Sequence stratigraphy, chemostratigraphy and facies analysis of Cambrian Series 2 – Series 3 boundary strata in northwestern Scotland

Globally, the Series 2 – Series 3 boundary of the Cambrian System coincides with a major carbon isotope excursion, sea-level changes and trilobite extinctions. Here we examine the sedimentology, sequence stratigraphy and carbon isotope record of this interval in the Cambrian strata (Durness Group) of NW Scotland. Carbonate carbon isotope data from the lower part of the Durness Group (Ghrudaidh Formation) show that the shallow-marine, Laurentian margin carbonates record two linked sea-level and carbon isotopic events. Whilst the carbon isotope excursions are not as pronounced as those expressed elsewhere, correlation with global records (Sauk I – Sauk II boundary and Olenellus biostratigraphic constraint) identifies them as representing the local expression of the ROECE and DICE. The upper part of the ROECE is recorded in the basal Ghrudaidh Formation whilst the DICE is seen around 30m above the base of this unit. Both carbon isotope excursions co-occur with surfaces interpreted to record regressive–transgressive events that produced amalgamated sequence boundaries and ravinement/flooding surfaces overlain by conglomerates of reworked intraclasts. The ROECE has been linked with redlichiid and olenellid trilobite extinctions, but in NW Scotland, Olenellus is found after the negative peak of the carbon isotope excursion but before sequence boundary formation

Ordovician climate changes in the northern subtropics: The δ18O record from the Tunguska Basin, Siberia

Oxygen isotopes from bioapatite (conodonts) have been used for several decades to reconstruct the Palaeozoic climate history. During the Ordovician, conodont-based δ18Ophos studies have revealed a general cooling trend throughout the system. The δ18Ophos data from Estonia confirm this long-term shift but also demonstrate that against the background of a generally cooling climate in the pre-Hirnantian, the Late Ordovician was quite unstable, with several episodes of sea surface temperature (SST) decrease and increase of different magnitude and duration. In the sedimentary sequence, these cooling events are reflected by major sea-level lowstands. Several of these are also recognizable in the Tunguska Basin of Siberia. We have recently studied the δ18Ophos record from two Middle and Upper Ordovician sections in Siberia. Comparisons of the results with data from the Baltic region have revealed differences but also some similarities in the δ18Ophos trends, even though these two regions were located on different palaeocontinents, Siberia and Baltica. Both were geographically separated and display different Early Palaeozoic histories with respect to their environmental conditions. Siberia was located in low equatorial latitudes from the Cambrian onwards and remained there through the Ordovician and Silurian, whereas Baltica drifted from high southern latitudes to low latitudes from the late Cambrian and reached the southern subtropics in the Late Ordovician (late Sandbianâearly Katian). Despite Siberiaâs location at low northern subequatorial latitudes, the upper Middle (starting from the Darriwilian) and the Upper Ordovician in the Tunguska Basin are assumed to be represented by cool-water deposits (various calcareous siltstones with interbeds of micritic and/or bioclastic limestone). The onset of cool-water conditions is explained by plate-tectonic reorganization, resulting in the upwelling of cold oceanic waters along the southern margin of the palaeocontinent and their penetration into the epicontinental seas. Our δ18Ophos data generally fluctuate around 17.5â° (VSMOW), indicating that SST was relatively stable. This general state is interrupted by six cooling episodes, but unlike in Baltica, no general trend of SST change in any direction (decrease or increase) is evident. In the Baltic region, a general cooling trend prevailed, and SST decreased continuously during the pre-Hirnantian Late Ordovician. In addition to this general trend, seven cooling events (CE) are observed, named (from the oldest upwards) the Late Kukruse, Haljala, Keila, Early Nabala, Vormsi, Early Pirgu, and Middle Pirgu CEs. Comparison of the δ18Ophos curves from the Tunguska and Baltoscandian basins shows that five of these CEs are reflected by brief intervals of higher δ18Ophos values also in the former one. The Keila CE is identified in the lower Mangazea Formation (Fm), the Early Nabala CE in the upper Mangazea Fm, and the Vormsi CE in the uppermost Mangazea Fm. In addition, the Haljala CE probably corresponds to an interval in the lowermost Mangazea Fm and the Early Pirgu CE to a small δ18Ophos peak in the lowermost Dolbor Fm. The most pronounced CE in Siberia is recorded in the upper Darriwilian, in the lower UstâStolbovaya Fm, and apparently reflects the Middle-Darriwilian Ice Age. The recognition of the same CEs in successions on two different palaeocontinents is clear evidence that they are not some regional phenomena but the result of global climatic perturbations. The recently acquired δ18Ophos data allow an improvement in the dating of the Siberian strata and their correlation with successions on other palaeocontinents. Previously, the MDICE, Upper Kukruse Low, GICE, and an interval probably including Rakvere (KOPE) and Saunja carbon isotopic events (CIE) were recognized in the Siberian δ13Ccarb record. Now, the position of the Keila CE just below an increase in δ13Ccarb in the Tunguska Basin confirms the identification of this CIE as GICE and allows the correlation of this level (lower Baksian Regional Stage) with the Keila Regional Stage in Estonia. In addition, the prolonged CIE between the Early Nabala and Vormsi CEs as identified in the Tunguska Basin apparently corresponds to the Saunja (Waynesville) CIE. This also means that the small peak in the δ13Ccarb curve below this CIE correlates with the Rakvere CIE

