Suboxic Conditions Research Articles

High primary productivity in an Ediacaran shallow marine basin influenced by strong seasonal to perennial upwelling

AbstractA significant area of late Neoproterozoic–early Cambrian seafloor hosted a ferruginous to euxinic condition as a result of expanded primary productivity-driven pumping of organic matter into subsurface water column and weak water column mixing in the concomitant sea. However, the cause and extent of increased marine primary productivity during this time interval remain unknown. To estimate the primary productivity in a late Neoproterozoic sea, this study investigated the Sirbu Shale, Vindhyan Supergroup, for trace elements, organic carbon isotopes and total organic carbon (TOC). Among the trace elements, cadmium (Cd), known for extremely low concentration in crustal rocks but higher abundance in biogenic organic matter, was the key parameter in the palaeoproductivity estimation. The Cd enrichment in the Sirbu Shale samples is comparable to that in modern marine sediments of the oxygen minimum zones in Chilean margins, Arabian Sea and Gulf of California characterized by high primary productivity and seasonal upwelling. In terms of Cd enrichment, the lower section of the Sirbu Shale was deposited under suboxic conditions, while the upper section was deposited under a relatively less reducing condition. Cd/Mo ratios > 0.36 in the shale sample indicate that the palaeoproductivity was strongly influenced by the nutrient supply through sea-shelf upwelling. Using non-detrital enrichment of Cd in Sirbu Shale samples, we calculated that the TOC exported to the floor of Sirbu Shale palaeodepositional setting through primary productivity ranged from 0.71 to 10.16%.

Early diagenetic REE migration from Fe-Mn nodules to fish teeth in deep sea sediments

In deep sea sediments, rare earth elements and yttrium (REYs) are predominantly carried by bioapatite and Fe-Mn (oxyhydr)oxides. According to recent studies, REYs can migrate into bioapatite after the dissolution of Fe-Mn (oxyhydr)oxides. However, the REY migration mechanism from Fe-Mn (oxyhydr)oxides to bioapatite are still unclear. Here, we integrated in situ trace element and Nd isotope analyses to elucidate the geochemical characteristics of fish tooth-bearing Fe-Mn nodules from the Northwest Pacific Ocean. Our Fe-Mn nodule samples can be classified into different morphological and geochemical types of the different periods in early diagenesis: (I) completely overgrown around bioapatite; II) overgrown around half of the bioapatite. Type I nodules grow around a bioapatite core, which has high LaN/SmN (0.41–0.50, avg. 0.46) and LaN/YbN (0.49–0.90, avg. 0.62). The ΣREY content is non-uniform in the two bioapatite types (WP-01 & WP-02) in Type I nodules, i.e., 475–2150 ppm and 272–431 ppm, respectively. The REY results show that the bioapatite is mainly adsorbed REY in early diagenesis. Type II nodules were covered both inside and at the tip of the tooth. The LaN/SmN (0.34–0.38, avg. 0.36) and LaN/YbN (0.26–0.61, avg. 0.37) ratios of the bioapatite samples are relatively low. Meanwhile, the ΣREY content is very uneven (229–3521 ppm). The REY data show that the bioapatite is mainly substituted during strong early diagenesis. The intercrystalline pores in some authigenic apatite may have closed upon recrystallization, causing REY redistribution and fractionation. The Y/Ho vs. ΣREE/Th plot shows a higher REY proportion from pore water in type II nodules than in type I ones. The widespread distribution of high Mn content and the gradual decrease of REY content from the nodule rim to core reflect the transport of REY from dissolved Fe-Mn (oxyhydr)oxides to bioapatite during early diagenesis under suboxic conditions. From type I to type II nodules, the mean εNd value of bioapatite increases from −6.69 to −5.56. The comparison of εNd values between REY-rich mud and bioapatite indicate that the REYs were derived from the bottom water and pore water in deep sea sediments. Our findings comprehensively document the transport and enrichment mechanisms of REY in early diagenetic processes from Fe-Mn (oxyhydr)oxides to bioapatite, which we argued to be relatively localized.

