Were There Anoxic Oceans During the Cryogenian NeoProterozoic?

Integrated geochemical-petrographic insights from component-selective δ238U of Cryogenian marine carbonates

Authors:

Hood et al

Abstract:

Emerging geochemical proxies have improved our understanding of the broad-scale history of Earth's oxygenation. However, paleoredox work does not always include extensive consideration of sample preservation and paleoenvironmental setting. This is particularly an issue with marine carbonates, which although being potentially ideal ocean redox archives, are commonly altered during diagenesis. Here we provide new insight into the robustness of uranium isotopes (238U/235U ratios: δ238U values) as paleoredox tracers by determining texture-specific δ238U values from a well-described Cryogenian (Balcanoona) reef complex in South Australia. We found high variability in δ238U values between different carbonate components, even within a single sample. Petrographically, the best-preserved components from the Balcanoona reef are marine cements, which have a mean δ238U value of –0.23‰, essentially unfractionated from riverine inputs. These values are interpreted as reflecting a marine system with widespread anoxic and iron-rich settings. Less-well-preserved phases have δ238U values spanning almost the entire extent of the documented isotopic range. This integrated petrographic-geochemical work demonstrates the need for petrographic analysis and careful sample selection on a case-by-case basis in future carbonate metal isotope geochemistry.

The Implications of a Stromatolite From the Olenekian (Spathian) Triassic of China

Upper Lower Triassic stromatolite from Anhui, South China: Geobiologic features and paleoenvironmental implications

Authors:

Luo et al

Abstract:

Widespread stromatolites and other microbialite deposits characterize Lower Triassic marine successions worldwide. This study documents a stromatolite deposit, 1.1 m thick, from the upper Spathian (Lower Triassic) of the Susong area, South China. The stromatolite comprises distinct laminated domes in the basal part and columns at the upper part. Dark laminae are loosely spaced and interlayered with thicker light colored laminae. Diffusive dark colored laminae are composed of peloidal micrite that grade locally into microclotted structures, and yield copies of bacteria clump-like and coccoid-like spheroid aggregates. The former are characterized by cloudy, micrite nuclei rimmed by coarse-grained, euhedral sparry calcite crystals, while the latter are comprised of solid calcite crystal nuclei coated with rather thin micrite envelopes. The cloudy, micrite nuclei resemble bacteria clump-like structures observed in present-day travertine. Both the coccoid-like spheroids and bacteria clump-like structures are surrounded by coarse-grained euhedral calcite crystals, suggesting a similar accretion mechanism. Both spherical structures therefore could be crucial in the accretion of the Susong stromatolite. The laminated/microclotted structures are interpreted as the result of variation in timing of lithification relative to the timing of the decay of microbes. Micro-analysis also unravels the common occurrence of authigenic micro-quartz crystals in association with Fe-bearing illite clay minerals in the stromatolite columns. Their coalescing nature with each other, together with the associated pyrite grains, strongly support the formation of micro-quartz crystals from microbial reduction of an Fe-bearing smectite precursor by sulfate reducing bacteria. A comparison of the Susong stromatolite with its counterparts from the upper Lower Triassic strata in Dajiang, South China reveals many similarities in stromatolite microstructures, suggesting that a harsh, euxinic–anoxic environment resulting in the bloom of sulfate reducing bacteria most likely extended into the latest Spathian in South China.

Oceanic Redox Conditions Across the Ediacaran/Cambrian Boundary

Redox variations and organic matter accumulation on the Yangtze carbonate platform during Late Ediacaran–Early Cambrian: Constraints from petrology and geochemistry

Authors:

Gao et al

Abstract:

In order to understand redox variations and organic matter accumulation on the Yangtze carbonate platform during the Late Ediacaran–Early Cambrian, petrological and geochemical studies of several wells were carried out in this work. Our data suggest that depositional environments were dominated by oxidizing bottom water conditions during the late Ediacaran, and evolved to anoxic conditions, triggered by blooms of microbial organisms in surface waters during the earliest Cambrian. Subsequently, massive release of H2S derived from both anaerobic recycling of organic matter and, probably, hydrothermal venting promoted a sulfidic ocean. The discovery of a Ni-Mo sulfide ore layer in the basal Cambrian implies that such a sulfidic condition spread onto the Yangtze carbonate platform interior during the late Cambrian Stage 2. Further, transgressive flooding led to widespread black shale deposition and persistently anoxic conditions, as indicated by geochemical proxies. During the late Ediacaran to earliest Cambrian, local and widespread phosphogenesis indicates that organic matter accumulation was intimately associated with microbial (especially cyanobacterial) blooms driven by phosphorus cycling. The organic matter accumulations in early Cambrian black shales, however, were in connection with anoxic bottom water conditions and intermittent replenishment of recycled organic phosphorus to surface waters.

