Sections In South China Research Articles

Zinc isotope perspective on global carbon cycling during the onset of the late Paleozoic icehouse

The onset of the late Paleozoic icehouse coincided with a large global carbon-cycle perturbation (represented by the mid-Tournaisian carbon isotope excursion [TICE]), although the underlying cause of this isotopic event remains uncertain. Zinc (Zn), an essential micronutrient for plankton, has an isotopic composition (δ66Zn) in seawater that is sensitive to subtle fluctuations in marine carbon (C) cycling. Here, we investigated C cycling during the TICE using paired carbonate δ66Zn-δ13C and organic δ13C records from two widely spaced sections in South China. These records reveal coupling between δ66Zn and δ13C over an ∼4 m.y. interval in the form of two positive excursions (Peaks I and II) separated by a negative shift (interpeak). Peak I is attributed to enhanced marine primary productivity (MPP), which was stimulated by increased micronutrient supply linked to enhanced silicate weathering and/or upwelling. The interpeak may record oxidation of remobilized organic matter from continental shelves exposed by glacio-eustatic sea-level fall, which, in turn, promoted MPP again and resulted in Peak II. Thus, the TICE records complex climate−carbon cycle feedbacks that may have led to large-scale organic carbon burial and driven stepwise cooling, marking the onset of a sustained late Paleozoic icehouse climate.

Re‐Evaluating Water Column Reoxygenation During the End Permian Mass Extinction

AbstractOcean anoxia is considered a key driver of the end‐Permian mass extinction (EPME). However, it is much debated whether there was an ocean reoxygenation phase during, and in the aftermath, of the EPME. Evidence for ocean reoxygenation is often inferred from the absence of framboidal pyrite in some boundary marine sediments (termed the “framboid gap”). To reconstruct ocean redox evolution across the EPME, we investigated the carbon isotopic, sedimentological, and redox records of the Ruichang and Ehtan sections in South China. These documents two negative δ13Ccarb excursions and the development of anoxia associated with deepening leading up to the Permian‐Triassic boundary. Above the level at which most siliceous organisms became extinct, pyrite framboid and iron proxies indicate that water column redox conditions were predominantly oxygenated but sporadically anoxic/ferruginous [non‐sulfidic, free Fe(II) in the water] at Ruichang, while ferruginous conditions were more widely developed at Ehtan. These contrasting redox states are characteristic of a dynamic ocean redox landscape in the extinction interval. The “framboid gap” is seen in strata deposited under both oxic and ferruginous conditions, suggesting that the availability of decomposable organic matter for sulfate reduction additionally controlled framboid genesis. Our data confirm that oxygenated conditions were developed in some deep water basins during the EPME.

Global correlation of the Guadalupian–Lopingian transition and associated events using a quantitative conodont-based biostratigraphic scheme

The Guadalupian–Lopingian boundary (GLB) interval contains global-scale changes in biota and environment. The establishment of a uniform time scale and a robust system of global correlation are critical in order to unravel the tempo of these events. Conodonts are key biostratigraphic tools; however, major discrepancies exist in conodont taxonomy and correlation in this interval. In this paper, we compile conodont data from 17 well-studied sections in South China, Japan, Oman and North America, comprising 206 occurrences and 100 species, based on previous publications and our own revisions, to test the reproducibility and correlation value of formerly established interval zones. The Unitary Association method (UAM), a quantitative stratigraphic method for the construction of biozones, is applied to construct a robust conodont biozonation for the GLB. Eleven unitary associations (UAs) and eight unitary association zones (UAZs) are recognized to establish biozones and the GLB was constrained within the UAZ5. According to the reproducibility of these UAZs, the correlation of the Lopingian is generally better than the Guadalupian. Apparent contradictions revealed by the UAZs suggest that biostratigraphic and/or taxonomic issues are present within the Clarkina liangshanensis, C. leveni, C. transcaucasica interval zones. The interval between UAZ4 and UAZ5 records the conodont turnover event from Jinogondolella to Clarkina in the paleoequatorial region. Our results suggest that the UAZs are more reliable for correlation. By contrast, some taxonomic and phylogenetic issues are present in the interval zones based on the conodont succession around the GLB. Although at a coarser stratigraphic resolution, removing all the contradictions enables UAZs to better correlate global sections compared with interval zones. The newly established UAZs indicate that the previously reported isotope excursions around the GLB in different regions may be temporally inconsistent.

