Hirnantian Stage Research Articles

Carbon- and sulfur-isotope geochemistry of the Hirnantian (Late Ordovician) Wangjiawan (Riverside) section, South China: Global correlation and environmental event interpretation

Detailed geochemical analyses (δ13Ccarb, δ13Corg, δ34Ssulfide, and abundance of sulfide, carbonate and organic carbon) were performed on samples from the Wangjiawan (Riverside) section, close to the GSSP (Global Stratotype Section and Point) for the Hirnantian stage of the Ordovician. New data show two increases in carbonate content coincident with two glacial pulses that reduced detrital input. The new δ34Ssulfide data show distinct changes in this section, with relatively high values in the Kuanyinchiao Formation, a pattern observed throughout the Yangtze Platform. However, there is no consensus on the cause of these changes. The new δ13Ccarb data show a sharp rise and peak in the extraordinarius zone, below the previously published δ13Corg peak in the persculptus zone. A compilation of the new results with other sections indicates the Hirnantian carbon-isotope excursion starts near the pacificus–extraordinarius boundary and elevated values remain until the end of the excursion in the persculptus zone for both δ13Corg and δ13Ccarb. The controversy over correlating Hirnantian graptolite zones with chitinozoan zones can also be addressed. The new δ13Ccarb data also allow direct comparison with Hirnantian δ13Ccarb data from Anticosti Island and the Baltic region, which are zoned by chitinozoan fossils. This comparison favors a correlation of the taugourdeaui and scabra chitinozoan zones with the extraordinarius graptolite zone.

Hirnantia sagittifera(Brachiopoda) andMucronaspis mucronatas.l. (Trilobita) in the Upper Ordovician of the East Baltic: taxonomy and distribution

The brachiopod Hirnantia sagittifera (M’Coy) and trilobite morphs of the genus Mucronaspis from the topmost Ordovician Porkuni Stage of the central East Baltic are described and compared with those from the Hirnantian Stage of other regions. These important Hirnantian taxa occur in the Livonian Tongue of the Central Baltoscandian Facies Belt of the Baltic Basin, where the Porkuni Stage is represented by the non-graptolitic Kuldiga and Saldus formations. Hirnantia sagittifera appears in the lowermost part of the Porkuni Stage and is rather widely distributed in the basin in spite of its rare finds in each locality. Our study of trilobites of the genus Mucronaspis has enabled us to observe morphological changes in its exoskeleton in time and to identify a succession of five morphotypes (morphs). In some characteristics these morphs are similar to those of different alleged species of the genus Mucronaspis (M. olini, M. danai, M. ganabina, M. mucronata) but they cannot be definitely assigned to any of these species due to some variances. However, here for the first time a stratigraphically ordered collection is presented, which deserves attention in revising the taxonomy of highly variable Mucronaspis. The described brachiopods and trilobites occur mainly in the strata correlated with the Normalograptus extraordinarius graptolite Biozone. However, the uppermost finds of both taxa come from strata correlated with the N. persculptus Biozone.

More about the Ordovician–Silurian transition beds at Mirny Creek, Omulev Mountains, NE Russia: carbon isotopes and conodonts

Profound environmental and biodiversity changes take place in the Ordovician-Silurian boundary interval. The Mirny Creek and Neznakomka River bank sections discussed in this paper expose the upper Katian-lower Rhuddanian part of the boundary beds. The succession consists of carbonate rocks, partly with bioherms, alternating with argillaceous and siltstone packages that are well dated by graptolites. Microfossils are rare, especially in the Hirnantian, but conodonts provide some useful markers just below and above the Hirnantian stage boundaries. The Hirnantian δ 13 C trend in the Mirny Creek section is the stratigraphically longest described so far and it has a highly specific shape. The trend commenced at the first appearance datum of Normalograptus extraordinarius or slightly below this level. The main peak occurs near the middle of the N. persculptus Biozone. Samples from the Neznakomka River suggest a somewhat wider peak interval than at Mirny Creek. Detailed comparison of the Mirny and Stirnas (Latvia) δ 13 C curves shows a general similarity despite great specific features of both trends. Correlation of the δ 13 C trends from China, Baltica and North America with that at Mirny Creek reveals a great variety of shapes of the carbon isotope curve. However, its rising limb commenced, if represented, everywhere close to the beginning of the N. extraordinarius Biozone or in terms of the Baltic succession, at the bottom of the Porkuni Regional Stage. Most likely a general shape of the HICE trend is pyramidal, which is peaking in the early N. persculptus Biochrone. Differences in the values and shape of an actual curve at different localities depend on local environmental conditions, sometimes modifying the global signal rather strongly.