Potentially NO Aptian and Albian Cretaceous Atmospheric CO2 changes during oceanic anoxic events

Aptian and Albian atmospheric CO2 changes during oceanic anoxic events: Evidence from fossil Ginkgo cuticles in Jilin Province, Northeast China

Authors:

Sun et al

Abstract:

The Early Cretaceous was a time with super-greenhouse conditions and episodic global oceanic anoxic events. However, relative timing of atmospheric CO2 emissions and oceanic anoxic events, and their causal relationships remain matters of debate. Using the stomatal index approach, well-preserved fossil cuticles of Ginkgo from the Lower Cretaceous Changcai Formation, eastern Jilin, and from the Lower Cretaceous Yingcheng Formation, central Jilin, Northeast China, were investigated to reconstruct atmospheric CO2 concentrations during the Aptian and earliest Albian (Early Cretaceous). The results indicate that the CO2 concentrations reached 1098–1142 ppmv (Carboniferous standardization) or 970–1305 ppmv (regression function) during the Aptian and earliest Albian. Our estimates of palaeoatmospheric CO2 concentrations during the earliest Albian (OAE 1b) are slightly higher than the data between the early Aptian Selli (OAE 1a) and the middle Aptian Fallot OAEs; this may indicate the absence of any great emissions of CO2 during the latest Aptian and earliest Albian.

Metazoans (Animals) Really do Need Oxygen

n 2010, a research team garnered attention when it published evidence of finding the first animals living in permanently anoxic conditions at the bottom of the sea. But a new study, led by scientists at the Woods Hole Oceanographic Institution (WHOI), raises doubts.

One alternative scenario is that cadavers of multicellular organisms were inhabited by bacteria capable of living in anoxic conditions, and these "bodysnatchers" made it seem that the dead animals were living, said Joan Bernhard, a geobiologist with WHOI and the lead author of the new study published in the December 2015 issue of the scientific journal BMC Biology.

Bernhard and Virginia Edgcomb, her colleague at WHOI, led an expedition in 2011 that returned to the site of the initial findings: a deep hypersaline anoxic basin (or DHAB) two miles deep in the Mediterranean Sea. DHABs are curious phenomena. They exist in depressions on the seafloor, where long-buried salt deposits become exposed to seawater and dissolve into the sea. The hypersaline water is extremely dense and remains separated, like oil and water, from surrounding normal seawater. It forms "lakes" on the seafloor, tens to hundreds of meters deep, that are extremely salty and devoid of oxygen.

"We have known for a long time that some metazoans inhabit extreme anoxic habitats on a periodic or even semi-permanent basis," Bernhard said. "But scientists have thought that metazoan's high-energy activities, such as reproduction, would require oxygen. If these loriciferans spend their whole lives and reproduce in a zero-oxygen environment, we would have to reconsider our concepts of animal metabolism. It was important to revisit the DHABs to confirm and understand those previous remarkable findings."

In the 2010 study published in the same journal, researchers from Polytechnic University of Marche and the Natural History Museum of Denmark, led by Roberto Danovaro, analyzed samples collected from a Mediterranean DHAB called L'Atalante. They reported finding multicellular animals (or metazoans), including previously unknown species of a type of tiny animals called loriciferans.

The contrast between conditions at the seafloor and at the surface makes it nearly impossible to recover live specimens, and so in the past, metazoan specimens collected from DHABs have been "interpreted as the result of a rain of cadavers that sunk to the anoxic zone from adjacent oxygenated areas," according to the Danovaro study.

But the scientists conducted experiments with fluorescent tags, taken up only by metabolically active organisms, which gave indications that the loriciferans had been alive. In addition, a few loriciferans appeared to have reproductive structures called oocytes (or eggs), indicating that the organisms were reproducing.

Intrigued by these findings, Bernhard and Edgcomb returned to L'Atalante and other nearby DHABs in 2011 to further investigate aboard the research vessel Atlantis. They collected sediment and water from the edges of three brine pools with different chemical compositions, using WHOI's remotely operated vehicle Jason to visually guide carefully targeted push-core samples. Samples were taken from points in the upper, middle, and lower levels of the layer of water immediately overlying the brine lake. This so-called "interface zone" is where normal seawater at the top transitions to the brine at the bottom, becoming more concentrated and anoxic the closer to the brine. The highly dense, saline, chemical-laden and oxygen-depleted water in all three pools was too dense for Jason to fully penetrate. Control samples from nearby sediment and water of normal oxygen and salinity were also collected.

link.

Sadly, this seems to be not true.

Carnian/Norian Boundary Environmental Changes Were NOT Caused by Methane Hydrates or Volcanic Eruptions

Paleoenvironmental changes across the Carnian/Norian boundary in the Black Bear Ridge section, British Columbia, Canada

Authors:

Onoue et al

Abstract:

The Black Bear Ridge section in northeastern British Columbia, Canada, consists of a continuously exposed sequence of Upper Carnian through Lower Norian (Upper Triassic) continental margin strata. The section has been proposed as a candidate Global Stratotype Section and Point (GSSP) for the Carnian/Norian boundary (CNB). In order to assess Late Carnian to Early Norian environmental changes recorded in the section, we examined stratigraphic variations in 87Sr/86Sr, δ13C, and δ18O values, and also values of redox sensitive elements (V, Ni and Cr), in the CNB interval. The study section is located along the north shore of Williston Lake, northeastern British Columbia. The Black Bear Ridge section was deposited in a distal ramp environment on the passive western margin of the North American craton.