Millennial-scale sedimentary evolution of carbonate platforms during the Permian–Triassic boundary hyperthermal event

The millennial-scale response of shallow-water carbonate platforms to the climatic upheaval at the Permian–Triassic boundary (PTB) remains unclear. This study presents three carbonate platform sections across the Permian–Triassic (P–T) boundary in South China and the Lhasa terrane. A total of 14 types of carbonate microfacies were identified, which show that the Liangfengya and Taiping sections in South China were deposited in an inner ramp, and the Wenbudangsang section on the Lhasa terrane was deposited in an outer ramp. Bulk carbonate carbon isotope patterns through the sections are generally characterized by negative excursions (−2.2 to −3.5‰) during the PTB hyperthermal event. These excursions can be divided into four phases (NC1 to NC4), with NC1 before the negative carbon isotope excursion (NCIE) event, NC2 during the initiation phase of the NCIE, NC3 during the body of the NCIE, and the NC4 marking the recovery stage of the NCIE. The pattern of sedimentary changes in each section through the PTB hyperthermal event, and the relationship between these changes and the carbon isotope stages, was established. In the studied sections, the onset of the NCIEs (NC2 stage) preceded evidence for a carbonate production crisis, with the occurrence of anachronistic facies developed after this crisis but prior to the NC3 stage. In detail, our data show that the carbonate production crisis occurred 11 kyr after the onset of the NCIE, and 47 kyr before the first Permian–Triassic mass extinction (PTME), and that the carbonate production crisis may have been closely related to the environmental stresses associated with greenhouse gas emission from the Siberian Traps Large Igneous Province (STLIP).

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.

A short-term warming interval during the apex of the Late Paleozoic Ice Age: Evidence from geochemical and magnetic records from South China

The Late Paleozoic Ice Age (LPIA) was the longest-lasting icehouse in the Phanerozoic. Previous studies indicated a climate warming interval in the earliest Permian—an apex of the LPIA. To better understand hydroclimatic changes and carbon cycling within this time interval, we conducted a series of high-resolution geochemical and rock magnetic analyses, including magnetic measurements (e.g., magnetic susceptibility, anhysteretic remanent magnetization, hysteresis loops, temperature-dependent susceptibility), ratio of iron to potassium (Fe/K), and carbonate carbon isotope (δ13Ccarb) from a deep-marine section in South China. Calibrated to the published astronomical time scale, a negative δ13Ccarb excursion (−1‰) was identified during 298.4–297.9 Ma, and it was time-equivalent with the warming interval, recognized by a decrease in conodont apatite oxygen isotopes in South China, low-latitude sea-level transgression, and high-latitude deglaciation. The negative δ13Ccarb excursion also coincided with a magmatic pulse of the Skagerrak-Centered Large Igneous Province (LIP), indicating that the perturbation in carbon cycling is likely associated with the release of volcanic CO2. Furthermore, cyclostratigraphic analysis on multiple paleoclimatic proxies, including ferrimagnetic susceptibility, gamma-ray, and Fe/K, suggested that the negative excursion of δ13Ccarb also overlapped with the maximum of the eccentricity cycle. It implied that orbital configurations may amplify the feedback of the LIP, causing this short-term warming interval.

