Cambrian Ocean Research Articles

Redox dynamics of later Cambrian oceans

A growing body of evidence suggests that the deep oceans during the early Paleozoic Era were widely oxygen deficient, despite evidence for increased marine oxygenation during the Neoproterozoic. However, the temporal and geographic extents and dynamics of reducing marine conditions within these oceans are not well understood. Here, we investigate marine redox history during the Drumian through the earliest Jiangshanian International Stages of the Cambrian Period, using concentrations of redox-sensitive metals (vanadium, uranium, and molybdenum), iron speciation, and Mo isotope stratigraphy of the Alum Shale Formation of Scandinavia. These data suggest a major perturbation occurred in trace metal cycling during the later Cambrian Period that was linked to a transient change in marine redox conditions coincident with the well-known Steptoean Positive Isotope Excursion or SPICE. The δ98Mo measurements of the Alum shale show systematic variations during the interval that contains the SPICE which are broadly consistent with a transient expansion of sulfidic, reducing marine environments — indicating a significant exacerbation of an already-common condition during the Cambrian Period. Additionally, iron speciation data record a local transition from predominantly anoxic, ferruginous (Fe+2 containing) to anoxic, euxinic (sulfide containing) water column conditions near the initiation of the SPICE. Trace metal abundances, however, appear to decline well before the start of the SPICE, suggesting an earlier initiation of the global expansion of reducing environments. More broadly, our data and modeling support the notion that significant portions of the oceans remained oxygen deficient throughout the later portion of the Cambrian, and that these oceans were also prone to transient intervals of more reducing conditions similar to the Oceanic Anoxic Events of the Mesozoic.

Depositional environment and hydrothermal controls on organic matter enrichment in the lower Cambrian Niutitang shale, southern China

The purpose of this research was to examine paleoenvironments, hydrothermal activity, and seawater restriction of the lower Cambrian Niutitang Formation shale gas reservoir in the eastern Xuefeng uplift and to determine factors affecting organic matter (OM) enrichment. In the studied borehole Xiangan 1 well in western Hunan Province, the Niutitang Formation can be subdivided into the Niu1, Niu2, and Niu3 Members based on geological and geochemical features, including trace element enrichment, lithology, and fossil content. Total organic carbon values of the Niutitang Formation are variable, averaging 1.5 wt. % in the Niu1 Member, 12.7 wt. % in the Niu2 Member, and 5.1 wt. % in the Niu3 Member. Paleoclimatic changes were responsible for changes in biota, which impacted patterns of OM enrichment. Climate proxy (chemical index of alteration) and productivity proxies (biogenic Ba, Cu/Al, and Ni/Al) consistently indicate higher paleoproductivity in the Niu2 Member. The Niu1 and Niu2 Members may be affected by hydrothermal events, whereas hydrothermal activity was absent during deposition of the Niu3 Member. Hydrothermal activity may provide nutrients and silica but may also enhance the reducing condition of the water column. In addition, hydrothermal events may have possibly influenced biological survival in the different environments, which in turn increased their reproduction within the early Cambrian ocean and affected OM production. Redox proxies (Mo and U enrichment factors) imply that the Niu1, Niu2, and Niu3 Members were deposited in suboxic, euxinic, and ferruginous environments, respectively. Redox conditions, strongly restricted water environments, and hydrothermal events were conducive to OM enrichment during the early Cambrian.

A juvenile-rich palaeocommunity of the lower Cambrian Chengjiang biota sheds light on palaeo-boom or palaeo-bust environments.

The fossil record, including the record of Burgess Shale-type deposits, is biased towards late ontogenetic stages. Larval stages, juvenile and subadult specimens exist but are very rare and often preserved as phosphatic fossils, resulting in biased population structures. Here, we report a new Burgess Shale-type Lagerstätte from Haiyan, China. The Haiyan palaeocommunity is extraordinary in that it is rich in fossils of early and middle ontogenetic stages of various phyla, with eggs also commonly found in the studied interval. This Lagerstätte also hosts a considerable number of new taxa-many related to later biotas of Gondwana and Laurentia. We propose that the deposit may either preserve one of the earliest nurseries in the fossil record or, alternatively, records several attempted invasions. Our study highlights the complexity of biotas and their interactions in the lower Cambrian ocean and calls for a better understanding of the mechanisms responsible for the observed spatial variation of fossil community composition in the Cambrian.

