Stable Isotope Excursion Research Articles

Assessing environmental change associated with early Eocene hyperthermals in the Atlantic Coastal Plain, USA

Abstract. Eocene transient global warming events (hyperthermals) can provide insight into a future warmer world. While much research has focused on the Paleocene–Eocene Thermal Maximum (PETM), hyperthermals of a smaller magnitude can be used to characterize climatic responses over different magnitudes of forcing. This study identifies two events, namely the Eocene Thermal Maximum 2 (ETM2 and H2), in shallow marine sediments of the Eocene-aged Salisbury Embayment of Maryland, based on magnetostratigraphy, calcareous nannofossil, and dinocyst biostratigraphy, as well as the recognition of negative stable carbon isotope excursions (CIEs) in biogenic calcite. We assess local environmental change in the Salisbury Embayment, utilizing clay mineralogy, marine palynology, δ18O of biogenic calcite, and biomarker paleothermometry (TEX86). Paleotemperature proxies show broad agreement between surface water and bottom water temperature changes. However, the timing of the warming does not correspond to the CIE of the ETM2 as expected from other records, and the highest values are observed during H2, suggesting factors in addition to pCO2 forcing have influenced temperature changes in the region. The ETM2 interval exhibits a shift in clay mineralogy from smectite-dominated facies to illite-rich facies, suggesting hydroclimatic changes but with a rather dampened weathering response relative to that of the PETM in the same region. Organic walled dinoflagellate cyst assemblages show large fluctuations throughout the studied section, none of which seem systematically related to CIE warming. These observations are contrary to the typical tight correspondence between climate change and assemblages across the PETM, regionally and globally, and ETM2 in the Arctic Ocean. The data do indicate very warm and (seasonally) stratified conditions, likely salinity-driven, across H2. The absence of evidence for strong perturbations in local hydrology and nutrient supply during ETM2 and H2, compared to the PETM, is consistent with the less extreme forcing and the warmer pre-event baseline, as well as the non-linear response in hydroclimates to greenhouse forcing.

Ba stable isotope excursions induced by multiple hyperthermal events: A potential new index for transient global warming

Ba stable isotope excursions induced by multiple hyperthermal events: A potential new index for transient global warming

U–Pb Geochronology and Stable Isotope Geochemistry of Terrestrial Carbonates, Lower Cretaceous Cedar Mountain Formation, Utah: Implications for Synchronicity of Terrestrial and Marine Carbon Isotope Excursions

The terrestrial Lower Cretaceous Cedar Mountain Formation, Utah, is a critical archive of paleoclimate, tectonics, and vertebrate ecology and evolution. Early Cretaceous carbon cycle perturbations associated with ocean anoxia have been interpreted from this succession, as expressed in stable carbon isotopes. However, refining the timing of the observed stable isotope excursions remains a key challenge in understanding how marine anoxia affects the Earth system, and is ultimately recorded in the terrestrial realm. The geochronology and geochemistry of a terrestrial carbonate near the base of this succession, which potentially records the Ap7 global carbon isotope excursion, is studied here. Petrographic and geochemical analyses are used to test plausible mechanisms for U incorporation into the calcite lattice in this sample. Using these methods, the hypothesis that the incorporation of U was at or close to the timing of carbonate precipitation is evaluated. U–Pb geochronology of calcite indicates a plausible Early Cretaceous age. However, comparison of the new U–Pb ages of calcite with detrital zircon maximum depositional ages immediately beneath the studied sample indicates a disparity in the apparent sedimentation rates if both types of geochronologic information are interpreted as reflecting the timing of sediment deposition. The totality of data supports an early, and high-temperature, diagenetic timing of U incorporation, with potential for minor leaching of U in subsequent fluid–rock interaction. The most likely mechanism for U transport and immobilization in these samples is hydrothermal fluid–rock interaction. Therefore, the radiometric ages, and corresponding stable isotope composition of U-bearing carbonate domains in this sample, indicate early subsurface fluid–rock interactions and not a record of atmosphere–soil geochemical reactions.