Impact of orthophosphate on the light-independent production of reactive oxygen species during microbially-mediated Fe redox cycles

Although light-independent production of reactive oxygen species (ROS) in subsurface environments has been increasingly documented, the processes controlling ROS transformations at redox interfaces remain poorly understood. The Fe(III)-reducing facultative anaerobe Shewanella oneidensis promotes light-independent ROS production in the presence of Fe(III) when alternately exposed to oxic and anoxic conditions and represents a good model of microbial processes at redox interfaces. In this study, S. oneidensis was incubated under three different orthophosphate concentrations from low natural conditions to high growth-promoting conditions to identify the processes regulating the light-independent production of ROS during redox oscillations and determine the impact of this important nutrient on ROS generation. Ferryl complexes were not significant intermediates in the redox cycling of Fe in this system. Instead, hydrogen peroxide (H2O2) and hydroxyl radicals (•OH) represented the main ROS produced in a process that is periodic during redox oscillations. Hydroxyl radicals were mainly produced during the aerated phases in concentrations inversely proportional to orthophosphate concentrations as a result of the complexation of Fe(III) and precipitation of oxidation-resistant vivianite. In relatively low orthophosphate concentration compared to the total reactive sites available on the Fe(III) oxides, •OH was produced at the mineral surface. Increased cell metabolic activities in the highest orthophosphate medium may have increased the •OH production efficiency during the aerated phases by enhancing O2•− formation and/or Fe(III) reduction. Unexpectedly, H2O2 was mainly detected under Fe(III) reduction conditions with net production rates that were proportional to orthophosphate concentrations, except when the solubility of vivianite was exceeded. Hydrogen peroxide is proposed to originate from the reduction of dissolved O2 by Fe(II) produced during dissimilatory Fe(III) reduction in microaerobic conditions. Precipitation of vivianite likely affects the availability of Fe(II) for ROS reactions during redox cycles, thus decreasing overall H2O2 production rates. The rise in metabolic activity, outcompetition of Fe(III) hydrolysis by orthophosphate complexation, and potential dimerization of surface generated •OH are proposed to result in the accumulation of H2O2 under suboxic conditions.

Organic Geochemistry of Crude Oils from the Kohat Basin, Pakistan

The crude oils from the reservoirs of Mela-01 and Mela-04 wells located in the Kohat Basin, Pakistan, were geochemically analyzed to evaluate the origin, depositional conditions, and thermal maturity of the rock units and possible facies from which these oils were sourced. Gas chromatography-mass spectrometry (GC-MS) was performed on the samples to obtain biomarker and non-biomarker parameters. Analyzed non-biomarker parameters, including carbon preference index (CPI), terrigenous to aquatic ratio (TAR), isoprenoids pristane to phytane (Pr/Ph), and biomarker parameters, including steranes and dibenzothiophene/phenanthrene (DBT/P) of aromatic compounds, were utilized in the present study to achieve the objectives. Most of these parameters suggest a mixed source of organic matter (marine/terrestrial) with sub-oxic conditions in the source rocks for the analyzed oil samples in the studied wells from Mela oilfield, Kohat Basin. Furthermore, the CPI and different biomarker parameters such as steranes C29 S/S+R, ββ/αα+ββ), moretane to hopane (M29/C30H), pentacyclic terpanes C27 (Ts/Ts+Tm), H32 (S/S+R) hopanes, and aromatic methylphenanthrene index (MPI) indicate that the analyzed oils have originated from thermally mature rocks falling in the oil window. As the studied Kohat Basin has multiple source rocks and contributes to the major petroleum production of the country, the present investigations reveal that its okthe Mela oils were generated by the strata of mixed organic matter that were deposited in marine sub-oxic conditions. Furthermore, this study suggests that this stratum would also have been produced in unexplored surrounding areas such as Tirah, Orakzai, and the Bannu Depression.

The formation environment of Carboniferous karstic bauxite deposits in the central North China Craton: Insights from nitrogen isotopes and geochemical proxies

Over 5 billion tons of karstic bauxites are hosted in the Late Carboniferous Benxi Formation in the North China Craton (NCC). However, the bauxite formation environment in the central NCC remains unknown. The geological, mineralogical, geochemical, nitrogen concentrations, and δ15N isotope evidence of the Huilong and Shangwutou bauxite deposits in the central NCC was conducted, aiming to clarify the formation environments. Typically, bauxites in the central NCC contains three layers from bottom to top: Fe-bearing claystone/clayey ironstone, bauxite ore, and bauxitic claystone. The Fe-bearing claystone/clayey ironstone is composed of kaolinite, goethite, and hematite; the bauxite ore is composed of diaspore, kaolinite, but also minor hematite, ilmenite, and svanbergite; and the bauxitic claystone consists of kaolinite, but also some diaspore and goethite. Diaspores coexisted extensively with hematite, anatase, and svanbergite, forming cryptocrystalline matrix of bauxites, suggesting that they may be formed during early diagenesis. Geochemical discriminant diagrams also indicated that two deposits were primarily affected by the hydrogenous deposition and early diagenesis, and no sedimentary recirculation. The ∑REEs concentrations and fractionation (Prn/Ybn) features along the profiles mainly concentrated in the bottom bauxite ore, which reflected that the pH values could be alkaline in bottom bauxite ore, and decreased weakly acidic for the central bauxite ore to bauxitic claystone along the profiles upwards. Combining the δ15N data with two geochemical redox proxies, and the widely coexistence of diaspore and hematite, indicated that the two deposits mainly formed under suboxic conditions, and thus denitrification was the predominant process in nitrogen cycling. The δ15N values are positively correlated with paleosalinity proxies, indicating that the higher saline bottom water may lead to a strong vertical density differential and water-column stratification, conducing to forming suboxic restricted pore-water environments.

