Euxinic Water Column Research Articles

Anatomy of the late Famennian Dasberg event in a deep shelf of southern Euramerica: Oxygenation and productivity in a restricted basin during a progressive long-term cooling

The late Famennian Dasberg event is described as a series of global hypoxic and transgressive events associated with global faunal turnover. The event, recorded on a deep shelf of the Rhenohercynian basin from southern Euramerica, was investigated in the Holy Cross Mountains of Poland using integrated high-resolution geochemical, mineralogical, and palynological studies. The data revealed the progressive restriction of the intrashelf basin resulting from intense regional block tectonics likely connected with the Late Devonian Variscan tectonic activity. This led to weak chemocline ventilation, the development of anoxic conditions, and the deposition of two organic-rich Dasberg black shale (DBS) horizons. The DBS was deposited in an environment characterized by the constant contribution of detrital components from a common source area. A slight change in terrestrial input may have been driven by modest bathymetric changes associated with the tectonics and stronger winds delivering charcoal and terrestrial components (i.e., miospores and phytoclasts). A supply of nutrients from land and delivery of crucial biolimiting elements (i.e., nitrogen and phosphorus) from deeper waters stimulated primary productivity, as recorded in phytoplankton blooms. The δ13Corganic values in the DBS reflect the incorporation of primary biomass from mainly marine photoautotrophs into sedimentary organic matter. Episodic delivery of toxic sulphides to the photic zone was detected by small-sized framboids and biomarkers, which record the appearance of green sulphur bacteria that photosynthesized in euxinic water column. The activity of phototrophic sulphide-oxidizing bacteria could have led to hyper-enrichment of Zn (715–1002 ppm) in the Lower DBS. The diachronous appearance of the DBS horizons in Euramerica and Gondwana, and regionally marked extinction of benthic fauna, suggest that anoxia developed in restricted Black Sea–like basins formed by intensive tectonic activity and continental plate convergence.

Marine redox and nutrient dynamics linked to the Cambrian radiation of animals

Abstract The early Cambrian witnessed an increase in metazoan ecosystem complexity, likely linked to enhanced oxygen and nutrient availability. However, while improved stratigraphic and geochemical records suggest that the Cambrian explosion occurred under highly dynamic redox conditions, mechanistic links to nutrient availability and early Cambrian evolutionary innovations are poorly constrained. Here, we report paleoredox and nutrient data for two drill cores documenting late Cambrian Stage 2 to Stage 3 (ca. 522–514 Ma) strata from the Yangtze block, South China. The development of water-column euxinia during Cambrian Stage 2 led to extensive recycling of P, fueling elevated primary production and hence an increase in atmospheric and shallow-marine oxygen concentrations. The resulting expansion of oxygenated shelf area promoted sedimentary P retention and, in combination with a diminished supply of P from upwelling, drove a transition to oligotrophic shallow-marine conditions during Cambrian Stage 3. Reduced primary production and limited water-column oxygen consumption allowed for the stabilization of oxygenated continental shelf habitats that supported a burgeoning biotic complexity.

A volatile sulfur sink aids in reconciling the sulfur isotope mass balance of closed basin lakes

Sedimentary rocks from the early Eocene Green River Formation comprise the largest known lacustrine oil shale deposits, contain remarkably well-preserved fossils, and provide a unique record of climate evolution across the Early Eocene Climate Optimum, a period of high atmospheric CO2. The depositional environment of these intermountain lakes spanned from relatively fresh and fluvially influenced to expanded and stratified saline closed basin lakes under the influence of the Laramide orogeny and alternating humid and arid climatic conditions. As the surface area of the lakes expanded, alkalinity and salinity increased, with depositional cycles that linked to evaporative cycles and marked by an increasing abundance of organic matter-rich shales (TOC > 10 wt%). Simultaneously, an intriguing 20 ‰ positive shift in the sulfur isotope composition of sulfide minerals and organic sulfur is observed. Given that this trend cannot be simply explained by a change in the source of sulfate delivered to the basin, the evolution of biogeochemical sulfur cycling and the balance of fluxes in response to basin evolution remain unresolved. Here, we combine the sulfur isotope compositions of pyrite, organic sulfur, and carbonate-associated sulfate and molecular proxies of euxinia in samples from the Uinta basin's depocenter. We find that organic matter-rich sediments reflect deposition in a stratified water column with enhanced burial of pyrite and sulfurized organic matter, while organic-lean facies present evidence of, at least transiently, euxinic conditions reaching the photic zone during arid conditions presumably because of evaporation. As the lake became both saline-stratified and euxinic, we observe that δ34S values of all measured sulfur-bearing sedimentary proxies increase and evolve along a 1:1 line, a trend independent of facies that we interpret as reflecting a sulfate-limited system despite saline conditions. The isotopic mass balance of sulfur fluxes implies the existence of a sink of sulfur depleted in 34S that is spatially decoupled from burial in the depocenter. Modeling sulfur biogeochemical processes in a saline stratified lake system allows us to estimate that at least 50 % of the sulfur entering the lake could have been lost from the upper part of the euxinic water column where the fractionation factor imparted by microbial sulfate reduction is expressed. We propose that the overall isotopic enrichment of the system was caused by H2S degassing during arid climate intervals, presumably enhanced by transient water column mixing events. Further, episodic intrusion of euxinic bottom waters into the upper part of the water column might have triggered mass fish and plankton mortality, consequently facilitating the formation of these exceptionally fossiliferous and organic matter-rich rocks. Our study finds that volatile outgassing may be an underappreciated mechanism for the sulfur mass balance of stratified lacustrine systems.