Organic carbon accumulation at the northern Gondwana paleomargin (Tunisia) during the Toarcian Oceanic Anoxic Event: Sedimentological and geochemical evidence

During the early Toarcian (Early Jurassic; ~183 Ma) global environmental changes promoted the expansion of oxygen-deficient conditions across vast shelf areas around the globe. Ocean margin deoxygenation favored the preservation of organic matter in shelf sediments and is considered to reflect a Toarcian Oceanic Anoxic Event (T-OAE). Higher-level global drivers controlled temperature fluctuations, sea level evolution, oceanic circulation patterns, marine primary productivity, and stable carbon isotope excursions. Lower-level factors, controlled by individual basin bathymetry and paleogeography, operated on local to regional scale and here regulated organic matter accumulation. Accordingly, the T-OAE has been described as global event with a strong regional character. During the Toarcian, preferentially organic matter-lean sediments have been deposited at the northern Gondwana paleomargin. Enhanced connectivity to the Tethys Ocean and low marine primary productivity are considered to have been major factors preventing the formation of oxygen-deficient conditions and organic matter preservation. We here report on early Toarcian sediments from the Châabet El Attaris Section (Tunisia), located at the northern Gondwana paleomargin that episodically became enriched in organic matter. Bio- and chemostratigraphy both confirm that organic matter-rich sediments preferentially formed during the T-OAE but also predate the T-OAE. Sedimentological and micropaleontological characterization in concert with bulk, molecular and stable isotope geochemical investigations reveal that accumulation of marine organic matter occurred under anoxic, but not under euxinic, conditions. Limitation in the supply of reactive iron regulated organic matter sulfurization indicating that during most of the depositional history the levels of free H 2 S in bottom waters were rather low but limited to the sediment pore water. We conclude that the chemocline was either situated within the sediment or close to the sediment water interface. Spatiotemporally-restricted occurrence of organic matter enriched sediments and anoxic pore waters was promoted by restriction in water column ventilation in a local graben structure. • Organic matter-enriched strata in Tunisia predates but maximizes in the T-OAE. • Organic matter preservation occurred under anoxic but non-euxinic conditions. • Formation of oxygen deficient conditions was promoted by local paleobathymetry and basin restriction.

Stable isotope excursions in the Jurassic carbonate System, Imbricated Zone, Northern Iraq

Stable isotope excursions in the Jurassic carbonate System, Imbricated Zone, Northern Iraq

Middlesex/punctata Event in the Rhenish Basin (Padberg section, Sauerland, Germany) – Geochemical clues to the early-middle Frasnian perturbation of global carbon cycle

A positive carbon stable isotope excursion of about 3‰ is documented in the topmost lower Frasnian at Padberg, eastern Rhenish Massif, as a muted record of the worldwide early−middle Frasnian isotopic perturbation ( punctata Event; up to 6–8‰ shift in both δ 13 C carb and δ 13 C org elsewhere), comparable with the Appalachian δ 13 C curve. This German isotopic signature occurs in a 12 m thick calciturbidite succession and correlates well with the three-step chemostratigraphic pattern known from the Holy Cross Mountains, Poland. It is especially clear in the δ 13 C org shifts, whilst δ 13 C carb (and elemental geochemical) proxies are partly biased by post-sedimentary alterations. The New York State, Polish, Nevada and Padberg conodont successions place the onset of the major positive δ 13 C excursion slightly beneath the early–middle Frasnian boundary, with Ancyrodella nodosa (previously Ad. gigas form 1) as the main conodont guide species, and coincident with the Middlesex transgression and spread of cold, nutrient-rich, poorly oxygenated water masses. In the light of geochemical proxies, enhanced primary production and oxygen deficiency occurred evidently in the Rhenish Basin during the punctata Event. Moderate Hg enrichments in the early Middlesex/ punctata Event interval suggest a volcanic signature. However, conclusive data from other regions are required to differentiate between effects of the regionally well-known synsedimenary magmatism and of a possible global volcanic trigger for the biogeochemical perturbation. • The early−middle Frasnian (E-MF) isotopic perturbation is recognized in the topmost Pa. transitans Zone in Rhenish Massif • The onset of the major positive δ 13 C excursion is documented beneath the E-MF boundary, with Ad. nodosa as the conodont guide species • Enhanced primary production and oxygen deficiency occurred during the Middlesex / punctata Event in the Rhenish basin • Volcanic signature detected in the Padberg Formation