Vertically migrating Isoxys and the early Cambrian biological pump.

The biological pump is crucial for transporting nutrients fixed by surface-dwelling primary producers to demersal animal communities. Indeed, the establishment of an efficient biological pump was likely a key factor enabling the diversification of animals over 500 Myr ago during the Cambrian explosion. The modern biological pump operates through two main vectors: the passive sinking of aggregates of organic matter, and the active vertical migration of animals. The coevolution of eukaryotes and sinking aggregates is well understood for the Proterozoic and Cambrian; however, little attention has been paid to the establishment of the vertical migration of animals. Here we investigate the morphological variation and hydrodynamic performance of the Cambrian euarthropod Isoxys. We combine elliptical Fourier analysis of carapace shape with computational fluid dynamics simulations to demonstrate that Isoxys species likely occupied a variety of niches in Cambrian oceans, including vertical migrants, providing the first quantitative evidence that some Cambrian animals were adapted for vertical movement in the water column. Vertical migration was one of several early Cambrian metazoan innovations that led to the biological pump taking on a modern-style architecture over 500 Myr ago.

Bioturbation feedbacks on the phosphorus cycle

Bioturbation—sediment mixing and ventilation by burrowing animals—provides one of the most prominent examples of how animals shape their surroundings. A critical but longstanding question is when and how in Earth's history bioturbators began to similarly influence marine biogeochemistry. Recent work has proposed that even though the development of well-mixed sediments was a protracted process, the initial rise of bioturbators led to a decrease in marine phosphate levels, global productivity crises and ultimately deoxygenation events in the early Cambrian oceans. Herein, using a diagenetic model, we provide a new view of how bioturbators impact the global phosphorus cycle. Our work focuses on incorporating more realistic representation of bioturbation with a more complete and mechanistic parameterization of benthic phosphorus cycling than has previously been used to explore the early Paleozoic. We find, in contrast to previous modeling studies, that bioturbation does not uniformly or unidirectionally mediate increased phosphorus burial. Enhanced biodiffusion of sedimentary particles can mediate enhanced phosphorus burial. In contrast, bioirrigation—nonlocal transport of solutes via burrows—enhances phosphorus recycling and may therefore stimulate rather than stymy productivity. Given evidence from the geologic record that bioirrigation rather than biodiffusion was predominant during the ramping up of bioturbation (through the early Paleozoic), the emergence of bioturbation is unlikely to have driven deoxygenation events.

Biogeochemical status of the Paleo-Pacific Ocean: clues from the early Cambrian of South Australia

The Ediacaran–Cambrian transition was a time of profound reorganisation of the biosphere, coinciding with the assembly of Gondwana and increasing atmospheric and oceanic oxygenation. The lower Cambrian marine sediments of the Stansbury Basin, South Australia, were deposited at low northern paleolatitudes on the western margin of the emerging Pacific Ocean. Here we report results of a multi-pronged investigation employing trace and rare earth element (REE) abundances, total organic carbon (TOC) contents and pyritic sulfur isotopic compositions (δ34Spy) in carbonaceous shales from three formations within the Normanville and Kanmantoo groups (sequences Є1.2 to Є2.2). TOC ranges from ≤0.5 wt% in the Emu Bay Shale, through 0.4–1.9 wt% in the Talisker Formation, to 0.2–2.6 wt% in the Heatherdale Shale. Covariance is demonstrated between trace elements and organic matter, with comparative uranium enrichment in the Heatherdale Shale likely linked to increased primary productivity. Heavy REEs and yttrium are typically more depleted than light REEs. Provenance appears to evolve through the upper Normanville sediments into the Kanmantoo Group, corroborating published detrital zircon interpretations that suggest an increasing dominance of southerly derived sources. The prevailing paleoredox regime for the Heatherdale Shale and the Talisker Formation was dysoxic, evolving into progressively more reducing conditions up section, the latter exhibiting a secular decline in δ34Spy (from +10 to −11‰ VCDT). Conversely, redox proxies concur in demonstrating that the basal fossiliferous Emu Bay Shale accumulated beneath an oxic water column. Comparison of trace element and REE distributions with those of slightly older sequences on the Yangtze Platform, South China, reveals striking similarities, implying that the trace element chemistry of the Paleo-Pacific and Asian oceans was homogeneous. KEY POINTS Trace and rare earth element, total organic carbon and sulfur isotopic data provide a multi-faceted profile of shales from three formations in the Stansbury Basin. The prevailing paleoredox regime for the Heatherdale Shale and Talisker Formation was dysoxic, whereas the basal Emu Bay Shale accumulated beneath an oxic water column. The strong covariance between TOC and normalised Mo, V and U in these shales highlights the evolving bioproductivity and oxygenation of the early Cambrian ocean. Similar element signatures in lower Cambrian black shales, South China, reflect the homogeneous biogeochemistry of the Paleo-Pacific and Asian oceans.