Mineralogical, geochemical and isotopic features of tuffs from the CFDDP drill hole: Hydrothermal activity in the eastern side of the Campi Flegrei volcano (southern Italy)

Abstract A 506 m drill-hole has been recently drilled in the framework of the Campi Flegrei Deep Drilling Project (CFDDP) and the International Continental Scientific Drilling Program (ICDP) with the intention of coring the subsurface in the eastern sector of the Campi Flegrei caldera. The borehole, located in the western district of the Neapolitan city (Bagnoli Plain) 3 km to the east of the most active volcanic area and about 5 m above sea level, is now targeted for monitoring purposes. This paper reports the results obtained from the analysis of two short cores collected at depths of − 443 and − 506 m below the ground level. The cores sampled two pre-caldera tuffs. Observations performed by optical and scanning electron microscopy, energy dispersive spectroscopy and powder X-ray diffraction were used to achieve data on the primary lithology, both primary and secondary mineralogical assemblages, and the relationship between texture and secondary mineralization. Sr isotope ratios were determined on selected primary feldspars, whereas δ13C and δ18O analyses were performed on carbonates from veins and filled-voids in tuffs. Our results provide information on the hydrothermal system in the eastern sector of the caldera that was not among the goals in the previous drilling programs. Secondary mineralization suggests a saline hydrothermal environment characterized by fluids that progressively evolved from boiling toward more alkaline and cooler conditions. A paleo-temperature of ca. 160 °C has been inferred from authigenic mineral occurrences and calculated on the basis of equilibria between cored calcites and fluids presently emitted at the surface, by using carbon and oxygen isotope data. The temperature measured at the bottom of the drilling is about 80 °C

An abrupt extinction in the Middle Permian (Capitanian) of the Boreal Realm (Spitsbergen) and its link to anoxia and acidification

The controversial Capitanian (Middle Permian, 262 Ma) extinction event is only known from equatorial latitudes, and consequently its global extent is poorly resolved. We demonstrate that there were two, severe extinctions amongst brachiopods in northern Boreal latitudes (Spitsbergen) in the Middle to Late Permian, separated by a recovery phase. New age dating of the Spitsbergen strata (belonging to the Kapp Starostin Formation), using strontium isotopes and d13C trends and comparison with better-dated sections in Greenland, suggests that the first crisis occurred in the Capitanian. This age assignment indicates that this Middle Permian extinction is manifested at higher latitudes. Redox proxies (pyrite framboids and trace metals) show that the Boreal crisis coincided with an intensification of oxygen depletion, implicating anoxia in the extinction scenario. The widespread and near-total loss of carbonates across the Boreal Realm also suggests a role for acidification in the crisis. The recovery interval saw the appearance of new brachiopod and bivalve taxa alongside survivors, and an increased mollusk dominance, resulting in an assemblage reminiscent of younger Mesozoic assemblages. The subsequent end-Permian mass extinction terminated this Late Permian radiation

