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Studies of Mesoproterozoic ocean chemistry have converged on a widely accepted scenario where a stable shallow layer of oxic waters occurred over deeper anoxic ferruginous waters, with mid-depth euxinic waters restricted to continental margins. However, several lines of evidence suggest shallow waters may not always have remained oxic, but fluctuated between oxic, anoxic, and euxinic conditions. Here, we present new Hg-C-S geochemistry data of ca. 1.5–1.4 Ga sediments in the Yanliao Basin to better understand the redox structure of shallow water environments, especially for the photic zone. In this study, positive Δ 199 Hg values suggest atmospheric Hg(II) deposition into oxic surface waters, whereas abnormally negative Δ 199 Hg values indicate enhanced sequestration of gaseous Hg(0) and photo-reduction of reduced sulfur-bound Hg(II) in response to photic zone euxinia. Our highly fractionated Δ 199 Hg values reveal a new scenario, in which the extent of euxinia is more dynamic than generally thought, and may expand from the mid-depth to the photic zone, likely linked to the superimposed operation of enhanced oxidative weathering input of SO 4 2 − and high OM flux in the water column. Given that euxinic waters may accelerate the sequestration of bio-essential elements, it is suggested that the dynamic euxinia of shallow water played a significant role in delaying the diversification of eukaryotes in the Mesoproterozoic oceans. Our study also highlights that Hg isotopes may act as a promising new proxy to trace redox changes in the surface ocean over geological time. • Negative Δ 199 Hg suggest photo-reduction of sulfur-bound Hg(II) in the photic zone. • The extent of euxinia was dynamic from the mid-depth to the photic zone. • Dynamic euxinia of shallow waters may delay the diversification of eukaryotes. • Hg isotopes may act as a new proxy to record redox changes in the surface ocean. [ABSTRACT FROM AUTHOR]

The Ediacaran–Cambrian (E–C) transition witnessed profound biological and oceanic changes, among which the appearance of explosive radiation of skeletonized animals is considered as the most remarkable one. Although the prominent rise of oxygen concentration in atmosphere and ocean is considered to be the major trigger of the "Cambrian Explosion", the links between the oxygenated process of deep ocean and the diversification of Cambrian metazoans are still under debate. In this study, δ15N and δ13C org values, TOC and TN contents, as well as trace elements concentrations of fresh shale core samples were analyzed to explore the redox condition and nitrogen cycle of deep oceans and their relationships with biological evolution. The Mo concentrations, Mo–U covariations and Th/U ratios of our samples indicate suboxic/anoxic waters in the mid-depth ocean during the late Ediacaran to Cambrian Fortunian age, widespread anoxic/euxinic waters with temporary sulfidic waters during Cambrian Age 2 to early Age 3, and suboxic waters again during the middle to late Age 3. The positive δ15N values from late Ediacaran to Cambrian Age 2 indicate a large NO 3 − reservoir with a deep chemocline. In contrast, the low δ15N values (<0‰) in the deepwater slope regions suggest a dominant biological N 2 -fixation, and/or partial NH 4 +-assimilation in the photic zone during Cambrian Age 3. Integrated δ15N values of this study and early publications suggest a continuous redox stratified paleo-ocean with anoxic/suboxic deep waters from late Ediacaran to Cambrian Age 3. It is also worthy to note that although the deep ocean may not experience substantial oxygenation during the main phase of Cambrian diversification, the expanding oxic surface waters and supply of bioavailable fixed nitrogen may satisfy complex high O 2 -demanding animals, and thus at least have promoted the Cambrian biodiversity in shallow waters. • Positive δ15N values reflect bioavailable NO 3 − in surface oxic waters and a deepening chemocline. • The lower δ15N (<−2‰) may be related to partial NH 4 +-assimilation in the photic zone. • Nitrogen isotopes indicate a redox stratified ocean from late Ediacaran to Cambrian Age 3. • The expanding oxic surface waters and supply of bioavailable NO 3 − may promote the Cambrian biodiversity. [ABSTRACT FROM AUTHOR]