The strata across the CNB display a positive shift in δ13C values and a corresponding increase in the redox indices V/(V + Ni) and V/Cr. The synchronous increase in δ13C values and redox indices suggests that burial rates of marine organic carbon increased in response to the development of anoxic conditions in the water column. An increase in δ13C values in carbonate rocks across the CNB has also been reported from Upper Triassic sections in the western Tethys (e.g., in the Pizzo Mondello section, Sicily), which suggests that the development of anoxic conditions within the CNB interval was widespread, affecting both the Panthalassan Ocean and Tethyan Sea. The geochemical data from this study, as well as from research into conodont biostratigraphy in the Black Bear Ridge section, show that the onset of oceanic anoxic conditions may have been responsible for the faunal turnover event at the CNB. The cause of this anoxic event is unknown, but the 87Sr/86Sr and δ13C isotope data largely exclude the possibility that the event was triggered by dissociation of methane hydrates and degassing related to large-scale volcanic activity.

Evidence of Anoxic Oceans at in the Lopingian Permian South China

Lopingian (Upper Permian) trace fossils from the northern Penglaitan Section, Laibin, Guangxi, South China and their environmental implications

Authors:

Ding et al

Abstract:

The end-Permian mass extinction has been widely documented to be accompanied with oceanic anoxia, which was considered as one of the most plausible killing mechanisms. However, it is still unclear when anoxia began to occur and how widely affected during the pre-extinction interval. In this study, Lopingian bottom-water oxygen level changes around the Wuchiapingian–Changhsingian boundary (WCB) at the northern Penglaitan section in Guangxi are analysed based on trace fossil assemblages and ichnofabric indices. Detailed bed-by-bed ichnological analyses confirm the presence of trace fossils Chondrites intricatus, C. isp., Palaeophycus isp., Planolites isp. A, P. isp. B, Thalassinoides isp. A, T. isp. B, Trichichnus linearis, Zoophycos isp., and other bioturbational structures. The ichnofabric indices around the WCB are mostly 1-2, but rapidly increase to 4-5 in some short intervals. The distinct fluctuations of ichnofabric indices suggest that the oxygen level of sediments was generally deficient, but the dysoxic or anoxic periods were punctuated by several short oxic intervals. Thus, it can be inferred that the benthic organisms were under secularly environmental stresses around the WCB at the northern Penglaitan section. The anoxic conditions unfavourable for aerobiosis in the deep basin may start in the late Wuchiapingian. However, more sections need to be studied to determine whether this is a local or regional phenomenon.

Did a Magma Injection Into Organic Sediments at the Barremian/Aptian Boundary Release Carbon Gases, Trigger Oceanic Anoxic Event 1a?

The Early Cretaceous Barents Sea Sill Complex: Distribution, 40Ar/39Ar geochronology, and implications for carbon gas formation

Authors:

Ploteau et al

Abstract:

Mafic igneous rocks of Cretaceous age (80–130 Ma) scattered around the Arctic Ocean are commonly referred to as the High Arctic Large Igneous Province (HALIP). We have mapped out the distribution of HALIP igneous rocks in the Barents Sea region over the past decade based on integrated seismic–gravity–magnetic interpretation, field work, review of publications, and analyses of new and vintage borehole and field samples. The mapping reveals abundant igneous rocks in the northern and eastern Barents Sea covering an area of ~ 900,000 km2 with a conservative volume estimate of 100,000 to 200,000 km3 of intrusions. The igneous province is dominated by sheet intrusions injected into Triassic and Permian sedimentary rocks. Hydrothermal vent complexes are rare, and only two potential vent complexes have been identified on seismic data in the eastern Barents Sea. We have further done extensive radiometric dating of the igneous samples in the Barents Sea region. New 40Ar/39Ar dating of thirteen samples from Svalbard reveal ages of crystallization and alteration. The large age span (60–140 Ma for the raw ages) is likely due to partial or complete overprint of the K/Ar system in plagioclase, and the age of the magma emplacement is better represented by U/Pb TIMS ages. Only one of our 40Ar/39Ar analyses of plagioclase yielded a statistically valid age that is in line with the recently published U/Pb TIMS ages of 122–125 Ma. The new data clearly document that relying on published data from the K/Ar system can lead to erroneous conclusions on the age of crystallization in this province without a careful use of additional 40Ar/39Ar degassing data (i.e., K/Ca). We propose that the magmatism on Svalbard and Franz Josef Land represents a distinct magmatic event near the Barremian/Aptian boundary (125 Ma) in the Barents Sea. This Early Cretaceous Barents Sea magmatism resulted in the formation of the BSSC (Barents Sea Sill Complex). BSSC age rocks are also present in Arctic Canada (Sverdrup Basin) and on Bennett Island (New Siberia Islands). The massive injection of hot magma into potentially organic-rich sediments in the eastern and northern Barents Basin caused rapid organic matter maturation and formation of thermogenic gas and oil in contact aureoles. We estimate that up to 20,000 Gt of carbon were potentially mobilized, corresponding to 175 trillion barrels of oil equivalent. The production rates and fate of the carbon gases are uncertain. However, we speculate that rapid release of aureole greenhouse gases (methane) may have triggered the Oceanic Anoxic Event 1a (OAE1a) and the associated negative δ13C excursion in the Early Aptian. Some of the methane may also be trapped in the vast hydrocarbon gas accumulations found in the east Barents Basin.

