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Ancient manganese (Mn) deposits are primarily characterized by the presence of Mn(II) carbonates that likely formed by the diagenetic reduction of precursor Mn(IV) oxides. As such, Precambrian sedimentary Mn deposits have been used as a line of evidence for the evolution of oxygen in Earth’s surface environments. However, recent studies have shown that these Mn(II)-carbonates have the ability to directly accumulate within anoxic water columns, where free oxygen does not play a role in their formation. This alternative pathway casts uncertainty on the robustness of using ancient Mn deposits to constrain the redox fabric of the past marine water columns. Here, we investigate the Wafangzi Mn and Fe ore deposit from the 1.45 billion-year-old Tieling Formation, North China. The deposit contains Mn(II,III) mineral phases (hausmannite, braunite) as inclusions, or unreacted residues, trapped within Mn(II) carbonate (Ca-rhodochrosite). Some nodules and oolites of Mn(II) and Fe(II)-carbonate phases are also present and display a banded structure with concentric rings. Mn(III) oxide (manganite) is present in a paragenetic assemblage along with hematite and replacement textures with braunite. The negative carbon isotope composition (δ13C, –7‰ to –4‰) from Mn(II) carbonate samples in the Wafangzi Mn deposit which are distinct from that of contemporaneous seawater (~0‰), along with petrographic and speciation analyses, collectively suggest that the Mn(II,III)- and Fe(II)-bearing mineral phases formed through the diagenetic reduction of primary Mn(IV)/Fe(III) minerals coupled to the oxidation of organic matter. Therefore, the Wafangzi Mn deposit suggests the presence of sufficiently oxygenated marine waters, overlying anoxic ferruginous deeper waters with a transitional manganous water layer that could have driven the redox cycling of Mn, Fe, and C. Given the contemporaneous economic Mn deposits in the 1.45 Ga Ullawarra Formation in Western Australia, our findings imply the existence of a transient, and perhaps widespread, pulsed oxygenation event in the mid-Proterozoic oceans.

Banded iron formations (BIFs) are iron-rich marine chemical sedimentary rocks, and their mineralogy and geochemistry can be used to gain insights into ancient ocean chemistry and biospheric evolution. Magnetite is the major iron-bearing mineral in many BIFs (particularly in the Archean) and is variably interpreted to be of primary, early diagenetic, or metamorphic origin. Different genetic interpretations for magnetite lead to divergent pictures of the Precambrian Earth system and its evolutionary models through time. The Baizhiyan Formation of the Neoarchean Wutai Group (Shanxi, North China) features magnetite-bearing, Algoma-type BIFs deposited ca. 2.52 Ga, in the lead-up to a major period of global iron formation deposition in the Paleoproterozoic. Abundant magnetite crystals found in the silica-rich bands of these BIFs show euhedral, hexagonal morphology. We suggest that this hexagonal magnetite likely represents pseudomorphs after green rust, a mixed-valence iron hydroxy-salt formed in the water column. The rare earth element composition of the BIFs shows negligible to slightly positive Ce anomalies (CeSN/CeSN* = 1.03 ± 0.07), which is characteristic of a dominantly anoxic water column. The presence of positive Eu anomalies (EuSN/EuSN* <3.9) suggests a substantial influence from proximal hydrothermal fluids. The co-occurrence of siderite layers associated with the magnetite-bearing strata may indicate iron cycling associated with ferruginous bottom seawater conditions. Geochemical signatures of the Baizhiyan BIFs are consistent with the interpretation that the magnetite was transformed from metastable green rust. This green rust could have formed via several processes, including the partial oxidation of Fe(II) by molecular oxygen/photoferrotrophs, the reaction of settling ferrihydrite with Fe(II)-rich hydrothermal fluids under anoxic conditions, or local dissimilatory iron reduction. In all cases, the contribution of primary green rust to BIF formation requires iron redox cycling, and similar pseudomorphs in the form of hexagonal magnetite may be more common in the geological record. Our findings support the models in which green rust was an important primary constituent of the Precambrian iron cycle, and the potential interactions of green rust with other elements (e.g., phosphorus) should be taken into consideration when reconstructing Precambrian biogeochemical cycles.