Groundwater red beds in Holocene fluvial sediments as a product of iron and manganese redox cycling; Morava River, Czechia

ABSTRACTModern fluvial deposits can be affected by early diagenetic element mobilization at redox boundaries between oxygenated and oxygen‐free zones near the groundwater level. The visible product of redox transformation of Fe is the sediment colour. Distinct red and black sediment layers have been studied in eight shallow (<4.2 m depth) sections in the floodplain of Morava River, Czechia. The aim was to investigate the composition, origin, rates of formation and stratigraphic significance of the red strata using bulk‐rock analytical methods, radiocarbon dating, optical and scanning electron microscopy coupled with in situ (energy‐dispersive X‐ray scanning electron microscopy and laser ablation inductively coupled plasma mass spectrometry) geochemistry. The coloured layers, in places well‐cemented, developed in permeable sands and gravels above the water table, close to the boundary with overlying less permeable floodplain sandy silts. Their colour is due to Fe and Mn oxyhydroxides (goethite, haematite, todorokite and birnessite) coatings of framework grains. Black, Mn‐rich layers occur stratigraphically higher than the red, Fe‐rich ones. The coatings are a few‐hundred microns thick at maximum, often botryoidal, composed of alternating Fe‐rich and Mn‐rich laminae enriched in As, Mo, Sb, P, Cu and U. The coatings formed under suboxic conditions due to Fe and Mn oxyhydroxide cycling, driven partly by microbial activity, at the groundwater‐related redox boundary which resulted in the distinct vertical arrangement of the black and red layers. Being markedly enriched in As bound to Fe‐oxyhydroxides and exposed to fluctuating redox conditions, the coloured strata can potentially release As to groundwater and represent serious environmental hazards. Radiocarbon ages suggest that the groundwater‐type red beds can occur extremely quickly, over a few hundred years. It is assumed that, under suitable circumstances, this reddening can be preserved in the rock record, and represent a specific model for the development of continental red beds.

P inputs determine denitrifier abundance explaining dissolved nitrous oxide in reservoirs

AbstractReservoirs are important sites for nitrogen processing, especially those located in agricultural and urban watersheds. Nitrogen inputs promote N2O production and emission, but the microbial pathways controlling N2O have been seldom studied in reservoir water columns. We determined N2O concentration in the water column of 12 reservoirs during the summer stratification and winter mixing. We explored the potential microbial sources and sinks of N2O by quantifying key genes involved in ammonia oxidation (bacterial and archaeal amoA) and denitrification (nirS and nosZ). Dissolved N2O varied up to three orders of magnitude (4.7–2441.2 nmol L−1) across systems, from undersaturated to supersaturated values (37%–24,174%) depending on reservoirs and depths. N2O concentration depended on nitrogen and oxygen availabilities, with the lowest and highest N2O values at suboxic conditions. Ammonia‐oxidizing archaea dominated over ammonia‐oxidizing bacteria but were not related to the dissolved N2O. In contrast, the abundance of the nirS gene was significantly related to N2O concentration, and three orders of magnitude higher than amoA abundance. Denitrifying bacteria appeared consistently in the water column of all reservoirs. The nirS and nosZ genes appeared in oxic and suboxic waters, but they were more abundant in suboxic waters. The nitrate concentration, and nirS and nosZ relative abundances explained the dissolved N2O. Besides, nirS abundance was related positively with total phosphorus and cumulative chlorophyll a, a proxy for fresh organic matter. Therefore, P inputs, not just N inputs, promoted N2O production by denitrification in the water column of reservoirs.

