Termination of a continent-margin upwelling system at the Permian-Triassic boundary (Opal Creek, Alberta, Canada)

Models of mass extinctions caused by greenhouse warming depend on the ability of warming to affect the oxygenation of the ocean, either through slowing circulation or changes in biological productivity and the organic carbon budget. Opal Creek, Alberta, Canada is a biostratigraphically continuous Permian–Triassic Boundary (PTB) section deposited in deep water on an outer shelf setting in the vast and understudied Panthalassic Ocean, along the western margin of Pangaea. The latest-Permian extinction is here represented as the disappearance of the previously dominant benthic fauna (siliceous sponges). On the basis of nitrogen and reduced sulfur isotopes as well as productivity-sensitive trace elements, the Middle Permian at Opal Creek is interpreted as a highly productive coastal upwelling zone where vigorous denitrification and sulfate reduction occurred in a mid-water oxygen minimum. Similar conditions appear to have continued into the latest Permian until the onset of a euxinic episode represented by a discrete pyrite bed and several trace element indicators of high productivity. This euxinic pulse is followed by the extinction of benthic fauna and a shift in nitrogen and sulfur isotopes to more normal marine values, suggesting the cessation of coastal upwelling and the consequent weakening of the mid-water oxygen minimum. The Lower Triassic appears to be a dysoxic, relatively unproductive environment with a bottom water oxygen minimum. Rhenium–osmium isotope systematics show a minimum of radiogenic Os near the main extinction event, which may be due to volcanic input, and increasingly radiogenic values approaching the PTB, possibly due to increased continental erosion. The Opal Creek system demonstrates that, while the biogeochemical crisis in the latest Permian was capable of impacting the coastal upwelling modality of ocean circulation, a transient increase in productivity likely drove the system toward euxinia and, ultimately, extinction

From marine bands to hybrid flows: sedimentology of a Mississippian black shale

Organic‐rich mudstones have long been of interest as conventional and unconventional source rocks and are an important organic carbon sink. Yet the processes that deposited organic‐rich muds in epicontinental seaways are poorly understood, partly because few modern analogues exist. This study investigates the processes that transported and deposited sediment and organic matter through part of the Bowland Shale Formation, from the Mississippian Rheic–Tethys seaway. Field to micron‐scale sedimentological analysis reveals a heterogeneous succession of carbonate‐rich, siliceous, and siliciclastic, argillaceous muds. Deposition of these facies at basinal and slope locations was moderated by progradation of the nearby Pendle delta system, fourth‐order eustatic sea‐level fluctuation and localized block and basin tectonism. Marine transgressions deposited bioclastic ‘marine band’ (hemi)pelagic packages. These include abundant euhaline macrofaunal tests, and phosphatic concretions of organic matter and radiolarian tests interpreted as faecal pellets sourced from a productive water column. Lens‐rich (lenticular) mudstones, hybrid, debrite and turbidite beds successively overlie marine band packages and suggest reducing basin accommodation promoted sediment deposition via laminar and hybrid flows sourced from the basin margins. Mud lenses in lenticular mudstones lack organic linings and bioclasts and are equant in early‐cemented lenses and in plan‐view, and are largest and most abundant in mudstones overlying marine band packages. Thus, lenses likely represent partially consolidated mud clasts that were scoured and transported in bedload from the shelf or proximal slope, as a ‘shelf to basin’ conveyor, during periods of reduced basin accommodation. Candidate in situ microbial mats in strongly lenticular mudstones, and as rip‐up fragments in the down‐dip hybrid beds, suggest that these were potentially key biostabilizers of mud. Deltaic mud export was fast, despite the intrabasinal complexity, likely an order of magnitude higher than similar successions deposited in North America. Epicontinental basins remotely linked to delta systems were therefore capable of rapidly accumulating both sediment and organic matter

Oceanic response to Pliensbachian and Toarcian magmatic events: Implications from an organic-rich basinal succession in the NW Tethys