No Sharp Delineation of Oxic/Anoxic Regions Across Cambrian/Ordovician Boundary PaleoOceans

Redox conditions across the Cambrian–Ordovician boundary: Elemental and isotopic signatures retained in the GSSP carbonates

Authors:

Azmy et al

Abstract:

Lime mudstone samples (rhythmites) were collected at high resolution from outcrops of the Cambrian–Ordovician GSSP boundary section at Green Point (western Newfoundland, Canada). The sequence (~ 45 m-thick) consists of slope carbonates with alternating shale and siltstone interbeds, and it spans the boundary located between the Martin Point and Broom Point members of the Green Point Formation (Cow Head Group). Samples were extracted from micritic rhythmites by microdrilling and subsequently screened using petrographic and geochemical criteria to evaluate their degree of preservation. Although the δ13Corg profile (− 29.7 to − 25.6‰ VPDB) shows insignificant variations, the TOC values (0.1 to 4.1%) exhibit a generally upward decreasing trend. A negative δ13Ccarb excursion, reflecting a sealevel rise, marks a geochemical anomaly that correlates with an increase in the N contents (0 to 2.9%) of organic matter and the δ15Norg values (− 0.6 to + 6.0‰), which suggests a change to more reducing oceanic conditions. The U contents vary from 0.1 to 3.0 ppm and the δ238U values (− 0.97 to − 0.18‰) generally decrease with the negative δ13Ccarb excursion. The U isotopic variations suggest a widespread increase in reducing conditions associated with sealevel rise during this interval. The investigated sedimentary rocks were slope carbonates where dysoxic conditions likely dominated throughout the entire section. Therefore, the changes in the TOC, N, δ15Norg, and δ238U profiles across the boundary are not as sharp as would be expected by a local change from oxic shallow-water to dysoxic/anoxic deep-water settings.

Persistent Oceanic Anoxia, Mass (?) Extinction Separate Cambrian, Ordovician Explosions

Persistent oceanic anoxia and elevated extinction rates separate the Cambrian and Ordovician radiations

Authors:

Saltzman et al

Abstract:

Recurrent mass extinction events (at "biomere"—a biostratigraphic unit—boundaries) characterize the middle Cambrian to Early Ordovician (Tremadocian) time interval that is between the major Cambrian and Ordovician radiations of animal life. A role for anoxia in maintaining elevated extinction rates in the late Cambrian has been proposed based on coincidence of an extinction with positive excursions in δ13Ccarb and δ34SCAS (CAS—carbonate-associated sulfate). Here we examine an Early Ordovician extinction event at the base of the North American Stairsian Stage (upper Tremadocian), and demonstrate concurrent onset of positive excursions in δ13C and δ34S inferred to reflect enhanced organic matter burial under anoxic waters. Sea-level rise may have brought anoxic waters onto the shelf to initiate extinctions. The evidence for δ13C excursions and elevated extinction rates appears to wane in the Tremadocian, consistent with progressive oxygenation of the oceans reaching a threshold that helped facilitate initial stages of the Great Ordovician Biodiversification Event.

NeoTethys Seawater was Oxygenated, but had Increasingly and Rapidly Warming Temperatures Just Before the Permian Extinction

Neotethys seawater chemistry and temperature at the dawn of the end Permian mass extinction

Authors:

Garbelli et al

Abstract:

The end of the Permian was a time of great death and massive upheaval in the biosphere, atmosphere and hydrosphere. Over the last decades, many causes have been suggested to be responsible for that catastrophe such as global warming, anoxia and acidification. The Gyanyima limestone block was an open ocean seamount in the southern Neotethys at subtropical latitude, and it affords us insight into open-ocean oceanographic changes during the end of the Permian After careful screening using multiple tests, we reconstructed carbonate/seawater curves from the geochemical data stored in pristine brachiopod shell archives from the shallow water limestone of the Changhsingian Gyanyima Formation of Tibet. The reconstructed strontium isotope curve and data for the late Changhsingian is relatively invariant about 0.707013, but in the upper part of the succession the values become more radiogenic climaxing at about 0.707244. The 87Sr/86Sr curve and trend is similar to that observed for the Upper Permian succession in northern Italy, but dissimilar (less radiogenic) to whole rock results from Austria, Iran, China and Spitsbergen. The Ce/Ce* anomaly results. ranging from 0.310 to 0.577 for the brachiopods and from 0.237 to 0.655 for the coeval whole rock before the event, and of 0.276 for whole rock during the extinction event, suggest normal redox conditions. These Ce* values are typical of normal open-ocean oxic water quality conditions observed in modern and other ancient counterparts. The biota and Ce* information clearly discounts global anoxia as a primary cause for the end-Permian biotic crisis. Carbon isotopes from brachiopod shells and whole rock are relatively invariant for most of the latest Permian interval, which is in stark contrast to the distinct negative carbon isotope excursion observed near and about the event. Estimates of seawater temperature at shallow depth fluctuated from 22.2 to 29.0 °C up to unit 8-2, and then gradually rise from 29.7 °C in unit 8-13 to values exceeding 35 °C at a stratigraphic level about 120 ky before the Permian-Triassic boundary, and just before the onset of the extinction interval. This dramatic increase in seawater temperature has been observed in global successions from tropical to mid latitude and from restricted to open ocean localities (e.g., northern Italy, Iran). The brachiopod archive and its geochemical proxies from Tibet support the paradigm that global warming must have been an important factor of the biotic crisis for the terrestrial and marine faunas and floras of the late Paleozoic world.

