that, when properly studied, all of the fossil groups considered can provide useful biostratigraphical information. The subdivision of the Přídolí Series into two stages is discussed. Bio-chemostratigraphical data confirm the late Ludfordian age of the Kuressaare Formation and its correlatives. The bio- and chemostratigraphical testing of biozonal indices suggests some tentative correlations with other areas, in particular the type Ludlow area, and enables identification of the Silurian–Devonian boundary in the East Baltic

Conodont biostratigraphy and sedimentary history in the upper Tremadoc at Uuga, Cape Pakri, NW Estonia

The upper Tremadocian boundary beds at Cape Pakri, NW Estonia, consist of an extremely friable glauconitic sandstone, which presents a challenge to detailed biostratigraphy. A combination of sedimentological and biostratigraphical criteria has served to clarify the tempo and mode of the processes that formed the sandstone and explain its relationships to strata immediately below and above it. Apatitic conodont elements, which abound in all these sediments, are particularly well suited to tracing the geological history of the surrounding sediment, since they can be repeatedly included in the sediment, eroded and redeposited, often leaving telltale marks on the elements which are nevertheless identifiable. By separating the indigenous elements from those that had been redeposited, we could place the local upper boundary of the Tremadocian at slightly more than 1 m above the base of the c. 4 m-thick sandy deposit. We showed that the sandstone, where 58-97% of the conodont elements have been redeposited, had been formed during four successive phases of sand deposition. The entire sandstone unit belongs to the Paroistodus proteus Zone. In the sandy and clayey Varanguan beds of the Paltodus deltifer Zone that underlie the sandstone, less than 50% of the conodont elements had been redeposited. The upper part of the section consists of limestone beds belonging to the Oepikodus evae Zone, where the redeposited portion of the conodont elements decreases upwards