Characterising the economic Proterozoic Glyde Package of the greater McArthur Basin, northern Australia

The greater McArthur Basin is an informal term for a Palaeo-to-Mesoproterozoic sedimentary system that consists of terranes from the McArthur Basin, Birrindudu Basin, and the Tomkinson Province. These spatially distant basins are interpreted to connect in the subsurface based on geophysical, lithological, and geochronological evidence. The coeval sedimentary units of the greater McArthur Basin were subdivided into non-genetic depositional ‘packages’ bookended by regional unconformities. In ascending order, these packages are the: Redbank, Goyder, Glyde, Favenc, and Wilton Packages. The ca. 1660–1610 Ma Glyde Package is the focus of this study and includes the economically important Barney Creek Formation, found in the McArthur Basin sensu stricto. The Barney Creek Formation hosts the world-class, sediment-hosted, Zn-Pb-Ag McArthur River deposit. Importantly, it is also a key petroleum source rock and unconventional hydrocarbon reservoir, containing Australia’s geologically oldest oil and gas discoveries and forming a part of the McArthur Petroleum Supersystem. Consequently, identifying chronostratigraphically similar units elsewhere in the greater McArthur Basin is important for explorers in finding analogous economic resources.In situ Rb–Sr geochronological results of the Barney Creek Formation shales sourced from borehole LV09001 yielded ages of 1634 ± 59 Ma and 1635 ± 67 Ma. Shale samples from Fraynes Formation in borehole Manbulloo S1 were dated at 1630 ± 57 Ma and 1636 ± 42 Ma using the same approach. These ages are in good agreement with U–Pb ages of tuffaceous layers from the same units, suggesting that they represent their early burial histories and not secondary, post-depositional events. These results indicate that the Fraynes Formation and the Barney Creek Formation are direct chronostratigraphic equivalents, with ages within analytical error of each other.In addition to the geochronological similarities, the δ13Ccarb,87Sr/86Sr, and δ88/86Sr isotopic constraints from both units also display parallel geochemical fingerprints up-section. These include a positive δ13Ccarb excursion of ∼2.0 ‰, a trend towards more crustal-dominated 87Sr/86Sr ratios, and a negative δ88/86Sr excursion of ∼-0.25 ‰. These findings further support the application of isotopic chemostratigraphy as a powerful tool to correlate distal carbonaceous rocks in the basin system. Importantly, these geochemical fingerprints also show that the McArthur Group and the Limbunya Group experienced similar changes in palaeoenvironments during the evolution of the basin system. However, trace element data collated in this study indicates that they may have recorded different, heterogeneous palaeoredox histories. Geochemical models based on redox-sensitive trace elements V and Mo suggest that the Fraynes Formation sustained a much more euxinic water column as opposed to the Barney Creek Formation. These differences may have implications for the accumulation and preservation of base metals and hydrocarbons within the sediment.

A stable and moderate nitrate pool in largely anoxic Mesoproterozoic oceans and implications for eukaryote evolution

The Mesoproterozoic (1.6–1.0 Ga) is a particularly important interval in Earth history that witnessed the initial radiation of eukaryotic life. Ocean anoxia and nutrient limitation has been invoked to explain the delayed expansion of more complex, multicellular eukaryotes during the Mesoproterozoic. However, pulsed oxygenation events and elevated nitrate availability have been identified in shallow marine settings. In deeper waters, however, nitrate availability and potential links to eukaryotic evolution remain poorly constrained. Here, we report an integrated geochemical study based on multiple proxies conducted on two new drill-core sections from the ∼1460 Ma Hongshuizhuang Formation and the ∼1380 Ma Xiamaling Formation, North China. Iron speciation and redox-sensitive trace element systematics suggest dominantly ferruginous to weakly euxinic water column conditions during deposition of both units. Our data are consistent with previous inferences of persistently anoxic deep water conditions in Mesoproterozoic oceans. Exclusively positive δ15N (+3.1 ± 0.8 ‰) values reveal the operation of partial denitrification in the water column, implying a relatively stable and moderate nitrate pool in offshore surface waters. Furthermore, a compilation of the Mesoproterozoic sedimentary δ15N record suggests a global nearshore to offshore oceanic nitrate gradient, with minimal but still bioavailable nitrate in offshore environments. The overall size of the nitrate pool was therefore apparently smaller than that of eukaryote-dominated modern oceans. Coupling Mesoproterozoic sedimentary δ15N data with fossil and biomarker records, we propose that this moderate level of nitrate may have been able to sustain a relatively low abundance of eukaryotic primary producers in the marine ecosystem, but was still not sufficient to trigger eukaryotic diversification and the rise of eukaryotes to ecological dominance.