Terminal Ordovician carbon isotope stratigraphy and glacioeustatic sea-level change across Anticosti Island (Quebec, Canada)

Globally documented carbon isotope excursions provide time-varying signals that can be used for high-resolution stratigraphic correlation. We report detailed inorganic and organic carbon isotope curves from carbonate rocks of the Ellis Bay and Becscie Formations spanning the Ordovician-Silurian boundary on Anticosti Island, Quebec, Canada. Strata of the Anticosti Basin record the development of a storm-dominated tropical carbonate ramp. These strata host the well-known Hirnantian positive carbon isotope excursion, which attains maximum values of ~4.5‰ in carbonate carbon of the Laframboise Member or the Fox Point Member of the Becscie Formation. The excursion also occurs in organic carbon, and δ^(13)C_carb and δ^(13)C_org values covary such that no reproducible Δ^(13)C (= δ^(13)C_carb – δ^(13)C_org) excursion is observed. The most complete stratigraphic section, at Laframboise Point in the west, shows the characteristic shape of the Hirnantian Stage excursion at the global stratotype section and point (GSSP) for the Hirnantian Stage in China and the Silurian System in Scotland. We therefore suggest that the entire Hirnantian Stage on Anticosti Island is confined to the Laframboise and lower Fox Point Members. By documenting discontinuities in the architecture of the carbon isotope curve at multiple stratigraphic sections spanning a proximal to distal transect across the sedimentary basin, we are able to reconstruct glacioeustatic sea-level fluctuations corresponding to maximum glacial conditions associated with the end-Ordovician ice age. The combined litho- and chemostratigraphic approach provides evidence for the diachroneity of the oncolite bed and Becscie limestones; the former transgresses from west to east, and the latter progrades from east to west. The sea-level curve consistent with our sequence-stratigraphic model indicates that glacioeustatic sea-level changes and the positive carbon isotope excursion were not perfectly coupled. Although the start of the isotope excursion and the initial sea-level drawdown were coincident, the peak of the isotope excursion did not occur until after sea level had begun to rise. Carbon isotope values did not return to baseline until well after the Anticosti ramp was reflooded. The sea-level–δ13Ccarb relationship proposed here is consistent with the “weathering” hypothesis for the origin of the Hirnantian δ^(13)C_carb excursion.

Strain analysis of Late Ordovician tectonic events in the In-Tahouite and Tamadjert Formations (Tassili-n-Ajjers area, Algeria)

The Tassili-n-Ajjers area in southern Algeria is located in the south-east of the Saharan platform. It is bounded to the east by the Tihemboka N–S trending structural system, the Hoggar shield to the south and to the north by the Illizi Basin. During the Palaeozoic, the Saharan Platform was part of the northern passive margin of the Gondwana supercontinent and the tectono-stratigraphic evolution of sedimentary systems in the Algerian Sahara was typical of the Gondwana stable cratonic platform. During the Late Ashgill (Hirnantian), the North African platform was located near the South Pole and an icecap developed over much of Africa and South America. Field observations, in the study area, together with integrated strain analysis and previous studies provide evidence for a number of Late Ordovician deformation phases. An extensional event of probably Late Caradoc–Early Ashgill age (?) was associated with the development of planar domino-style normal faults and listric faults that occurred prior to the infill of glacial palaeovalleys. This is observed in the pre-glacial In-Tahouite Formation. Glaciotectonic over the Tassilli-n-Ajjers is related to the Late Ashgillian (Hirnantian stage) Taconic event. Probably, the Taconic unconformity is a combination of traditional tectonic movements and glaciotectonics. Many soft-sediment deformation structures are attributed to glaciotectonic process. A systematic description of these deformation structures and strain analysis show that there are two types of deformation, pure shear at the top of the Hirnantian syn-glacial Tamadjert Formation and simple shear in the basal part and there are two types of glaciotectonic deformation. The first type at the base is the subglacial deformation and is characterized by simple shear strain and extensional tectonics. The second type at the top is characterized by pure shear strain and compressional tectonics and consists on proglacial deformation. The glacial palaeovalleys show a gravity-sliding mechanism affecting the unconsolidated deposits.