Ultra-shallow-marine anoxia in an Early Triassic shallow-marine clastic ramp (Spitsbergen) and the suppression of benthic radiation

Lower Triassic marine strata in Spitsbergen accumulated on a mid-to-high latitude ramp in which high-energy foreshore and shoreface facies passed offshore into sheet sandstones of probable hyperpycnite origin. More distal facies include siltstones, shales and dolomitic limestones. Carbon isotope chemostratigraphy comparison allows improved age dating of the Boreal sections and shows a significant hiatus in the upper Spathian. Two major deepening events, in earliest Griesbachian and late Smithian time, are separated by shallowing-upwards trends that culminated in the Dienerian and Spathian substages. The redox record, revealed by changes in bioturbation, palaeoecology, pyrite framboid content and trace metal concentrations, shows anoxic phases alternating with intervals of better ventilation. Only Dienerian–early Smithian time witnessed persistent oxygenation that was sufficient to support a diverse benthic community. The most intensely anoxic, usually euxinic, conditions are best developed in offshore settings, but at times euxinia also developed in upper offshore settings where it is even recorded in hyperpycnite and storm-origin sandstone beds: an extraordinary facet of Spitsbergen's record. The euxinic phases do not track relative water depth changes. For example, the continuous shallowing upwards from the Griesbachian to lower Dienerian was witness to several euxinic phases separated by intervals of more oxic, bioturbated sediments. It is likely that the euxinia was controlled by climatic oscillations rather than intra-basinal factors. It remains to be seen if all the anoxic phases found in Spitsbergen are seen elsewhere, although the wide spread of anoxic facies in the Smithian/Spathian boundary interval is clearly a global event

From Alpine-type sulfides to nonsulfides in the Gorno Zn project (Bergamo, Italy)

Recent exploration of the Gorno Zn-Pb-Ag deposit in northern Italy identified 3.3 Mt of sulfides at 4.9% Zn, 1.3% Pb, and 27.2 g/t Ag (indicated+inferred resources), and a further mineralized nucleus of mixed sulfides-nonsulfides in the Val Vedra area, currently under evaluation. The ores are hosted in Triassic limestone and shale. Sulfides (sphalerite, Ag-bearing galena, minor pyrite, and chalcopyrite) paragenetically follow Mn-Fe-bearing saddle dolomite and sparry calcite. The mineral association, and the carbon and oxygen isotope ratios of the sparry calcite (avg. δ13C = 1.0 ± 0.6‰ V-PDB; avg. δ18O = 19.63 ± 1.25‰ V-SMOW), are in agreement with precipitation from hydrothermal fluids in a deep burial setting. Sulfide emplacement occurred before the Alpine orogeny, likely during the Early-Middle Jurassic, in analogy to other Alpine-type deposits. The nonsulfide ore formed at the expense of sulfides, and mainly consists of smithsonite, hydrozincite, hemimorphite, and cerussite. The C-O-isotope values of the early generations of Zn-carbonates are characterized by δ18O between 24.1 and 26.8‰ V-SMOW and δ13C ratios between − 3.1 and 1.7‰ V-PDB. The later generations have lower δ18O (21.9 to 23.9‰) and lower δ13C (− 6.2 to − 3.9‰). These compositions, as those measured on cerussite (δ13C = −6.3 and − 7.7‰; δ18O = 14.0 and 15.3‰), agree with the formation of the nonsulfides in a supergene environment, under climatic conditions warmer than today. The δ18O decrease from early to late generations suggests progressive involvement of meteoric water sourced from higher altitudes. These characteristics indicate that the nonsulfides formed during the exhumation of the Gorno area from Miocene to Pliocene

