Robert J. Newton

Large shifts in the isotopic composition of seawater sulphate across the Permo–Triassic boundary in northern Italy

Carbonate-associated sulphate (CAS) extracted from a Permo–Triassic succession at Siusi in northern Italy is shown to preserve a true seawater-sulphate isotope record. Two periods of increasing δ34S and δ18O in CAS provide evidence for increased oceanic anoxia in the Late Permian and the Early Triassic. These two anoxic episodes are separated by an event characterised by the addition of isotopically light sulphur and oxygen to the oceanic sulphate pool. Simple mass balance calculations suggest that this sulphate originates from the reoxidation of bacterially derived H2S during oceanic mixing, rather than a volcanogenic source. A dramatic fall in CAS-δ18O directly above the P–T boundary, not accompanied by a large change in CAS-δ34S, records an oceanic deoxygenation event probably caused by the release of methane from gas hydrates, subsequently recorded in the carbonate-carbon isotope record. The decline of Early Triassic oceanic anoxia is not recorded by a fall in CAS-δ34S, but is preserved by declining CAS-δ18O. This is because of an increase in the flux of reactive iron to the oceans during the Early Triassic anoxic episode, triggered by the demise of land plants. This permanently removes a greater proportion of light sulphur from the oceanic sulphate reservoir as pyrite, and means that the heavy residual sulphate-sulphur isotope signature of Griesbachian anoxic seawater is preserved as a geochemical ‘fossil’ until the beginning of the Middle Triassic.

The Permo-Triassic transition in Spitsbergen; delta 13 C org chemostratigraphy, Fe and S geochemistry, facies, fauna and trace fossils

New δ 13 C org analyses of two boundary sections between the late Permian Kapp Starostin Formation and the early Triassic Vardebukta Formation of western Spitsbergen confirm field evidence that their contact is a conformable one. Thus, contrary to previous reports, some Spitsbergen sections contain a complete record of the environmental and faunal changes during the crisis interval of the end Permian mass extinction. No environmental deterioration is recorded in the late Permian until near the end of the terminal Changxingian Stage, whereupon the abundant siliceous sponge fauna of the Kapp Starostin Formation disappears along with the deep-burrowing fauna responsible for the Zoophycus trace fossil. A low diversity dysaerobic trace fossil assemblage is briefly developed before a transition to finely laminated, pyritic facies immediately beneath the Permo-Triassic boundary. Analysis of the S/C ratios from the laminated strata suggests that free H 2 S was present in the water column (euxinic conditions) even in relatively nearshore settings subject to storm sandstone deposition. The mass extinction crisis in Spitsbergen is therefore coincident with the extensive development of oxygen-poor conditions in the water column and compares closely, both in timing and nature, with the crisis seen in lower latitude Tethyan settings. However, the subsequent aftermath and recovery in the Boreal sections of Spitsbergen was more rapid than in Tethys. Thus, a shoreface sandstone body within the Dienerian Stage contains an appreciable diversity of fauna (by the standards of the early Triassic), including bryozoans, calcareous algae and deep infaunal bivalves, that suggests the marine ecosystem recovery began earliest in higher palaeolatitudes.

Volcanism, mass extinction, and carbon isotope fluctuations in the middle permian of China

Middle Permian Extinction A major extinction in the Middle Permian 260 to 270 million years ago preceded the huge end-Permian extinction. Wignall et al. (p. 1179) present a detailed analysis of the Middle Permian event from rocks in southwest China. The extinction coincided with extensive nearby volcanic eruptions. A major drop in carbon isotope values followed the extinction event, implying massive disruption of the carbon cycle. Fossiliferous rocks from southwest China show that a major extinction in the Middle Permian coincided with extensive volcanic eruptions. The 260-million-year-old Emeishan volcanic province of southwest China overlies and is interbedded with Middle Permian carbonates that contain a record of the Guadalupian mass extinction. Sections in the region thus provide an opportunity to directly monitor the relative timing of extinction and volcanism within the same locations. These show that the onset of volcanism was marked by both large phreatomagmatic eruptions and extinctions amongst fusulinacean foraminifers and calcareous algae. The temporal coincidence of these two phenomena supports the idea of a cause-and-effect relationship. The crisis predates the onset of a major negative carbon isotope excursion that points to subsequent severe disturbance of the ocean-atmosphere carbon cycle.