Chitinozoan biostratigraphy of a new Upper Ordovician stratigraphic framework for Anticosti Island, Canada

Thickness and facies variations in the Upper Ordovician succession of Anticosti Island have made correlations between the western and eastern ends of the island difficult. This chitinozoan study reveals that most strata of the Ellis Bay Formation at the eastern end of the island correlate with the Vaureal Formation at the western end. In the eastern end, chitinozoans typical of the Ellis Bay Formation occur only in the upper part of the Lousy Cove Member, whereas chitinozoans diagnostic of the mid-upper Vaureal occur in the rest of the formation. Moreover, the eastern Schmitt Creek and Mill Bay members of the Vaureal Formation are older than the uppermost Vaureal eponym members in the western end. New east-west correlations, supported by geochemical, sedimentological, and sequence stratigraphic data, are proposed. Comparison with coeval Upper Ordovician successions suggests a Hirnantian age for the gamachiana Zone that occurs in the lower members of the Ellis Bay Formation. It also indicates that the Hirnantian Isotopic Carbon Excursion (HICE) on Anticosti begins in the D. anceps Zone, just below the base of the Hirnantian Stage, reaches a first peak in the lower N. extraordinarius Zone, a second modest peak high in the N. extraordinarius Zone, and a third, more important one, in the lower part of the N. persculptus Zone. These results are in agreement with the biostratigraphic and chemostratigraphic data at the Global Boundary Stratotype Section and Points (GSSPs) for the base of the Hirnantian and Silurian in south-central China and at Dob9s Linn in Scotland.

Did changes in atmospheric CO 2 coincide with latest Ordovician glacial–interglacial cycles?

The Late Ordovician Hirnantian Stage (∼ 444 million years ago) was one of three time periods during the past half billion years in which large continental glaciers formed over Earth's polar regions. The effects of this glaciation were far-reaching and coincided with one of the largest marine mass extinction events in Earth history. The cause of this ice age is uncertain, and a paradoxical association with evidence for high atmospheric CO 2 levels has been debated. Precise linkages between sea level, ice volume, and carbon isotope ( δ 13C carb and δ 13C org) proxy records of pCO 2 have been poorly understood due in part to uncertainties in stratigraphic correlation and the interpretation of globally important sections. Although correlation difficulties remain, recent Hirnantian biostratigraphic studies now allow for improved correlations. Here we show that consistent trends in both δ 13C carb and δ 13C org from two well-dated stratigraphic sequences in Estonia and Anticosti Island, Canada coincide with changes in Late Ordovician (Hirnantian) climate as inferred from sea level and the extent of ice sheets. The integrated datasets are consistent with increasing pCO 2 levels in response to ice-sheet expansion that reduced silicate weathering. Ultimately, the time period of elevated pCO 2 levels is followed by geologic evidence of deglaciation.

Correlation of Ordovician diamictites from Argentina and South Africa using detrital zircon dating

Abstract: The results of detrital zircon U–Pb geochronology suggest a maximum depositional age of 485 ± 7.2 Ma for a glacial diamictite from the Sierra del Volcán in eastern Argentina (Tandilia System). Earlier interpretations associated the deposit with the Neoproterozoic ‘snowball Earth' hypothesis. The data allow direct correlation, for the first time, between Early Palaeozoic deposits in both South America and South Africa connecting the glacial deposits from southern Bolivia to central Argentina with those in South Africa (Pakhuis Formation). On the basis of these results, a new distribution map of glacial cover, corresponding to the Hirnantian stage, can be developed. Supplementary material: Sampling and analytical techniques, data, a simplified map and BSE images are available at http://www.geolsoc.org.uk/SUP18384 .