Mo isotope records from Lower Cambrian black shales, northwestern Tarim Basin (China): Implications for the early Cambrian ocean

AbstractThe widely developed black shales deposited during the early Cambrian recorded paleoenvironmental information about coeval seawater. Numerous studies have been conducted on these shales to reconstruct the paleomarine environment during this time period. However, most research has been conducted on stratigraphic sections in South China, and equivalent studies of sections from other cratons are relatively rare. Here, we report Mo isotopic compositions as well as redox-sensitive trace-element and iron (Fe) speciation data for black shales of the Lower Cambrian Yuertusi Formation from the Tarim block (i.e., a small craton). The Fe speciation data show high FeHR/FeT and Fepy/FeHR ratios, indicating roughly sustained euxinic bottom-water conditions during their deposition. Based on Mo isotopic compositions (δ98/95Mo), we further classified the euxinic black shales into two intervals: a lower interval (0–21.3 m) and an upper interval (21.3–32.3 m). The lower interval is characterized by variable Mo isotopic compositions (−2.12‰ to +0.57‰, mean = –0.52‰ ± 0.72‰), with an obvious negative excursion in its middle portion. The overlying upper interval has relatively heavy δ98/95Mo values up to +1.42‰ (mean = +0.62‰ ± 0.37‰). We ascribe δ98/95Mo differences in the lower and upper intervals to inadequate aqueous H2S concentrations for quantitative thiomolybdate formation under euxinic conditions. The most negative Mo isotope excursion may have been caused by upwelling hydrothermal inputs during a transgression, consistent with significantly elevated total organic carbon (TOC) contents, Mo and U enrichments, and Fe supply. Relatively positive δ98/95Mo values in the upper interval have roughly similar variations with other coeval sections, indicating such variations were common for early Cambrian euxinic deposits, and they were most likely caused by local differences in [H2S]aq. Compilation of Mo isotope data from the early Cambrian and earlier times further indicates relatively oxygenated seawater, especially the deep-marine areas during the early Cambrian before reaching a state like modern seawater.

Variable redox conditions as an evolutionary driver? A multi-basin comparison of redox in the middle and later Cambrian oceans (Drumian-Paibian)

The middle to later Cambrian (Drumian-Jiangshanian Ages, 505–490 Ma) was a time of unique evolutionary dynamics that remain enigmatic. This interval records unusually high rates of faunal turnover that produced a “plateau” within the broader trajectory of rapidly increasing biodiversity seen across the Cambrian Explosion and Great Ordovician Biodiversification Event (GOBE). The oceans during this time are generally thought to have been less oxygenated than later in the Phanerozoic, yet knowledge of oceanic redox structure and the influence this exerted upon the biosphere remains limited. Importantly, this interval also encompasses two large carbon cycle perturbations—the DICE and SPICE events— that are thought to involve the expansion of anoxic and, more specifically euxinic regions in the ocean. Despite this supposition, direct characterization of redox conditions across this time remains limited. Here we explore these conditions using new and previously published Fe-speciation data from seven basins distributed across five paleocontinents representing a range of depositional conditions. Our analysis reveals anoxia was a common and persistent feature of deeper-water environments and that it was generally absent from shallower-waters across this timespan. An exception to this broad pattern is seen during the SPICE when these deeper-water anoxic conditions expanded into shallower-water environments. These anoxic conditions were dominantly ferruginous and rare instances of euxinia were spatiotemporally limited to environments of high productivity, low clastic sedimentation and high sulfate availability within a generally low-sulfate ocean. Intriguingly, during these events faunal turnover was concentrated in inner-shelf areas suggesting a mechanistic link to the variable redox conditions characteristic of these environments. More broadly this instability in nearshore environments appears a likely cause of the high rates of faunal turnover seen across the later Cambrian and into the Early Ordovician, but further detailed paleontological and redox investigation of these environments are needed to adequately evaluate this view.