Changes in palaeoclimate and palaeoenvironment in the Upper Yangtze area (South China) during the Ordovician–Silurian transition

The Ordovician–Silurian transition was a critical period in geological history, during which profound changes in climatic, biotic, and oceanic conditions occurred. To explore the provenance, palaeoclimate, and palaeoredox conditions in the Sichuan Basin during the Late Ordovician–early Silurian interval, we conducted mineralogical, geochemical, and isotopic analyses of three formations (Wufeng, Guanyinqiao and Longmaxi formations) in the Xindi No. 2 well. The ternary and bivariate diagrams indicate that the provenance is mainly felsic igneous rocks and originated mainly from a collisional setting, presumably due to an active continental margin. The chemical index of alteration (CIA) values in the lower Wufeng and Longmaxi formations are relatively high (67.48–73.57), indicating a warm and humid climate. In contrast, the CIA values declined rapidly (58.30–64.66) during the late Katian to early Hirnantian, which had a fluctuating cold and dry climate and was interrupted by a transient warm and humid climate. The palaeoredox indices (Mo concentrations and Moauth/Uauth, U/Th, V/Cr, Ni/Co, and V/V + Ni values) during the Late Ordovician–early Silurian indicate two cycles of water column euxinia. The first cycle occurred during Wufeng Formation deposition, with bottom waters evolving from oxic-suboxic to suboxic-anoxic. Most samples show relatively low redox-sensitive trace element concentrations during the Guanyinqiao Formation, pointing to oxic-suboxic conditions. The second cycle, during the late Hirnantian, transitioned from oxic to euxinic water conditions. Our δ13Corg data are comparable to previously reported records and exhibit a strong correlation between the Hirnantian isotopic carbon excursion (HICE), climate change, and redox conditions. We suggest that the variations in the δ13C values are related to two elements: (1) increased photosynthetic activity under oxic water conditions, and (2) increased carbonate weathering exposed by the glacio-eustatic sea- level. In addition, the high δ13Corg values might indicate a more shelf-proximal setting during Xindi No. 2 well deposition. The δ13Corg isotopic data effectively constrain the timing of the Late Ordovician mass extinction (LOME) and the evolution of the temporal changes in the climatic and ocean redox conditions, suggesting an apparent stratigraphic coincidence between climate and redox fluctuations and two-phase extinctions, which implies a strong causal relationship. The LOME was systematically driven by the combination of cooler glacial temperatures, glacio-eustatic sea-level fluctuations, and anoxic water conditions that caused the two pulses of extinction in the Yangtze shelf sea.

Mo isotope composition of the 0.85 Ga ocean from coupled carbonate and shale archives: Some implications for pre-Cryogenian oxygenation

This study addresses marine palaeoredox conditions of the mid-Neoproterozoic by analysing the Mo isotope, trace element, and U-Th-Pb isotope compositions of shallow water microbial carbonate, deep water pelagic carbonate, and shale from the Stone Knife Formation (SKF) in NW Canada. The U-Th-Pb isotope SKF systematics of reef microbialite carbonates, and the moderately expressed negative Ce anomalies are consistent with the presence of dissolved O2 in the surface waters. Thirteen of 14 analysed samples yield a depositional 206Pb/238U regression age of 0.850 ± 0.028 Ga. The Mo isotope data (δ98Mo) are distinct for the microbial and pelagic carbonates and the deeper water shales, with the isotopically heaviest black shales 0.4 ‰ lighter than the heaviest carbonate. The bulk digestion carbonate δ98Mo data scatter widely, ranging up to 1.64 ‰, and are not reproducible between repeat digestions. The spread in shallow-water carbonate δ98Mo cannot be attributed to a single origin (e.g., admixture of silicate-hosted Mo) and probably reflects a combination of factors, including the complex pathway of Mo into microbial carbonates. Regardless, we propose a minimum δ98Mo of 1.64 ‰ for the 0.85 Ga ocean, similar to other Neo- and Mesoproterozoic estimates from studies of proxies other than black shale. Our new black shale δ98Mo data agree with most previous results from 1.8 to 0.7 Ga shales. If interpreted as reflecting seawater, this would mean a minimum oceanic δ98Mo composition of only 1.29 ‰, implying a limited oxic reservoir compared to the modern Mo budget in agreement with previous studies. This study’s results suggest that the discrepancy could also be explained by a systematic offset between δ98Mo compositions of black shales and the overlying water columns, regardless of depositional environment, akin to the relative depth distribution of δ98Mo in modern euxinic water columns such as the Black Sea. If valid, an implied heavier seawater δ98Mo throughout the Proterozoic would indicate that the magnitude of the Mo oxic sink remained relatively stable throughout the Proterozoic, shifting the apparent expansion of oxygen towards the younger boundary of the interpreted onset of the NOE (ca. 1.0–0.54 Ga).

