Marine Redox Conditions 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.

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.

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.

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.

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.

Uranium isotopes in non-euxinic shale and carbonate reveal dynamic Katian marine redox conditions accompanying a decrease in biodiversity prior to the Late Ordovician Mass Extinction

The Late Ordovician mass extinction (LOME) is the first major mass extinction event in the Phanerozoic. However, global biodiversity started to decline during the Katian (prior to the LOME) and coeval global ocean redox conditions are not well understood. The deposition of Katian organic-rich sedimentary rocks, namely the calcareous Collingwood Member and overlying siliciclastic Rouge River Member, was triggered by the concurrent Taconic Orogeny in southern Ontario (Canada) near the interface between the epicratonic Appalachian and Michigan basins. In this study, we used geochemical elemental proxies and uranium isotope compositions (δ238U) to reconstruct local and global ocean redox states during deposition of both units at seven drill core localities throughout southern Ontario. Local redox conditions are revealed by several proxies (redox sensitive trace metals, Fe speciation, and Corg:P ratios). Collectively, these proxies suggest spatiotemporal redox variations for the Collingwood Member: O2-deficient conditions (non-euxinic) for the deep waters of the Appalachian and Michigan basins and mostly oxic-suboxic conditions for the shallow waters. In contrast, predominantly oxic-suboxic conditions are suggested for the Rouge River Member.Global ocean redox conditions are inferred from the δ238U of both units. The coeval seawater δ238U value during Collingwood Member deposition is estimated to be from ∼−0.87‰ to ∼−0.64‰, based on both carbonate δ238U (samples leached with 1 N HCl) and δ238Ubulk-non-detrital data (corrected from the bulk δ238U). Particularly, the δ238Ubulk-non-detrital data of the Collingwood Member from multiple cores (both shallow and deep waters) are positively correlated with elemental redox proxies, suggesting a local redox control on δ238U offsets between sediments and water columns. In contrast, the δ238Ubulk-non-detrital of the suboxic Rouge River Member (average = −0.32 ± 0.12‰) from several cores do not correlate with local redox proxies. The coeval seawater δ238U during Rouge River Member deposition is estimated between −0.62 ± 0.12‰ and −0.42 ± 0.12‰ assuming a δ238U offset of 0.1–0.3‰ as observed between modern suboxic sediments and seawater. For this range of seawater δ238U, a three-sink U isotope mass balance model suggests a contraction of global euxinic seafloor area from deposition of the Collingwood Member (0.5–31.6%; median = 1.0–14.2%) to the Rouge River Member (0.2–2.0%; median = 0.3–1.0%), which could have been associated with the concurrent Taconic Orogeny. Collectively, global marine redox proxy data (i.e., δ98Mo, δ238U) from this and previous studies imply dynamic Katian ocean redox conditions during the decrease of metazoan biodiversity prior to the LOME. As a similar extent of global ocean euxinia during the LOME2 also occurred episodically during the Katian but did not result in LOME2-like mass extinctions, it is suggested that other factors (e.g., climate), besides expanded ocean euxinia, could have contributed to the second phase of the LOME.

Mass Extinction Coincided With Expanded Continental Margin Euxinia During the Cambrian Age 4

AbstractEarth's biosphere witnessed the first major extinction event in the Phanerozoic during the Cambrian Age 4, with a genera loss up to ∼45%. The traditional view suggested that marine anoxia was the main cause of the biotic crisis, yet recent geochemical investigations yielded highly debated opinions on marine redox states during the Cambrian Age 4. Herein, we supplement new geochemical evidence for expanded marine euxinia at the extinction intervals on the Yangtze Platform, South China. Most importantly, modern‐level sedimentary δ98Mo (∼+2.34‰) records were most parsimoniously explained by transitory expansion of continental margin euxinia and concomitant intensification of sedimentary Mo sequestration via Fe‒Mn shuttles in the global ocean. The results clarify global marine redox conditions during the Cambrian Age 4, and lend firm support to a causal link between expanded marine euxinia and the extinction event.