Panthalassa Ocean was Suboxic/Hypoxic, not Anoxic Across the Spathian Olenekian/Anisian Triassic

Redox conditions in the end-Early Triassic Panthalassa

Authors:

Takahashi et al

Abstract:

This study focuses on an upper Lower Triassic (Spathian) to lowermost Middle Triassic (Anisian) section representing the central Panthalassic deep sea. Analysed organic carbon isotope ratio (δ13Corg) records from the section demonstrate that lower values in the Spathian increase by up to 6‰ at the Spathian–Anisian transition. This trend accords with the carbonate carbon isotope (δ13Ccarb) record from shallow-water carbonate sections. Most horizons during late Spathian–early Anisian show features of redox conditions of not fully oxic but dysoxic conditions, inferred from low Mn, U, V, Mo and euhedral pyrite-dominated occurrences. Conversely, in the end-Spathian black-coloured beds and underlying siliceous claystone beds, relatively higher concentrations of redox-sensitive elements such as U, V, Mo and abundant pyrite framboids are detected. As enrichment factors of redox-sensitive elements are not much higher than the typical anoxic–sulphidic trend and large pyrite framboids are found, these trends suggest suboxic rather than strong anoxic conditions. These oxygen-poor conditions coincide with carbon isotope minimum values at the late Spathian. At the same time, reducing seawater conditions have been also reported in from continental sections. These coincidences imply global environmental perturbations that may have been related to the delayed recovery of life after the end-Permian mass extinction.

Evidence Barremanian/Aptian Cretaceous Global Warming Tied to ‘Oceanic Anoxic Event 1a’

The Barremian and Aptian stepwise development of the ‘Oceanic Anoxic Event 1a’ (OAE 1a) crisis: integrated benthic and planktic high-resolution palaeoecology along the Gorgo a Cerbara stratotype section (Umbria-Marche Basin, Italy)

Authors:

Patruno et al

Abstract:

The Barremian and Aptian were times of global plate reconfiguration and profound changes in the ocean-climate system, culminating in the worldwide deposition of lower Aptian black shale layers (OAE1a). Based on high-resolution lithostratigraphic and micropalaeontological analyses, precursor conditions and timing of the anoxia are here reconstructed along a 33.06 m thick section in the proposed Barremian/Aptian boundary GSSP stratotype at Gorgo a Cerbara (Umbria-Marche Basin, Italy).

A non-uniform history of benthic foraminiferal diversification is interrupted by the Selli Level (= OAE1a) and by three turnover points for both benthic and planktic organisms, each highlighted by prominent breaks in lithology and outcrop morphology. The first two points, at ~ 1.4 Myr and ~ 20-50 Kyr prior to the onset of the OAE1a, correspond respectively to the nannoconid (= bathypelagic calcareous nannofossils) ‘decline’ and ‘crisis’ events, and separate: (a) pelagic limestone/chert-claystone lithological cycles of the uppermost Maiolica Formation, showing frequencies reminiscent of orbital eccentricity and containing an oligotrophic Barremian benthic and planktic assemblage, dominated by k-selected nannoconids and Rhizammina; (b) greenish-grey cherty marls of the lowermost Marne a Fucoidi Formation, associated to declining sedimentation rates and eutrophic early Aptian assemblages dominated by radiolarians and lituolid benthic foraminifera; and (c) anoxic radiolarites and shales of the carbonate-free Selli Level. High magnitude and frequency assemblage fluctuations occur between the nannoconid crisis (~ 25 cm below the lowermost OAE1a black shale layer) and the Selli Level base, with suggestions of bottom-water acidification. At ~ 2.5 Myr after the OAE1a end, a third turnover point in the basal reddish member of the Marne a Fucoidi, highlights ‘middle’ Aptian assemblages characterized by a bloom of early macroperforate planktic foraminifera (Hedbergella spp.) and appearances of new benthic species, marking a shift towards better oxygenation.

OAE1a oxygen and pelagic carbonate factory crises are here suggested to have been step-wise and linked to global warming conditions.