A potential linkage between excess silicate-bound nitrogen and N2-rich natural gas in sedimentary reservoirs

Molecular nitrogen (N2) released from sedimentary rocks during metamorphism is an important component of the biogeochemical nitrogen cycles. However, the importance and variability of this metamorphic N2 flux from rock nitrogen to Earth's surface environment remains largely unexplored. Here we present a comprehensive bulk rock C-N and N2 concentration dataset from the lower Cambrian shale across the Yangtze Block. The results reveal a spatial trend of excess silicate-bound nitrogen in the lower Cambrian shale throughout the Yangtze Block, which is interpreted as partial assimilation of ammonium (NH4+) with high concentrations of NH4+ accumulating in the euxinic water column and in sediment pore waters at shelf and slope environments during sedimentation. The remarkable spatial coupling between silicate-bound nitrogen in bulk rock shale and N2 concentration in modern shale reservoirs indicates that the high proportion of silicate-bound nitrogen may act as an important control on the formation of N2-rich gas in shale reservoirs during metamorphism. These N2-rich reservoir rocks may have affected the surface environment through tectonic movement over Earth's history. Our results therefore identify a novel linkage in the nitrogen cycle and provide evidence for the importance of metamorphism on the return of rock nitrogen back to the surface environment. We further reveal that the metamorphic N2 gas flux from the geosphere to the atmosphere is dependent on environmental conditions during sediment deposition.

Genesis and Distribution of Pyrite in the Lacustrine Shale: Evidence from the Es3x Shale of the Eocene Shahejie Formation, Zhanhua Sag, East China.

Pyrite is a common mineral in sedimentary rocks and is widely distributed in a variety of different morphologies and sizes. Pyrite is also widely distributed in the Es3x shale of the Eocene Shahejie Formation in the Zhanhua Sag, Bohai Bay Basin. A combination of geochemical and petrographic studies has been applied to address the formation and distribution of pyrite in the Es3x shale. The methods include thin section analysis to identify the representative samples of the shale-containing pyrite, total organic carbon (TOC) content analysis, X-ray fluorescence, X-ray diffraction, electron probe micro-analysis, and field emission scanning electron microscopy (FE-SEM) coupled with the energy dispersive spectrometer, to observe the characteristics, morphology, and distribution of pyrite in the lacustrine shale. The content of pyrite in the Es3x shale ranges from 1.4 to 11.2% with an average content of 3.42%. The average contents of TOC and total organic sulfur (TS) are 3.48 and 2.53 wt %, respectively. Various types of pyrites are observed during the detailed FE-SEM investigations including pyrite framboids, euhedral pyrite, welded pyrite, pyrite microcrystals, and framework pyrite. Pyrite framboids are densely packed sphere-shaped masses of submicron-scale pyrite crystals with subordinate large-sized euhedral crystals of pyrite. Welded pyrite forms due to the overgrowth and alteration of pyrite crystals within the larger pyrite framboids. Pyrite microcrystals are the euhedral-shaped microcrystals of pyrite. The framework pyrite is also observed and is formed due to the pyritization of plant/algal tissues. Based on the growth mechanism, the pyrites can be divided into syngenetic pyrites, early diagenetic pyrites, and late diagenetic pyrites. The presence of pyrite, especially the abundance of pyrite framboids, suggests that the environment during the Es3x shale deposition in the lacustrine basin was anoxic. Their dominant smaller size suggests the presence of an euxinic water column, which is consistent with the lack of in-place biota and high TOC contents. This research work not only helps to understand the pyrite mineralization, role of organic matter, and reactive iron in pyrite formation in the shale but also helps to interpret the paleoredox conditions during the deposition of shale. This research work can also be helpful to other researchers who can apply these methods and conclusions to studying shale in other similar basins worldwide.