Lower to Middle Triassic conodont biostratigraphy and carbonate carbon isotope chemostratigraphy of the Zuodeng area, Guangxi, South China, and its relevance for stratigraphic correlation from the Induan to the Anisian

The Nanpanjiang Basin in South China is a vital area for studying the Early Triassic marine ecosystem recovery in the aftermath of the end-Permian mass extinction (EPME). The Zuodeng sections (A, B, and C) are located on the Debao platform, one of the isolated shallow-water platforms within the Nanpanjiang Basin. This area preserves a relatively complete conodont succession from the Lower Triassic to the lower Middle Triassic. Here, a total of 12 conodont zones and 4 subzones are established, namely, the Hindeodus postparvus Zone, the Hindeodus sosioensis Zone, the Neoclarkina krystyni Zone, the Neoclarkina discreta Zone, the Neospathodus dieneri Zone (including the Neospathodus dieneri M1 and Neospathodus dieneri M3 subzones), the Neospathodus cristagalli Zone, the Neospathodus pakistanensis Zone, the Novispathodus waageni eowaageni Subzone, the Novispathodus waageni waageni Subzone, the Neospathodus yangtzeensis-Novispathodus shani Zone, the Novispathodus pingdingshanensis Zone, the Triassospathodus homeri Zone, the Neospathodus triangularis Zone and the Chiosella timorensis Zone.The Permian-Triassic boundary is placed at the base of Bed 48 in the Zuodeng A section, coinciding with the base of the widespread microbialite beds and the carbon isotope excursion. The Induan-Olenekian boundary is placed at the bases of Bed 86 and Bed 21 in the Zuodeng A and Zuodeng B sections, respectively, where Nv. ex gr. waageni occurs at or slightly below the peak of the positive δ13Ccarb excursion. The Smithian-Spathian boundary is recognized at the bases of Bed 92 and Bed 33 in the Zuodeng A and Zuodeng B sections, respectively, based on the first occurrence (FO) of Nv. pingdingshanensis near the middle point of the positive δ13Ccarb excursion. The Olenekian-Anisian boundary is drawn at the top of Bed 5 in the Zuodeng C section based on the FO of Ch. timorensis. The conodont zones recognized in the Zuodeng area correlate well with sections in South China and elsewhere worldwide. The Ns. yangtzeensis-Nv. shani Zone is newly established, and the recognition of this zone in several sections significantly improves the accuracy of stratigraphic subdivision and correlation of the Smithian. The conodont zones in the Zuodeng area thus provide a high-precision time scale for future studies on marine ecosystem recovery in the aftermath of the EPME and environmental events during the Early Triassic.

Towards deciphering the source of rare earth elements in kerogen: Seawater or diagenetic porewater origin?

The composition of rare earth elements (REEs) in kerogen has long been hypothesized to preserve signatures of shallow seawater, assuming that kerogen primarily originates from biomass inhabiting the shallow photic zone. In this study, we examine the composition of REEs in kerogen samples obtained from the Ediacaran Dongkanshang and Fengtan sections in South China. Through a meticulous screening process, we provide evidence that kerogen samples exhibiting patterns of heavy rare earth element (HREE) enrichment are free from acid-insoluble mineral contamination and preserve unaltered REE signatures. This study reveals that kerogens display low total rare earth element (ΣREE) concentrations. As kerogen samples exhibit higher levels of HREE enrichment, the Ce/Ce*, Eu/Eu*, and Y/Ho ratios show an upward trend. Moreover, all the kerogen samples exhibit chondritic Y/Ho ratios (with an average of 32). These findings imply that REEs in kerogens are of diagenetic origin and influenced by the ambient redox conditions. Our results suggest that the distribution patterns of REEs in kerogens derive from porewater within methanogenic zones, as opposed to the oxic seawater photic zone. Consequently, the formation of kerogen could potentially occur within a similar domain as the methanogenic zones, where kerogens can assimilate REEs from pore fluids and incorporate them into their macromolecular structures.