Sedimentary records suggest that the mid-Proterozoic (ca. 1.8–0.8 Ga) was persistently characterized by a greenhouse climate despite significantly lower solar luminosity compared to modern levels. To maintain greenhouse conditions, the partial pressure of carbon dioxide (pCO2) must have remained elevated, possibly indicative of key differences in the complexities of the carbon cycle compared to the modern. Modeling has suggested that high pCO2was likely maintained by elevated rates of “reverse weathering:” marine authigenic clay formation, a process that consumes alkalinity and generates CO2. This process is kinetically slow in modern marine environments, yet is hypothesized to have been enhanced during the mid-Proterozoic due to the greater availability of important species for clay authigenesis such as silica and ferrous iron. This hypothesis is testable using the geological record, because enhanced reverse weathering would lead to the formation of abundant marine authigenic clays. However, the distribution of marine authigenic clays in the Proterozoic sedimentary record has not been paid sufficient attention. In this study, we report the presence of authigenic clays (glauconite and berthierine) from the Xiamaling Formation (ca. 1.4 Ga), North China. The glauconite-berthierine horizons occur as millimeter- to centimeter-thick laminae interbedded with muddy siltstone and feature detrital grains supported by the clay matrix. In places, these layers were partially reworked to form soft and cohesive intraclastic sands, suggesting a syndepositional origin. We hypothesize that marine iron cycling in the iron- and silica-rich mid-Proterozoic oceans may have facilitated the formation of authigenic iron-rich clay during the deposition of the Xiamaling Formation. The formation of iron-hydroxides on the seafloor—and the local increase in pH caused by subsequent dissimilatory iron reduction—could have resulted in the absorption of SiO2, Al(OH)3, and Fe(OH)2to form soft, cohesive and noncrystalline Fe(OH)3-SiO2-Al(OH)3-Fe(OH)2gels. These gels would have subsequently converted to glauconite/berthierine through aging. The transformation from glauconite-rich layers to berthierine-rich laminae was likely facilitated by a greater availability of Fe(II), and therefore higher Fe(II)/TFe and Fe/ Si ratios. We suggest that the relatively rapid formation of syndepositional, seafloor berthierine and glauconite layers in the basal Xiamaling Formation is the result of enhanced reverse weathering during this time. This study provides an important geological support for carbon cycle models that invokes enhanced reverse weathering rates in the mid-Proterozoic ocean that may have helped to maintain a high-baseline pCO2during this time.

Previous studies suggest that the atmospheric and shallow seawater oxygen levels during the mid‐Proterozoic may have remained persistently unbalanced due to the low atmospheric oxygen levels and that, under these conditions, the oxygen concentrations of shallow seawater would largely reflect local oxygenation caused by primary productivity. As a result, in a microbialite‐rich setting, it may be difficult to differentiate oxygenation caused by local primary productivity from overall oxygenation of the surface environment. To address this issue, we conducted an integrative study of the ∼1.44 Ga stromatolitic carbonates of the Fengjiawan Formation in the Xiong’er Basin, southern North China, using techniques of sedimentology, mineralogy and geochemistry. The results show that the Fengjiawan Formation is dominated by water‐column precipitated carbonate mud (now dolomitized), pointing to moderately oxidized marine conditions. A significant and stable negative Cerium (Ce) anomaly (as low as 0.42) is identified in a long interval (>50 m in thickness) of this formation. This Ce anomaly is lower than the minimum values both recorded in the Great Oxidation Event (GOE; minimum 0.76) and in the Neoproterozoic Oxygenation Event (NOE; minimum 0.52), suggesting a significant oxygenation process rather than an oxygen oasis. Some intervals in this formation show I/(Ca + Mg) values higher than the Precambrian background value of 0.5 μmol/mol, providing a further support for shallow seawater oxygenation. This study, together with the enhanced oxygenation identified in the time‐equivalent Tieling Formation, indicates a multi‐basin oxygenation event in North China at ∼1.44 Ga. The ∼1.44‐billion‐year‐old Tieling Formation of the Yanliao Basin in the northern part of North China records a significant increase