Holocene methane emissions in the South-Western part of the Okhotsk Sea (on the basic of carbon isotopes in the tests of benthic foraminifera)

We report several negative carbon stable isotope excursions in benthic foraminifera in a gas-bearing core LV50-05, collected from an area of active methane venting on the Eastern Slope of Sakhalin Island, the Okhotsk Sea. Within the study area, four methane events (ME) are established in Holocene: short-term ME-1 (700-900 yr BP) and ME-2 (from 1200 to 1400 yr BP), long-term ME-3 (2500-4700 yr BP) and ME-4 (7400-10000 yr BP).

Holocene Methane Emissions in the Southwestern Sea of Okhotsk: Evidence from Carbon Isotopes in Benthic Foraminifera Shells

Several negative stable carbon isotope excursions are revealed in benthic foraminifera from the LV50-05 gas-bearing core collected from an area of active methane venting on the eastern slope of Sakhalin Island, Sea of Okhotsk. Within the study area, four methane events (ME) were established to have taken place during the Holocene, short-term ones—ME-1 (700–900 yrs BP) and ME-2 (1200–1400 yrs BP)—and long-term ones—ME-3 (2500–5400 yrs BP) and ME-4 (7400–10 000 yrs BP).

Trigger recognition of Early Cretaceous soft‐sediment deformation structures in a deep‐water slope‐failure system

The triggers of soft‐sediment deformation structures (SSDS) that are present in the deep‐water slope‐failure system of the Early Cretaceous Lingshandao Formation on Lingshan Island have been analysed on the basis of SSDS in 5 outcrops. The sediments in these outcrops are interpreted to have been deposited on a near‐shore submarine fan. The inner and middle fans were observed in Yangjiaodong, and the outer fan outcropped in Qiancengya, Dengta, and Chuanchang. The inner and middle fans are characterized by coarse‐grained debrites, turbidites, and rare SSDS, whereas the outer fan by fine‐grained debrites, turbidites, deformed sediments, and abundant SSDS. Based on sedimentary composition, element analysis and palaeo‐geographical indication of SSDS, the source area was supposed to be uplifted Sulu Orogen in an active continental margin. Combination with sedimentary environment, tectonic setting, and the space distribution of different scaled SSDS, seismicity was proposed to be the trigger of larger scaled SSDS, and small‐scale SSDS may be triggered by gravity loading, slump‐derived shear stress, and traction effect. The seismicity may respond to active tectonism and volcanism. Besides, time consistence between the Lingshandao Formation (~131–121 Ma), cooling events, sea‐level change, stable isotope excursion, and palaeo‐seismicity events suggested a catastrophic event, which may be related to two Early‐Cretaceous Anoxic Events (Faraoni Event, ~131 Ma; OAE 1a, ~120 Ma).

Gas hydrate destabilization and methane release events in the Krishna–Godavari Basin, Bay of Bengal

Methane release events have been linked to global warming, alteration of the carbon cycle and influence on biota. However, unequivocal evidence of paleomethane release events are limited. We report several negative carbon stable isotope excursions in planktic and benthic foraminifera in a core (MD161-8) from the Krishna-Godavari (K–G) Basin, Bay of Bengal. The most negative δ13C spikes are recorded during the marine isotope stages MIS-4 and at the transition of MIS-5 to 4. Occurrence of highly 13C depleted (average δ13C = −48 ± 2.4‰ VPDB) authigenic high magnesian calcite are also reported within this time window from the core MD161-8. In the present work an unequivocal explanation for the observed 13C depletion in the marine planktic and benthic foraminifera is difficult to achieve solely from the optical/electron microscopy or C–O stable isotope ratio analyses due to possible influence of diagenetic alteration. We attribute the observed episodic methane expulsion events, as inferred from the negative δ13C excursions and earlier reports on the occurrence chemosynthetic bivalves and Mo concentration anomaly to the destabilization of the base of gas hydrate stability zone (BGHSZ). Sea level drop and shale tectonics induced focused fluid flow are the two possible causes of hydrate destabilization discussed here. Shale tectonics were possibly responsible for creating fault systems which acted as the conduit for gas flow through the sediment column and subsequent seepage. Shale and salt tectonics in the passive continental margins being a globally observed phenomenon, its role as an important driving force for enhanced methane emission needs detailed investigation to understand the climatic perturbations through geologic time. Additional evidence of methane emission from site MD161-15 further supports the link between shale tectonics and methane emission.