Molecular and isotopic signatures of indigenous and non-indigenous hydrocarbons in Campanian sediments from the Tano Basin, Ghana

The Campanian source strata of the Tano Basin, Ghana, have garnered exploration interest based on the recent discoveries of hydrocarbon shows in some offshore wells. This study reports on the bulk geochemical composition (Total organic carbon content and Rock-Eval pyrolysis parameters), molecular markers, and stable carbon isotope (δ13C) composition of eleven Campanian sediments from TP–1 and Ankobra–1 well, a shallow and deep-water well in the Tano Basin respectively. Based on the analysis of total organic carbon content and Rock-Eval pyrolysis parameters (S1, S2, and Tmax), it was found that the TP-1 Campanian sediments contain indigenous hydrocarbons. Contrariwise, the Campanian samples from the Ankobra-1 well are mainly composed of non-indigenous hydrocarbons, possibly originating from offshore drilling fluid contaminants. Distribution of saturated and aromatic hydrocarbons, including normal alkanes, acyclic isoprenoids, steranes, naphthalenes, phenanthrenes, dibenzothiophene, dibenzofuran, fluorenes, and their alkyl derivatives, as well as stable carbon isotope (δ13C) values, were analyzed to gain insight into the biological source input, depositional environment, and thermal maturity of the organic matter (OM) in the sediments. The molecular and isotope results indicated that the OM in the indigenous hydrocarbons is from mixed organic sources, with a predominance of terrestrial plants, whereas lower marine organisms dominate the non-indigenous hydrocarbons. Moreover, the molecular ratios obtained from our analysis indicated that the OM present in the indigenous hydrocarbons was deposited under suboxic conditions in a marginal marine environment. Contrarily, the non-indigenous hydrocarbons exhibit a strong association with marine carbonate facies. Additionally, the maturity assessment based on biomarker ratios, indicated that the OM within the Campanian sediments from TP-1 well is in the early phase of oil generation whereas the OM in the non-indigenous hydrocarbons is in the peak-late mature phase. In conclusion, our geochemical findings have revealed the hydrocarbon potential and heterogeneity of hydrocarbons within the Tano Basin's Campanian source strata induced by non-indigenous hydrocarbons. The geochemical data obtained from our study provide evidence of the potential influence of non-indigenous hydrocarbons on the misrepresentation of source input, depositional environment, and thermal maturity of organic matter (OM) in source rock analysis.

Influence of hydrothermal and upwelling events on organic matter accumulation in the gas-bearing lower Cambrian shales of the middle Yangtze Block, South China

With onoing studies on the diversification of animals during the early Cambrian in the Yangtze Block, resource effects associated with environmental changes have started to be elucidated, in which regional sedimentary events might have played an important role. The gas-rich Yichang area in the southern Huangling anticline is a potential area for shale gas development and provides a more suitable target for unravelling the debates on organic matter (OM) accumulation in the Low Cambrian marine black shales. This study integrated elemental geochemical and mineralogy time-series analyses based on core samples drilled in Yichang area on the shallow-water platform to constrain the evolution of the depositional environments and consider the forcing function behind OM enrichment in the Shuijinguo Formation. Distinctive TOC and Cu/Al peaks within the black shale deposited in the Shuijingtuo Member I suggest intensification of bioproductivity level. The chemical index of alteration (CIA), regarded as an indicator of paleoclimatic variation, increased in the upsection, and together with increasing Sr/Cu and quartz/feldspar ratio values, suggest a trend toward warming conditions and a moderate degree of chemical weathering. The geochemical proxies for the degree of water mass restriction (Mo/TOC and Co × Mn) and paleoredox (MoEF, UEF, U/Th and Corg/P) indicate that the shallow-water shelf experienced weakly-to moderately-restricted water conditions under an anoxic/euxinic environment in the Shuijingtuo Member I and transited to a moderately-restricted water mass with suboxic conditions during the late Shuijingtuo and Shipai periods. A positive Eu anomaly within the Shuijingtuo Member I provided robust evidence of hydrothermal activity. We propose here for an opening of deposition conditions on the shallow-water platform which results in enhanced upwelling and hydrothermal activity (Si, Ba, et.), thereby ultimately disrupting the original nutrient cycling for the metabolism and prompting the OM accumulation in the epicontinental sea during the early Cambrian.

