Marine Redox Research Articles

Synchrotron-based P K-edge XANES spectroscopy reveals the transition of phosphorus cycling in the early Cambrian ocean

Phosphorus, as a long-term limiting nutrient, is essential for the rapid animal diversification in the early Cambrian ocean. However, the widespread occurrence of phosphorites during this period indicates anomalous phosphorus cycling mechanisms that remain inadequately explored. To investigate the interplay between marine redox conditions and phosphorus cycling, we used geochemical proxies and synchrotron-based P K-edge X-ray Absorption Near Edge Structure (XANES) spectroscopy to analyze the Cambrian Terreneuvian Yurtus Formation (Fortunian to Stage 2) from the Tarim Block. Redox-sensitive elements, iron speciation and nitrogen isotopes reveal an expansion of the surface oxygenated waters, despite persistent ferruginous bottom waters. Moreover, a shift of phosphorus speciation was observed in bedded cherts via P K-edge XANES, suggesting a transition of phosphorus cycling. Specifically, the samples exhibit low total phosphorus contents with a high rate of carbonate-fluorapatite authigenesis in the lower Yurtus Formation. This is followed by an increase in the proportion of terrestrial-derived fluorapatite and iron-bound phosphorus, corresponding to the expansion of surface oxygenated waters. Our findings reveal a significant shift in dominant phosphorus speciation in the early Cambrian marine sediments, which is intricately linked to the coupled phosphorus‑iron‑carbon‑oxygen cycles in the Cambrian ocean.

Isotopically light Mo in sediments of methane seepage controlled by the benthic Fe–Mn redox shuttle process

Methane seepage has been extensively observed in various continental margin settings. It has profound effects on the marine redox environment and the molybdenum (Mo) cycles in marine sediments. Therefore, there has been much recent attention on the redox-sensitive behavior of Mo in methane seepage environments. However, the characteristics of the Mo isotope composition in the cold-seep system remain poorly understood. In this study, we performed geochemical analyses, including Mo content and isotope composition, on sediment samples (core QDN-MS6) from the “Haima” cold-seep deposit area in the South China Sea. The analysis reveals a significant concentration of authigenic pyrite in the mid-section of QDN-MS6 core (373–403 cm). Moreover, the δ34S value in this interval is notably elevated with high total sulfur/total organic carbon ratio. Additionally, the sediments in the mid-section exhibits substantial enrichment in Mo (enrichment factors of Mo ranging from 5.29 to 39.32). This implies that the sediments in the mid-section are influenced by sulfate-driven anaerobic oxidation of methane. Most notably, the sediments in the mid-section displayed distinct low δ98Mo values (with an average of −0.7‰). After careful consideration, we ruled out the influence of organic matter, an oxic environment, a weakly sulfidic environment, and incomplete removal of thiopolybdate as contributing factors. Based on δ56Fe-Fe/Al ratios, (Mo–U) enrichment factors, and As enrichment factors, we propose that the “benthic Fe-Mn redox shuttle process” is the primary cause of the observed light δ98Mo signatures in sediments. This newly identified mechanism sheds light on Mo isotope cycling in methane seepage environments and enhances our understanding of the Mo isotope cycling process.

Acceleration of phosphorus weathering under warm climates.

The release of phosphorous (P) via chemical weathering is a vital process that regulates the global cycling of numerous key elements and shapes the size of the Earth's biosphere. It has long been postulated that global climate should theoretically play a prominent role in governing P weathering rates. Yet, there is currently a lack of direct evidence for this relationship based on empirical data at the global scale. Here, using a compilation of temperature and P content data of global surface soils (0 to 30 cm), we demonstrate that P release does enhance at high mean annual surface temperatures. We propose that this amplification of nutrient supply with warming is a critical component of Earth's natural thermostat, and that this relationship likely caused expanded oceanic anoxia during past climate warming events. The potential acceleration of phosphorus loss from soils due to anthropogenic climate warming may pose threats to agricultural production, terrestrial and marine ecosystems, and alter marine redox landscapes.

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.

