Jaak Nõlvak

Polar front shift and atmospheric CO2 during the glacial maximum of the Early Paleozoic Icehouse

Our new data address the paradox of Late Ordovician glaciation under supposedly high pCO2 (8 to 22× PAL: preindustrial atmospheric level). The paleobiogeographical distribution of chitinozoan (“mixed layer”) marine zooplankton biotopes for the Hirnantian glacial maximum (440 Ma) are reconstructed and compared to those from the Sandbian (460 Ma): They demonstrate a steeper latitudinal temperature gradient and an equatorwards shift of the Polar Front through time from 55°–70° S to ∼40° S. These changes are comparable to those during Pleistocene interglacial-glacial cycles. In comparison with the Pleistocene, we hypothesize a significant decline in mean global temperature from the Sandbian to Hirnantian, proportional with a fall in pCO2 from a modeled Sandbian level of ∼8× PAL to ∼5× PAL during the Hirnantian. Our data suggest that a compression of midlatitudinal biotopes and ecospace in response to the developing glaciation was a likely cause of the end-Ordovician mass extinction.

Biological interpretationand paleobiodiversity of a cryptic fossil group: The “chitinozoan animal”

Chitinozoans are used to evaluate, at the species level the variations of diversity within a cryptic fossil group from their emergence to their ultimate demise. The chitinozoans appear to be a group suitable for such investigations because of their fairly restricted range, world-wide distribution and rather limited number of described species. However, before using this group, it was necessary to demonstrate that chitinozoans represent an ontogenic stage (eggs) in the life-cycle of a cryptic group (i.e. virtually non fossilisable soft bodied organisms). The eggs register the variations of diversity affecting the group and are therefore valid indicators of species diversity. Our results reveal the dramatic diversification of the group in its early stage, reflecting the incorporation of numerous innovations and an accelerated radiation phase lasting twenty millions years, from the late Tremadoc to the Llanvirn. The end-Ordovician faunal crisis and its Eo-Silurian recovery are also well documented. The decline of the group occurred as early as the Lower Devonian and its extinction at the very end of the Devonian, after a brief increase in diversity during the Givetian. However, depending on the time units used, the shape of the biodiversity curves varies significantly and some artefacts may be present. At the series level, only the major tendencies are recorded, whereas at the stage level, additional second-order signals, not firmly related to abiotic events, are revealed.

Carbon isotope stratigraphy in the latest Ordovician of Estonia

Abstract On the basis of bulk carbonate carbon isotope data from six drill cores and the type section of the Porkuni Stage of Estonia, δ13C curves were compiled, which altogether cover nearly the whole succession of the latest Ordovician (Hirnantian) —well known as a time of short-lived glaciation and mass extinction. In pre-Porkuni and Silurian rocks, δ13C values were low (from −0.2‰ to 1.7‰). For the Porkuni Stage three intervals of the curve were established: (1) lower — a positive shift up to 6.7‰, (2) middle part with variable values from 3.3‰ to 5.4‰, and (3) upper phase with slightly decreased values from 3.2‰ to 3.6‰. The correlation with the Dobs Linn (Scotland) kerogen δ13C curve shows that the first two intervals coincide with the Normalograptus extraordinarius graptolite Biozone, the third one with the N. persculptus Biozone.

Chitinozoan dating of Ordovician impact events in Sweden and Estonia. A preliminary note

Abstract Four impact craters of Ordovician age are known from Sweden and Estonia. Chitinozoans from impact and post-impact sediments in these craters have been investigated. Three craters (Kardla, Tvaren and Lockne) were created at different occasions in the earliest Caradoc, and one crater (Granby) probably in late Arenig.

New data on Ordovician stable isotope record and conodont biostratigraphy from the Viki reference drill core, Saaremaa Island, western Estonia

The Viki drill core is one of the reference sections for the Estonian bedrock succession, characterizing strata of Mid Ordovician through early Silurian age in the eastern part of the Baltoscandian basin. In this article, we present the hitherto missing Ordovician stable carbon isotope curve and Middle Ordovician conodont biostratigraphy from the Viki core to complement the previous lithological, geochemical, geophysical and biostratigraphic studies. For the first time, the Hirnantian sulfur isotope excursion is reported from the eastern Baltic region, showing high amplitude and tight coupling with carbon cycling.

Biostratigraphy and taxonomy of three-dimensionally preserved nemagraptids from the Middle and Upper Ordovician of Baltoscandia

Abstract Biostratigraphically significant finds of two important species of Nemagraptus in the Middle and Upper Ordovician carbonate sections of Baltoscandia are reviewed. Nemagraptus subtilis is found in the lower part of the Uhaku Regional Stage, which is correlated with the Hustedograptus teretiusculus graptolite zone. Nemagraptus gracilis is restricted to the upper part of the Kukruse Stage in the Central and South Estonian and western Latvian sections. This level correlates with the Nemagraptus gracilis graptolite Zone. The vertical distribution of these taxa is similar to, although less complete than, their ranges in the classic black shale succession in Scania, southern Sweden. The first three-dimensionally preserved specimens of N. subtilis are illustrated and described. Nemagraptus subtilis can be differentiated from N. gracilis by its lack of cladial branches, more slender stipes, shorter sicula, and upward orientation of the first two thecae.