The Bächental bituminous marls (Bächentaler Bitumenmergel) belonging to the Sachrang Member of the Lower Jurassic Middle Allgäu Formation were investigated using a multidisciplinary approach to determine environmental controls on the formation of organic-rich deposits in a semi-restricted basin of the NW Tethys during the Early Jurassic. The marls are subdivided into three units on the basis of mineralogical composition, source-rock parameters, redox conditions, salinity variations, and diagenetic processes. Redox proxies (e.g., pristane/phytane ratio; aryl isoprenoids; bioturbation; ternary plot of iron, total organic carbon, and sulphur) indicate varying suboxic to euxinic conditions during deposition of the Bächental section. Redox variations were mainly controlled by sea-level fluctuations with the tectonically complex bathymetry of the Bächental basin determining watermass exchange with the Tethys Ocean. Accordingly, strongest anoxia and highest total organic carbon content (up to 13%) occur in the middle part of the profile (upper tenuicostatum and lower falciferum zones), coincident with an increase in surface-water productivity during a period of relative sea-level lowstand that induced salinity stratification in a stagnant basin setting. This level corresponds to the time interval of the lower Toarcian oceanic anoxic event (T-OAE). However, the absence of the widely observed lower Toarcian negative carbon isotope excursion in the study section questions its unrestricted use as a global chemostratigraphic marker. Stratigraphic correlation of the thermally immature Bächental bituminous marls with the Posidonia Shale of SW Germany on the basis of C27/C29 sterane ratio profiles and ammonite data suggests that deposition of organic matter-rich sediments in isolated basins in the Alpine realm commenced earlier (late Pliensbachian margaritatus Zone) than in regionally proximal epicontinental seas (early Toarcian tenuicostatum Zone). The late Pliensbachian onset of reducing conditions in the Bächental basin coincided with an influx of volcaniclastic detritus that was possibly connected to complex rifting processes of the Alpine Tethys and with a globally observed eruption-induced extinction event. The level of maximum organic matter accumulation in the Bächental basin corresponds to the main eruptive phase of the Karoo-Ferrar large igneous province (LIP), confirming its massive impact on global climate and oceanic conditions during the Early Jurassic. The Bächental marl succession is thus a record of the complex interaction of global (i.e., LIP) and local (e.g., redox and salinity variations, basin morphology) factors that caused reducing conditions and organic matter enrichment in the Bächental basin. These developments resulted in highly inhomogeneous environmental conditions in semi-restricted basins of the NW Tethyan domain during late Pliensbachian and early Toarcian time

Trace metals as paleoredox and paleoproductivity proxies: An update.

International audienc

Hydrothermal influences in the Late Triassic Lagonegro Basin (southern Italy): evidence of contemporaneous mid-ocean-ridge spreading in the western Tethys

The south-western branch of the Tethys Ocean was interested, during the Late Triassic, by the opening and spreading of an oceanic basin, the Ionian Ocean. Its north-western termination was characterized by carbonatic/siliceous deposition inside the Lagonegro Basin, which was imposed on transitional crust. Three stratigraphic sections of the Lagonegro Basin sedimentary record, representing a proximal-to-distal transect (e.g., Casacci et al., 2016) from the continental margin, have been investigated to evaluate the interaction of this basin with the adjacent Ionian Ocean. This ocean was actively spreading since the Middle Triassic (Passeri et al., 2014) but no investigation has been done on the influence of the ridge activity on its peripheral portion situated northward, represented by the pelagic sediments of the Lagonegro Basin. Three major inputs into the sediment were recorded: detrital clay, biogenic silica and carbonates, and most of the samples here studied consist of mixtures of these components. In order to discriminate the chemical signature of the main fractions, bulk and partition geochemical investigations have been carried out in relation of the three main components of the sediments. The rare earth elements (REEs) reflect different pattern for each sediment fraction with the SiO2-bio group showing middle REEs enrichment, suggesting Fe oxyhydroxy influence; the clays group shows a flat pattern typical of shale from siliciclastic input; the carbonate group indicates heavy REEs enrichment and Ce negative anomaly, pointing out seawater influence. Enrichments in hydrothermally derived elements, from the submarine weathering of middle oceanic ridge basalts (MORB), were found in both the clay and carbonate fractions indicating a moderate mixing of the two end-member solutions, hydrothermal fluids and seawater, during the deposition of the Lagonegro Basin sediments. The tectonics of the basin and the different setting for each section left the shallower and more proximal section to the continental margin, without any record of the hydrothermal influence. The results can be considered as a first geochemical evidence of the hydrothermal activity related to the Ionian Ocean ridge, in the Lagonegro Basin deposits

Stepwise deforestation during the Permian-Triassic boundary crisis linked to rising temperatures

Although the trajectory of the marine mass extinction at the ∼252-Ma Permian-Triassic (P-Tr) boundary has been well studied, details of the coeval collapse of terrestrial ecosystems remain murky. Here, we use hydrocarbon biomarker compositions and other geological records (i.e., organic carbon isotopes (δ13Corg), charcoal abundance, and Hg content) from a tropical peatland succession in southwestern China to reconstruct in detail the history of terrestrial ecosystem collapse during the P-Tr crisis. Our high-resolution hydrocarbon biomarker records reveal that this collapse proceeded in a stepwise manner with increasing intensity as the crisis unfolded. We recognize three discrete crisis stages: Stage I within the uppermost Xuanwei Formation and Stages II and III within the lowermost Kayitou Formation. Stage I, the early crisis stage, is marked by a significant decline in terrestrial biomass (continuing into the later stages), as recorded by reduced C29 steranes relative to total steranes and a concomitant reduction in the ratio of pristane to phytane (Pr/Ph). Stage II, the main crisis stage, records intensified soil erosion and sediment flux as revealed by rising dibenzofuran (DBF) content and high hopane/sterane ratios, the disappearance of coal seams, a sharp negative shift in δ13Corg, and peak concentrations of charcoal reflecting increased wildfire incidence. Stage III, the late crisis stage characterized by enhanced soil erosion, corresponds to peak values of Hg and Hg/TOC but no charcoal peak, suggesting intensified volcanism and a return to a humid climate. These stages closely follow temperature records, which show a stepwise rise during the crisis interval, implying that the deforestation process was strongly influenced by punctuated rises in temperature and/or its attendant effects (e.g., climate aridification). This P-Tr transition scenario suggests that global warming can trigger deforestation and reduce terrestrial carbon storage, thus serving as a positive climate feedback, with important implications for present-day climate change