Paleoenvironmental reconstruction preceding and during the early Aptian Oceanic Anoxic Event 1a in southern Tibet, eastern Tethys

The early Aptian Oceanic Anoxic Event (OAE 1a, ∼120 Ma) represents an episode of severe paleoenvironmental disturbance during the Cretaceous, marked by the widespread deposition of organic matter (OM)-rich sediments. The OAE 1a has been extensively studied in the Pacific, Boreal, and western and southern Tethyan regions. However, limited studies of the OAE 1a were conducted in the eastern Tethys (especially in southern Tibet). Here, high-resolution organic and inorganic geochemical data are used to reconstruct paleoenvironmental and paleoclimatic conditions during the OAE 1a in the Gucuo area of southern Tibet, eastern Tethys. Enrichment factors of redox-sensitive Mo and U elements and the Th/U ratio revealed oxic-to-suboxic bottom water oscillations preceding and at the base of OAE 1a. A shift from oxic to suboxic condition occurred prior to the OAE 1a, followed by a change from suboxic to oxic at the base of OAE 1a. Based on multiple geochemical proxy indicators and plots, a warm-humid climate in response to global warming was prevalent during the early Aptian. The ratios of B/Ga, Sr/Ba, Rb/K, and CaO/(CaO + Fe) revealed freshwater-dominated salinity conditions prior to and at the base of the OAE 1a. Low salinity conditions during the deposition suggested that massive freshwater runoff took place into the ocean due to a warm-humid climate and related hydrological cycles at this time. OM production was controlled by a high surface water bioproductivity driven by enhanced freshwater input that promoted fluvial delivery of micronutrients and accelerated hydrological cycles. However, oxic-suboxic redox conditions played a negative role in oxidizing and decomposing available labile OM in the water column, which led to moderate organic carbon burial at this time.

Influence of low oxygen concentrations on biological transformations of trace organic chemicals in sand filter systems

Managed aquifer recharge systems for drinking water reclamation are challenged by trace organic chemicals (TOrCs) since some of them are poorly retained. Although a lot of research has been done to investigate biological transformation of TOrCs in sand filter systems, there are still uncertainties to predict the removal. A laboratory column system with two different filter sands was set up to test TOrC transformation, the influence of low oxygen concentrations as well as the adaptation and influence of spiked TOrC influent concentrations. Bioactivity was quantified with the fluorescence tracer resazurin. In the experiment, a low elimination performance in the first column segment, defined as lag zone, was observed, implying incomplete adaptation or inhibiting co-factors. To assess these lag zones and to determine the dissipation time DT50 for 50% removal, a modified Gompertz model was applied. For acesulfame, formylaminoantipyrine, gabapentin, sulfamethoxazole, and valsartan acid DT50 of less than 10 h were observed, even when influent oxygen concentrations decreased to 0.5 mg/L. In general, TOrC transformations in technical sand with lower bioactivity and especially valsartan acid transformation responded very sensitive to low influent oxygen concentrations of 0.5 mg/L. However, in well adapted sand originating from soil aquifer treatment (SAT) with sufficient bioactivity, TOrC removal was hardly affected by such suboxic conditions. Furthermore, increasing the influent concentrations of TOrCs to 10 μg/L was found to promote adaptation especially for acesulfame and sulfamethoxazole. Benzotriazole, carbamazepine, diclofenac and venlafaxine were recalcitrant under the applied experimental conditions.

Anthropogenic perturbation enhances the release of geogenic Mn to groundwater: Evidence from hydrogeochemical characteristics

High geogenic Mn groundwater is widespread around the world and has also proved to be harmful to human health, especially to the IQ of Children. The natural release of Mn from aquifer sediments in slightly reducing condition is believed to be the primary cause. However, there isn't enough evidence to prove that anthropogenic activities promote the reductive release of Mn. Here a Historical Petrochemical Waste Storage Site (HPWSS) was studied to evaluate its impact on groundwater quality. Significantly elevated Mn, as well as elevated TDS, anionic surfactants, and organic pollutants, were found in the shallow aquifer (9-15 m) groundwater compared to the surrounding area. The Mn was believed to be generated in-situ, while others are caused by anthropogenic pollution. The good correlations between Mn and NH4+, HCO3−, I, As, Co, V, Ti, respectively, showed the Mn mobilization was mainly attributed to the reductive dissolution of Mn oxides/hydroxides. The potential processes leading to this enhanced Mn release are discussed, including 1) the infiltration of high salinity water which solubilized sediment organic matter (OM); 2) the anionic surfactants that promoted the dissolution and mobilization of surface-derived organic pollutants as well as sediment OM. Any of these processes may have provided a C source to stimulate the microbial reduction of Mn oxides/hydroxides. This study showed the input of pollutants could change the redox and dissolution conditions of the vadose zone and aquifer, causing a secondary geogenic pollution risk in groundwater. Since Mn is easily mobilized in suboxic condition as well as its toxicity, the enhanced release due to anthropogenic perturbation merits more attention.