Widespread marine euxinia along the western Yangtze Platform caused by oxygen minimum zone expansion during the Capitanian mass extinction

The development of widespread marine anoxic and/or euxinic conditions has been proposed as a likely driver of the mid-Capitanian mass extinction. However, the driving mechanisms and spatiotemporal evolution of anoxia/euxinia remain poorly constrained. In order to decipher changes in marine redox conditions and their possible influence on the mid-Capitanian biotic crisis, we applied multiple geochemical indicators to three sections across a shelf-to-basin transect in the Middle Permian Kuhfeng and Lower Yinping formations of the Lower Yangtze Basin, South China. Our integrated Fe speciation and redox-sensitive trace element data suggest that euxinic waters dynamically coexisted at intermediate depths on the western margin of the Yangtze Platform, with oxygenated surface waters and ferruginous deeper waters providing compelling evidence for a redox structure similar to an oxygen minimum zone (OMZ). The synthesis of a five-stage spatiotemporal evolution of redox conditions, coupled with changes in upwelling and hydrographic restriction, indicates particularly intensified euxinia and an expanded OMZ across the shelf-basin transect during the middle Capitanian. Long-lasting anoxia-euxinia was likely maintained by enhanced phosphorus cycling, with sluggish ocean circulation due to collapsed upwelling and enhanced restriction under climate warming intensifying the euxinic conditions. Through a comparison of available global data, we infer that widespread anoxia-euxinia was prevalent in the middle Capitanian due to expanded OMZ conditions. Expanded anoxia and euxinia in shelf and slope environments occurred concurrently with an ongoing biotic crisis suggesting that these redox changes were a contributory factor.

A Five‐stage Evolution of Earth's Phosphorus Cycle

AbstractPhosphorus (P) is a key biological nutrient and probably the ultimate limiter of marine productivity during Earth history. In recent years, a wealth of new knowledge has revolutionized our understanding of the global P cycle, yet its long‐term evolution remains incompletely documented. In this paper, we review the effects of three major controlling factors on the long‐term evolution of the global P cycle, i.e., tectonics, marine redox conditions, and bio‐evolution, on the basis of which a five‐stage model is proposed: Stage I (&gt;∼2.4 Ga), tectonic‐lithogenic‐controlled P cycling; Stage II (∼2.4 Ga to 635 Ma), low‐efficiency biotic P cycling; Stage III (∼635 Ma to 380 Ma), transitional biotic P cycling; Stage IV (∼380 Ma to near‐modern), high‐efficiency biotic P cycling; and Stage V (Anthropocene), human‐influenced P cycling. This model implies that the earlier‐proposed Ediacaran reorganization of the marine P cycle may represent only the start of a ∼250–Myr–long transition of the Earth's P cycle (Stage III) between the low‐efficiency biotic mode of the Proterozoic (Stage II) and the high‐efficiency biotic mode of the Phanerozoic (Stage IV). The development of biologically‐driven, high‐efficiency P cycling may have been a key factor for the increasing frequency and volume of phosphorite deposits since the late Neoproterozoic.

Persistent late Tonian shallow marine anoxia and euxinia

The increase in the complexity of eukaryotic life during the Tonian Period (ca. 1000 to 720 Ma) is commonly associated with the oxygenation of the Earth’s ocean–atmosphere system, yet the timing and duration of these redox changes remain uncertain. In particular, it is unclear how shallow marine environments, which were likely crucial habitats for early eukaryotic organisms, responded to Neoproterozoic oxygenation. This study uses trace and rare earth element geochemistry to determine shallow marine redox conditions during the deposition of the late Tonian (ca. 760 Ma) Devede Formation, northern Namibia. In this unit, although carbonate phases (microbialite, former aragonite and high Mg calcite marine cements, and primary dolomite marine cements) generally exhibit no Ce/Ce* anomaly, rare negative values (up to 0.55) are consistent with short-lived periods of oxygenation. In contrast, the dominantly positive Eu/Eu* anomalies of the same phases suggest that basinal conditions were anoxic. These phases also exhibit low concentrations of redox-sensitive (U, V, Mo) and chalcophile (Co, Cu, Cd, Zn, Pb) elements, suggesting that euxinic conditions were predominant during the deposition of the Devede Formation. Integration of this data (Ce/Ce* and Eu/Eu*) with that of analogous late Tonian (ca. 840 to 731 Ma) carbonate strata reveals that shallow marine settings were characterised by persistent anoxia and euxinia, suggesting that any potential increase in atmospheric O2 ca. 800 Ma (e.g. the Bitter Springs Anomaly) was insufficient to facilitate resilient ocean oxygenation. These findings suggest that the late Tonian oceans were likely challenging environments for complex (e.g. eukaryotic) life, and add to a growing body of evidence that the spatially variable redox conditions of late Proterozoic shallow marine settings likely reflect the complex nature of the Neoproterozoic Oxygenation Event.