Stable isotope stratigraphy and larger benthic foraminiferal extinctions in the Melinau Limestone, Sarawak

Important long-ranging groups of larger benthic foraminifera (LBF) are known to have become extinct during a period of global cooling and climate disruption at the Eocene–Oligocene transition (EOT) but the precise timing and mechanisms are uncertain. Recent study showed unexpectedly that the LBF extinction in Tanzania occurs very close to the Eocene/Oligocene boundary, as recognised by the extinction of the planktonic foraminiferal Family Hantkeninidae, rather than at the later period of maximum global ice growth and sea-level fall, as previously thought. Here we investigate the same phase of extinction in the Melinau Limestone of Sarawak, on the island of Borneo, Malaysia one of the most complete carbonate successions spanning the Eocene to Lower Miocene. Assemblages of LBF from the Melinau Limestone were studied extensively by Geoffrey Adams during the 1960s–80s, confirming a major extinction during the EOT, but the section lacked independent means of correlation. By analysing rock samples originally studied by Adams and now in the Natural History Museum, London, we provide new bulk stable isotope (δ13C and δ18O) records. This enables us to identify, albeit tentatively, the level of maximum stable isotope excursion and show that the LBF extinction event in the Melinau Limestone occurs below this isotope excursion, supporting the results from Tanzania and indicating that the extinction of LBF close to the Eocene/Oligocene boundary may be a global phenomenon.

Cenomanian–Turonian carbon isotope stratigraphy of the Western Canadian Sedimentary Basin

The Cenomanian–Turonian boundary was characterized by distinctive positive carbon isotope excursions that were related to the formation of widespread oceanic anoxia. High-resolution geochemical proxies (TOC, CaCO3, δ13Corg, and δ13Ccarb) obtained from bulk rock, planktic foraminifers, and inoceramids from four marine marlstone-dominated stratigraphic sections in the Western Canada Sedimentary Basin (WCSB) were used to establish a regional carbon isotope stratigraphic framework and to investigate paleoenvironmental variability in four different depositional settings. Compared to background δ13Corg, (<−27‰) and δ13Ccarb (<2‰) values which were correlative to stable isotope excursions during Oceanic Anoxic Event (OAE) II worldwide, the δ13Corg (>24‰), and δ13Ccarb (>4‰) derived from inoceramid prisms in the studied sections within WCSB, were elevated during the Late Cenomanian–Early Turonian. During this interval, TOC and CaCO3 values which increased sporadically to >40% and 7%, respectively, were not consistent enough to be used for stratigraphic correlations. Based on the δ13Corg excursions, two bentonite beds were regionally correlated across this portion of the Western Interior Seaway (WIS). The eruption associated with the “Red” bentonite occurred approximately coeval with the maximum δ13Corg-excursion during OAE II in the Neocardioceras juddii Zone, whereas the “Blue” bentonite coincides with the termination of OAE II in the latest Watinoceras devonense zone. During the Late Cenomanian–Early Turonian in the WCSB, benthic foraminifers were sparse or totally absent, indicating the existence of fully anoxic bottom-water conditions. Planktic foraminifera were common in the well-oxygenated surface waters. A benthic oxic zone characterized by several agglutinated species occurs in the eastern part of the WSCB at the beginning of OAE II in the Sciponoceras gracile zone. The termination of the OAE II in the WCSB coincides with the first occurrence of small ammonites (Subprionocyclus sp.) in the western part of the basin.