Thermodynamic sensitivity of ammonia oxidizers-driven N2O fluxes under oxic-suboxic realms

In terrestrial ecosystems, the nitrogen dynamics, including N2O production, are majorly regulated by a complex consortium of microbes favored by different substrates and environmental conditions. To better predict the daily, seasonal and annual variation in N2O fluxes, it is critical to estimate the temperature sensitivity of different microbial groups for N2O fluxes under oxic and suboxic conditions prevalent in soil and wetlands. Here, we studied the temperature sensitivity of two groups of ammonia oxidizers, archaea (AOA) and bacteria (AOB), in relation to N2O fluxes through both nitrification and nitrifier-denitrification pathways across a wide temperature gradient (10–55 °C). Using square root theory (SQRT) and macromolecular rate theory (MMRT) models, we estimated thermodynamic parameters and cardinal temperatures, including maximum temperature sensitivity (TSmax). The distinction between N2O pathways was facilitated by microbial-specific inhibitors (PTIO and C2H2) and controlled oxygen supply environments (oxic: ambient level; and suboxic: ∼4%). We found that nitrification supported by AOA (NtA) and AOB (NtB) dominated N2O production in an oxic climate, while only AOB-supported nitrifier-denitrification (NDB) majorly contributed (>90%) to suboxic N2O budget. The models predicted significantly higher optimum temperature (Topt) and TSmax for NtA and NDB compared to NtB. Intriguingly, both NtB and NDB exhibited significantly wider temperature ranges than NtA. Altogether, our results suggest that temperature and oxygen supply control the dominance of specific AOA- and AOB-supported N2O pathways in soil and sediments. This emergent understanding can potentially contribute toward novel targeted N2O inhibitors for GHG mitigation under global warming.

Benthic foraminiferal response to the Oceanic Anoxic Event 2 (Late Cretaceous) and evidence of bottom water re‐oxygenation during the Plenus Cold Event at Clot Chevalier (Vocontian Basin, SE France)

The Cenomanian-Turonian boundary interval is characterized by environmental perturbations related to the Oceanic Anoxic Event 2 (OAE 2) that severely affected the marine biota, including benthic and planktonic foraminifera. We present a continuous high-resolution benthic foraminiferal record in combination with published planktonic foraminiferal and geochemical data across the Cenomanian-Turonian boundary interval in the Clot Chevalier section (Vocontian Basin, SE France) with the aim to interpret paleoceanographic changes in bottom waters. Benthic foraminiferal assemblages are characterized by low diversity and indicate an outer shelf-upper bathyal environment. Changes in composition and abundance of the benthic assemblages throughout the stratigraphic section allow identification of seven distinct environments. In the middle Cenomanian, below the OAE 2, benthic foraminifera show high species diversity and abundance of infaunal and epifaunal taxa, suggesting an environment characterized by oxic conditions at the seafloor. The microfossil assemblages in the upper Cenomanian within the OAE 2 interval are dominated by radiolaria (> 50%) followed by benthic calcareous taxa (Gavelinella sp., Gyroidinoides sp., Praebulimina elata, Tappanina laciniosa). Benthic agglutinated taxa (Ammobaculites sp., Ammodiscus cretaceus, Gaudryina sp., Textulariopsis bettenstaedti) show high abundance in the lowest part of the OAE 2 and disappear close to the top of the interval. The assemblage composition and the highest values of Total Organic Carbon (TOC) registered in this interval suggest eutrophication and suboxic conditions at the seafloor. However, at ∼1 m above the onset of the OAE 2, the repopulation by inferred oxic benthic foraminiferal taxa (i.e., Frondicularia sp. and Ramulina aculeata) suggests ventilation of bottom water masses, recording the Plenus Cold Event (PCE). In the lowermost Turonian, the assemblages exhibit high species diversity and abundance with the re-appearance of calcareous taxa, indicating oxic conditions, and of agglutinated taxa associated to moderate oxygen concentrations. Results show that the distribution of benthic foraminifera follows fluctuations in oxygenation and carbon export flux at the seafloor interpreted as the environmental changes associated to the onset of the OAE 2 and of the PCE across the Cenomanian-Turonian boundary interval.