Enrichment pattern of tungsten in sediments under methane seepage environments: Applicability as a proxy for tracing and reconstructing (paleo-)methane seepage

The release of hydrogen sulfide by sulfate-driven anaerobic oxidation of methane (SD-AOM) during methane seepage activities has a strong impact on the evolution of marine redox conditions and the cycles of redox-sensitive elements (e.g. Mo and U). Consequently, the sedimentary enrichment or depletion of these elements is often used to trace and reconstruct (paleo-)methane seepage activity. In recent years, tungsten (W) has shown promise as an indicator of marine redox changes, but its applicability to trace methane seepage activity is unknown. This study identified significant differences in the W content of sediments in methane seepage environments compared to suboxic environments not influenced by methane seepage and oxic environments in the South China Sea. The sediments subjected to methane seepage were characterized by low W contents, similar to those of euxinic Black Sea sapropels. It is believed that the sulfidic environment resulting from SD-AOM is the primary cause of W depletion in sediments subjected to methane seepage. By examining W's interaction with its twin element, Mo, we present WMo enrichment patterns in various environments. In methane seepage environment (sulfidic environments), sediments had high MoEF (> 10) and low WEF (< 2) with a high Mo/W molar ratio (> 10); in suboxic environments not influenced by methane seepage, sediments exhibited a WMo enrichment pattern similar to the UCC (the MoEF and WEF values are predominantly below 1 and 2, respectively, with a low Mo/W molar ratio, <1.5); in oxic environments (enrichment of FeMn (oxy)hydroxides), sediments exhibited high MoEF (2 < MoEF < 10) and high WEF (> 2), with a medium Mo/W molar ratio (1.5–10). This further indicates that the Mo/W molar ratio has great potential in tracing and reconstructing (paleo-)methane seepage activities.

Shallow-water redox changes and nitrogen cycles in the early Cambrian Nanhua Basin, South China

Widespread oceanic anoxia has been frequently invoked for metazoan extinctions during the Cambrian. However, this hypothesis remains to be debated. In this study, we measured total organic carbon (TOC), total bulk nitrogen (TNbulk), iron speciation, major and trace elements, and isotopic compositions of sedimentary bulk nitrogen (δ15Nbulk) for the Shiyantou Formation in the Meishucun section. Our results suggest ferruginous conditions and high productivity in the lower member (LM: 0–16.0 m), dominantly suboxic conditions and low productivity in the middle member (MM: 16.0–23.7 m), and oxic conditions and low productivity in the upper member (UM: 23.7–28.0 m). Low δ15Nbulk values (average 1.0 ‰) in the LM suggest quantitative assimilation of NH4+ and strong nitrogen fixation as a consequence of high primary productivity and widespread water-column reducing conditions in the Nanhua Basin. δ15Nbulk values close to 0 ‰ in the MM suggest intense nitrogen fixation owing to quantitative denitrification and anammox in an oligotrophic environment. Low δ15Nbulk values (average 1.2 ‰) in the UM are indicative of marine nitrogen limitations in an oligotrophic environment. These findings suggest a similar range of low δ15Nbulk values might result from different environments. In comparison with marine redox conditions and δ15Nbulk data from the other coeval sections in the Nanhua Basin, we propose that widespread anoxic conditions might cause the extinction of small shelly faunas. More importantly, the oligotrophic environment and anoxic conditions might delay the recovery of small shelly faunas in the shallow and deep waters of the Nanhua Basin, respectively.

Statistical estimation of the early to middle Ediacaran ocean redox architecture in the Yangtze block of South China