Assessing offsets between the δ13C of sedimentary components and the global exogenic carbon pool across early Paleogene carbon cycle perturbations

Negative stable carbon isotope excursions (CIEs) across the Paleocene–Eocene thermal maximum (PETM; ∼56 Ma) range between 2‰ and 7‰, even after discounting sections with truncated records. Individual carbon isotope records differ in shape and magnitude from variations in the global exogenic carbon cycle through changes in (1) the relative abundance of mixed components with different δ13C within a measured substrate, (2) isotope fractionation through physiological change, and (3) the isotope composition of the carbon source. All three factors likely influence many early Paleogene δ13C records, especially across the PETM and other hyperthermal events. We apply these concepts to late Paleocene–early Eocene (∼58–52 Ma) records from Lomonosov Ridge, Arctic Ocean. Linear regression analyses show correlations between the δ13C of total organic carbon (TOC) and two proxies for the relative contribution of terrestrial organic components to sediment TOC: the branched and isoprenoid tetraether index and palynomorphs. We use these correlations to subtract the terrestrial component from δ13CTOC and calculate marine organic matter δ13C. The results show that the magnitude of the CIE in δ13CTOC across the PETM is exaggerated relative to the magnitude of the CIE in δ13CMOM by ∼3‰ due to increased contributions of terrestrial organic carbon during the event. Collectively, all carbon isotope records across the PETM and other major climate–carbon cycle perturbations in Earth's history are potentially biased through one or more of the above factors. Indeed, it is highly unlikely that any δ13C record shows the true shape and magnitude of the CIE for the global exogenic carbon cycle. For the PETM, we conclude that CIE in the exogenic carbon cycle is likely &lt;4‰, but it will take additional analyses and modeling to obtain an accurate value for this CIE.

The first record of the “Mid-Barremian” Oceanic Anoxic Event and the Late Hauterivian platform drowning of the Bilecik platform, Sakarya Zone, western Turkey

The late Hauterivian platform drowning has been recorded in the Hauterivian/Barremian boundary beds in western Turkey. This drowning event recorded by presence of hardgrounds, glauconite- and phosphate-bearing facies over the platform carbonates is followed up by pelagic carbonates.“Ammonitico Rosso” like reddish colored, thick-bedded limestones include multiple hardgrounds, abundant ammonites, belemnites and bivalves just below the Hauterivian/Barremian boundary. The hardground surfaces contain macro and micro-scale manganese and iron encrustations, micro- and macro-boring structures, calcite mineralization and glauconite minerals. Geochemical analysis indicates that Mn encrustation in the hardground is related to hydrothermal and/or hydrogenetic-detrital and associated with continental margin occurrence.A red pelagic limestone succession with planktonic foraminifera overlies the last hardground surface at the Hauterivian–Barremian boundary and is followed up by the alternation of gray colored bioturbated sandy/silty limestones with radiolaria and black shales or sandy/silty mudstones. A black shale level including ammonite, manganese, iron, pyrite and glauconite minerals takes place around the early Barremian–late Barremian boundary. At the top of the sections, there is a succession composed of alternation of thicker sandy limestone and shale/marl covering the black shale interval.The record of “mid-Barremian” black shale interval over the platform carbonates and the overlying pelagic succession with radiolaria indicate that the “mid-Barremian” Oceanic Anoxic Event took place after the drowning of the platform. Stable δ13C and δ18O isotope excursions have been detected within black shale zone and display parallel pattern with European sections. This study presents the first record of the mid-Barremian anoxic oceanic event on the Sakarya Zone of Pontides in Turkey.

Relation of Phanerozoic stable isotope excursions to climate, bacterial metabolism, and major extinctions

Conspicuous global stable carbon isotope excursions that are recorded in marine sedimentary rocks of Phanerozoic age and were associated with major extinctions have generally paralleled global stable oxygen isotope excursions. All of these phenomena are therefore likely to share a common origin through global climate change. Exceptional patterns for carbon isotope excursions resulted from massive carbon burial during warm intervals of widespread marine anoxic conditions. The many carbon isotope excursions that parallel those for oxygen isotopes can to a large degree be accounted for by the Q10 pattern of respiration for bacteria: As temperature changed along continental margins, where ∼90% of marine carbon burial occurs today, rates of remineralization of isotopically light carbon must have changed exponentially. This would have reduced organic carbon burial during global warming and increased it during global cooling. Also contributing to the δ(13)C excursions have been release and uptake of methane by clathrates, the positive correlation between temperature and degree of fractionation of carbon isotopes by phytoplankton at temperatures below ∼15°, and increased phytoplankton productivity during "icehouse" conditions. The Q10 pattern for bacteria and climate-related changes in clathrate volume represent positive feedbacks for climate change.