of oxygen levels. However, it remains uncertain whether this represents a localized marine oxygen oasis or an extensive atmospheric oxygenation. To settle this problem, we conducted an integrated study of the stromatolitic carbonates in the time‐equivalent Fengjiawan Formation of the Xiong’er Basin, southern North China, in terms of sedimentology, mineralogy and geochemistry. Our study shows that the Fengjiawan Formation is dominated by water‐column precipitated carbonate mud with negative Cerium anomaly and iodine enrichment indicative of significant increase of oxygen levels in shallow marine environment. Combined with the oxygenation evidence from the Tieling Formation, we propose the occurrence of a possible cross‐basin oxygenation event at ∼1.44 billion years ago. Combined dolostone I/(Ca + Mg) and Ce anomaly can be used as a robust proxy to reconstruct shallow seawater redox conditionsIntegrated study of dolostone from the ∼1.44 Ga Fengjiawan Formation, Xiong’er Basin, indicates oxygenated shallow marine conditionsTogether with the time‐equivalent Tieling Formation, Yanliao Basin, the study suggests a multi‐basin oxygenation event at ∼1.44 Ga Combined dolostone I/(Ca + Mg) and Ce anomaly can be used as a robust proxy to reconstruct shallow seawater redox conditions Integrated study of dolostone from the ∼1.44 Ga Fengjiawan Formation, Xiong’er Basin, indicates oxygenated shallow marine conditions Together with the time‐equivalent Tieling Formation, Yanliao Basin, the study suggests a multi‐basin oxygenation event at ∼1.44 Ga

Enhanced continental phosphorus input into the ocean has been suggested as a potential trigger for the transient oxygenation events during the mid‐Proterozoic; however, the response of phosphorus cycling to these marine oxygenations remains unclear. Here, we report the changes in phosphorus cycling associated with a ∼1.7 Ga transient oxygenation. Abundant authigenic aluminum phosphate–sulfate mineral svanbergite (SrAl3(PO4) (SO4) (OH)6; 8.02 ± 4.92 wt%) is identified within the ∼1.7 Ga Yunmengshan ironstones from the Xiong'er Basin, North China and other contemporaneous basins. This observation provides new evidence to support the suggestion that early diagenetic aluminum phosphate‐sulfate minerals could have represented a critical sink of marine phosphorus during the Proterozoic. We suggest that atmospheric oxygenation and concomitant changes in porewater redox chemistry may have enhanced the formation of early diagenetic phosphates, leading to a negative feedback on the oceanic phosphorus reservoir and atmospheric oxygen levels. It becomes increasingly clear that multiple transient oxygenation events likely punctuated the low background oxygen world during the mid‐Proterozoic. This may imply that a negative feedback could have inhibited a secular rise in atmospheric oxygen, though the deoxygenation mechanisms remain unclear. Phosphorus (P) availability regulated primary production and therefore controlled the atmospheric oxygen levels during this time, and oxygenation would in turn affect phosphorus cycling. Here we investigate the P cycling during a ca. 1.7 Ga transient oxygenation using mineralogical and geochemical methods. The results show an enhanced phosphorus burial during this transient oxygenation. The elevated phosphorus and sulfate inputs from the enhanced continental weathering and oxygenation may have promoted the formation of aluminum phosphate‐sulfate minerals. This, in turn, reduced the bio‐availability of phosphorus in the marine environment, ultimately limiting marine productivity and leading to a negative feedback on the oxygenation event. This study highlights that phosphorus cycling pathway, which was previously overlooked, may have played a role in the deoxygenation during the intermittent oxygenation events in the mid‐Proterozoic. Abundant phosphorus and sulfur precipitation as authigenic svanbergite is associated with the ∼1.7 Ga transient oxygenationThe oxygenation resulted in suboxic and acidic porewater conditions rich in P‐ and sulfate, facilitating the authigenesis of svanbergiteThe enhanced P and S burial as svanbergite represents a significant but overlooked negative feedback during the transient oxygenation Abundant phosphorus and sulfur precipitation as authigenic svanbergite is associated with the ∼1.7 Ga transient oxygenation The oxygenation resulted in suboxic and acidic porewater conditions rich in P‐ and sulfate, facilitating the authigenesis of svanbergite The enhanced P and S burial as svanbergite represents a significant but overlooked negative feedback during the transient oxygenation