Using whole rock and in situ pyrite chemistry to evaluate authigenic and hydrothermal controls on trace element variability in a Zn mineralized Proterozoic subbasin

The mid-Proterozoic stratigraphy of the McArthur Basin (Australia) contains some of the most well-preserved sedimentary rocks of Precambrian age, which are also host to giant, clastic dominant (CD-type) massive sulfide Zn deposits. The most recently discovered CD-type deposit (the Teena deposit) is located in the Teena subbasin and hosted by the 1.64 Ga Barney Creek Formation. The Teena subbasin, therefore, provides the perfect natural laboratory for evaluating authigenic and hydrothermal controls on trace element (TE) variability, both of which contribute to paleoenvironmental reconstructions and ore deposit models. As the Teena deposit formed beneath the paleoseafloor, this also provides the opportunity to evaluate TE zonation around a fossilized subseafloor replacement hydrothermal system. In situ laser ablation inductively coupled mass spectrometry (LA-ICP-MS) has been used to define compositional end members in diagenetic and hydrothermal pyrite. The overgrowth of hydrothermal sulfides on diagenetic pyrite is associated with TE anomalism (Tl, Pb, As, Zn) that extends > 100 meters above the main high grade sulfide mineralization the Teena subbasin. The vertical zonation in TEs is consistent with the infiltration of hydrothermal fluids into overlying hangingwall sediments that were undergoing diagenesis. Bulk rock lithogeochemical data record covariation between total organic carbon (TOC) and a suite of TEs (Mo, Co, Ni, V). We suggest this was caused by local hydrographic factors during deposition of the Barney Creek Formation. High TOC/P molar ratios, resulting from regeneration of P in a euxinic water column, are associated with an interval overlying the main maximum flooding surface in the subbasin. The relationships between TOC, P and TEs resemble the redox architecture of a silled basin rather than an open marine margin. Sulfidic conditions developed during periods of high productivity, which were linked to nutrient supply that was enhanced by connectivity with surrounding water masses. The evidence of redox bistability, involving a delicate balance between ferruginous (anoxic, non-sulfidic) and euxinic (sulfidic) conditions, is consistent with recent models for other mid-Proterozoic sedimentary units. Nevertheless, there was a strong localised (101 km2) control on the authigenic and hydrothermal TE chemistry of the Barney Creek Formation in the Teena subbasin, which highlights a key challenge when extrapolating from data collected in partially restricted intracontinental marine settings.

Spatiotemporal Evolution and Genesis of the Late Ordovician—Early Silurian Marine Euxinia in Northeastern Upper Yangtze Basin, South China

Marine redox conditions and their dynamic changes were a major factor that controlled the formation of black shale and caused the late Ordovician marine extinction in the Upper Yangtze Basin (South China). However, the spatiotemporal variation and potential controlling factors of marine redox conditions in this area remain unclear. We analyzed whole-rock geochemistry and pyrite sulfur isotopes (δ34Spy) of 47 shale samples from the Late Katian to Rhuddanian in a shelf-to-slope (Qianjiang Shaba section and Wc-1 well) region of northeastern Upper Yangtze Basin, and reconstructed water column redox conditions during the Late Ordovician–Early Silurian Transition. The geochemical characteristics of shale, including the ratio of elements, discriminant function and ternary diagram location in the study area suggest a passive continental margin sedimentary environment, wherein the terrigenous detritus is mainly derived from felsic igneous rocks in the upper crust, showing characteristics of near-source deposition. The redox indices (Fe speciation, Corg/P, UEF, and MoEF) showed that the development of anoxic water, especially euxinia, has obvious spatiotemporal heterogeneity. Under conditions of high availability of active organic carbon and limited sulfate supply, high active Fe input and strong biological irrigation in the shallow water area may effectively remove H2S produced by microbial sulfate reduction, conducive to the prevalence of ferruginous water columns. However, for this deep water area, the rapid accumulation rate of organic matter, decrease in dissolved Fe (caused by upwelling in the open sea), and seawater stratification (caused by the rising of sea level) promoted the development of a euxinic water column. This inference is supported by the covariant relationship between organic carbon accumulation rate, chemical index of alteration, Co × Mn, and δ34Spy. Our study highlights the potential control effects of sea level change, continental weathering and upwelling on the development of euxinic water columns.

Progressive development of ocean anoxia in the end-Permian pelagic Panthalassa

The end-Permian mass extinction (EPME) has been linked with the widespread development of oxygen-poor oceanic conditions. However, information on the spatial extent of anoxia in the Panthalassa super-ocean has been limited. This study reports oceanic redox records from a deep-sea chert succession (the Waiheke 1 section, WHK 1, New Zealand) that was located in southern mid-latitudes of Panthalassa. High-resolution carbon isotope (δ13C) correlation between Waiheke and the Permian-Triassic boundary (PTB) type section indicates that the EPME is recorded in a thin black claystone interbedded between siliceous mudstone beds at WHK 1. Pyrite-dominated enrichment in highly reactive iron, coupled with elevated U/Al and Mo/Al ratios, are prevalent through this black claystone bed and the overlying Permo-Triassic transition strata, suggesting the development of euxinic water column conditions. Similar redox variations across the EPME horizon have been reported from other Panthalassic deep-sea PTB sections. Comparison with these PTB sections indicates that euxinic conditions were widespread in low-latitude regions of the Panthalassan ocean, and such conditions developed earlier than in mid-latitude settings, up to 100,000 years before the EPME. This suggests there was a gradual expansion of ocean anoxia from low to middle-high latitude regions during the Permo-Triassic transition. The extent of ocean anoxia resulted in a decrease in the seawater inventory of redox sensitive trace metals (e.g., Mo), which is evident in the earliest Triassic strata of the studied section and other PTB sections. Panthalassic anoxia during the EPME coincides with extreme climate warming and the associated effects (e.g., changes in ocean circulation, marine eutrophication intensified by terrestrial weathering) were likely critical triggers for ocean deoxygenation.