Application of pyrite trace-metal and S and Ni isotope signatures to distinguish sulfate- versus iron-driven anaerobic oxidation of methane

The formation of authigenic pyrite in marine sediments involves multiple reactions between ferrous iron (Fe2+) and hydrogen sulfide (H2S). Ferrous iron is commonly provided through the reductive dissolution of Fe-(oxyhydr)oxides by organic matter (i.e., dissimilatory Fe reduction), dissolved sulfide (i.e., abiotic Fe reduction) or methane (i.e., Fe-AOM), whereas sulfide is supplied by organoclastic sulfate reduction (OSR) or sulfate-driven anaerobic oxidation of methane (SD-AOM). Since Rayleigh-type distillation operates widely in sediments of gas-hydrate-bearing zones, sulfur and nickel isotope compositions (i.e., δ34S and δ60Ni) cannot readily distinguish OSR- from SD-AOM-associated pyrite. However, these microbial pathways may yield different patterns of trace-element enrichment in pyrite. To better understand the linkage of trace-element patterns to specific microbial pathways (i.e., Fe reduction, Fe-AOM, OSR and SD-AOM), and to evaluate the use of S and Ni isotopic signatures as tracers for pyrite formation pathways in methane-rich sediments, we report pyrite-associated trace element and δ34S and δ60Ni isotope analyses of sediments from a gas hydrate borehole (Site GMGS4-SC-03) from the Shenhu area, Pearl River Mouth Basin, South China Sea. Pyrite formed in conjunction with Fe- and/or SD-AOM exhibits abundant framboidal overgrowths and extremely high δ34S (up to +142.8‰) and δ60Ni (up to +2.72‰), representing the highest stable S and Ni isotopic compositions of pyrite reported to date. These pyrite morphologies are enriched in Co and Ni, which may be a diagnostic signature of an SD-AOM pathway. By contrast, OSR-associated pyrite is enriched in Cu and Zn due to OSR-induced release of trace elements from decaying organic matter. In addition, the relationship of As to Cu and/or Zn can distinguish microbial Fe/Mn reduction from Fe/Mn-AOM, because microbial Fe/Mn reduction releases trace elements from both Fe/Mn-(oxyhydr)oxides (i.e., As) and organic matter (i.e., Cu and Zn), whereas Fe/Mn-AOM only releases trace elements from Fe/Mn-(oxyhydr)oxides. Furthermore, an observed covariation between As and either Co or Ni in most pyrite with high δ34S, indicates that this pyrite captured both As released during Fe/Mn-AOM and Co and Ni from SD-AOM. Thus, the high nickel isotope values measured in this study likely dominantly reflect release of isotopically heavy Ni from Fe- and Mn-(oxyhydr)oxides. Our results demonstrate that the trace-element composition of pyrite in gas-hydrate-bearing sediments can record the geochemical signature of the dominant microbial processes

Heterogeneous sulfide reoxidation buffered oxygen release in the Ediacaran Shuram ocean

Ediacaran (~635–539 Ma) carbonate rocks record the largest negative carbonate-carbon isotope excursion in Earth history, termed the Shuram Excursion (SE). This event has been attributed to anaerobic oxidation of dissolved organic carbon as a result of enhanced weathering inputs of sulfate to the ocean during the amalgamation of Gondwana. However, the effect of carbon–sulfur cycle interplay on the net redox state of the ocean–atmosphere system remains unclear, impeding our understanding of the co-evolution of life and the environment during the Ediacaran. Here, we generate high-resolution records of paired sulfate sulfur and oxygen isotopes, in addition to phosphorus concentrations, for the SE interval in South Australia (Parachilna Gorge) and South China (Jiulongwan and Xiang’erwan sections, Three Gorges), and we evaluate these data in the context of COPSE biogeochemical model simulations to assess net long-term redox changes. Our results support widespread H2S reoxidation in shelf areas during the SE, which would have buffered the net release of oxygen sourced from the burial of organic carbon and pyrite. Varying degrees of H2S reoxidation on different cratons likely contributed significantly to high spatial heterogeneity in both local oceanic redox state and nutrient availability, which characterized local oxygen-deficient conditions in an overall oxygenated SE shelf ocean, and likely affected the distribution of the Ediacaran Biota. Our study highlights the important role of H2S reoxidation in the coevolution of marine redox conditions and complex life during the critical Ediacaran period