Early to middle Ediacaran organic-rich black shales host several famous fossil biotas and provide key evidence for understanding the coevolution of multicellular organisms and palaeoceanic environment. The water column redox states play critical roles in organic-rich shale deposition. Amongst multiple geochemical redox indexes, iron-speciation chemistry (FeHR/FeT, Fepy/FeHR) in shales constitutes a reliable proxy to reconstruct the first-order redox framework of the ocean basin. However, iron-speciation is a local marine redox proxy, and only compiled data collected from multiple different sedimentary facies record statistically significant changes of oxidation states within a depositional basin. Here we compiled the spatiotemporal distribution of iron-speciation data in early to middle Ediacaran black shales of the Yangtze block in South China. New high-resolution analysis on two outcrops and one drill-core reflects overall ferruginous condition and dominant euxinic condition locally interrupted by oxic states, respectively, generally indicating pervasive anoxic depositional environment for the Member II shales of the Ediacaran Doushantuo Formation in the Lower Yangtze block. Compilation of fourteen Ediacaran sections from shallow-, slope- and deep-water facies demonstrates frequent spatiotemporal oscillation of the redox conditions throughout the Yangtze block. Widespread ferruginous states, accompanied with euxinic zones mainly focusing in lower slope facies, are detected during the early Ediacaran period, in contrast with more widespread euxinic settings in the middle Ediacaran period. Statistical data including new high-resolution results from the Lower Yangtze block show generally low enrichment for redox sensitive elements (RSEs, e.g., Mo, V and U) in the Member II shales probably due to their contemporary low seawater concentrations. An increase of RSEs concentrations occurs in the Member IV shales, which probably reflects more oxidized Earth surface environment during middle Ediacaran period. The increasing Earth oxygenation documents the occurrence of widespread euxinic states during deposition of the Member IV shales. Compilation of spatial distribution of TOC contents in black shales demonstrates their apparent relevance to spatial distribution of the euxinic rather than ferruginous states, which is likely due to the favorable role of sulfurization process on burial and preservation of organic matters. The euxinic states may also facilitate the exceptional preservation of the soft-bodied Ediacaran fossils.

Zinc isotopic evidence for enhanced continental weathering and organic carbon burial in the Early Silurian

The Late Ordovician and Early Silurian periods experienced glaciation and mass extinctions. However, debates exist regarding the factors that initiated the glaciation and the subsequent delay in biotic recovery. To uncover the relationships between paleoclimate, marine redox states, and biotic recovery in the Early Silurian, temporal variations in Zinc (Zn) isotopic compositions along with other multi-geochemical proxies were analyzed for a carbonate-dominated section in South China. A stratigraphic δ66Zn trend was found in studied section, indicating a widespread disturbance to the marine Zn cycle. A significant negative shift in δ66Zn by up to ∼0.7 ‰ was observed immediately following the Hirnantian Glaciation and HICE, which can be attributed to a combination of reduced burial efficiency of organic carbon, fast weathering of large igneous provinces and increased riverine Zn input. This was followed by a ∼ 0.4 ‰ rise in δ66Zn values, alongside a sharp decrease in δ13C values, indicating an increase in sulfide precipitation (e.g., ZnS, FeS) due to expansions of euxinia. Subsequently, a notable rise in δ66Zn values by up to ∼1.1 ‰ is interpreted as an increase in organic carbon burial, given the concurrent variation of δ66Zn and δ13C during this period. The findings suggest that the increase in organic carbon burial, a reduction in reverse weathering, or a combination of them contributed to the glaciation in Silurian. Furthermore, the weathering-induced high primary productivity led to anoxic water conditions in the latest Hirnantian and Rhuddanian. This prolonged marine anoxia in seawater thus caused the deposition of organic-rich sediments and hindered biotic recovery in the Rhuddanian.

Phosphate-associated sulfate in Lingula shells represents a potential archive for seawater sulfate composition