Marine red beds (MRBs) are common in sedimentary records, but their genesis and environmental implications remain controversial. Genetic models proposed for MRBs variably invoke diagenetic or primary enrichments of iron, with vastly different implications for the redox state of the contemporaneous water column. The Xiamaling Formation (ca. 1.4 Ga) in the North China Platform hosts MRBs that offer insights into the iron cycling and redox conditions during the Mesoproterozoic Era. In the Xiamaling MRBs, well-preserved, nanometer-sized flaky hematite particles are randomly dispersed in the clay (illite) matrix, within the pressure shadow of rigid detrital grains. The presence of hematite flake aggregates with multiple face-to-edge (“cardhouse”) contacts indicates that the hematite particles were deposited as loosely bound, primary iron oxyhydroxide flocs. No greenalite or other ferrous iron precursor minerals have been identified in the MRBs. Early diagenetic ankerite concretions hosted in the MRBs show non-zero I/(Ca+Mg) values and positive Ce anomalies (>1.3), suggesting active redox cycling of iodine and manganese and therefore the presence of molecular oxygen in the porewater and likely in the water column during their formation. These observations support the hypothesis that iron oxyhydroxide precipitation occurred in moderately oxygenated marine waters above storm wave base (likely <100 m). Continentally sourced iron reactivated through microbial dissimilatory iron reduction, and distal hydrothermal fluids may have supplied Fe(II) for the iron oxyhydroxide precipitation. The accumulation of the Xiamaling MRBs may imply a slight increase of seawater oxygenation and the existence of long-lasting adjacent ferruginous water mass.

The mid‐Proterozoic (∼1.8–0.8 Ga) ocean‐atmosphere system is hypothesized to have experienced fluctuations in redox conditions with transient oxygenation events. One of these happened at ∼1.4 Ga, and it is speculated that this event may link to the emplacement of large igneous province (LIP) at this time. However, direct evidence for this relationship remains to be proved. Here, we report Hg/TOC, P, and trace element concentrations across the ∼1.4 Ga oxygenation event in the Xiamaling Formation of North China. A prominent increase in Hg/TOC is slightly earlier than that of nutrient contents (especially P), pyrite and TOC abundances, suggesting that this distinct oxygenation event was likely the result of LIP activity at ∼1.4 Ga, which increased nutrient and sulfate supply from continental weathering to the ocean, sustaining elevated primary productivity, organic carbon and pyrite burial. This study indicates that LIP weathering could trigger transient oxygenation events during the mid‐Proterozoic. The mid‐Proterozoic (∼1.8–0.8 Ga) retains persistent low oxygen concentrations in atmosphere and shallow seawater but is occasionally punctuated by transient oxygenation events. Multiple lines of evidence from sedimentary geochemistry have argued a global oxygenation event at ∼1.4 Ga, and speculated that this event was closely related to large igneous province (LIP) activity at this time. Direct link between LIP activity and the oxygenation event, however, has rarely been addressed yet. As mercury in sedimentary rocks has been widely used to trace volcanism, we therefore report Hg/TOC, P, and trace element concentrations across the ∼1.4 Ga oxygenation event. The result shows that a prominent increase in Hg/TOC is clearly recognized in the shales that stratigraphically just underly the layers recording the oxygenation event, suggesting a close causal link between LIP emplacement and the oxygenation event. Accompanied with this Hg/TOC ratio peak, increased nutrient contents (especially P), pyrite and TOC abundances are also observed. Based on these observations, we proposed a model that the Xiamaling oxygenation event was the result of the LIP's emplacement at ∼1.4–1.35 Ga, which increased nutrient and sulfate supply from continental weathering to the ocean, sustaining elevated primary productivity, organic carbon and pyrite burial. High Hg/TOC ratios associated with large igneous province (LIP) emplacement were detected in the ∼1.4 Ga Xiamaling Formation shales of North ChinaHigh P content accompanied with this interval indicates enhanced phosphorous input from LIP weathering to the oceanA causal link between the LIP weathering and oxygenation event at ∼1.4 Ga was suggested High Hg/TOC ratios associated with large igneous province (LIP) emplacement were detected in the ∼1.4 Ga Xiamaling Formation shales of North China High P content accompanied with this interval indicates enhanced phosphorous input from LIP weathering to the ocean A causal link between the LIP weathering and oxygenation event at ∼1.4 Ga was suggested