Changes in shelf phosphorus burial during the Hirnantian glaciation and its implications

The Late Ordovician mass extinction occurred during an icehouse interval, accompanied by the glaciation of the supercontinent Gondwana, which was located at the South Pole at that time. As suggested by sequence stratigraphy of near- and far-field sedimentary successions as well as stable oxygen isotope studies, ice sheets reached their maximum extent in the late Hirnantian M. persculptus graptolite zone. As a result, the global sea-level dropped significantly during the Hirnantian Glacial Maximum (HGM). This led to exposure and erosion of sediments on the tropical shelves of Laurentia and Baltica. Where shelves remained submerged, water depths were probably very shallow. Local redox proxies, such as I/Ca ratios or iron speciation, indicate that shelf environments were well oxygenated. At the same time, stable uranium isotopes, measured on shallow-water carbonate samples, indicate a global expansion of the seafloor overlain by anoxic water. This implies that the observed increase in anoxia was confined to the open ocean and that there was a redox gradient between coastal and oceanic environments. Unfortunately, the lack of Late Ordovician deep water sedimentary records makes it impossible to directly measure open-ocean redox conditions. In general, Late Ordovician deoxygenation is in stark contrast to other oceanic anoxic events of the Phanerozoic, which occurred during greenhouse conditions and are associated with rising water temperatures. Under present interglacial conditions, with a relatively high sea-level, it is estimated that about 70% of the nutrient phosphorus delivered from the continents is retained in shelf sediments. Hence, shelf environments act as a nutrient filter. However, during times of low sea-level, this filter is switched off due to the bypassing of incoming riverine dissolved load through river canyons. As a result, excess phosphorus is released into the open ocean leading to eutrophication. This has previously been proposed for the Last Glacial Maximum (LGM) and is supported by geochemical data. We suggest that this scenario may also be applicable to the HGM and serve as an explanation for increasing anoxia during cold climatic conditions. To test this, we measured phosphorus concentrations across the HGM on carbonate samples collected from two low-latitude successions (Ruisseau aux Algues on Anticosti Island and Valga-10 core section from Estonia, both interpreted as shelf environments). To eliminate the detrital, non-reactive phosphorus fraction, we used the SEDEX sequential extraction method, which allows to separately measure reactive (Preact) and organic phosphorus (Porg). In order to evaluate the burial efficiency of phosphorus, we determined total organic carbon (C) concentration and calculated C/Porg and C/Preact ratios. We observe a decreasing trend in Preact towards the HGM and a minimum during the subsequent initial transgression. Low C/Preact and C/Porg in the range of the Redfield ratio indicate efficient burial under oxygenated conditions. Therefore, we can rule out that phosphorus regeneration, which is stronger under anoxic conditions, caused the observed minimum. This means that Preact is a direct measure of primary productivity and phosphorus burial. Using the Preact data and an estimate for the shelf area, we modelled the global burial flux of phosphorus into shelf sediments. Due to the overall reduction in shelf area during the HGM and the low Preact contents, the modelling suggests that the shelf burial flux was approximately halved. If the phosphorus input had remained constant across the interval, which is reasonable given that the erosion of exposed shelf sediments compensated for a reduction in continental weathering during the cold and arid climate, the excess phosphorus would have entered the open ocean stimulating primary productivity. Ultimately, enhanced productivity would have resulted in a high organic matter sinking flux and oxygen depletion by aerobic respiration. In summary, our data and modelling support the proposed scenario of an inefficient nutrient filter

Sequence Stratigraphy, Chemostratigraphy and Facies Analysis of Cambrian Series 2 - Series 3 Boundary Strata in Northwestern Scotland