Theory and classification of mass extinction causation.

Theory regarding the causation of mass extinctions is in need of systematization, which is the focus of this contribution. Every mass extinction has both an ultimate cause, i.e. the trigger that leads to various climato-environmental changes, and one or more proximate cause(s), i.e. the specific climato-environmental changes that result in elevated biotic mortality. With regard to ultimate causes, strong cases can be made that bolide (i.e. meteor) impacts, large igneous province (LIP) eruptions and bioevolutionary events have each triggered one or more of the Phanerozoic Big Five mass extinctions, and that tectono-oceanic changes have triggered some second-order extinction events. Apart from bolide impacts, other astronomical triggers (e.g. solar flares, gamma bursts and supernova explosions) remain entirely in the realm of speculation. With regard to proximate mechanisms, most extinctions are related to either carbon-release or carbon-burial processes, the former being associated with climatic warming, ocean acidification, reduced marine productivity and lower carbonate δ13C values, and the latter with climatic cooling, increased marine productivity and higher carbonate δ13C values. Environmental parameters such as marine redox conditions and terrestrial weathering intensity do not show consistent relationships with carbon-cycle changes. In this context, mass extinction causation can be usefully classified using a matrix of ultimate and proximate factors. Among the Big Five mass extinctions, the end-Cretaceous biocrisis is an example of a bolide-triggered carbon-release event, the end-Permian and end-Triassic biocrises are examples of LIP-triggered carbon-release events, and the Late Ordovician and Late Devonian biocrises are examples of bioevolution-triggered carbon-burial events. Whereas the bolide-impact and LIP-eruption mechanisms appear to invariably cause carbon release, bioevolutionary triggers can result in variable carbon-cycle changes, e.g. carbon burial during the Late Ordovician and Late Devonian events, carbon release associated with modern anthropogenic climate warming, and little to no carbon-cycle impact due to certain types of ecosystem change (e.g. the advent of the first predators around the end-Ediacaran; the appearance of Paleolithic human hunters in Australasia and the Americas). Broadly speaking, studies of mass extinction causation have suffered from insufficiently critical thinking-an impartial survey of the extant evidence shows that (i) hypotheses of a common ultimate cause (e.g. bolide impacts or LIP eruptions) for all Big Five mass extinctions are suspect given manifest differences in patterns of environmental and biotic change among them; (ii) the Late Ordovician and Late Devonian events were associated with carbon burial and long-term climatic cooling, i.e. changes that are inconsistent with a bolide-impact or LIP-eruption mechanism; and (iii) claims of periodicity in Phanerozoic mass extinctions depended critically on the now-disproven idea that they shared a common extrinsic trigger (i.e. bolide impacts).