Berthierine and chamosite are iron-rich clay minerals that share similar chemical compositions. Berthierine forms at low temperature (25–45 °C) during early diagenesis and may transfer to chamosite at temperatures of ≥70 °C. Because the formation of berthierine and chamosite requires significant amount of Fe2+supply, their presence in marine sediments is often used as a mineral proxy for ferruginous conditions in porewater. Recent studies reveal that the Precambrian oceans were characterized by pervasive ferruginous water-column conditions that may favor the formation of iron-rich clay minerals like berthierine and chamosite. To evaluate if ferruginous water-column conditions in the Precambrian ocean played a role on iron-rich clay mineral formation, we conducted an integrated petrographic, mineralogical, and geochemical study on the chamosite- and glauconite-bearing strata of the Mesoproterozoic Xiamaling Formation (~1.40–1.35 Ga) in North China. Petrographic, XRD, SEM, and EDS analyses show that the chamosites of the Xiamaling Formation was transferred from glauconite, with berthierine as an intermediate mineral phase during early diagenesis. Geochemical analyses indicate that a complete transformation from glauconite-dominated to chamosite-dominated end-members (samples) requires an addition of a large amount of Fe (16.9 wt%), Mg (2.4 wt%), and a small amount of Al (1.4 wt%), but a simultaneous release of Si (11.8 wt%) and K (6.0 wt%). Considering that the glauconite- and chamosite-bearing strata are devoid of iron-rich detrital minerals (e.g., biotite and iron oxides) and lack evidence of hydrothermal alteration, the required Fe2+for glauconite-berthierine-chamosite transformation was most likely from Fe2+-rich (ferruginous) seawater, which may have promoted glauconite-berthierine transformation at the very early diagenetic stage when Fe2+exchange between porewater and seawater was still available. This interpretation is consistent with the high FeHR/FeT(but low Fepy/FeHR), Fe/Al, and V/Al ratios from the hosting strata that support ferruginous depositional environments. Because most Precambrian strata have passed the oil window temperature (>50–150 °C), the preservation of berthierine would be rare and chamosite should be the representative iron-rich clay mineral. Thus, the abundance of chamosite in fine-grained, marine siliciclastic sediments may be used as a mineral indicator of ferruginous water-column conditions.

Marine carbonate fabrics are intrinsically related to ocean chemistry, physical processes and biological activity. Molar tooth structure (MTS), a globally distributed structure in Proterozoic carbonate sediments, has been widely studied for more than a century; yet its connections with physical and biological processes remain unclear. Using multiple techniques, we studied ∼1.57 Ga MTS and identified a connection between its occurrence and increased marine oxygenation. In our samples, the matrix surrounding MTS is typically dominated by carbonate mud with early diagenetic dolomite crystals. High I/(Ca + Mg) ratios (up to 4.1 μmol/mol) and negative Ce anomalies (∼0.8) detected in the matrix are consistent with the oxidative removal of inhibitors such as Fe2+and Mn2+in the water‐column that permitted carbonate “whiting” mud precipitation stimulated by cyanobacterial photosynthesis. This cohesive but not rigid seafloor carbonate mud was a prerequisite for synsedimentary MTS crack formation. Systematically higher carbon isotope (δ13C) values in MTS microspars, relative to host sediment, support origination of the cracks by methane degassing in the organic‐rich carbonate mud. Low, but non‐zero, I/(Ca + Mg) values of the MTS microspar suggest that the precipitation of the microspar that filled the MTS cracks was triggered by oxidative removal of residual Fe2+and Mn2+in porewater through mixing with overlying oxygenated seawater. We therefore propose that MTS formed under moderately oxygenated conditions and that its sporadic occurrence prior to ∼1.2 Ga reflects episodes of pulsed marine oxygenation in an overall anoxic setting. Molar tooth structure (MTS) has been identified in Precambrian carbonate sedimentary rocks for more than a century. However, its formation