Biome responses to a hydroclimatic crisis in an Early Cretaceous (Barremian–Aptian) subtropical inland lake ecosystem, Northwest China

The Early Cretaceous palaeoclimate was characterized by arid–humid alternation and accompanied by a series of short-term extreme environmental events, resulting in significant disturbances to the terrestrial ecosystem. However, the discontinuity of terrestrial geological records leads to a lack of detailed research on the coevolution mechanism of organisms and the environment. This study aims to investigate the response of the late Barremian–early Aptian subtropical lake biome structure to the hydroclimate crisis in the Yin'e Basin based on palynology, macropalaeontology, and geochemistry (i.e., major and trace elements, organic carbon isotopes, and biomarkers). The warm and semihumid climate promoted an increase in the abundance and degree of divergence in the vegetation, and the brackish–oxic conditions of the water column were conducive to the prosperity of aquatic organisms. The early aridification of the climate led to a decline in the lake level and a salinity crisis. The saline–suboxic conditions of the water column triggered the first aquatic crisis, while bacterial populations increased gradually. Cooling and continuous aridification led to a significant reduction in the types and amount of vegetation. The insufficient supply of nutrients and the continuous shrinking of the lake caused a second aquatic crisis, and several macroscopic organisms disappeared. The multivariate correlation analysis based on geochemical parameters showed that the palaeoclimate was the key driving factor for the evolution of palaeolakes and their conditions in the water column, and there was a close relationship among the ecological environmental indicators. The petrological and geochemical parameters suggest the existence of lacustrine hydrothermal activity, which could also have a direct impact on the ecological environment of the lake. This study of the Bayingebi Formation palaeolake in the Yin'e Basin provides a strong example of the close coupling changes in the palaeoclimate and lake ecosystem to understand the continental sedimentary response to Early Cretaceous palaeoclimate warming events.

Geochemical fingerprint—inter-basin chemostratigraphic correlation allowed for a more detailed stratigraphic subdivision of the Pliensbachian–Bathonian strata from the Carpathian Foredeep basement (Ukraine)

In this study, we combined the geochemical characteristics based on Rock-Eval pyrolysis and gas chromatography – mass spectrometry (GC–MS) results with the facies analysis. These surveys were conducted within grey to black claystones/mudstones intervals of the Podil’tsi and Kokhanivka formations, related to the Pliensbachian and Toarcian–Bathonian ages, respectively. The geochemical results revealed that the Podil’tsi Formation contains mixed marine/terrigenous, early-mature to mature organic matter. The deposition of this formation took place in dysoxic redox conditions of a sulphate-poor marine palaeoenvironment, with oxygen scarcity within the photic zone, as documented by green- and brown-pigmented Chlorobiaceae. Oleanane is present within the samples from the Podil’tsi Formation, which is uncommon within Lower Jurassic sedimentary rocks. The Kokhanivka Formation contains mostly early-mature, terrestrial organic matter, deposited in suboxic conditions of a sulphate-poor, fluvial–deltaic palaeoenvironment. The absence of aliphatic diterpenoids within the Middle Jurassic strata points to the low significance of conifers in the sediment supply area at this time. All of the Podil’tsi and most of the Kokhanivka formations are characterised by poor hydrocarbon potential. Only the middle part of the Kokhanivka Formation, built by brown, organic-rich claystones, shows fair-to-good hydrocarbon potential. Based on our results, a chemostratigraphic correlation of the Toarcian–Bathonian strata from the Carpathian Foredeep with the same strata from the neighbouring Polish Basin was performed. The juxtaposition of the geochemical and facies results suggests that the interval of brown organic-rich claystones, from the middle part of the Kokhanivka Formation can be related to the Middle–Upper Aalenian Age.

Seasonal variation in mercury and methylmercury production in vegetated sediment in the Dongtan wetlands of the Yangtze River Estuary, China

The seasonal cycling of mercury (Hg) in vegetated sediments in the Dongtan wetlands of the Yangtze River Estuary were determined, and microcosm incubation experiments were conducted to evaluate methylmercury (MeHg) production after Hg input. The results showed that the seasonal variations of total Hg and MeHg were very different. The enhanced activity of methylating bacteria could have been the main contributor to the elevated MeHg in the upper surface layer (0–12 cm), which was supported by the higher copy numbers of the hgcA gene in the surface sediment and the MeHg increase during sediment incubation following litterfall addition. Moreover, the incubation results showed that Hg addition greatly increased net MeHg production and that this increase remained under suboxic conditions, suggesting that the potential health risk of Hg in estuarine wetlands could exist for a long time under changing redox conditions.