Redox Conditions of the Late Ediacaran Ocean on the Southern Margin of the North China Craton

Previous studies have revealed dynamic and complex redox conditions of the late Ediacaran ocean. Integrated analyses of Ediacaran successions on different continents can help to better understand global ocean redox conditions. In this study, we used iron and redox-sensitive trace elements (RSTEs) geochemical analyses to present the detailed redox conditions of the late Ediacaran Dongpo Formation on the southern margin of the North China Craton (NCC). Paleoredox reconstruction reveals a dominantly anoxic late Ediacaran ocean punctuated by multiple transient oxygenation events across the southern margin of the NCC. These transient oxidation events in the NCC may have contributed to the appearance of the Ediacaran tubular fossil Shaanxilithes. Based on the assumption that local iron speciation data in a global framework can track the mean and variance of paleoredox conditions through time, we additionally analyzed about 3300 new and published iron speciation data from fine-grained clastic rocks to infer the global redox change in Ediacaran–Cambrian oceans. Our statistical analyses indicated dynamic Ediacaran marine redox conditions and stepwise early–middle Cambrian ocean oxygenation. The appearance and rise of the Ediacaran biota and the diversification of metazoans corresponded temporally with the middle Ediacaran global ocean oxygenation and the early–middle Cambrian stepwise oceanic oxygenation, respectively. Our results highlight the coevolutionary relationship between ocean redox conditions and early animals.

Constraining the onset and environmental setting of metazoan biomineralization: The Ediacaran Nama Group of the Tsaus Mountains, Namibia

The advent of animal (metazoan) biomineralization marks a fundamental transition in Earth's history, facilitating the diversification of novel body plans as well as the biological control of carbonate sediment production, resulting in a permanent restructuring of the global carbon cycle. The oldest known skeletal metazoans, the tubular ‘cloudinids’ e.g., Cloudina, appeared during the terminal Ediacaran of the Nama Group, Namibia, but precisely when, where, and why metazoans first acquired the ability to biomineralize and their timing relative to the regional δ13Ccarb profile is undocumented. Assessing possible environmental triggers for this key event therefore requires accurate constraint of the age, paleoenvironmental setting and the geochemical context of the earliest cloudinids.Here we present new stratigraphic, sedimentological, and geochemical (δ13Ccarb and δ18Ocarb) data from the oldest strata (Dabis Formation) of the Nama Group from the Witputs Sub-basin, exposed in the Tsaus Mountains of the Tsau Khaeb National Park (formerly Sperrgebiet). This constrains the oldest confirmed occurrence of Cloudina to limestones of the lower Kliphoek Member, ca. 551–550 Ma, that were deposited laterally-equivalent to the more proximal fossiliferous Kliphoek quartzite. These limestones have dominantly negative δ13Ccarb values that immediately precede recovery from the basal Nama negative δ13Ccarb excursion (BANE), and were deposited in shallow waters after a transition from semi-restricted, evaporitive-dolomitic to open marine carbonate settings. Regional correlation shows that Cloudina first appeared during an interval of dominantly low oxygen and unstable, regional marine redox conditions. We conclude that data do not support a long-term shift towards more stable, oxygenated conditions as a driver for the first appearance of skeletonization in metazoans, but do suggest that open marine carbonate settings with high carbonate supersaturation were required to support the calcifying Cloudina. In the Nama Group, the appearance of Cloudina also notably coincided with a major sea level lowstand, which we hypothesise may have enabled oxygenation of these more distal settings. Such an expansion of shallow marine oxygen may therefore have provided an evolutionary driver for innovations that were not metabolically costly, such as the onset of Cloudina biomineralization.

Heterogeneous sulfide reoxidation buffered oxygen release in the Ediacaran Shuram ocean

Ediacaran (∼635–539 Ma) carbonate rocks record the largest negative carbonate-carbon isotope excursion in Earth history, termed the Shuram Excursion (SE). This event has been attributed to anaerobic oxidation of dissolved organic carbon as a result of enhanced weathering inputs of sulfate to the ocean during the amalgamation of Gondwana. However, the effect of carbon–sulfur cycle interplay on the net redox state of the ocean–atmosphere system remains unclear, impeding our understanding of the co-evolution of life and the environment during the Ediacaran. Here, we generate high-resolution records of paired sulfate sulfur and oxygen isotopes, in addition to phosphorus concentrations, for the SE interval in South Australia (Parachilna Gorge) and South China (Jiulongwan and Xiang’erwan sections, Three Gorges), and we evaluate these data in the context of COPSE biogeochemical model simulations to assess net long-term redox changes. Our results support widespread H2S reoxidation in shelf areas during the SE, which would have buffered the net release of oxygen sourced from the burial of organic carbon and pyrite. Varying degrees of H2S reoxidation on different cratons likely contributed significantly to high spatial heterogeneity in both local oceanic redox state and nutrient availability, which characterized local oxygen-deficient conditions in an overall oxygenated SE shelf ocean, and likely affected the distribution of the Ediacaran Biota. Our study highlights the important role of H2S reoxidation in the coevolution of marine redox conditions and complex life during the critical Ediacaran period.