Large perturbations of the carbon and sulfur cycle associated with the Late Ordovician mass extinction in South China

High-resolution δ 13 C data of organic carbon from a continuous section of the Late Ordovician–Early Silurian reveal two positive δ 13 C excursions that are associated with the mass extinction in South China. The fi rst stratigraphic δ 34 S measurements on pyrite tied to well-established biostratigraphy indicate a large perturbation of the sulfur cycle, consistent with major sea-level changes related to the glaciation. The elevated δ 34 S values of pyrites and a large, short-lived negative δ 34 S excursion of ~20‰ associated with the decay of the glaciation suggest deep-water anoxia during the Hirnantian Stage, in contrast to the conventional view that the global oceans were oxygenated. We suggest that deep-water anoxia may have contributed to the Late Ordovician mass extinction in South China and possibly elsewhere.

Carbon isotopes and event stratigraphy near the Ordovician–Silurian boundary, Yichang, South China

The organic carbon isotope data through the unweathered Ordovician–Silurian boundary strata at the Wangjiawan Riverside section, which is c. 180 m southeast of the Wangjiawan North section, the GSSP for the base of the Hirnantian Stage (Upper Ordovician), show that a positive δ 13C org excursion begins just below the base of the Hirnantian Stage and peaks in the lower part of the N. extraordinarius Biozone. This is followed by an interval of slightly reduced δ 13C values and a second peak of 2‰ above pre-Hirnantian values, which occurs in the lower part of the N. persculptus Biozone (upper Hirnantian). The peaks in δ 13C org values can be correlated well with episodes of glacial expansion described from Africa and peri-Gondwanan Europe. Evidence from sedimentological, faunal, and geochemical data from South China, as well as the evidence of glacial sediments in North Africa, all suggests a short-lived glaciation of ~ 1 Ma in the Ordovician South Pole region, consisting of at least two major pulses. The first phase of gradual glacial expansion began just before the Hirnantian Epoch and the second phase ended with rapid melting in the late Hirnantian (early N. persculptus Biozone).

New chitinozoans from the historical type area of the Hirnantian Stage and additional key sections in the Wye Valley, Wales, UK

AbstractThe type locality for several core elements of the Hirnantia brachiopod fauna is a small disused quarry on the western slopes of Cwm Hirnant. There, the Hirnant Limestone Member of the Foel‐y‐Ddinas Mudstone Formation yields a new, well‐preserved chitinozoan assemblage, attributed to the Spinachitina taugourdeaui Biozone. This allows tight correlation with the Hirnantian of Baltica and Laurentia and neatly ties the chitinozoan zonation with the classical brachiopod fauna. Nearby, the chitinozoan assemblage from the Caradoc Cymerig Limestone Member at Gelli‐grîn belongs to the Spinachitina cervicornis Biozone?, and is identical to that recovered from the Burrellian in the Onny Valley, Welsh Borderland. A Silurian assemblage higher up section, discovered in the Cwm‐yr‐Aethnen Formation, is attributed to the globally recognized Eisenackitina dolioliformis Biozone. Attempts to integrate the chitinozoan and graptolite biozonation, in the central Welsh Rhayader area, were less successful. Copyright © 2008 John Wiley & Sons, Ltd.

SHRIMP zircon U-Pb dating from K-bentonite in the top of Ordovician of Wangjiawan Section, Yichang, Hubei, China

The Global boundary Stratotype Section and Point (GSSP) for the base of the Hirnantian Stage (the latest stage of the Ordovician System) is defined at a point 0.39m below the base of the Kuanyinchiao Bed in the Wangjiawan North Section, which is the upper most “golden spike” of the Ordovician. However, this “golden spike” is lack of reliable geochronology data. This article gives a sensitive high resolution ion microprobe (SHRIMPII) zircon U-Pb dating for a K-bentonite sample from the Kuanyinchiao Bed in the Wangjiawan North Section. The age of the K-bentonite sample is 443.2±1.6 Ma, that is to say, the isotopic age of the uppermost of Hirnantian Stage, the point of Ordovician-Silurian boundary, should be near to, but slightly younger than 443.2±1.6 Ma. This age is identical to the Ordovician-Silurian boundary age 443.7±1.5 Ma as declared by International Commission on Stratigraphy (ICS). So, this research provides some good geochronlogical data for the Hirnantian Stage and the Ordovician-Silurian boundary as well as the global correlation.