Tracking the evolution of seawater Mo isotopes through the Ediacaran–Cambrian transition

The rapid increase in animal diversity and abundance over the Ediacaran–Cambrian boundary has been attributed to, among other parameters, pronounced Earth surface oxygenation at that time. However, whether atmospheric oxygen elevated to modern levels, or whether the extent was smaller and oxygen remained relatively muted until the late Paleozoic is currently debated. Here, we present new Mo isotope datasets of drill core material for two key black shale units (the Ediacaran Doushantuo Formation and the lower Cambrian Jiumenchong Formation), located on the Yangtze Block. These data capture a 0.7‰ isotope fractionation between sediments experiencing quantitative and non-quantitative Mo removal. By identifying this offset, we provide integrated estimates for global seawater Mo isotope compositions during the deposition of these two formations (~1.2–1.4‰ and ~1.8–2‰, respectively). When paired with a mass balance model, our results indicate that a large proportion of the early Cambrian ocean was bathed under oxic waters. We also discuss how recent discoveries have provided insights into the geochemical processes underpinning the oxygenation of the oceans.

In siturare earth element analysis of a lower Cambrian phosphate nodule by LA-ICP-MS

AbstractRare earth elements (REE) in marine minerals have been widely used as proxies for the redox status of depositional and/or diagenetic environments. Phosphate nodules, which are thought to grow within decimetres below the sediment–water interface and to be able to scavenge REE from the ambient pore water, are potential archives of subtle changes in REE compositions. Whether their REE signals represent specific redox conditions or they can be used to track the overlying water chemistry is worth exploring. Throughin situlaser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS), we investigate the REE compositions of a drill-core-preserved phosphate nodule from the lower Cambrian Niutitang Formation in the Daotuo area, northeastern Guizhou Province, South China. REE distributions of the nodule show concentric layers with systematic decreases in Ce anomalies (Ce/Ce*) from the core to the rim. The lowest Ce/Ce* appears in the outer rim where REE concentrations are relatively high. These results are interpreted to reflect REE exchange with pore water at a very early stage or bathymetric variation during apatite precipitation. The origin of the shale-normalized middle REE (MREE) enrichment in our sample is less constrained. Possible driving factors include preferential MREE substitution for Ca in the apatite lattice, degradation of organic matter and deposition beneath a ferruginous zone. Although speculative, the last possibility is consistent with the chemically stratified model for early Cambrian oceans, in which dynamic fluctuations of the chemocline provided an ideal depositional context for phosphogenesis.

Oceanic chemistry recorded by cherts during the early Cambrian Explosion, South China