The marine sulfur cycle is highly sensitive to redox environments at the Earth's surface. Fluctuations in atmospheric pO2 level or marine redox environments are commonly associated with changes in seawater sulfate concentration, sulfate sulfur (S) and oxygen (O) isotopes. Thus, the marine S cycle through the Earth's ancient past provides a first-order constraint on the redox evolution of the Earth's surface. Reconstructions of the marine S cycle have been approached through analyses of various sulfate-bearing geological materials, such as sedimentary barite, evaporitic sulfate (e.g., gypsum), carbonate associated sulfate (CAS), and phosphate-associated sulfate (PAS) in phosphatic ores. These geological materials have their own pros and cons. In this study, we focus on PAS in Lingula brachiopods, which possess phosphatic shells and have a long evolutionary history since Cambrian times. Biogenic phosphate typically has a higher sulfate concentration than carbonate and phosphate ores, and is less vulnerable to diagenetic alteration. Here, we analyzed modern Lingula shells from the South China Sea and fossil Lingula shells collected from the Late Devonian Xiejingsi Formation of South China. Living and fossil Lingula shells have similar P/S ratios (PO43−/SO42−) of ∼76, and S and O isotopes of PAS (δ34SPAS and δ18OPAS) in each group show limited variations. For living Lingula shells, the mean values of δ34SPAS and δ18OPAS are +15.0 ± 1.5‰ and + 9.8 ± 0.4‰, respectively, which are ∼6‰ lower and comparable to those of modern seawater sulfate compositions (δ34SSW and δ18OSW-SO4), probably due to vital effects. For fossil Lingula shells, δ18OPAS (14.1 ± 0.9‰) is identical to coeval evaporite. The range of δ34SPAS is encompassed by the values of both coeval evaporite and CAS, but the mean value of δ34SPAS (29.5 ± 0.9‰) is higher. Thus, fossil Lingula shells show a good potential for reconstructing δ34SSW and δ18OSW-SO4 values.

Marine redox dynamics and biotic response to the mid-Silurian Ireviken Extinction Event in a mid-shelf setting

The early Silurian Llandovery–Wenlock boundary interval is marked by significant marine perturbations and biotic turnover, culminating in the Ireviken Extinction Event and the Early Sheinwoodian Carbon Isotope Excursion. Here, we apply multiple independent redox proxies to the early Wenlock Buttington section, which was deposited in a mid-shelf location in the Welsh Basin, UK. To account for the regional geochemical variability in marine sediments due to factors such as sediment provenance, we first define oxic baseline values for the Welsh Basin, utilizing deeper water, well-oxygenated intervals of late Llandovery age. Our approach documents unstable, oscillating redox conditions on the mid-shelf at Buttington. We suggest that these dynamic redox fluctuations are likely to relate to changes in the position of the chemocline or a migrating oxygen minimum zone. Benthic biota, such as trilobites, brachiopods, bivalves and gastropods, appear to have been relatively unaffected by fluctuating oxic-ferruginous conditions, but were more severely impacted by the development of euxinia, highlighting the inhibiting role of toxic sulfides. By contrast, the redox perturbations appear to have placed extreme stress on graptolites, causing many extinction losses regardless of the specific development of euxinia. Supplementary material: The geochemical data for the Buttington section is available at https://doi.org/10.6084/m9.figshare.c.7165009 Thematic collection: This article is part of the Chemical Evolution of the Mid-Paleozoic Earth System and Biotic Response collection available at: https://www.lyellcollection.org/topic/collections/chemical-evolution-of-the-mid-paleozoic-earth-system

Local depositional and diagenetic conditions control the positive δ34S excursion during the Ordovician-Silurian transition: Evidence from the Wufeng-Longmaxi formations in Sichuan Basin

The Ordovician-Silurian (OS) transition was characterized by significant changes in global climate, marine redox conditions, and biological evolution, which include global seawater euxinia, the onset of the Hirnantian glaciation, and large-scale extinctions. During this time, the marine sulfur cycle was disrupted, and the pyrite sulfur isotope (δ34Spy) showed a large positive excursion. This excursion is closely tied to the Hirnantian glaciation, with δ34Spy increasing at the start of and decreasing at the end of the glaciation. Although it is thought that variations in marine sulfate concentration and primary productivity are responsible for the positive excursion in δ34Spy, contemporary marine sediment δ34Spy data suggest that sedimentary processes can also lead to significant fluctuations in δ34Spy. To assess the impact of local changes in sedimentary conditions on this positive δ34Spy excursion event, we conducted detailed depositional and geochemical analyses of sedimentary pyrite found in black shales from the Wufeng-Longmaxi formations in the Weiyuan area of the Sichuan Basin, China. These sedimentary pyrites include three types: framboidal, euhedral, and laminated pyrite. Bulk sample sulfur isotope analyses revealed that δ34Spy values vary from −16.2‰ to 15.8‰ (mean = −3.8‰, n = 22). In situ analyses of δ34Spy show that different pyrite crystals exhibit a range of intragranular δ34Spy values that are likely indicators of diagenesis. Variations in bulk sample δ34Spy are strongly and negatively correlated with the proportion of framboidal pyrite, i.e., lower levels of framboidal pyrite content resulted in a higher bulk sample δ34Spy value, suggesting diagenetic alteration controls on δ34Spy. In addition, the δ34Spy excursion in the Wufeng-Longmaxi formations also appears to be compatible with lithological changes related to sedimentation rate. The δ34Spy values from the Guanyinqiao Bed, which consists of calcareous shales, are higher than those from black shales with a relatively low sedimentation rate. Our results suggest that the positive δ34Spy excursion during the OS transition during the Hirnantian glaciation is controlled not only by diagenetic alterations but also by local sedimentation rate.