and significance are still not fully understood. Precambrian ocean chemistry and low oxygen levels have previously been linked to MTS formation. Here, we use carbonate iodine data as a proxy for oxygenation, and carbon isotope data to decipher microbial processes. The iodine data indicate an oxygenated environment, and the carbon isotope data support previous suggestions that degassing of microbial methane was responsible for MTS crack formation. For the first time, we show that MTS occurred in oxygenated conditions. We propose that the oxidative removal of ions such as Fe2+and Mn2+that can inhibit carbonate precipitation was a key factor in the formation of the cohesive carbonate mud matrix of MTS, and in the rapid precipitation of the microspar cement that filled and preserved the MTS cracks. The relatively rare occurrence of MTS prior to 1,200 Ma could therefore be a sedimentary indicator of sporadic marine oxygenation when marine conditions were mostly anoxic. Petrographic and geochemical data constrain molar tooth structure (MTS) development in ∼1.57 Ga carbonates from North ChinaMicrobially induced water column carbonate mud and CH4degassing from the sediment are critical for MTS formationThe connection between MTS formation and marine oxygenation suggests MTS as a sedimentary redox proxy Petrographic and geochemical data constrain molar tooth structure (MTS) development in ∼1.57 Ga carbonates from North China Microbially induced water column carbonate mud and CH4degassing from the sediment are critical for MTS formation The connection between MTS formation and marine oxygenation suggests MTS as a sedimentary redox proxy

The mid‐Proterozoic (∼1.8–0.8 Ga) is characterized by low atmospheric oxygen and pervasive oceanic anoxia, with transient oxygenation events. One of the oxygenation events happened at ∼1.57 Ga, during which atmospheric O2may have reached ≥4% present atmospheric level (PAL) and thus promoted the evolution of multi‐cellular eukaryotes. The cause of this oxygenation event, however, remained unknown. Here, we report δ202Hg, ∆199Hg, P concentration, Rb/Al, and εNd(t) data across the ∼1.57 Ga oxygenation event in the Gaoyuzhuang Formation of North China. The positive δ202Hg, εNd(t), and negative Δ199Hg shifts, plus the accompanied peaks in Rb/Al, P, and I/(Ca + Mg), suggest that this oxygenation event was likely resulted from enhanced weathering of intermediate to mafic rocks, which may have promoted primary production via increasing P influx to the ocean and led to the rise in oxygen levels. The data indicate that enhanced weathering input could trigger transient oxygenation events during the mid‐Proterozoic. During the mid‐Proterozoic (∼1.8–0.8 Ga), the oxygen concentrations in atmosphere and shallow oceans were persistently low but occasionally interrupted by transient oxygenation events. The origin of these short‐lived oxygenation events, however, is not properly understood. To reveal the potential causes behind these oxygenation events, we take the ∼1.57 Ga oxygenation event as an example to perform mercury (Hg) isotope, εNd(t), major and trace element analyses. The results show that this oxygenation event is associated with high δ202Hg, εNd(t), Rb/Al and low Δ199Hg values, as well as an increase in phosphorus concentration. The most plausible explanation for these data is that enhanced weathering of intermediate to mafic rocks led to an increase of nutrient input (e.g., phosphorus) to the ocean, resulted in high ocean primary productivity and oxygen release, and triggered the transient oxygenation event. Hg‐enrichment recognized in the middle Gaoyuzhuang Formation, North China, coincides with the transient oxygenation event at ∼1.57 GaPositive δ202Hg, Rb/Al and negative Δ199Hg shifts suggest that the Hg enrichment was resulted from enhanced terrestrial weatheringEnhanced weathering increased phosphorous input and primary production in the ocean and triggered the oxygenation event at ∼1.57 Ga Hg‐enrichment recognized in the middle Gaoyuzhuang Formation, North China, coincides with the transient oxygenation event at ∼1.57 Ga Positive δ202Hg, Rb/Al and negative Δ199Hg shifts suggest that the Hg enrichment was resulted from enhanced terrestrial weathering Enhanced weathering increased phosphorous input and primary production in the ocean and triggered the oxygenation event at ∼1.57 Ga