Iron and manganese shuttle has no effect on sedimentary thallium and vanadium isotope signatures in Black Sea sediments

Thallium and V isotopes in organic-rich sediments have recently been explored for reconstructing marine oxygenation by tracking the burial of Mn oxides either locally or globally. The ‘Fe and Mn shuttle’ is a well known mechanism that can transport elements associated with Fe and Mn oxide phases formed under mildly oxidizing shallow water environments both laterally and vertically to euxinic (sulfidic water column) deep waters. The Fe and Mn shuttle has been demonstrated to strongly affect Fe and Mo isotope compositions of sediments accumulating beneath anoxic and euxinic settings. Similar to Mo, Tl and V are also enriched in oceanic ferromanganese crust and nodules, which impart the largest isotopic offsets from modern seawater for Tl and V. Thus, the mechanism of an Fe and Mn shuttle has the potential to, but currently unconstrained, affect Tl and V isotopic compositions (ε205Tl and δ51V) in organic-rich sediments in the modern and ancient records.We have used Black Sea sediments that are known to preserve enrichment and sedimentary isotope signatures for an Fe shuttle. This is due to an Fe transport from the more oxic margin to the deep euxinic basin and corresponding enrichment in the underlying sediments. These samples were used to test for a potential Fe and Mn shuttle effect on ε205Tl and δ51V in sediments deposited under oxic, suboxic, and euxinic water columns. Authigenic ε205Tl from all three depositional settings is indistinguishable (–2.7 ± 0.3, –2.4 ± 0.5, and –2.4 ± 0.3), which is within error of the Black Sea surface seawater (–2.2 ± 0.3). In contrast, oxic and suboxic sediment have similar δ51V values (–1.06 ± 0.33‰ and –1.04 ± 0.30‰), which are significantly more negative than euxinic sediments (–0.57 ± 0.06‰).All the sediments deposited under oxic, suboxic, and euxinic water columns in the Black Sea capture ε205Tl of the surface seawater because there is almost no permanent burial of Mn oxides in this basin. This finding is consistent with the indistinguishable ε205Tl between the surface seawater and the riverine input. Sedimentary δ51V appears to be controlled predominantly by the redox state of the depositional environments with little effect from an Fe or Mn shuttle. The lack of Fe and Mn shuttle effect on ε205Tl and δ51V in the Black Sea is likely due to the shallow (∼100 m) chemocline in the strongly, 2000-m deep redox-stratified water column of this basin and very slow deep-water renewal rate. Iron and Mn in oxides are not delivered to the sediments, instead, they are dissolved and captured in the water column as iron sulfide and reduced Mn mineral phases. Our results suggest that Fe and Mn shuttle transport mechanisms may not strongly affect Tl and V isotopes, at least in extreme cases of water column redox stratification. Further work is required to constrain the isotopic signals using other modern and, by inference, diverse ancient settings for additional shuttling mechanisms.

Redox dynamics of later Cambrian oceans

A growing body of evidence suggests that the deep oceans during the early Paleozoic Era were widely oxygen deficient, despite evidence for increased marine oxygenation during the Neoproterozoic. However, the temporal and geographic extents and dynamics of reducing marine conditions within these oceans are not well understood. Here, we investigate marine redox history during the Drumian through the earliest Jiangshanian International Stages of the Cambrian Period, using concentrations of redox-sensitive metals (vanadium, uranium, and molybdenum), iron speciation, and Mo isotope stratigraphy of the Alum Shale Formation of Scandinavia. These data suggest a major perturbation occurred in trace metal cycling during the later Cambrian Period that was linked to a transient change in marine redox conditions coincident with the well-known Steptoean Positive Isotope Excursion or SPICE. The δ98Mo measurements of the Alum shale show systematic variations during the interval that contains the SPICE which are broadly consistent with a transient expansion of sulfidic, reducing marine environments — indicating a significant exacerbation of an already-common condition during the Cambrian Period. Additionally, iron speciation data record a local transition from predominantly anoxic, ferruginous (Fe+2 containing) to anoxic, euxinic (sulfide containing) water column conditions near the initiation of the SPICE. Trace metal abundances, however, appear to decline well before the start of the SPICE, suggesting an earlier initiation of the global expansion of reducing environments. More broadly, our data and modeling support the notion that significant portions of the oceans remained oxygen deficient throughout the later portion of the Cambrian, and that these oceans were also prone to transient intervals of more reducing conditions similar to the Oceanic Anoxic Events of the Mesozoic.