Globally, the Series 2 - Series 3 boundary of the Cambrian System coincides with a major carbon isotope excursion, sea-level changes and trilobite extinctions. Here we examine the sedimentology, sequence stratigraphy and carbon isotope record of this interval in the Cambrian strata (Durness Group) of NW Scotland. Carbonate carbon isotope data from the lower part of the Durness Group (Ghrudaidh Formation) show that the shallow-marine, Laurentian margin carbonates record two linked sea-level and carbon isotopic events. Whilst the carbon isotope excursions are not as pronounced as those expressed elsewhere, correlation with global records (Sauk I - Sauk II boundary and Olenellus biostratigraphic constraint) identifies them as representing the local expression of the ROECE and DICE. The upper part of the ROECE is recorded in the basal Ghrudaidh Formation whilst the DICE is seen around 30m above the base of this unit. Both carbon isotope excursions co-occur with surfaces interpreted to record regressive-transgressive events that produced amalgamated sequence boundaries and ravinement/flooding surfaces overlain by conglomerates of reworked intraclasts. The ROECE has been linked with redlichiid and olenellid trilobite extinctions, but in NW Scotland, Olenellus is found after the negative peak of the carbon isotope excursion but before sequence boundary formation

Oxygen isotope compositions of conodonts – analytical challenges of in situ SIMS studies

Reliable deep-time environmental and climate reconstructions are needed to understand the drivers of Earthâs system evolution over geological time. Palaeozoic temperature estimates, including reconstructions of the climate change through the Ordovician, are based mainly on oxygen isotope (18O/16O; δ18OVSMOW) thermometry derived from carbonate rocks with fossils such as calcitic brachiopods and phosphatic conodonts that are often the best preserved repositories of environmental conditions. Palaeoenvironmental reconstructions are reliable only if the geochemical data is obtained using well-calibrated analytical tools. Most previous research devoted to oxygen isotope composition of conodonts has been conducted using the bulk method (gas source isotope ratio mass spectrometry (GS-IRMS)) that typically requires pooling several dozens of conodont elements for a single isotope ratio measurement. As such, studies of conodont-poor intervals and assessments of taxon-specific δ18O variability require extensive sample preparation and are challenging using the bulk method. Such challenges can be addressed by in situ secondary ion mass spectrometry (SIMS) analyses using only picogram sampling masses. However, several studies have reported inconsistencies between SIMS and GS-IRMS δ18O data for the same research material. We aim to solve this controversy by establishing a robust analytical protocol for conodont isotope analysis by SIMS. Here we present conodont data on Pterospathodus and Amorphognathus specimens extracted from Ordovician strata in Nurme and Mehikoorma-421 boreholes (Estonia). Oxygen isotope composition of conodonts was analysed by both SIMS and GS-IRMS, where we paid particular attention to four inorganic apatite reference materials in order to understand the offset between these two techniques that have been reported in the literature. While the results of GS-IRMS measurements conducted using high-temperature reduction of Ag3PO4 represent exclusively δ18O of phosphate-bound oxygen, SIMS analyses do not discriminate between different oxygen components (e.g., (PO4)3â, (SiO4)4â, (CO3)2â, and (OH)â) in apatite, inherently providing information on pooled isotope compositions. We conducted quantitative chemical analyses of selected conodont elements by electron probe microanalysis to assess to what extent matrix effects cause the offsets between the two isotope techniques. We also used scanning electron microscopy and white light optical profilometry to evaluate sample topography and porosity, which have a major impact on SIMS data quality. We collected oxygen isotope data using a CAMECA 1280-HR large geometry instrument at the Potsdam SIMS user facility over several months to determine reproducibility of the results and to optimise a routine measurement protocol. Our tests included a variety of instrumental settings, e.g., different raster parameters for both pre-sputtering and data collection, which yielded slightly differing results due to different instrumental mass fractionation. SIMS is a comparative method, and as such relies on reference materials that have been previously characterised by bulk methods, ideally provided by multiple laboratories. We noted that the inconsistent offsets between SIMS and GS-IRMS data obtained for a given conodont specimen (with SIMS δ18O values in most cases being higher) are linked to reference material measurements that are necessary for conodont data calibration and are often biased towards lighter δ18O values. Our tests show that such bias is even more significant when calibration is based on a single reference material characterised by a single GS-IRMS laboratory, which has been a common practice in past conodont studies