The regional stratotype section and point for the base of the Hirnantian Stage (the uppermost Ordovician) at Mirny Creek, Omulev Mountains, Northeast Russia

A complete Hirnantian sequence comprising the Normalograptus extraordinarius and N. persculptus biozones is well developed at the Mirny Creek section in the Omulev Mountains. The underlying beds are assigned to the Appendispinograptus supernus Biozone, and in the overlying strata the lower boundary of the Silurian is precisely defined at the base of the Akidograptus ascensus Biozone. Due to the completeness of the sedimentary and palaeontological record, the Mirny Creek section can be considered as a reference section for the Hirnantian Stage. The succession, about 100 m thick, is composed of calcareous siltstones and marls with pebble-shaped limestones, deposited at a high sedimentation rate in shallow shelf settings. The regional stratotype section and point (RSSP) for the lower boundary of the Hirnantian is established at the base of member 68, where Normalograptus extraordinarius first appears. This level can be precisely correlated with that at the GSSP section in Yichang and with the sections in Kazakhstan and North America. The position of the Ordovician-Silurian boundary is redefined and placed at the FAD of A. ascensus at the base of member 74.

The succession of Hirnantian events based on data from Baltica: brachiopods, chitinozoans, conodonts, and carbon isotopes

The Hirnantian (late Ordovician) environment was complex and dynamic. Understanding correct order of events and their precise correlation with a time scale are extremely important for development of different kinds of environmental interpretations. The lower boundary of Hirnantian Stage is officially defined by the lowest occurrence of Normalograptus extraordinarius, base of major positive carbon-13 isotope excursion, and beginning of a pronounced sea-level fall associated with onset of a major glaciation (ICS website). Our aim is to check if these events are synchronous, particularly how situation is with respect to Baltic. Thus several sections were analysed using mainly East Baltic data (drill cores), but also data from elsewhere, including brachiopod, chitinozoan, and conodont biostratigraphy combined with graptolite and carbon isotope data. Brachiopod faunas of Pirgu and Porkuni stages are rather similar to those of Ellis Bay Formation of Anticosti Island, Quebec, but Pirgu assemblage, as well as that of lower Ellis Bay Fm. lack key elements of Hirnantian faunas. The primary criterion quoted above is stable, but auxiliary data like isotope curve are less convincing. When latter is used, it should be specified, e.g. that carbon isotope excursion begins usually slightly earlier (in Diceratograptus mirus Biozone), but it might be conventionally placed into N. extraordinarius Biozone when a biostratigraphical proxy is available. The peak of excursion is in lower N. persculptus Biozone, but main increase in values takes place in N. extraordinarius Biozone. The Spinachitina taugourdeaui Biozone marks at many localities bottom of Hirnantian, and only recent chitinozoan-graptolite data from topmost Lousy Cove Member (on Anticosti Island) suggest a mid-Hirnantian age, which is at variance with common correlation schemes.

The Global Boundary Stratotype Section and Point (GSSP) for the base of the Hirnantian Stage (the uppermost of the Ordovician System)

The Global Boundary Stratotype Section and Point (GSSP) for the base of the Hirnantian Stage (the uppermost of the Ordovician System)

Carbon isotope chemostratigraphy in Arctic Canada: Sea-level forcing of carbonate platform weathering and implications for Hirnantian global correlation

Three sections through latest Ordovician strata in the Canadian Arctic Islands have been studied for carbon isotopes, derived from the organic matter (δ 13C org) and whole-rock carbonate (δ 13C carb) fractions. The sections are well constrained biostratigraphically using graptolites, lithostratigraphically and palaeogeographically. δ 13C org data appear to provide a signal that mainly reflects chemical changes in the seawater, whereas the δ 13C carb data seem to have been variably affected by sediment reworking and diagenesis. Results show that a positive δ 13C org excursion of 3–6‰ begins just below the base of the Hirnantian Stage and peaks in the lower part of the Normalograptus extraordinarius biozone of lower Hirnantian. This is followed by an interval of reduced δ 13C values and a second peak of similar magnitude, which occurs in the lower Normalograptus persculptus biozone (upper Hirnantian). These peaks appear to correlate well with episodes of glacial expansion described from West Africa. Global correlation between δ 13C curves suggests that the timing of peak positive excursions is not completely synchronous between different regions. In particular, the lower Hirnantian peak seen in Arctic Canada and some other areas appears to be suppressed in sedimentary successions from the circum-Iapetus region, where peak values occur in later Hirnantian time. Thus, no single, regional δ 13C curve can reliably serve as a benchmark for high-resolution, global correlation. These data provide support for the hypothesis that the positive δ 13C shifts seen in these sections and many others worldwide are the result of increased rates of weathering of carbonate platforms that were exposed during the glacio-eustatically controlled sea-level fall. This caused the isotope value of the C-weathering flux to shift towards the 13C-enriched carbonate end-member, increasing the δ 13C value of carbon transported by rivers to both epeiric seas and the oceans. Magnitude differences between Hirnantian δ 13C excursions in shallower and deeper water parts of epeiric sea basins, as well as between different regions, may be explained by water mass differentiation between those regions. The positive shift in the δ 13C value of the Hirnantian oceans is predicted to be about 2–3‰, which is about half the value of the larger excursions found in basin proximal settings of low latitude epeiric seas.