Anoxia was Gradual in Appalachian During Frasnian–Famennian Kellwasser Event

THE LAST GASP: TRACE FOSSILS TRACK DEOXYGENATION LEADING INTO THE FRASNIAN–FAMENNIAN EXTINCTION EVENT

Authors:

Boyer et al

Abstract:

The Frasnian–Famennian boundary is correlated with one of several Late Devonian extinction pulses that resulted in a significant decrease in diversity as well as ecological restructuring. This event is recognized within the globally correlated Upper Kellwasser interval that is well exposed and biostratigraphically well constrained in shale units of western New York State. The ichnological and geochemical signals of the interval stratigraphically below the Upper Kellwasser event at these localities provides insight into the onset of this important extinction event. Detailed analysis of ichnogeneric composition, relative size of burrow populations, amount of bioturbation, and trace metal concentrations vary in concert. Deep-penetrating, pyritized Skolithos burrows terminate abruptly at a thin, laminated black shale interval with enriched Mo levels, up to 31 ppm, and are overlain by an interval of gray-green bioturbated shales dominated by Chondrites. These textural and chemical shifts reveal that bottom-water oxygen levels decrease rapidly below the base of the Upper Kellwasser interval. Relative oxygen levels are interpreted to remain low through the Chondrites-dominated interval, with protracted stressed conditions followed by a gradual decrease to anoxic conditions within the Upper Kellwasser interval. These results suggest that, at least locally in the Appalachian Basin, bottom-water oxygen stress and/or fluctuating oxygen conditions were present leading up to the extinction event. This evidence does not support an instantaneous onset of anoxia as causal mechanism for extinction.

Evidence of Aptian Cretaceous Marine Anoxia From France

Three successive phases of platform demise during the early Aptian and their association with the oceanic anoxic Selli episode (Ardèche, France)

Authors:

Pictet et al


Abstract:

A stratigraphic and depositional model, constrained by biostratigraphy, geochemistry, total phosphorus contents, and bulk-rock mineralogy, is proposed for lower Aptian sediments from the Languedoc platform in Ardèche, SE France. The upper lower Aptian is documented by the Chabert Formation (upper Deshayesites forbesi Zone to upper Dufrenoyia dufrenoyi Subzone), deposited on a discontinuity surface on top of the Urgonian platform, recording a first emersion phase and consecutive drowning event. The Chabert Formation starts with the marly Violette Member, which passes into crinoidal limestone of the Rocherenard Member. The top of this member is associated with a second discontinuity, recording a further drowning phase, which is followed by the deposition of the glauconitic and partly phosphatic Picourel Member (upper Deshayesites grandis to upper Dufrenoyia dufrenoyi Subzone). A third erosive phase is documented by a phosphatic conglomerate (upper Dufrenoyia furcata Zone), which represents a lag deposit derived from underlying sediments. The formation of this conglomerate was associated with a substantial emersion phase. This emersion was followed by a drowning event reworking the phosphatic conglomerate into the base of the upper Aptian black marls (Frayol Formation). The carbon-isotope record shows a negative excursion which coincides with the onset of the early Aptian oceanic anoxic Selli episode (OAE 1a) in the middle/upper part of the Deshayesites forbesi Zone. Emersion phases were an important factor implied in the formation of the sequence boundaries, which were transformed into drowning unconformities during subsequent phases of significant transgressions. These phases were associated with the installation of higher trophic levels, transforming or impeding carbonate production. The first drowning phase preceded the onset of the Selli episode, suggesting that rapid sea-level change and associated environmental change were already an important element of the early Aptian before the major phase of environmental change during the Selli episode.

Evidence of Barremian/Albian Cretaceous Anoxic Oceans Interrupted by Turbulent Mixing

Are Early Cretaceous environmental changes recorded in deposits of the Western part of the Silesian Nappe? A geochemical approach

Authors:

Wójcik-Tabo et al

Abstract:

The organic matter (OM)-rich, relatively monotonous sequence of upper Barremian–Albian deposits in the western part of the Silesian Nappe in the Polish Outer Carpathians, was investigated, using geochemical proxies to show environmental changes. The amounts of organic carbon (0.4–4.32 wt.% of TOC) co-vary with the concentration of S and/or redox sensitive trace elements (RSTE: Mo, U, Co, V, As). Kerogen is present as Type III and mixed Type II/III. Maceral assemblages consist of vitrinite, inertinite and liptinite. The carbon isotopic composition of the organic matter (δ13Corg) varies between − 25.38 and − 21.31‰ with an outlier at − 15.14‰. During the Early Cretaceous the Proto-Silesian Basin was the site of OM accumulation in a detrital oceanic anoxic event as a result of increased sedimentation rates of terrestrial organic matter. Runoff of terrestrial organic matter was accompanied by the production of plankton on an occasional basis. The marine matter contribution is inferred from occurrence of siliceous microfauna, the domination of short-chain n-alkanes, the distribution of sterane and prevalence of cholestane, and enrichment in nutrient elements (P2O5, Ba, Ag, Cd). The bottom waters could have been dysoxic to anoxic, with a mid-water oxygen minimum zone (OMZ) developed owing to the oxidation of organic matter. Anoxia is inferred from the lamination, pyritization patterns, RSTE concentration and low Pr/Ph and C23t/C30 H ratios. Intervals enriched in TOC and RSTE in the Veřovice Formation record pulses of marine productivity, preceded by volcanic (teshenite) and hydrothermal activity. They are referable to the following episodes: the Taxy (Barremian–Aptian), early Aptian Selli, Fallot and Paquier during the late Aptian and at the Aptian–Albian boundary. The initiation of turbidite deposition during the Albian led to bottom ventilation. The Lhoty Formation records the late Albian Toolebuc Episode, marked by a TOC peak, which resulted from the detrital flux. Acidic solution was responsible for reducing conditions in the sediment.