Formation conditions of Jixian System cherts in the Qishan area, Ordos Basin: Implications for marine redox conditions and paleoecology

Abundant chert bands and nodules are discovered throughout the Mesoproterozoic Jixian System in the Ordos Basin. These cherts faithfully record the Jixian period oceanic conditions and paleoecology. However, the diagenetic mechanism of the cherts remains unclear and controversial. To understand the origin of these cherts, we performed a multitracer study by combining field reconnaissance, petrological analyses, Si isotope analysis, and major and trace element analysis of chert samples from the Qishan section. The results show that Jixian cherts had a high Al/(Al + Fe + Mn) value (~0.48), showed a flat distribution of rare earth elements in seawater, and most of the Fe/Ti values are <20. A weak negative Ce anomaly (~0.97) indicates the contribution of REY to weakly oxygenated seawater. The high isotopic composition of silicon (δ30Si = 0.74 ‰-1.35 ‰) and the average Siex value of 37.52 indicate that there is a source of biological silica, and the relationship between Eu/Eu* and Y/Ho and δ30Si shows that the hydrothermal and volcanic influences were less and correlated with seawater. The chert bands were closely related to biological activities. The chert nodules were influenced by hydrothermal activity. Secondary cherts were related to diagenesis, but the nature of siliceous fluids remains unchanged, still characterized by biogenic activity origins. The sea oxygen concentration in the Jixian period had increased, with periodic variations in the intensity of biological activities affecting the pH of the water body, or biological photosynthesis binding SiO2 colloid in water, affecting chert sediments, leading to frequent interlayering between dolomite and chert bands. Episodic hydrothermal fluids and late-stage diagenetic processes jointly influenced the formation of cherts. This study bears significant significance in enhancing our understanding of the sedimentary environment and the origin of cherts during the Jixian period in the Ordos Basin.

Synglacial carbonate records of snowball Earth ocean composition—Evidence from the Nantuo Formation, South China

Abstract The Cryogenian (ca. 717–635 Ma) snowball Earth glaciations ended with the precipitation of “cap” carbonate successions with negative carbon isotope (δ13Ccarb) values, which have been explained by the addition of various 13C-depleted carbon sources in the deglacial process. These arguments assumed that marine dissolved inorganic carbon (DIC) was enriched in 13C in the synglacial ocean. However, this assumption has not yet been tested, because the synglacial ocean chemistry is unknown. In this study, we carried out detailed analyses of the petrology, carbonate carbon (δ13Ccarb) and oxygen (δ18Ocarb) isotopes, organic carbon (δ13Corg) isotopes, major and minor elemental compositions (Ca, Mg, Mn, Fe, Sr), and iron speciation (total Fe, highly reactive Fe, pyrite Fe) of the carbonate layers (also called synglacial carbonate layers) from the Nantuo Formation (ca. 650–635 Ma) on the Yangtze block, South China. Petrographic observations indicated that the synglacial carbonate comprises dolomicrite, mud-crystal powder dolomite, lime dolomite, and dolomitic limestone, supporting an authigenic carbonate origin, and thus, it potentially recorded the ocean chemistry during the Marinoan ice age. The synglacial carbonate is characterized by extreme Mn enrichment, low Fe/Mn ratios, and low δ13Ccarb (−7‰) values. High Mn contents and low Fe/Mn ratios imply marine redox conditions favoring Mn2+ accumulation and Fe2+ oxidation, while low δ13Ccarb values might be attributed to CO2 degassing of submarine volcanoes as well as low primary burial during the glaciation. Since the δ13CDIC value of the synglacial ocean was lower than the δ13Ccarb values of most cap carbonates, we infer the addition of 13C-enriched DIC or removal of 12C during cap carbonate precipitation, such as through carbonate weathering or organic carbon burial. These findings provide new insights into the nature of Cryogenian glaciation, the origin of cap carbonates, and the aftermath of global glaciation.