The Sedimentary Geochemistry and Paleoenvironments Project.

The Sedimentary Geochemistry and Paleoenvironments Project.

A quantification of the effect of diagenesis on the paleoredox record in mid-Proterozoic sedimentary rocks

Abstract Iron speciation in ancient sedimentary rocks is widely used to reconstruct oceanic redox conditions over geological time, specifically to assess the extent of oxic, euxinic (anoxic containing sulfide), and ferruginous (anoxic containing iron) conditions. We explore how post-depositional sedimentary processes can skew particular geochemical signals in the rock record. One such process is when aqueous sulfide—including that produced in the sediment column—reacts with sedimentary iron, converting non-sulfide, highly reactive iron minerals to iron-sulfide minerals; this can lead to increased preservation of iron as pyrite and an overestimation of seafloor euxinia. We show that sedimentary rocks with higher (>5 wt%) total iron content are more buffered to this effect and thus are a more reliable indicator of true water-column euxinia. When considering this effect in the geological past, we estimate that true euxinia in the mid-Proterozoic may have been as much as fourfold less than previously thought—more in line with other recent paleoredox proxies not based on iron minerals. Marine iron and sulfate concentrations were more equivalent in Proterozoic–Neoproterozoic oceans, suggesting this time period was particularly susceptible to this post-depositional alteration, explaining the extent of euxinia suggested for this geological interval.

Anatomy of the Major Baltic Inflow in 2014: Impact of manganese and iron shuttling on phosphorus and trace metals in the Gotland Basin, Baltic Sea

The deep basins of the Baltic Sea experienced dramatic climate- and eutrophication-induced redox changes between oxic and euxinic water column conditions. Irregularly appearing Major Baltic Inflows (MBI) carrying North Sea waters to the Baltic Sea are partly able to oxygenate euxinic bottom waters within several weeks. After an absence of ~10 years, an exceptional strong inflow event in December 2014 reached the Gotland Basin in early March 2015 providing the opportunity of detailed studies on the impact of fast redox shifts on elemental cycles. Here we report on P and trace metal (Co, Mo, U, V and W) dynamics in the water column, pore water, and sediment of the central Gotland Basin over a 4.5 year-long observation period. These time series record the entire redox development including the primary MBI-related oxygenation, the transitional development to anoxia before a second inflow pulse, and the final re-establishment of euxinia. Samples obtained in August 2012 represent the previous euxinic stagnation period not affected by MBI's since nearly a decade. Concentration measurements and a budget calculation suggest only a limited impact of the MBI on the water column P inventory and the corresponding eutrophication level of the Gotland Basin. This is due to insufficient Fe availability preventing efficient trapping of phosphate during water column oxygenation. In addition, the fast recovery of reducing conditions allowed only a short-term storage of deposited Mn–Fe–P mineral phases in the surface sediment. Due to pronounced scavenging by Mn oxides, substantial water column depletion occurred intermittently for V and W, but again, re-establishing reducing conditions favored Mn oxide dissolution and the release especially of W back into the water column. Mo and U were less affected by Mn and Fe shuttling and Mo even showed an increasing water column inventory due to replenishment by intruding North Sea waters. In contrast, the water column inventory of Co remained on an approx. 50% level about 4.5 years after the inflow suggesting a significant sedimentary burial of Co within the course of the inflow event. Although sedimentary Mn carbonate layers, which were commonly formed in concert with the frequent inflows between ~1960 and 1980, were missing or only weakly developed after the single inflow events in 1994, 2003, and 2014, dated short cores from the study area reveal clear Co and less pronounced V enrichments during the latter inflows. Because Mo, U and W are synchronously depleted compared with their normal sapropel levels, the combination of these trace metals and Co may help to identify past MBIs not reflected by Mn carbonate presence.

The Mineralogical Evolution of the Clastic Dominant-Type Zn-Pb ± Ba Deposits at Macmillan Pass (Yukon, Canada)—Tracing Subseafloor Barite Replacement in the Layered Mineralization