The early Cambrian ocean was marked by significant redox changes, but the detailed redox evolution of seawater and its relationship to biological expansion are not fully understood. Widespread chert successions are present in lower Cambrian deposits on the Yangtze Block, South China, which have great potential to record oceanic chemistry. Here, we analyze rare earth elements (REE) and Si and Fe isotopes along three regional chert sections to constrain the origin of lower Cambrian cherts and the redox conditions of the early Cambrian ocean. The REE patterns and Si isotopes illustrate that lower Cambrian cherts at the Muyang (MY; Yanjiahe Formation) and Zunyi (ZY; Niutitang Formation) locales originated from the replacement of precursor carbonates and black shales, respectively, whereas cherts at Chuanyanping (CYP; Liuchapo Formation) locale originated from direct chemical precipitation from seawater and/or hydrothermal fluids. All cherts, except for several lower CYP cherts with hydrothermal contributions, primarily preserved seawater chemical signals. The Ce/Ce* and Y/Ho ratios indicate that these cherts were deposited near a suboxic/anoxic interface, where seawater may have been ferruginous overall, while more subtle redox changes, namely, ferruginous (less reducing) and ferruginous (more reducing), were identified by Fe isotopes. Based on new data and previous results, we propose that the early Cambrian ocean on the Yangtze Block was characterized by a stratified redox framework. Before ~535 Ma in the Cambrian, oxic seawater occurred only along the inner shelf (MSC section), below which seawater was primarily ferruginous (less reducing). During ~535–526 Ma, oxic seawater gradually expanded to the outer shelf (MY section) and a metastable ferruginous (more reducing) zone dynamically existed at shelf–slope locations. Temporal and spatial comparisons revealed that the water column on the Yangtze Block may have experienced a progressive deepening of the redoxcline during the early Cambrian (~535–526 Ma). The metastable redox zone may have been an analogue of the modern oxygen minimal zone (OMZ), which is associated with biological activity and high productivity. The occurrence and spatial fluctuation of the metastable ferruginous (more reducing) zone may have been associated with the evolution of small shelly fossils from assemblage 1 (SSF1) to assemblage 3 (SSF3) and their spatial expansion from shelf to slope. Therefore, accompanied by oceanic oxygenation, the occurrence and spatial fluctuation of an OMZ-like metastable zone may have regulated the biological diversification and distribution during the early Cambrian (~535–526 Ma).

Spatio-temporal evolution of ocean redox and nitrogen cycling in the early Cambrian Yangtze ocean

The early Cambrian was a critical interval for the Earth system, during which a rise in oceanic and atmospheric oxygen levels coincided with the rapid diversification of metazoans. A variety of contrasting models have been proposed for the spatiotemporal redox evolution of the early Cambrian ocean. These include the development of a well-oxygenated deep ocean at the base of Cambrian Stage 3 (commencing at ~521 Ma), or alternatively, persistent and widespread anoxic (ferruginous) conditions throughout the early Cambrian ocean. Here, we present redox sensitive trace element (RSTE), Fe speciation, and N and C isotope (δ15Nsed and δ13Corg) data for samples from a section (Zhongnancun) of the early Cambrian Niutitang Formation, which was deposited on the outer-shelf of the Yangtze Block, South China. The Fe speciation and RSTE data provide evidence of a transition from euxinic, through ferruginous, to oxic conditions during deposition of the Niutitang Formation. The combination of these new data with existing data from the inner-shelf to basin environment, implies regional redox stratification across the Yangtze Block during Cambrian stages 2 and 3, with oxic shallow waters above ferruginous deep waters, and spatial variability in the degree of mid-depth euxinia. Oxygenation of deeper waters may have occurred by early Cambrian Stage 4 (~514 Ma). A compilation of δ15N values from multiple early Cambrian sections of the Yangtze Block indicate that N2 fixation dominated the nitrogen cycle during late Cambrian Stage 2. Low δ15N values (<−2‰) preserved in shelf sections can be interpreted to represent partial assimilation of NH4+, where NH4+ was not a limiting nutrient. During the early-middle Cambrian Stage 3, more positive δ15N values (0 to +3‰) are recorded in shelf sections, with lower values (−2 to +2‰) recorded in slope-basin sections. The positive δ15N values observed in shelf sections are likely a consequence of partial denitrification in the water column, whereas coeval deeper water δ15N values of ~0‰ may reflect the dominance of N2 fixation. The distribution of δ15N values, combined with a gradient in δ13Corg values, is consistent with a stratified ocean model. The δ15N values of all sections are lower than those of the modern ocean, which may indicate that the nitrate concentration of the early Cambrian Yangtze ocean was generally low during Cambrian Stage 3. The observed gradient in δ15N values is similar to that observed in records from Mesoproterozoic oceans, suggesting that abundant nitrate availability may have been restricted to shelf environments. We propose that increased nitrogen availability in shelf settings may have contributed to the evolution of large-celled eukaryotic phytoplankton. This provided a positive feedback on ocean oxygenation, allowing for increased complexity in early animal ecosystems on the continental shelf, and ultimately deep water oxygenation.