Molybdenum Isotope Fingerprinting of Microbial Sulfate Reduction in Seep Carbonate Rocks

AbstractUnderstanding the interaction between molybdenum (Mo) and organic matter during microbial sulfate reduction is critical for the use of Mo to reconstruct marine redox conditions throughout Earth's history. However, little is known about Mo isotope fractionation and how it relates to organic matter remineralization during microbial sulfate reduction. Here, we report Mo abundances and isotopic (δ98Mo) compositions for bulk‐rock, non‐lithogenic and sequentially extracted fractions, including carbonate (carb), pyrite, and organic matter (OM), of seep carbonate rocks. Our data indicate that the difference between δ98Mocarb and δ98MoOM (Δ98Mocarb‐OM) displays significant variability in the studied samples, ranging between 0.72 and 1.01‰. Remarkably, the obtained Δ98Mocarb‐OM values indicate correlative trends with stable carbon isotope ratios and bulk abundances of (a) total organic carbon, (b) Mo, and (c) pyrite in seep carbonates, which we interpret as reflecting sustained adsorption of isotopically light Mo onto organic matter during enhanced sulfate reduction. On this basis, we put forward the concept that Δ98Mocarb‐OM of authigenic carbonate rocks can be used as a measure of the intensity of sulfate reduction and for reconstructing past interactions between Mo and organic matter in marine sediments.

Marine redox fluctuations during the Marinoan glaciation

The proliferation of eukaryotes preceding the Cryogenian Marinoan glaciation (650–635 Ma) and the subsequent radiation of early animals imply a conceivable linkage between global glaciation and complex life evolution. However, the marine redox condition remains enigmatic during the Cryogenian, impeding our understanding of the role of O2 on this biological innovation. To fill this gap, we comprehensively reported mineralogy, iron isotope (δ56Fe), and iron speciation in the Cryogenian Nantuo Formation at Liulongshan, South China. This section encompasses glacial marine and non-glacial deposits, where the redbeds situated in the middle of the section, in conjunction with the diamictites adjacent to them, divide the Nantuo Formation into three distinct glacial sedimentary cycles. Micro-particle hematite within the Nantuo Formation indicates that the primary Fe-oxide was authigenic and trapped in water column. Furthermore, the Fe isotopic compositions of these authigenic components (δ56FeHR) calculated by mass balance equation reveal the redox state of the ocean coinciding with dynamic glaciation. The lower diamictite shows an increasing trend of δ56FeHR, Fe/Al, and FeHR/FeT, indicative of gradually more anoxic seawater conditions coupled with ice advance. Although redbeds intervals exhibit higher FeHR/FeT ratios (∼ 0.51), traditionally indicative of anoxic environments, our integration of mineralogical evidence suggested that the increased Feox content in redbeds might represent increased oxidation of Fe(II) at the redox chemocline, rather than the redox conditions of the bottom waters. This interpretation is consistent with the observed decreasing trend in our Fe isotope data, as well as the sedimentological evolution. Subsequently, the positive and increasing δ56FeHR values suggest that the oceanic redox conditions shifted to anoxic as the second glacial episode re-emerged. These findings suggest dynamic fluctuations in the redox condition of the ocean during the Marinoan Ice Age. Specifically, the formation of the redbeds may reflect a locally oxic environment. A modern analogy resembling this situation is blood falls, where the oxidation of iron-rich brine plumes under the Antarctic ice sheets. Such subglacial environments could have potentially provided a habitable “refuge” for early life to survive during the Neoproterozoic glaciation.