Rare earth element signatures of Doushantuo cap dolostones capture an increase in oxygen in the anoxic Ediacaran ocean

The Rare Earth Element (REE) systematics of the post-Marinoan cap dolostones reflect the marine redox conditions and chemistry in the immediate aftermath of the snowball Earth. Rare earth elements and yttrium (REY) compositions in the Doushantuo cap dolostones that directly overlie Nantuo glacial diamictites in south China are determined from the inner shelf to the slope. In general, shale-normalized REY patterns (REYSN) of the cap dolostones show significant fractionations that are characterized by light REE depletion, slight middle REE enrichment relative to the light and heavy REEs, positive Eu anomalies, and slightly super-chondritic Y/Ho ratios. These dolostones, however, show no significant shale-normalized negative Ce anomalies. Such REYSN patterns are consistent with an extensive anoxic Ediacaran ocean in the shallow-to-deep water columns. The REE concentrations and normalized patterns in the cap dolostones vary spatially and stratigraphically. When compared to the shelf edge and the slope, cap dolostones from the inner shelf are distinguished by uniformly less fractionated REYSN patterns with an absence of Eu anomalies, and suppressed light REE depletion and MREE enrichment. These marked differences in the REYSN patterns indicate significant continental influences on the seawater chemistry within the restricted inner shelf (lagoon) during glacial meltdown. The temporal trends in the REY dataset of the Doushantuo cap dolostone reflect shifts in marine redox conditions. Dolostones from two thick shelf margin sections (5 m and 7 m thick, respectively) show almost identical stratigraphic trends in the REY concentrations, and the Ce, Y, and middle REE anomalies. The overall stratigraphic decrease in the Ce anomaly, accompanied by the increase in the Y anomaly and decrease in the middle REE anomaly towards the top of the rock unit, suggests that dissolved oxygen was increasing in the shallow water column during the deposition of the cap dolostones. The REE data in this study reveal that the oxygenation of the Ediacaran oceans started in the immediate aftermath of the snowball Earth (635–632 Ma).

Revisiting marine redox conditions during the Ediacaran Shuram carbon isotope excursion.

The Neoproterozoic carbonate record contains multiple carbon isotope anomalies, which are the subject of intense debate. The largest of these anomalies, the Shuram excursion (SE), occurred in the mid-Ediacaran (~574-567 Ma). Accurately reconstructing marine redox landscape is a clear path toward making sense of the mechanism that drives this δ13 C anomaly. Here, we report new uranium isotopic data from the shallow-marine carbonates of the Wonoka Formation, Flinders Ranges, South Australia, where the SE is well preserved. Our data indicate that the δ238 U trend during the SE is highly reproducible across globally disparate sections from different depositional settings. Previously, it was proposed that the positive shift of δ238 U values during the SE suggests an extensive, near-modern level of marine oxygenation. However, recent publications suggest that the fractionation of uranium isotopes in ferruginous and anoxic conditions is comparable, opening up the possibility of non-unique interpretations of the carbonate uranium isotopic record. Here, we build on this idea by investigating the SE in conjunction with additional geochemical proxies. Using a revised uranium isotope mass balance model and an inverse stochastic carbon cycle model, we reevaluate models for δ13 C and δ238 U trends during the SE. We suggest that global seawater δ238 U values during the SE could be explained by an expansion of ferruginous conditions and do not require a near-modern level of oxygenation during the mid-Ediacaran.