Silicification and Si cycling in a silica-rich ocean during the Ediacaran-Cambrian transition

The upper Ediacaran-lower Cambrian (551–526 Ma) sedimentary succession of the Yangtze Block, South China, hosts abundant chert that formed contemporaneously with the Neoproterozoic Oxidation Event and the Cambrian Explosion. The effect of biological evolution, especially the rise of silica-secreting organisms, on marine Si cycling during the Ediacaran-Cambrian (E-C) transition remains controversial. This paper describes results of a petrological and geochemical investigation aimed at elucidating the origin of the chert and silica biogeochemical cycling during the E-C transition. Our results reveal that the studied chert is dominated by quartz and carbonate. The widespread presence of relict carbonate in inferred peritidal chert deposits suggests that carbonate replacement was an especially common silicification pattern of this facies, whereas the lack of carbonate precursors in subtidal strata argues in favor of primary and biogenic silica precipitation in deposits of this facies. Silicon isotope compositions, relative abundances of Al, Fe, and Mn and rare earth element (REE) + Y concentrations suggest that seawater rather than high-temperature hydrothermal fluid was the primary silica source of the studied cherts. Secular variation of chert Si isotopes reveals that marine silica cycling was closely linked with biological activity. Although silicification was abiological during the Precambrian, it may have been facilitated by microbial activities, especially the widespread formation of cyanobacterial mats. The flourishing of siliceous sponges and radiolarians in the early Cambrian ocean may have served as a global silica sink thus enhancing biological control of the silica cycle after Cambrian time. The studied chert deposits not only demonstrate the silica-rich nature of the global ocean during the E-C transition, but also record the transition of silica cycling from abiological to biological processes and the effect of the evolutionary radiation of silica-secreting organisms during the Cambrian Explosion on silica cycling.

The nature of Early Palaeozoic Kwangsian orogenic event in the South China Block: constraints from detrital zircons in Cambrian strata

ABSTRACT The tectonic nature of Early Palaeozoic Kwangsian orogeny in the South China Block (SCB) is still debated, due to controversy whether there was a Cambrian ocean between the Yangtze Block and the Cathaysia Block. Similarly, continental affinities of the SCB with India and Australia in East Gondwana during the Cambrian are also controversial. Herein, we present detrital zircon U-Pb geochronology of Cambrian sandstone samples from the southern SCB. Cambrian samples (including our new samples and published samples) from the Cathaysia Block and the Yangtze Block show similar detrital zircon age spectra with one prominent age cluster of ~1000-900 Ma and several subordinate age groups, as well as similar cumulative proportion curves (CPC). Such signatures, together with other geological records, suggest that a Cambrian ocean did not exist between the Cathaysia Block and the Yangtze Block, and that Early Palaeozoic Kwangsian orogeny resulted from an intra-continental collision. Cambrian samples from the SCB have much more similar detrital zircon age spectra and CPC to those from Himalaya and Qiangtang than those from Australia, but detrital zircons in Cambrian samples from the SCB, Himalaya, Qiangtang and Australia have a similar prominent age group of ~1000-900 Ma. Thus, the SCB were likely located between Australia and India during the Cambrian, receiving sediments both from India and Australia.

Seawater carbon and strontium isotope variations through the late Ediacaran to late Cambrian in the Tarim Basin