A late Ordovician (Hirnantian) karstic surface in a submarine channel, recording glacio‐eustatic sea‐level changes: Meifod, central Wales

AbstractThe growth and decay of the end‐Ordovician Gondwanan glaciation is globally reflected by facies changes in sedimentary sequences, which record a major eustatic fall and subsequent rise in the Hirnantian Stage at the end of the Ordovician. However, there are different reported estimates of the magnitude and pattern of sea‐level change.Particularly good evidence for end‐Ordovician sea‐level change comes from a sequence at Meifod in central Wales, which has a karstified limestone unit within a channel incised into marine shelf sediments. Pre‐glacial (Rawtheyan) mudstones have a diverse fauna suggesting a mid‐to‐deep‐shelf water depth of c. 60 m. The channel, 20 m deep, was incised into these mudstones and partially filled with a mixture of fine sand and detrital carbonate. The taphonomy of bioclasts and intraclasts indicates that many had a long residence time on the sea floor or suffered diagenesis after shallow burial before being resedimented into the channel. The presence of carbonates on the Welsh shelf is atypical and they are interpreted as having accumulated as patches during a minor regression prior to the main glacio‐eustatic fall. Comparison of the carbon stable‐isotopic values of the bioclast material with the global isotopic record confirms that most of the material is of Rawtheyan age, but that some is Hirnantian. The resedimented carbonates lithified rapidly and formed a limestone, several metres thick, in the deepest parts of the channel. As sea‐level fell, this limestone was exposed and eroded into karstic domes and pillars with a relief of over 2 m. The overall, glacio‐eustatic, sea‐level fall is estimated to be in excess of 80 m.A succeeding sea‐level rise estimated to be 40–50 m is recorded in the laminated crust that mantles the karstic domes and pillars. The crust is formed of encrusting bryozoans, associated cystoids, crinoid holdfasts and clusters of the brachiopod Paromalomena, which is normally associated with mid‐shelf environments. Fine sands buried the karst topography and accumulated to fill the channel. In the sandstones at the base of the channel there is a Hirnantia fauna, while in the sandstones high in the channel‐sequence there is cross‐stratification characteristic of mid‐shoreface environments. This would indicate a fall of sea‐level of c. 30 m. The subsequent major transgression marking the end of the glaciation is not recorded at the Meifod locality, but nearby exposures of mudstones suggest a return to mid‐to‐deep‐shelf environments, similar to those that prevailed before the Hirnantian regression.The Meifod sequence provides strong evidence for the magnitude of the Hirnantian sea‐level changes and by implication confirm larger estimates for the size of the ice sheets. Smaller oscillations in relative sea‐level seen at Meifod may be local phenomena or may reflect eustatic changes that have not been widely reported elsewhere. Copyright © 2005 John Wiley & Sons, Ltd.

Patterns of ostracod migration for the ‘North Atlantic’ region during the Ordovician