Marine eutrophication within the Tarim Platform in sync with Middle to Late Ordovician climatic cooling

Previous work has proposed climatic cooling and atmosphere–ocean oxygenation as potential triggers for the Great Ordovician Biodiversification Event, with the suggestion of better oxygenated oceans during the Middle to Late Ordovician. However, recent studies have argued for spatial and temporal heterogeneity in marine redox state on several continents. Here we investigate a black-shale succession accumulated within the Tarim Platform via a combination of geochemical proxies to address these debates. Negative shifts in bulk nitrogen isotopes and synchronous increases in excess phosphorus suggest moderate-high marine primary production coinciding with the development of bottom-water anoxia, as indicated by enrichments in highly reactive iron and modest concentrations of redox-sensitive trace metals (Mo, U). Moreover, the occurrence of black shale correlates well with equivalent successions formed in deep-water marginal basins along several continents, including South China, North China, Laurentia and Baltica. This may suggest an expansion of marine anoxia in low-latitude zones of the late Darriwilian to early Sandbian oceans, probably as a result of enhanced upwelling in sync with climatic cooling. The extent and ultimate cause of marine anoxia requires further quantifying constraints at a global scale, which will enable potential links between global oceanic redox conditions and concurrent biotic changes to be evaluated in more detail. Supplementary material: Geochemical dataset for the Dawangou section (Table S1) is available at https://doi.org/10.6084/m9.figshare.c.7036552 Thematic collection: This article is part of the Chemical Evolution of the Mid-Paleozoic Earth System and Biotic Response collection available at: https://www.lyellcollection.org/topic/collections/chemical-evolution-of-the-mid-paleozoic-earth-system

Organic Matter Accumulation in the Upper Permian Dalong Formation from the Lower Yangtze Region, South China

AbstractThe Late Permian was marked by a series of important geological events and widespread organic‐rich black shale depositions, acting as important unconventional hydrocarbon source rocks. However, the mechanism of organic matter (OM) enrichment throughout this period is still controversial. Based on geochemical data, the marine redox conditions, paleogeographic and hydrographic environment, primary productivity, volcanism, and terrigenous input during the Late Permian in the Lower Yangtze region have been studied from the Putaoling section, Chaohu, to provide new insights into OM accumulation. Five Phases are distinguished based on the TOC and environmental variations. In Phase I, anoxic conditions driven by water restriction enhanced OM preservation. In Phase II, euxinic and cycling hydrological environments were the two most substantial controlling factors for the massive OM deposition. During Phase III, intensified terrestrial input potentially diluted the OM in sediment and the presence of oxygen in bottom water weakened the preservation condition. Phase IV was characterized by a relatively higher abundance of mercury (Hg) and TOC (peak at 16.98 wt%), indicating that enhanced volcanism potentially stimulated higher productivity and a euxinic environment. In Phase V, extremely lean OM was preserved as a result of terrestrial dilutions and decreasing primary productivity. Phases I, II and IV are characterized as the most prominent OM‐rich zones due to the effective interactions of the controlling factors, namely paleogeographic, hydrographic environment, volcanism, and redox conditions.

Paleo-marine redox environment fluctuation during the early Cambrian: Insight from iron isotope in the Tarim Basin, China