The radiation of early animals during the Cambrian explosion was accompanied by widespread perturbations in oceanic and atmospheric conditions. However, the cause-effect relationships between evolutionary innovations, carbon (C) cycling, and continental weathering are still a matter of debate. Further paleoenvironmental reconstructions of poorly studied paleo-continents covering the Precambrian/Cambrian (PC/C) transition may improve the correlation of the events and their impact on the Earth system on a global scale. The Tarim Basin was presumably located proximal to the well-studied South China craton during the Ediacaran-Cambrian boundary and is expected to contribute equally to our paleoenvironmental reconstructions. Here we apply carbon and strontium (Sr) isotope records (δ13C and 87Sr/86Sr) obtained from well-preserved marine carbonates spanning the late Ediacaran to the late Cambrian from the Keping region in the northwestern Tarim Basin, NW China. By comparison to other carbonate successions from various paleo-continents, our obtained seawater 87Sr/86Sr curve in this study shows a stepwise first-order increase to more radiogenic values superimposed by second-order presumably regional variations. Our obtained δ13C variations exhibit four prominent negative and three positive excursions identical to the global δ13C record. Based on existing paleontological evidence and our new Tarim δ13C and 87Sr/86Sr curves, we propose a stratigraphic correlation from the late Ediacaran to the late Cambrian between the Tarim Basin and other paleo-continents. Besides, our paleoenvironmental reconstruction reveals both regional and global sea-level changes that may have controlled the influx of radiogenic Sr and presumably the influx of nutrients controlling primary production and the rate of organic carbon burial in the Tarim Basin at the time of deposition. These feedbacks may have ultimately influenced the rapid diversification of metazoans and the oxygenation of the Cambrian ocean and atmosphere approaching-present pO2 levels.

Constraints on the Ediacaran-Cambrian boundary in deep-water realm in South China: Evidence from zircon CA-ID-TIMS U-Pb ages from the topmost Liuchapo Formation

The placement of the Ediacaran-Cambrian boundary in deep-water realm in South China and the high-precision temporal framework for the Ediacaran-Cambrian transition in this region have not yet been completely solved. Recently, we have found two K-bentonite beds in the top of the Liuchapo Formation at the Pingyin section, Guizhou Province. It provides an opportunity for constructing the temporal framework of the transitional strata on the Yangtze Platform in South China and for determining the Ediacaran-Cambrian boundary in this area. In this study, we conducted high-precision CA-ID-TIMS U-Pb dating on zircons from the two K-bentonites. The ages of the two K-bentonites were precisely constrained at 536.40±0.47/1.1/1.2 Ma (2σ, n=7, MSWD=0.92) and 541.48±0.46/1.1/1.2 Ma (2σ, n=8, MSWD=1.3). Combining the geochronological results with organic carbon isotope data of chert in the topmost Liuchapo Formation from the section, we suggest that the EdiacaranCambrian boundary should be consistent with a significant negative organic carbon isotope excursion between the two K-bentonites. The scheme of the Ediacaran-Cambrian boundary in this study is of great significance for global correlation, and further for unravelling the information of the terminal Ediacaran-early Cambrian ocean.

Single subduction zone for the generation of Devonian ophiolites and high‐P metamorphic belts of the Variscan Orogen (NW Iberia)

AbstractWithin the Variscan Orogen, Early Devonian and Late Devonian high‐P belts separated by mid‐Devonian ophiolites can be interpreted as having formed in a single subduction zone. Early Devonian convergence nucleated a Laurussia‐dipping subduction zone from an inherited lithospheric neck (peri‐Gondwanan Cambrian back‐arc). Slab‐retreat induced upper plate extension, mantle incursion and lower plate thermal softening, favouring slab‐detachment within the lower plate and diapiric exhumation of deep‐seated rocks through the overlying mantle up to relaminate the upper plate. Upper plate extension produced mid‐Devonian suprasubduction ocean floor spreading (Devonian ophiolites), while further convergence resulted in plate coupling and intraoceanic ophiolite imbrication. Accretion of the remaining Cambrian ocean heralded Late Devonian subduction of inner sections of Gondwana across the same subduction zone and the underthrusting of mainland Gondwana (culmination of NW Iberian allochthonous pile). Oblique convergence favoured lateral plate sliding, and explained the different lateral positions along Gondwana of terranes separated by Palaeozoic ophiolites.