• The Yangtze Block was strongly restricted at the beginning of Cambrian. • The negative δ 98/95Mo values reflect preferential adsorption light Mo by Mn oxides in the stratified late Ediacaran paleo-ocean. • The variation of sedimentary δ 98/95Mo values draw the outline of 'sandwich'-like oceanic redox model. • The adsorption of isotopically light Mo by Mn oxides may play a role in driving locally heterogeneous heavy seawater δ 98/95Mo values. To explore paleo-ocean redox variations and Mo biogeochemistry across the Ediacaran–Cambrian transition (E-C), continuous fresh core shale samples of Ediacaran to Cambrian were collected from a well drilled in the upper slope region of the Yangtze Block in South China and analyzed for δ 98/95Mo and δ13C org values, TOC, as well as the concentrations of trace and rare earth elements. The Mo–U covariations, Th/U and Re/Mo ratios as well as published iron speciation data indicate a redox stratified ocean with oxic surface waters and anoxic/euxinic deep waters prevailing across the E-C transition. During the time of Cambrian Fortunian and Stage 2, the bottom water in the upper slope became oxic due to the deepening oxycline. Afterwards, widespread anoxia and/or euxinia occurred during latest Age 2 and early Age 3. The Mo concentrations and Mo/TOC ratios suggest that the Yangtze Block was well connected with the open ocean during the late Ediacaran, and then became strongly restricted at the beginning of Cambrian. Following a large-scale transgression during Cambrian Age 2, its connection with the open ocean was enhanced again. The negative δ 98/95Mo values may reflect preferential adsorption of isotopically light Mo by Mn oxides particles above the chemocline and their subsequent shuttling to euxinic sediments in the redox stratified late Ediacaran ocean; whereas the relatively low δ 98/95Mo values during the Cambrian Fortunian and Age 2 indicate the decrease of δ 98/95Mo Seawater values in the restricted basin. The operation of local Mn particulate shuttles could also be stimulated by inflow of oxic waters into restricted basin during marine transgression, resulting in negative δ 98/95Mo values. In conjunction with previous published Mo isotope data, the variations of sedimentary δ 98/95Mo values from shallow shelf to deepwater slope regions draw the outline of 'sandwich'-like model with euxinic waters (e.g., [H 2 S] aq > 11 μM) sandwiched by oxic surface waters and anoxic deep waters during the E-C transition. The shuttling of isotopically light Mo to euxinic sediments by Mn oxides in the redox stratified Ediacaran-Cambrian ocean may play a role in driving heterogeneous heavy seawater Mo isotopes and variable sedimentary δ 98/95Mo values. [ABSTRACT FROM AUTHOR]

To understand the evolution of oceanic redox conditions and organic matter (OM) accumulation in the shelf and slope regions of the Upper Yangtze Platform during Cambrian Stage 3, high-resolution geochemical data were obtained from two wells respectively representing shelf and slope environments. Based on redox proxies, widespread anoxic/euxinic waters with intermittent sulfidic water columns were formed from shallow shelf to deep water slope and basinal regions during Stage 2 and early Stage 3; then, widespread anoxic waters were reduced and oxic waters expanded during the middle Stage 3, after which fluctuating suboxic and oxic (occasionally anoxic) environments became predominant during late Stage 3. The presence of hyalophane and positive Eu anomalies suggest strong Ba-rich hydrothermal activities in the shelf and deep basin regions during early and middle Stage 3. The Ba-rich hydrothermal fluids and upwelling events enhanced the primary productivity in surface waters and subsequently promoted organic matter enrichment. During Stage 2 and early Stage 3, the high primary productivity driven by coastal upwelling and hydrothermal activity were the main factors controlling OM enrichment; during middle Stage 3, the local anoxic water condition became responsible for OM enrichment when oxic waters began to gradually expand; during late Stage 3, both primary productivity and redox conditions became the main control factors for OM enrichment. According to the analysis of primary productivity, redox conditions, hydrothermal activity and upwelling events, an evolution model is proposed to account for the paleo-environmental variation and the OM enrichment from shallow shelf to slope regions during Cambrian Stage 3. • Oceanic redox and OM accumulation during Cambrian Stage 3 were investigated. • Ba-rich hydrothermal activities occurred in the shelf and deep basin regions during early to middle Stage 3. • Stepwise expansion of oxic waters spread from the shallow shelf to slope regions during middle Stage 3. • Hydrothermal activities and upwelling events promoted the high primary productivity in surface waters. • Hyalophane may originate from reactions between Ba-rich fluids and K-feldspar. [ABSTRACT FROM AUTHOR]