The importance of the relationship between scale and process in understanding long-term DOC dynamics

Concentrations of dissolved organic carbon have increased in many, but not all, surface waters across acid impacted areas of Europe and North America over the last two decades. Over the last eight years several hypotheses have been put forward to explain these increases, but none are yet accepted universally. Research in this area appears to have reached a stalemate between those favouring declining atmospheric deposition, climate change or land management as the key driver of long-term DOC trends. While it is clear that many of these factors influence DOC dynamics in soil and stream waters, their effect varies over different temporal and spatial scales. We argue that regional differences in acid deposition loading may account for the apparent discrepancies between studies. DOC has shown strong monotonic increases in areas which have experienced strong downward trends in pollutant sulphur and/or seasalt deposition. Elsewhere climatic factors, that strongly influence seasonality, have also dominated inter-annual variability, and here long-term monotonic DOC trends are often difficult to detect. Furthermore, in areas receiving similar acid loadings, different catchment characteristics could have affected the site specific sensitivity to changes in acidity and therefore the magnitude of DOC release in response to changes in sulphur deposition. We suggest that confusion over these temporal and spatial scales of investigation has contributed unnecessarily to the disagreement over the main regional driver(s) of DOC trends, and that the data behind the majority of these studies is more compatible than is often conveyed.

Contrasting Deep-water Records from the Upper Permian and Lower Triassic of South Tibet and British Columbia: Evidence for a Diachronous Mass Extinction

Abstract Remarkably different Late Permian–Early Triassic marine records are seen in sections from the western deep-water margin of Pangea (Ursula Creek, British Columbia) and the high paleolatitude, southern margin of the Neotethyan Ocean (Selong, South Tibet). The Ursula Creek section reveals the progressive decline of seafloor oxygen values in the Changxingian Stage (loss of bioturbation, authigenic U enrichment, appearance of pyrite framboid populations), followed by the persistent development of euxinic conditions in the latest Changxingian and throughout the Early Triassic; an event that coincides with the disappearance of a siliceous sponge fauna and the loss of diverse radiolarian populations. The Selong section, which was located on a distal passive margin, records regression and erosion in the mid-Changxingian, followed by a phase of deepening that began in the late Changxingian. The boundary interval is associated with a marked diversity increase due to the appearance of equatorial taxa (foraminifera, brachiopods, and sponges), suggesting warming without extinction in marine waters at high southern paleolatitudes. Only in the late Griesbachian Stage are the diverse Permian holdovers eliminated, again at a level showing evidence for dysoxia (thinly-bedded, authigenic U-enriched, pyrite-rich limestone). Thus, the end-Permian mass extinction is seen to be diachronous by half a million years or more, with late Changxingian extinction in Panathalassa coinciding with diversity increase associated the migration of warm-water taxa into the high southerly paleolatitudes regions of Neotethys.

An Indicator of Water-Column Anoxia: Resolution of Biofacies Variations in the Kimmeridge Clay (Upper Jurassic, U.K.)

ABSTRACT An Indicator of Anoxicity (IA), defined as the ratio (Pyrite Fe + Oxide Fe)/Total Fe, can be used to recognize sediments deposited under an anoxic water column. Modern marine sediments deposited under oxygenated bottom waters mainly have IA values 0.5. IA values in the central Kimmeridge Clay (Upper Jurassic, UK) for fissile, organic-rich shales indicate that deposition occurred principally from anoxic bottom waters with infrequent oxygenation events.