Early Palaeozoic cooling events: peri-Gondwana and beyond

Abstract The short-lived end-Ordovician Hirnantian glaciation allied to marine mass extinction is variously considered as a short-lived event or as the peak of long-drawn-out climatic cooling through at least late Ordovician–early Silurian times. Evidence from Early Palaeozoic facies, faunas and stable isotope excursions used to interpret climatic cooling events ranges farther, from late Mid-Cambrian to late Silurian times. Glacigenic sediments, structures and geomorphology provide direct evidence of glacial episodes. Cool-water carbonate deposition, which is particularly widespread during the late Ordovician Boda event in high-latitude peri-Gondwana–Gondwana, and beyond into mid–low palaeolatitudes, is interpreted as indicating global cooling, not warming as has been proposed. Such carbonates also characterize mid-latitude continents widely at horizons earlier in the Ordovician, and more locally in the mid-Silurian in high-latitude Gondwana. Cool-water carbonate mounds have distinctive facies-controlled mound faunas across palaeocontinents. Other facies evidence for palaeoclimates includes black shale deposition, including deglacial organic-rich ‘hot shales’, which indicate transgression in epeiric seas, and sea-level curves interpreted from facies and faunal successions. Correlation is shown between facies evidence and positive C isotope excursions, from which cyclicities are apparent. The possible interface of orbitally controlled rhythms is considered against evolving palaeobiogeography, and changes in global sea level and in p CO 2 . Facies and faunal evidence from peri-Gondwanan terranes (Armorica, Central Europe, Alborz) is assessed with that from Gondwana (mostly North Africa, South America) and correlatives in Avalonia, Baltica and Laurentia to establish a wider picture of early Palaeozoic cooling events.

Record of environmental changes in the Triassic–Jurassic boundary interval in the Zliechov Basin, Western Carpathians

The Fatra and Kopieniec formations in the Zliechov Basin comprise a record of several environmental crises that may have contributed to the global Triassic–Jurassic (T–J) boundary events. The diversity of the benthic fauna decreases at the base of the ‘transition beds’, the uppermost member of the Fatra Formation. This assemblage comprises many species that do not appear in younger strata. In ‘cycle 13’ of the Fatra Formation, stable isotope excursions correspond to significant lithological changes in the sequence. The negative δ 13C excursion is correlated with a positive δ 18O peak and the beds containing the excursion are overlain by a thin layer with unusual lithological and mineral composition. This layer is composed of small calcitized microspheres showing complex alteration during diagenesis. The origin of this layer, traceable over tens of kilometres, is problematic (impact ejecta, volcanic glass, or altered aragonitic particles). An increase in terrigenous sediment recorded by sandy beds in the uppermost part of the Fatra Formation indicates increased run-off. A sharp lithological boundary between the Fatra and the Kopieniec formations marks a sudden termination of carbonate sedimentation, followed by deposition of the ‘boundary clay’. The negative δ 13C anomaly recognized in this succession may correspond to the isotopic signature at the T–J boundary which has been described from many sections in the world.

Assessing mechanisms of environmental change: Palynological signals across the Late Ludlow (Silurian) positive isotope excursion ( δ13C, δ18O) on Gotland, Sweden