A review of Ordovician neritic ostracods from the ‘North Atlantic’ region including Europe and North America identifies over 100 genera (including 44 palaeocopes and 31 binodicopes) which show a complex pattern of migration between two or more of the palaeocontinents Gondwana, Ibero-Armorica, Perunica, Avalonia, Baltica and Laurentia. Many dispersals were relatively slow, and the migration of a genus between palaeocontinents often took the duration of one or more graptolite biozones. Over 70 migrations appear to have occurred more rapidly, including those of Pseudulrichia, a genus which dispersed to five palaeocontinents within the duration of three graptolite biozones. Longevity clearly facilitated the chances of migration, as the most widespread genera such as Vannieria, Platybolbina, Medianella and Euprimites, are often the most long-ranging. Low migration rates prior to the Llanvirn are, at least in part, related to low ostracod taxonomic diversity. Greatly increased diversity from the late Llanvirn coincided with a much higher rate of migration. Coupled with the spread of carbonate–mudstone shelf marine facies in Laurentia during the early and mid Caradoc, this resulted in the migration of up to 18 Baltic-origin genera to Laurentia. Relative to overall ostracod diversity, migration rates were generally higher during periods of lower global sea level, suggesting that ostracod dispersal may have been aided by mid-ocean islands or outer-shelf carbonate platforms, which provided more extensive island-hopping routes during periods of low sea level. The palaeogeographical convergence of Avalonia, Perunica and Baltica, and subsequently of Avalonia and Baltica with Laurentia, in low latitudes and warm surface waters, is suggested by increasing ostracod migration between these palaeocontinents from the late Llanvirn onwards. This culminated, during the Ashgill, in numerous species–level links. Baltica may have been the source area for more than 40 migrant genera, reflecting its high-diversity faunas and its intermediate palaeogeographical position between Laurentia and Avalonia. Several ostracod genera used Baltica as a staging-post in migrations between Avalonia and Laurentia. Migrations continued during the late Ashgill Hirnantian Stage (24 migrations), especially between Laurentia, Baltica and Avalonia (up to 19 migrations of genera), suggesting close geographical proximity for these palaeocontinents. Some ostracods, particularly the binodicopes Pseudulrichia, Klimphores, Kinnekullea, Aechmina and Spinigerites, could occupy outer-shelf and cooler-water benthic palaeoenvironments. They were part of a widespread deep-shelf fauna from the mid Caradoc onwards, for which distances or climatic barriers were less of an obstacle for trans-oceanic migration. None of these ostracods were bathyal.

An organic carbon isotope record of Late Ordovician to Early Silurian marine sedimentary rocks, Yangtze Sea, South China: Implications for CO2 changes during the Hirnantian glaciation

Studies of δ 13 C of marine sedimentary organic carbon in six clastic sections with sediments deposited from the latest Ordovician to earliest Silurian in the Yangtze Sea on the South China Plate, show a positive δ 13 C org excursion of up to 4‰ in the latest Ordovician Hirnantian Stage, corresponding in time to a major continental glaciation on Gondwana. Various factors that might be responsible for the δ 13 C org increase are considered, including input and contribution from exogenic carbon sources, secondary processes (such as heterotrophy and thermal maturation), and changes in Yangtze Sea surface-water temperature, productivity, primary producers, dissolved CO 2 concentrations and the isotopic composition of dissolved inorganic carbon. Current data are consistent with the interpretation that the observed δ 13 C org increase may have resulted from a decrease in the surface-water dissolved CO 2 in the Yangtze Sea during the Hirnantian glaciation, probably reflecting a significant decrease in glacial ocean/atmosphere p CO 2 levels at that time.

High-Resolution carbon isotope stratigraphy of the basal Silurian Stratotype (Dob's Linn, Scotland) and its global correlation

Since its designation as the Global Stratotype Section and Point (GSSP) for the base of the Silurian System, the choice of Dob's Linn, Southern Scotland, has received criticism due to the difficulties of relating its well-constrained graptolite biostratigraphy to shallow-water sequences elsewhere. Kerogen samples from across the Ordovician-Silurian boundary interval at Dob's Linn have yielded carbon stable-isotope signatures consistent with those recorded elsewhere, in particular showing a clear positive δ 13 C excursion in the terminal Ordovician. The architecture of the δ 13 C curve from Dob's Linn enables very high-resolution stratigraphic subdivision and direct correlation between the deep water Dob's Linn section and time-equivalent carbonate shelf deposits. An integrated stratigraphic scheme using isotope stratigraphy and biostratigraphy of graptolites, conodonts and shelly faunas has been constructed. This direct correlation shows that the shallow water successions, including the former stratotype candidate at Anticosti Island, are generally incomplete, with hiatuses related to the rapid sea-level changes during the Hirnantian stage. This confirms and greatly increases the global utility of Dob's Linn as a boundary stratotype.