Organic carbon isotope (δ13Corg) data from two well-preserved sections across a shallow-to-deep water transect of the late Ediacaran-Early Cambrian Yangtze Platform in South China show significant temporal and spatial variations. In the shallow-water Jiulongwan-Jijiapo section, δ13Corgvalues of the late Ediacaran Dengying Formation range from −29‰ to −24‰. In the deep-water Longbizui section, δ13Corgvalues from time-equivalent strata of the Dengying Formation are mostly between −35 and −32‰. These new data, in combination with δ13Corgdata reported from other sections in South China, reveal a 6‰-8 shallow-to-deep water δ13Corggradient. High δ13Corgvalues (>−30‰) occur mostly in shallow-water carbonate rocks, whereas low δ13Corgvalues (<−32‰) dominate the deep-water black shale and chert. The large temporal and spatial δ13Corgvariations imply limited buffering effect from a large dissolved organic carbon (DOC) reservoir that was inferred to have existed in Ediacaran-Early Cambrian oceans. Instead, δ13Corgvariations between platform and basin sections are more likely caused by differential microbial biomass contribution to total organic matter. High δ13Corgvalues (>−30‰) documented from shallow-water carbonates are within the range of typical Phanerozoic δ13Corgdata and may record the isotope signature of organic matter from primary (photosynthetic) production. In contrast, low δ13Corgvalues (<−32‰) from deep-water sections may have resulted from higher chemoautotrophic or methanotrophic biomass contribution to bulk organic matter in anoxic environments. The δ13Corgdata provide indirect evidence for ocean stratification and episodic chemocline fluctuations in the Ediacaran-Early Cambrian Yangtze Platform.

Recent geochemical and paleontological studies have revealed a significant ocean oxygenation episode and an evolutionary leap of eukaryotes at the onset of the Mesoproterozoic. However, the potential role of nitrogen availability and its interaction with other nutrients in these environmental and biological events have not been investigated. Here we present an integrated study of nitrogen isotopes (δ15N), organic carbon isotopes (δ13Corg), and major and trace element concentrations from Member III of the Gaoyuzhuang Formation in the central North China Craton where the earliest macroscopic multicellular eukaryotic fossils were reported. The enrichments of redox‐sensitive elements (Mo, U, and V), coupled with Mo‐U covariations, δ13Corg, and I/(Ca + Mg), indicate that the Gaoyuzhuang Member III in the study area was deposited in largely suboxic‐anoxic environments with ephemeral occurrences of euxinia. These data reinforce previous inferences of a strongly redox stratified ocean during the early Mesoproterozoic, but a pulsed oxygenation event may have resulted in deepening of the chemocline. The high δ15N values from the study section are interpreted as a result of aerobic N cycling and the presence of a fairly stable nitrate pool in the surface oxic layer, possibly due to the combined effects of oxygenation and low primary productivity. Increased availability of nitrate could have contributed to the expansion of eukaryotic life at this time. However, our data also suggest that nitrate alone was not the only trigger. Instead, this evolutionary leap was likely facilitated by multiple environmental factors, including a rise in O2levels and increasing supplies of phosphorus and other bio‐essential trace elements. The oldest macroscopic eukaryotes (algae) first appeared in ancient oceans about 1.56 billion years ago. Before that, only simple and microscopic fossils have been discovered. The reason behind this evolutionary leap is, however, still puzzling scientists. Here we use multiple proxies to study the change of nutrient and oxygen levels during this critical period. Our results demonstrate the availability of the nutrient nitrogen (in form of nitrate) in the oxic surface ocean owing largely to restrictive consumption by organic carbon decomposition, providing favorable N substance for bio‐utilization. Furthermore, we also found evidence for the joint increase of O2level, seawater phosphorus, and some bio‐essential trace elements along with the occurrence of these earliest algae. Our study provides important clues for untangling the triggers of biological innovation at circa 1.56 Ga. Integrated data of δ15N, δ13Corg, and major and trace element contents are reported from the circa 1.56 Ga Gaoyuzhuang Formation in North ChinaThe preservation of an aerobic N cycling signal in a stratified ocean is attributed to joint effects of O2rise and low primary productivityThe evolutionary leap of eukaryotes at circa 1.56 Ga was triggered by expanding reservoirs of multiple essential nutrients