Upper Katian (Ordovician) bentonites in the East Baltic, Scandinavia and Scotland: geochemical correlation and volcanic source interpretation

Altered volcanic ash interbeds (bentonites) in the upper Katian of Baltoscandia indicate significant volcanic activity in neighbouring tectonically active areas. Katian bentonites in the East Baltic can be reliably correlated using sanidine phenocryst composition. Ratios of immobile trace elements TiO 2 , Nb, Zr and Th to Al 2 O 3 enable extension of the correlations to Scandinavia, where late diagenetic alterations could have caused recrystallization of sanidine phenocrysts. At least seven volcanic eruptions were recognized in Baltoscandian sections. Several bentonites found in deep-sea sediments are absent in shallow-sea sediments, indicating extensive breaks in sedimentation and erosion during late Katian and Hirnantian times. The areal distribution pattern of Katian bentonites in Baltoscandia indicates a volcanic source from the north or northwest (present-day orientation) from the margins of the Iapetus Palaeo-Ocean. Signatures of ultra-high-pressure metamorphism in the Seve Nappe (Central Sweden) and intrusions in the Helgeland Nappe Complex in Central Norway have been proposed as potential sources of the magmas that generated the volcanic ashes deposited in the East Baltic in Katian times. Geochemical similarities between Baltoscandian and Dob9s Linn bentonites from southern Scotland suggest a common volcanic source in Katian times.

Middle to Late Ordovician graptolite and chitinozoan biostratigraphy of the Kandava-25 drill core in western Latvia

This paper presents integrated graptolite and chitinozoan biostratigraphical data for the upper Middle and lower Upper Ordovician of the Kandava-25 drill core in Latvia, a core that overwhelmingly contains the most diverse graptoloid fauna in the East Baltic carbonate-dominated sections. Graptolites are abundant in two intervals in the Kandava-25 core. The older strata (Stirnas and Furudal formations) contain the graptolites Pseudamplexograptusdistichus, Proclimacograptusangustatus, Archiclimacograptuscaelatus, Archiclimacograptus sebyensis, Archiclimacograptusosloensis n. sp., Dicaulograptushystrix, Haddingograptuseurystoma, Haddingograptus luperus, Haddingograptus maennili n. sp., Hustedograptusteretiusculus, Hustedograptus vikarbyensis, Gymnograptuslinnarssoni, Dicellograptusvagus, Dicellograptus intortus, Dicellograptus gurleyi, Dicranograptusirregularis and Nemagraptussubtilis. These species indicate that the lower interval belongs to the P. distichus and D. vagus zones, zones that correlate with the more widely recognizable Didymograptusmurchisoni (upper part) and H.teretiusculus zones. Thus, the lower interval is late Dariwillian (Lasnamägi and Uhaku regional stages) in age. A second graptolite-rich interval begins in the middle of the Dreimani Formation and contains Nemagraptusgracilis, Hustedograptusuplandicus, Orthograptuswhitfieldi, Orthograptus calcaratusincisus, Oepikograptusbekkeri, Dicellograptussextans, Dicellograptus salopiensis, Normalograptusrotundatus and Archiclimacograptusbalticus n. sp. This fauna is Sandbian (Kukruse and Haljala regional stages) in age. Chitinozoans are abundant and well preserved throughout the Kandava-25 core. Six chitinozoan biozones and five subzones can be recognized in the upper Middle and lower Upper Ordovician strata of the core. The co-occurrence of biostratigraphically important graptolites and chitinozoans in the Kandava-25 core provides direct tie-points for Ordovician graptolite and chitinozoan biozonations. http://zoobank.org/urn:lsid:zoobank.org:pub:CC9E068C-E56C-4197-89B8-E30C067B69CB

Chitinozoan biostratigraphy in the Haljala Regional Stage, Upper Ordovician: a high-resolution approach from NE Estonia

The distribution of chitinozoans in the Haljala Stage (Upper Ordovician, Sandbian) from Kerguta-565, Piilsi-729 and Vasknarva-639 drill cores in NE Estonia was examined from 128 closely spaced samples. The studied chitinozoan fauna was diverse – altogether 14 chitinozoan genera and 39 species were recorded. Based on chitinozoan ranges in the studied sections, 13 species were identified as biostratigraphically most valuable. The high-resolution sampling used in this study has allowed us to refine the stratigraphic usefulness and extent of distribution of several chitinozoan key species as well as elaborate internal biostratigraphy of the Haljala Stage.

Integrated Upper Ordovician–lower Silurian biostratigraphy of the Grötlingbo-1 core section, Sweden

The biostratigraphical data obtained from the Grötlingbo-1 core section (southern Gotland, Sweden) enable recognition of most of the regional stages known from the Sandbian (Upper Ordovician) to lower Sheinwoodian (Wenlock, Silurian) interval in the East Baltic. The strata corresponding to the Upper Ordovician Keila–Nabala stages have markedly reduced thicknesses in the Grötlingbo-1 core section. Gaps of considerable duration were recognised in some parts of the section: lithological evidence suggest that only lower part of the Pirgu Stage (corresponds to all or part of the Jonstorp Formation) is represented in the section; in the Llandovery, strata corresponding to the Aspelundia? fluegeli and Distomodus staurognathoides Conodont Zones (CZs), and to the Pterospathodus eopennatus CZ (equivalent to the Conochitina alargada and Eisenackitina dolioliformisChitinozoan Zones (CtZs)) are missing. Graptolites from the core indicate that the sedimentary record above the Aeronian to middle Telychian gap recommences in the lower part of the Oktavites spiralis Graptolite Zone (GZ). Co-occurrences of conodonts, graptolites and chitinozoans agree with earlier data suggesting correlation between the lower boundaries of the Cyrtograptus lapworthiGZ and Pterospathodus amorphognathoides amorphognathoides CZ, and between the lower and middle parts of the Ireviken Event and the upper Cyrtograptus murchisoni GZ.