Abstract Clastic dominant-type massive sulfide deposits are well preserved in Upper Devonian carbonaceous mudstones in the Macmillan Pass district (Yukon, Canada). The Macmillan Pass deposits have been considered to be type examples of sedimentary exhalative Zn-Pb mineralization, whereby sulfides precipitated when hydrothermal fluids were vented into a euxinic (H2S-bearing) water column. We propose a new mineralization model, documenting the mineralogical evolution of layered mineralization. We show that textures previously interpreted to form via depositional processes actually formed by subseafloor replacement of diagenetic barite. Mineral assemblages associated with barite dissolution in the layered mineralization include (1) barium carbonate phases (witherite, barytocalcite, and norsethite), which are intergrown with Zn-Pb sulfides and represent localized Ba mass transfer, and (2) barium feldspar (celsian, hyalophane) that is abundant in the mudstones surrounding the layered mineralization. The barium feldspar formed following transport of Ba in low-sulfate fluids on the margins of the subseafloor replacement system. This resulted in whole-rock Ba enrichments (up to 5 wt %) in mudstones 15 m below and above the layered mineralization. High Ba in these surrounding mudstones is coupled with decreasing K/Al ratios, indicative of secondary illite and kaolinite. The source(s) of fluids related to the diagenetic (barite, barytocalcite) and hydrothermal (ankerite) assemblages can be constrained using Sr isotopes. Whereas highly radiogenic 87Sr/86Sr values (>0.714) in ankerite correspond with host-rock alteration within the vent complex, the overlying barite and barytocalcite preserve lower 87Sr/86Sr values (<0.714), providing evidence of mixing between a radiogenic fluid (likely a formation water) and Late Devonian seawater. The complex mineralogy and paragenesis contained within the layered mineralization are linked to a protracted history of diagenetic and hydrothermal fluid events, all of which took place in and peripheral to a subseafloor replacement hydrothermal system.

Development of Iron Speciation Reference Materials for Palaeoredox Analysis

The development and application of geochemical techniques to identify redox conditions in modern and ancient aquatic environments has intensified over recent years. Iron (Fe) speciation has emerged as one of the most widely used procedures to distinguish different redox regimes in both the water column and sediments, and is the main technique used to identify oxic, ferruginous (anoxic, Fe(II) containing) and euxinic (anoxic, sulfidic) water column conditions. However, an international sediment reference material has never been developed. This has led to concern over the consistency of results published by the many laboratories that now utilise the technique. Here, we report an interlaboratory comparison of four Fe speciation reference materials for palaeoredox analysis, which span a range of compositions and reflect deposition under different redox conditions. We provide an update of extraction techniques used in Fe speciation and assess the effects of both test portion mass, and the use of different analytical procedures, on the quantification of different Fe fractions in sedimentary rocks. While atomic absorption spectroscopy and inductively coupled plasma‐optical emission spectrometry produced comparable Fe measurements for all extraction stages, the use of ferrozine consistently underestimated Fe in the extraction step targeting mixed ferrous–ferric minerals such as magnetite. We therefore suggest that the use of ferrozine is discontinued for this Fe pool. Finally, we report the combined data of four independent Fe speciation laboratories to characterise the Fe speciation composition of the reference materials. These reference materials are available to the community to provide an essential validation of in‐house Fe speciation measurements.

Tracing water column euxinia in Eastern Mediterranean Sapropels S5 and S7

Sapropels S5 and S7 formed in the semi-enclosed Eastern Mediterranean Sea (EMS) during peak interglacial periods MIS5e and MIS7a, respectively, are considered among the most strongly developed Quaternary sapropels. This study investigates the redox dynamics of the water column during their formation, via Fe isotope and Fe speciation studies of cores taken at 2550 m depth at site ODP-967. Both sapropels show an inverse correlation between δ56Fe and FeT/Al, with slopes mostly matching that found for the Black Sea, pointing to a benthic shelf to basin shuttle of Fe and subsequent precipitation of Fe sulphides in euxinic bottom waters. An exception to these Black Sea-type trends occurs during the later, peak, stages of S7, where the negative δ56Fe - FeT/Al slope shallows. Fe speciation studies reveal that the dominant highly reactive Fe phase (FeHR) in the sapropels is pyrite, with Fe (oxyhydr)oxides forming the second major mineral component. Correspondingly, FeHR/FeT plots show increased strengthening of anoxic water conditions during the passage from pre-sapropel sediment into the sapropel. Nevertheless, despite the evidence for euxinic conditions from both Fe isotopes and high Mo concentrations in the sapropel, Fepy/FeHR ratios remain below values commonly used to identify water column euxinia. This apparent contradiction is ascribed to the sedimentary preservation of a high flux of crystalline Fe (oxyhydr)oxide minerals to the basin, which resulted in a relatively low degree of sulphidation, despite the presence of euxinic bottom waters. Thus, the operationally defined ferruginous/euxinic boundary for EMS sapropels is better placed at Fepy/FeHR = 0.6, which is somewhat below the usually ascribed lower limit of 0.7. Consistent with the significant presence of crystalline Fe (oxyhydr)oxides, the change in the δ56Fe - FeT/Al slope during peak S7 is ascribed to an enhanced monsoon-driven flux of detrital Fe(III) oxides from the River Nile into the EMS basin and comcomitant diagenetic sulphidation. Euxinic water column conditions in sapropel S5 and S7 are interpreted here to reflect the positive balance between dissolved sulphide formation and rates of reductive dissolution of Fe (oxyhydr)oxide minerals. Both of these parameters in turn depend on the extent to which water overturn times are reduced and export productivity increased during sapropel formation.