Suboxic conditions prevailed during the Toarcian Oceanic Anoxic Event in the Alpine-Mediterranean Tethys: The Sogno Core pelagic record (Lombardy Basin, northern Italy)

Suboxic conditions prevailed during the Toarcian Oceanic Anoxic Event in the Alpine-Mediterranean Tethys: The Sogno Core pelagic record (Lombardy Basin, northern Italy)

Redox Changes during the Past 100 ka in the Deeper Eastern Arabian Sea: A Study Based on Trace Elements and Multivariate Statistical Analysis

The temporal distribution of trace elements in a sediment core (SK117/GC-08) indicates minor changes in oxygenation during the last 100 kilo years in the bottom waters of the deeper eastern Arabian Sea. The high values of Mn, Co, Cu, Mn/Al, Co/Al, Cu/Al, V/Cr, and V/(V + Ni) in the sediments during interglacials and interstitials collectively indicate oxic conditions during warm periods. The high values of Cr, Ni, V, Mo, Cr/Al, Ni/Al, and Ni/Co in sediments during glacials and stadials collectively indicate dysoxic to suboxic conditions during the colder last glacial maximum and during the entire marine isotope stage two. The bottom waters have never experienced anoxic conditions. Multivariate statistics showed the attribution of the trace elements in two factors: cluster 1 (Co, Cu, Mn) was enriched during oxic conditions and cluster 2 (Cr, Mo, Ni, V) was enriched during dysoxic to suboxic conditions. Oxygenation conditions are mainly driven by variations in monsoon-controlled surface water productivity and changes in the flux of circumpolar deep water. The dysoxic to suboxic bottom water conditions developed at the core location during colder climates are very well synchronised with an increased organic matter flux. The main factor that controls the accumulation of the organic-rich sediments in the eastern Arabian Sea during a glacial is the increase in the supply of organic matter from increased primary productivity in the surface waters, controlled by winter monsoon winds, and localized convective mixing. During warmer interglacials and interstadials, the core location has remained well ventilated.

Critical assessment of U, Ba and Ni as redox and productivity proxies in organic-rich sediments underneath dynamic, highly productive waters

Sediments of the continental shelf anoxic zones of the Benguela upwelling system (BUS) and the Peruvian upwelling system are present-day hotspots of trace element accumulation. However, contribution of the lithogenic trace element fraction and early diagenetic transformation processes are poorly constrained. The identification of source and accumulation mechanisms is necessary for the validation of trace elements as proxies for productivity and redox cycling, notably in highly dynamic upwelling systems such as the BUS. Here, we analyzed redox- and biosensitive elements (U, Ba, Ni), lithogenic tracers (Al, Ti, Zr), total organic carbon (TOC) and P in eleven short sediment cores from the Namibian shelf (ca. 22–25°S) as well as grain size fractionated hinterland samples from the Tsauchab Valley at Sossusvlei (∼25°S). These samples help to constrain the lithogenic trace element contributions, a prerequisite for a realistic interpretation of marine authigenic trace element data. Our findings corroborate previous findings that Zr is a sensitive tracer of the coarse fraction in marine sediment for dust input and/or sediment reworking. Hence, the combined analysis of the coarse fraction (Zr), organic matter, and P enrichments (TOC/P) differentiates between a calm, organic matter-rich central shelf environment and a more energetic phosphorite-rich southern shelf environment. A significant correlation of U with P in cores from the southern shelf corroborates previous findings of U incorporation into apatite, the initial formation of which requires oscillating oxic to sulfidic redox conditions. Non-apatite associated U dominates in the cores from the central shelf, where it is only enriched in the buried, anoxic parts of the sediment. This also supports oscillating oxic to sulfidic redox conditions in surface sediment, where U may be recycled. Hence, U accumulates either (i) in apatite under oscillating oxic to sulfidic conditions or (ii) in buried anoxic sediment sections. This strongly questions the use of U as indicator of suboxic conditions but rather anoxic conditions. While lithogenic fractions of U were negligible, the lithogenic fractions of Ba and Ni were found to be elevated. The calculation of authigenic Ba when accounting for lithogenic Ba from the Sossusvlei fine fractions gives more realistic authigenic Ba values. The high Ba enrichment in the BUS is interpreted as uptake of Ba by diatoms and/or nucleation of barite in P-rich diatom remains, as suggested previously. Given the shallow water depth of the cores, barite formation must already take place in subsurface waters (<50 m water depth) as opposed to previous suggesting greater water depths. The lithogenic background of Ni strongly varies from 23°S to 25°S based on Sossusvlei data and the literature. When authigenic Ni contents are calculated accordingly and plotted versus TOC, more systematic trends are seen (compared to total Ni versus TOC). Essentially all cores show an increase in the authigenic Ni/TOC ratio with sediment depth suggesting better preservation and retention of Ni as compared to TOC consistent with previous observations in sediments from the Peruvian upwelling system. This trend corroborates the use of Ni as a productivity indicator for upwelling sediments.