Coniacian/Santonian Cretaceous Marine Moroccan Records of the Oceanic Anoxic Event 3

Marine palynology of the Oceanic Anoxic Event 3 (OAE3, Coniacian – Santonian) at Tarfaya, Morocco, NW Africa – transition from preservation to production controlled accumulation of marine organic carbon

Author:

Prauss

Abstract:

The paper presented is the first comprehensive, fully quantitative, high resolution study of marine palynology from an OAE3 black-shale environment. It is based on 175 m core spanning the upper Turonian to lower Santonian at Tarfaya, Morocco, NW Africa, which has been sampled from centimetre to 3 m intervals. The results are integrated and discussed with lithology and geochemistry data to (1) distinguish between potential changes in production and preservation of total organic carbon (TOC) accumulation and (2) constrain the stratigraphic position of the Oceanic Anoxic Event 3 (OAE3).

The succession is characterized by increased total organic carbon (TOC), varying between 1% and 19% (average about 6%). Distinct black-shale horizons of variable thickness appear episodically throughout the succession, with higher frequency in the late Turonian. Higher TOC contents do not strictly correlate to lithologic black-shales or peaks of a specific taxon of organic-walled algae. The palynomorph spectrum is strongly dominated by organic-walled algae, with the ratio of terrigenous sporomorphs to organic-walled algae (t/m index) varying between zero and 0.05 (average 0.01). The dominance of algal organic matter is corroborated by the prevalence of Type I kerogen identified using Rock-Eval pyrolysis. Dinocyst diversity is low, with the absolute taxa number varying from 7 to 27 between single samples. The peridinioid/gonyaulacoid ratio of dinocysts (p/g ratio) shows strong fluctuations, varying between 1 and 283 (average of about 100).

The upper Turonian interval is dominated by Bosedinia spp., a dinocyst taxon formerly described as abundant only in lacustrine sediments from the Oligocene and Miocene of SE Asia. This dominance is episodically modified by the increase of the warm-temperate waters dinocysts fraction, here mainly represented by the genera Alterbidinium, Isabelidinium and Spinidinium. Within the Coniacian-Santonian, black-shale horizons are limited in number and are concentrated within the upper Coniacian to lower Santonian interval. The dinocysts show alternating, prominent peak abundances of Palaeohystrichophora spp. and the warm-temperate water dinocysts fraction, here mainly represented by the genera Trithyrodinium and Chatangiella. However, a final episode of increased proportions of Bosedinia spp. is confined to a 5 m thick black-shale horizon closely spanning the Coniacian-Santonian boundary.

Changes in the ratio of total sulphur to total organic carbon (TS/TOC) reflect fluctuating oxygen contents of bottom waters throughout the late Turonian to Santonian. These are significantly parallelled by the alternation of dinocysts assemblages suggestive of enhanced upwelling and water column stratification respectively, probably reflecting changes in the mode of TOC accumulation. Accordingly, preservation largely prevails during the late Turonian interval and changes towards increased production within the Coniacian-Santonian. However, a final preservation-event is probably represented by the black-shale horizon closely spanning the Coniacian-Santonian boundary (top Dicarinella concavata foraminifera zone), which may reflect an episodic shutdown of a major upwelling cell. It is thus proposed, that the “culmination” of the OAE3 at Tarfaya may represent intermittent preservation of TOC within an otherwise high productivity environment related to a global cooling trend.

Environmental Impacts During Biotic Recovery After the Permian/Triassic Mass Extinction

Environmental controls on marine ecosystem recovery following mass extinctions, with an example from the Early Triassic

Authors:

Wei et al

Abstract:

The recovery of marine ecosystems following a mass extinction event involves an extended interval of increasing biotic diversity and ecosystem complexity. The pace of recovery may be controlled by intrinsic ecosystem or extrinsic environmental factors. Here, we present an analysis of changes in marine conditions following the end-Permian mass extinction with the objective of evaluating the role of environmental factors in the protracted (~ 5-Myr-long) recovery of marine ecosystems during the Early Triassic. Specifically, our study examines changes in weathering, productivity, and redox proxies in three sections in South China (Chaohu, Daxiakou, and Zuodeng) and one in northern India (Mud). Our results reveal: 1) recurrent environmental perturbations during the Early Triassic; 2) a general pattern of high terrestrial weathering rates and more intensely reducing marine redox conditions during the early Griesbachian, late Griesbachian, mid-Smithian, and (more weakly) the mid-Spathian; 3) increases in marine productivity during the aforementioned intervals except for the early Griesbachian; and 4) stronger and more temporally discrete intervals of environmental change in deepwater sections (Chaohu and Daxiakou) relative to shallow and intermediate sections (Zuodeng and Mud). Our analysis reveals a close relationship between episodes of marine environmental deterioration and a slowing or reversal of ecosystem recovery based on metrics of biodiversity, within-community (alpha) diversity, infaunal burrowing, and ecosystem tiering. We infer that the pattern and pace of marine ecosystem recovery was strongly modulated by recurrent environmental perturbations during the Early Triassic. These perturbations were associated with elevated weathering and productivity fluxes, implying that nutrient and energy flows were key influences on recovery. More regular secular variation in deepwater relative to shallow-water environmental conditions implies that perturbations originated at depth (i.e., within the oceanic thermocline) and influenced the ocean-surface layer irregularly. Finally, we compared patterns of environmental disturbance and ecosystem recovery following the other four “Big Five” Phanerozoic mass extinctions to evaluate whether commonalities exist. In general, the pace of ecosystem recovery depends on the degree of stability of the post-crisis marine environment.