• Organic petrology and biomarkers in the Lower Cretaceous Jiufotang Formation from the Kazuo Basin were investigated. • Terrigenous organic matter was diluted by detrital minerals and phytoplankton was the most important organic matter source. • Anoxic bottom water may be necessary for organic matter enrichment in lake systems. • Climate changes controlled the organic matter enrichment model during deposition of the Jiufotang Formation. The Lower Cretaceous Jiufotang Formation, situated in western Liaoning Province, China, has attracted attention for abundant Jehol Biota fossils and the potential to improve understanding of the relationships among climate change, evolution of terrigenous organisms, and organic matter (OM) enrichment in lake systems during the Early Cretaceous. Based on organic petrology and biomarker analyses, this study investigated the depositional environment, OM sources, and enrichment mechanism of Jiufotang mudstone in the Kazuo Basin. Maceral composition shows that the OM source of Jiufotang mudstone shifted from mainly aquatic to terrigenous, and biomarkers such as abundant diterpenoids and retene indicate that terrigenous higher plants were mainly conifers during deposition of the Jiufotang Formation. Pr/Ph ratios (0.07–0.65) show that the studied section was deposited in anoxic bottom water that may be necessary for OM enrichment in lake systems. Gravity flows were accompanied by input of many clastic minerals and oxygenation of bottom waters, which is unfavorable for OM enrichment. Therefore, the best OM enrichment model for lake systems includes anoxic bottom water, stable water column stratification, low sedimentation rates, and high phytoplankton contributions. The climate changed from relatively dry to humid, which drove changes in the OM enrichment model in the lake. More importantly, the climate and terrigenous plant information obtained by biomarkers is useful to reconstruct the ecological environment of the Jehol Biota. [ABSTRACT FROM AUTHOR]

• New nutrient proxy reveals oceanic P peaks during the Hirnantian glaciation and deglaciation. • The HICE was caused by enhanced organic carbon burial linked to more efficient P cylcing. • Persistent volcanisms and less CO 2 consumption via silicate weathering terminated the glaciation. • Post-glacial carbon cycle and climate depend on the duration of intensive volcanisms and oceanic nutrient availability. The Hirnantian Stage of the Late Ordovician coincides with a positive carbon isotope excursion (HICE, ∼+6 ‰), a major glaciation, increased volcanic activity, expanded marine anoxia, and one of the largest mass extinctions of the Phanerozoic. The origin of the HICE is debated, with proposed mechanisms favoring enhanced low-latitude carbonate weathering and/or increased efficiency of organic carbon burial. To test those hypotheses, we assembled new and published δ13C carb and δ13C org data and shale phosphorus and carbonate-associated phosphate concentrations from diverse depositional settings on several continents. We then evaluated these results using an integrated carbon cycle model derived from GEOCARB, and a coupled oceanic carbon and phosphorus cycle model. This approach yielded quantitative tests of the response of the global carbon cycle to changes in volcanic degassing, silicate and carbonate weathering rates, and organic carbon burial, individually and in combination, to determine which forcings yield signals most aligned with observed δ13C carb and δ13C org records. On this basis, the most plausible scenario for the HICE involves a ∼50 % decrease in weathering of silicates and organic carbon at higher latitudes combined with enhanced carbonate weathering (f wcarb = 88 %) at lower latitudes. Synchronous, globally enhanced burial of organic carbon was due to more efficient nutrient cycling caused by stronger thermohaline circulation and increased volcanism that further enriched oceanic dissolved inorganic carbon (DIC) in 13C. The influence of increased input of 13C-depleted carbon from volcanism and metamorphism may have been overwhelmed by these drivers of the positive δ13C shift. The Hirnantian Glaciation was likely initiated by secular enhancement of continental weathering due to the expansion of early plants but was terminated by increased volcanism and decreased CO 2 consumption by silicate weathering during the cooling. This scenario is consistent with the timeline of known geological events and offers insights into the mechanisms responsible for the associated biotic crisis. Specifically, the onset of the Hirnantian Glaciation induced lower global temperatures, resulting in sea-level fall and a loss of habitat space, while its termination led to enhanced organic carbon export and deep-water anoxia, all of which likely contributed to the Late Ordovician mass extinction. [ABSTRACT FROM AUTHOR]