Carbon and nitrogen isotope disturbances and an end-Norian (Late Triassic) extinction event

Major perturbations of organic carbon and nitrogen isotope ratios from a Norian-Rhaetian (Late Triassic) boundary section in British Columbia coincide with an extinction of the dominant, deep-water invertebrate fauna of the Late Triassic (monotids and most ammonoids). The carbon isotope excursion is attributed to the development of widespread oceanic stagnation that favored organic-rich shale deposition. The coincident nitrogen isotope excursion suggests that progressively more nitrate-limited productivity forced a change to nitrogen-fixing cyanobacteria populations as ocean stagnation created nutrient-starved conditions. The biotic crisis and geochemical events of the Norian-Rhaetian boundary predate the latest Rhaetian (end-Triassic) mass extinction. Thus, the Late Triassic interval was marked by more than one extinction event.

An evaluation of the use of the chromous chloride reduction method for isotopic analyses of pyrite in rocks and sediment

Abstract The use of chromous chloride reduction as a preparation technique for isolation of pyrite sulphur for isotopic analysis has been assessed. A small but consistent fractionation is introduced by reduction of pyrite to H 2 S, enriching the yield in 34 S by 0.55 ± 0.14‰.

Turbidite depositional influences on the diagenesis of Beecher's Trilobite Bed and the Hunsrück Slate; sites of soft tissue pyritization

Chemical signatures of enrichment of highly reactive iron, and framboid size distributions, are reported in turbidite sediments that host soft tissue pyritization (Beechers Trilobite Bed, Upper Ordovician, and the Hunsrück Slate, Lower Devonian). These signatures demonstrate that the sediment of Beechers Trilobite Bed was enriched in highly reactive iron prior to turbidite transport but that no enrichment was present in the Hunsrück Slate. Turbidite transport and re-sedimentation altered framboid size distributions. Small diameter framboids (< 5 μm) that formed in the sediment at the pre-transport site were lost during transport due to oxidation and/or size sorting. Larger diameter framboids (∼5 –15 μm) that formed at the pre-transport site were transported without alteration. The oxidation of the original small framboid population formed highly reactive iron (oxyhydr)oxides that were reduced during post-transport suboxic diagenesis to produce porewaters rich in dissolved iron. In some turbidites a bimodal framboid population resulted where a minor population of small diameter framboids, produced by limited sulfate reduction at the post-transport site, was added to the transported large diameter population. Soft tissue pyritization in this setting was facilitated by the presence of suboxic, iron rich porewaters where dissolved sulfide formed during soft tissue decay, confining iron sulfide precipitation to the decay site.

Black shales on the basin margin: a model based on examples from the Upper Jurassic of the Boulonnais, northern France

Abstract Organic-rich shales were deposited during two phases of relative sea-level rise in the Kimmeridgian (Late Jurassic) basin margin sections of the Boulonnais (N. France). Pyrite framboid analysis indicates that bottom waters were euxinic during substantial periods of organic-rich shale formation, and Th/U ratios and degree of pyritisation of Fe suggest severe oxygen-restriction, whilst palaeoecological and sedimentological evidence suggests that seafloor oxygenation events were frequent. Interbedded, hummocky cross-stratified sandstones indicate deposition above storm wave base. Correlation of lateral facies changes reveals that the black shales record deposition beneath a nearshore zone of oxygen-poor waters that contracted in lateral extent during parasequence formation. The Boulonnais black shales are similar to other varieties of transgressive black shale, in that they are developed upon surfaces of marine starvation, but they differ in that they pass down-dip into better oxygenated facies. They are therefore classified as transgressive, nearshore (TN) black shales that do not record the ‘feather edge’ of basinal conditions. Deposition is interpreted to have occurred beneath a small volume of sub-thermocline bottom waters that were readily de-oxygenated, whilst anoxia was less readily developed in more offshore settings due to the greater volume of sub-thermocline water. Why such conditions should pertain only during initial stages of base-level rise remains obscure.