The Ediacaran to Cambrian period is generally considered to be the vital transition in the history of marine redox environment and life evolution on earth. The ocean oxygenation levels during this transition period are still debated. Since iron is widely involved in biogeochemical cycles and undergoes redox cycling both in the seawater and sediments, it has become a significant proxy to reconstruct paleo-marine environment. In order to constrain the paleo-marine redox state in the early Cambrian, the iron isotope composition of bulk rock (δ56FeT) is interpreted combining with iron-speciation, redox sensitive elements and pyrite sulfur isotope (δ34Spy) of Yuertusi Formation in Tarim Block. The δ56FeT values varies from −0.39 ‰ to 0.48 ‰, with an average of 0.07 ‰, mainly controlled by pyrite mineral facies in this study. Based on the mechanism of pyrite generation in different redox condition, it is proposed that the marine environment of the lower Cambrian in the Tarim basin is dominated by anoxic with intermittent euxinic state. The dynamic evolution of redox environment can be divided into three intervals. The gradual decrease of δ56Fe in Interval I indicates the paleo-marine environment changed from anoxic ferruginous to euxinic, and the paleo-marine sulfate reservoir decreased to a limited level, which might be attributed to abundant burial of organic matter and pyrite. For Interval II, δ56Fe values first increase to evident positive because of partial oxidization then decreased to that of seawater (about 0 ‰) due to complete oxidization. In Interval III, the continuous decrease of δ56Fe values infers a sustaining oxidization. In summary, the paleo-marine environment of the lower Cambrian Yuertusi Formation evolved from anoxic ferruginous to euxinic and then oxidized continuous. Iron isotope statistics from geological historical periods indicate that seawater was relatively oxidized after the NOE event but did not reach the oxidation levels of present-day seawater.

Nitrogen isotopic compositions of organic-rich shales (∼560 Ma) in the Chengkou region, South China: Implications for a stable and relatively large nitrate reservoir of the late Ediacaran ocean

The late Ediacaran period is a critical transitional period in Earth's history, marked by the evolution of the Ediacara biota (∼574 to 538.8 Ma). The marine redox state and nutrient level, having a close relationship with bioevolution, can be reflected by sedimentary nitrogen isotopes (δ15N). Doushantuo Formation Member IV, a widespread organic-rich interval during this critical period of South China, may provide important clues for nitrogen (N) cycling. However, high-resolution δ15N research on Member IV is still lacking. In this study, a drill core from the Chengkou sub-basin, with the well-developed Doushantuo Member IV, was carefully studied base on its N isotopic composition. The implications for the N cycling characteristics are discussed using a combination of proxies for bottom water redox conditions and basin openness. The total nitrogen (TN) concentrations (up to 0.79 wt%) of these organic-rich samples are substantially higher than those of other counterpart profiles. The TOC content shows a positive linear relationship with TN concentration, and a significant positive TN-intercept (∼0.21 wt%) represents the addition of inorganic N produced by the remineralization of sinking organic N. The high TN concentrations may also be due to the flourishing and enhanced burial of primary producers. The δ15N value in the Doushantuo Member IV progressively decreases from ∼8.2‰ to ∼3.2‰ (from bottom to top), a result of the changes in basin openness and the enhancement of N2-fixation. Compilation shows the δ15N and δ13Corg values are approximately covariant, and both have significant temporal and spatial variations at this member of South China, indicating a strong heterogeneity in the water column redox environments and changes in the type of primary producers. A mode δ15N value of 4‰ indicates a stable and relatively large nitrate reservoir in the late Ediacaran ocean.

Globally limited but severe shallow-shelf euxinia during the end-Triassic extinction

One of the most severe extinctions of complex marine life in Earth’s history occurred at the end of the Triassic period (~201.4 million years ago). The marine extinction was initiated by large igneous province volcanism and has tentatively been linked to the spread of anoxic conditions. However, the global-scale pattern of anoxic conditions across the end-Triassic event is not well constrained. Here we use the sedimentary enrichment and isotopic composition of the redox-sensitive element molybdenum to reconstruct global–local marine redox conditions through the extinction interval. Peak δ98Mo values indicate that the global distribution of sulfidic marine conditions was similar to the modern ocean during the extinction interval. Meanwhile, Tethyan shelf sediments record pulsed, positive δ98Mo excursions indicative of locally oxygen-poor, sulfidic conditions. We suggest that pulses of severe marine de-oxygenation were restricted largely to marginal marine environments during the latest Triassic and played a substantial role in shallow-marine extinction phases at that time. Importantly, these results show that global marine biodiversity, and possibly ecosystem stability, were vulnerable to geographically localized anoxic conditions. Expanding present-day marine anoxia in response to anthropogenic marine nutrient supply and climate forcing may therefore have substantial consequences for global biodiversity and wider ecosystem stability.