Multiple Isotopic Signatures in Toarcian Jurassic Oceanic Anoxic Event

Multiproxy geochemical analysis of a Panthalassic margin record of the early Toarcian oceanic anoxic event (Toyora area, Japan)

Authors:

Kemp et al

Abstract:

The early Toarcian oceanic anoxic event (OAE) was a significant palaeoenvironmental perturbation that led to marked changes in ocean chemistry and climate, and which also had a long-lasting impact on marine ecosystems. The global significance of the event has been recognised from the widespread occurrence of a ~ 3-7‰ negative excursion in the carbon-isotope (δ13C) composition of marine organic and inorganic matter and terrestrial plant material. This feature of the event is indicative of a pronounced perturbation to the global carbon cycle; an inference further supported by widespread evidence for seawater deoxygenation and elevated rates of organic carbon burial. Nevertheless, the precise palaeoenvironmental impacts of this event from sections outside of the Boreal and Tethyan realms are uncertain. Here, we present the results of a multiproxy geochemical study of an expanded record of the early Toarcian event from the northwest Panthalassa Ocean margin exposed in southwest Japan (Toyora area, Yamaguchi prefecture). Our results indicate that in the studied succession, organic matter enrichment persisted through the early Toarcian event, but elemental redox proxies do not support persistent seawater anoxia. Analyses of terrigenously derived major and trace element abundances coupled with sedimentological observations reveal an increase in coarse-grained sediment close to the onset of a ~ − 4‰ excursion in δ13Corg, and coincident with an inferred increase in terrestrial organic matter flux. These observations are consistent with previously published evidence for a marked strengthening of hydrological cycling and increased runoff that occurred contemporaneously with abrupt warming at the onset of the carbon isotope event.

Environmental Changes in the Western Interior Seaway Across the Cenomanian/Turonian Cretaceous Boundary

Paleoenvironmental and paleoceanographic changes across the Cenomanian–Turonian Boundary Event (Oceanic Anoxic Event 2) as indicated by foraminiferal assemblages from the eastern margin of the Cretaceous Western Interior Sea

Authors:

Elderbak et al

Abstract:

Two sites near the eastern margin of the Cretaceous Western Interior Sea (WIS) were investigated. The section at Cuba, Kansas (CK) is ~ 630 km east of the GSSP for the Cenomanian/Turonian boundary at Rock Canyon, Colorado (RC), and the section near Sioux City, Iowa (SCI) is ~ 315 km northeast of the Cuba site. Surprisingly, planktic foraminifera dominate all the studied samples despite the relative proximity of the sites to the paleo-shoreline and presumed neritic water depths. Such dominance suggests inhospitable benthic environments, and benthic foraminiferal assemblages indicate that seafloor oxygen in the WIS decreased eastward. Therefore, the hypothesis of an influx of freshwater into the WIS from the western margin forming a fresh water cap that diminished eastward is not supported. Benthic foraminifera are scarce in most studied samples and species diversity is low. In the CK section, diversity and abundance increase abruptly in the uppermost Cenomanian Benthonic Zone at the initiation of the δ13Corg positive excursion marking the onset of Oceanic Anoxic Event (OAE 2). In the SCI section, however, the Benthonic Zone is recognized only by presence of Nodosaria bighornensis that has a stratigraphic range restricted to this interval; other benthic taxa are very rare or absent. The favorable conditions of the Benthonic Zone were short-lived. While relatively diverse planktic foraminiferal assemblages, including keeled species, characterize the initial δ13C excursion at the CK site, dwarfed specimens of Heterohelix and Hedbergella dominate the assemblages at the more proximal Sioux City site. Furthermore, most of the biostratigraphic events recognized in the basin center RC section can be traced into the CK section, but not into the SCI section, including the Rotalipora spp. and Globigerinelloides bentonensis extinctions, the Heterohelix shift event, and the brief benthic recovery event. Correlation between eastern WIS sites shows that the SCI section is thicker, suggesting a major sediment source to the east. This may explain the inhospitable benthic conditions and development of a ‘dead zone’ (seasonal hypoxia) analogous to the modern shelf of the northern Gulf of Mexico proximal to active discharge of the Mississippi River. Development of estuarine circulation in the WIS provided the eastern part of the basin with input of calcareous plankton from the south including planktic foraminifera. Foraminiferal assemblages, total organic carbon (TOC), and δ13C data of the studied sections suggest a two-fold history of Greenhorn transgression and OAE 2 development. The initial phase is a global signal characterized by a positive δ13C excursion, low TOC values, and high foraminiferal species diversity. The second phase is characterized by overprinting by local conditions in the WIS, including high fluvial input, relatively high TOC values, and low foraminiferal species diversity. These findings suggest that the eastern margin records unique depositional and biotic environments further revealing the dynamic and complex nature of the WIS and its sedimentary cycles.