Evidence for high organic carbon export to the early Cambrian seafloor

Oxygenation of the early Cambrian ocean is commonly ascribed to high organic export to the sediment due to the rise of algae and filter-feeding animals, but direct evidence of elevated export fluxes has been lacking to date. Here, we report an integrated proxy dataset (U-Mo isotopes, Fe speciation, and major and trace elements) for lower Cambrian black shales at Yuanjia on the Yangtze Platform (South China). These shales are characterized by high iron speciation ratios and Mo and U enrichments, indicating persistently euxinic water conditions during their deposition, but a broad range of δ98Mo values (−0.10‰ to +1.94‰) implies strongly variable watermass sulfidity. High and variable nutrient element (Cu, Zn, Ni, Cd) enrichments at Yuanjia are consistent with elevated and variable organic productivity and export fluxes from the euphotic zone to the early Cambrian seafloor. The Yuanjia shales also exhibit super-heavy δ238U values (to +0.84‰) that cannot be explained by U reduction in sediment porewaters. We hypothesize that these values record a U reduction process within the water column, where U isotope fractionation can be much larger than in sediment porewaters. Such processes may have operated within a benthic organic flocculent layer at the sediment-water interface, a feature that is characteristic of high productivity systems with anoxic bottom waters. Furthermore, the coupling between δ238U, nutrient elements, and δ98Mo suggests that elevated productivity levels were driven likely by episodic upwelling in early Cambrian oceans, supplying abundant nutrients for productivity and influencing H2S concentrations in the water column. These observations, along with high organic carbon accumulation rates (5600 mg cm−2 kyr−1), are direct evidence of high organic export fluxes to the early Cambrian seafloor, thus providing key support for the hypothesis that enhanced organic export and burial triggered a major atmospheric-oceanic oxygenation event that stimulated the subsequent explosion of Cambrian life.

Multiple S-isotopic constraints on paleo-redox and sulfate concentrations across the Ediacaran-Cambrian transition in South China

Changes in oceanic chemical conditions are frequently invoked as a primary driver of biodiversification and mass extinction events in Earth history. During the Ediacaran–Cambrian transition, two major biological events, including the extinction of the Ediacaran biota and the Cambrian explosion, occurred approximately coincident with perturbations of the carbon and sulfur cycle. These dramatic biological changes and coeval geochemical events are commonly thought to be linked, but their causal relationships remain controversial. Here we present multiple S-isotopic compositions of pyrite (δ34Spy and Δ33Spy) from the Xiaotan section spanning the Ediacaran-Cambrian transition interval in South China. Our isotopic results show that negative Δ33Spy values, together with positive δ34Spy values, occurred across the Ediacaran-Cambrian boundary. The negative Δ33Spy data suggest mixing of sulfide from deep waters during the upwelling of sulfidic waters into oxygenated shallow waters. The coincidence between the negative Δ33Spy and the approximate timing of the extinction of the Ediacaran biota implies that paleo-redox perturbations may have contributed to the disappearance of the Ediacaran biota. Negative Δ33Spy values coupled with positive δ34Spy values were also observed in the Cambrian Zhongyicun Member when the phosphorites were deposited, suggesting a strong linkage between the upwelling of nutrient-rich sulfidic deep waters and the deposition of the phosphorites in oxygenated shelf areas. In addition, using box models and stoichiometry calculations, we quantitatively estimated elevated burial rates of organic matter and pyrite based on the parallel positive δ13Ccarb, δ34Spy and Δ33Spy excursions observed at Xiaotan within Cambrian Stage 2. Our mass balance modelling results provide an independent constraint on sulfate concentrations which were 4 mM or lower in the early Cambrian oceans. The probable increase in oxygen levels resulting from the enhanced burial of pyrite and organic matter could have provided a more habitable environment for early animals and thus paved the way for the radiation of early Cambrian animals.

Trace element and mineral composition of Ediacaran-early Cambrian strata in South China and their geochemical implications

Ediacaran-early Cambrian is a critical interval in the evolution of atmosphere-ocean environment and biodiversity. The continuous Ediacaran-Cambrian strata in South China provide a good opportunity for investigating the associated environment with its ecological evolution. In this study, outcrop sedimentary rock samples collected from the Dongkanshang and Fengtan sections on the Yangtze Block were analyzed for their mineral and trace element compositions. The results showed some heterogeneity in the geochemistry of the Ediacaran-early Cambrian ocean in South China. Based on the U/Th, V/(V+Ni), V/Cr, and Ni/Co ratios, the middle-deep water was predominated by anoxic or euxinic condition with interferences from several transient oxidation events. The basinal Fengtan section indicates a more reduced depositional environment when compared with the slope Dongkanshang section. This shows that the seawater oxygenation was a gradual process that expanded from shallow to deep water.