The Silurian is characterised by strong environmental changes, as indicated by several pronounced positive δ 13C and δ 18O excursions. The mechanisms responsible for these isotopic shifts are a matter of much debate. The purpose of the present study is a quantitative high-resolution evaluation of the palynomorph distribution across the Late Ludlow (Ludfordian) isotope excursion on Gotland. Marine and terrestrial palynomorphs have been chosen as palaeoenvironmental indicators because they are widely distributed and exceptionally well preserved. Moreover, the combined analysis of marine and terrestrial palynomorphs allows the reconstruction of contemporaneous palaeoenvironmental change in the marine and terrestrial realms. To correlate our results with the carbon isotope stratigraphy, we have sampled only localities with published isotope data from diagenetically unaltered brachiopod shells. Our results show that fluctuations in the composition of the acritarch assemblages are closely correlated with the stable isotope development. Low abundances of acritarchs occur in times of high stable isotope values and vice versa, indicating that an increase in marine productivity cannot have been the reason for the positive δ 13C excursion. The results are in good agreement with climatic models for the Silurian assuming alternating humid and arid climatic conditions in low latitudes. Times of high stable isotope values correspond to arid climatic conditions in low latitudes, with low input of terrestrial nutrients resulting in impoverished acritarch and conodont communities, whereas times of low stable isotope values correlate with humid climate, high nutrient input, and abundant and diverse acritarch communities. The major change in acritarch communities took place during the increase of the stable isotope values after hemipelagic planktonic or nektonic organisms (graptolites, conodonts) have been affected. This indicates that environmental changes connected with the stable isotope excursion first affected deeper-water settings and later the photic zone. The absolute abundance of terrestrial spores closely mirrors the marine δ 13C and δ 18O development, showing that both the marine and the terrestrial realm were synchronously affected by the climatic changes. The high abundance of spores during the isotope excursion might be explained by increased aeolian input.

Silurian carbonate platforms and extinction events—ecosystem changes exemplified from Gotland, Sweden

Recent and ancient carbonate platforms are major marine ecosystems, built by various carbonate-secreting organisms with different sensitivity for environmental change. For this reason, carbonate platforms are excellent sensors for changes in contemporaneous marine environments. A variety of ecosystem changes in carbonate platforms have previously been recognised in the aftermath of mass extinction events. This paper addresses how two Silurian extinction events among graptolites, conodonts, and pentamerid brachiopods can be related to changes in the style of carbonate production and general evolution of low latitude carbonate platforms in a similar way as previously reported from the major five mass extinctions of the Phanerozoic. Strata formed on Gotland during the Mulde and Lau events share remarkably many similarities but are strikingly different in composition compared to other strata on the island. The event-related strata is characterised by the sudden appearance of widespread oolites, deviating reef composition, flat-pebble conglomerates, abundant micro- and macro-oncoids, stromatolites, and other microbial facies suggesting decreased bioturbation levels in contemporaneous shelf seas. Importantly, these changes can be tied to high-resolution biostratigraphic frameworks and global stable isotope excursions. The anomalous intervals may therefore be searched for elsewhere in order to test their regional or global significance.

The effects of the final stages of the Late Ordovician glaciation on marine palynomorphs (chitinozoans, acritarchs, leiospheres) in well Nl-2 (NE Algerian Sahara)

Palynomorph assemblages, especially chitinozoans and acritarchs, from the Upper Ordovician of well Nl-2 (north-east of the Algerian Sahara) are studied in order to precisely date the ultimate effects of the Late Ordovician glaciation and to document the impact of this major climatic stress on the diversity of the palynoplankton. References are made to stable isotope excursions and to global eustatic sea level variations in order to improve the local age determination. The Hassi el Hadjar Formation, i.e. glacio-marine diamictites, is interpreted as a transgressive event resulting from the melting of the northern Gondwana ice cap. It yields poorly preserved and moderately diverse chitinozoans of late Hirnantian age. Acritarchs are more abundant in the lower part of these “microconglomeratic clays”, but display a low diversity and are badly preserved throughout the whole formation. Reworked individuals are recorded in both groups. The marine sediments of the M'Kratta Formation of latest Hirnantian age contain better preserved, more abundant and more diverse palynomorph assemblages, especially in the Upper Member. The composition of this palynoplankton indicates a fairly good faunal and phytoplankton recovery after the early Hirnantian climatic stress. The extinction of the Ordovician forms, and the appearance of Silurian type taxa occur only in the uppermost Hirnantian, i.e. following with a slight delay the glacial event. The overlying black shales of Wenlock age (lower part of the Oued Mehaiguène Formation) are indicative of marine anoxic environments. They yield a virtually exclusive, but enormous number of Tasmanacea. The very peculiar composition of this palynoplankton seems to be independent of the Late Ordovician glaciation and is most likely related to the factors that, later, generated and maintained anoxic conditions in this area. A new species of chitinozoan, Spinachitina oulebsiri sp. nov. from the latest Hirnantian M'Kratta Formation, is described and illustrated.