Cadmium isotopic evidence for reduced deep-water marine primary productivity during the end-Permian mass extinction

Marine primary productivity is a key control on the stability of marine ecosystems. Decreasing marine primary productivity might have led to the end-Permian mass extinction, although this is debated. However, the changes in primary productivity at different seawater depths during the end-Permian mass extinction remain poorly constrained. We investigated the Cd isotopic systematics of shallow-water (Dajiang) and deep-water (Shangsi) sedimentary sections in South China. Combined with previously published data from an intermediate–deep-water section (Meishan), we were able to reconstruct the depth changes in marine primary productivity. In the Dajiang section, prior to the mass extinction, the Cd isotopic compositions of seawater (δ114/110CdSW) varied from +0.73‰ to +0.92‰, with an average of +0.83‰±0.13‰ (2SD; n=11). During and after the mass extinction, δ114/110CdSW varied from +0.73‰ to +0.91‰, with an average of +0.81‰±0.12‰ (2SD; n=12). In the Shangsi and Meishan sections, prior to the mass extinction, δ114/110CdSW varied from +0.47‰ to +0.85‰, with an average of +0.66‰±0.16‰ (2SD; n=35). During and after the mass extinction, δ114/110CdSW varied from −0.19‰ to +0.36‰, with an average of −0.02‰±0.27‰ (2SD; n=27). During the end-Permian mass extinction, the marine primary productivity recorded by Cd isotopes in the relatively deep-water sections was considerably reduced, which may have caused the destruction of relatively deep-water marine ecosystems. We suggest that upward expansion of sulfidic and anoxic deep water, possibly due to the volcanic eruption of the Siberian Traps large igneous province, was one of the causes of the decrease in marine primary productivity. However, the marine primary productivity did not change in the shallow-water section during the end-Permian mass extinction, although changes in the types of marine primary producers are evident from the fossil record. This change in the types of primary producers may have contributed to the mass extinction in shallow-water platform environments. Vertical variations in the changes in primary productivity indicate that the environmental factors leading to the extinction event mainly began in deep waters during the end-Permian mass extinction.

Late Cambrian geomagnetic instability after the onset of inner core nucleation

The Ediacaran Period marks a pivotal time in geodynamo evolution when the geomagnetic field is thought to approach the weak state where kinetic energy exceeds magnetic energy, as manifested by an extremely high frequency of polarity reversals, high secular variation, and an ultralow dipole field strength. However, how the geodynamo transitioned from this state into one with more stable field behavior is unknown. Here, we address this issue through a high-resolution magnetostratigraphic investigation of the ~494.5 million-year-old Jiangshanian Global Standard Stratotype and Point (GSSP) section in South China. Our paleomagnetic results document zones with rapid reversals, stable polarity and a ~80 thousand-year-long interval without a geocentric axial dipole field. From these changes, we suggest that for most of the Cambrian, the solid inner core had not yet grown to a size sufficiently large to stabilize the geodynamo. This unusual field behavior can explain paleomagnetic data used to define paradoxical true polar wander, supporting instead the rotational stability of the solid Earth during the great radiation of life in the Cambrian.

Palaeo-environmental significance of fibrous carbonate cement in Marinoan cap carbonates

Marinoan cap carbonates are the first sedimentary rocks deposited after the Marinoan Snowball Earth glaciation and likely record the marine geochemical signals across the Cryogenian-Ediacaran boundary. The nature of the parent fluids, the possible precursor mineralogies and the dolomitisation models of these carbonates are debated to the present day. Cap carbonates and their fibrous dolomite cement samples from the Jiulongwan and Huajipo sections in South China were analysed for their petrography and geochemistry. Data obtained shed light on the deposition and alteration history of cap carbonates. Two types of fibrous dolomite cement precipitated penecontemporaneously with platform carbonate sediment deposition: (i) isopachous fascicular-optic dolomite fabrics with a characteristic botryoidal habit and mottled cathodoluminescence colour; they likely formed via an aragonitic precursor; and (ii) radial-slow dolomite cement, representing a direct marine precipitate. Growth zones are recognised under the SEM and visible in alternating dull- and bright-red zones under the cathodoluminescence microscope. Basal cap carbonates (Ce anomalies: 0.85 ± 0.11; Mn: 3324.3 ± 558.2 ppm) and the base (Ce anomalies: 0.83 ± 0.22; Mn: 2349.2 ± 1276.4 ppm) of fascicular-optic dolomite cement from platform facies indicate anoxic marine seawater with high Mn concentrations. Based on the geochemical data of fibrous dolomite and blocky calcite cement, the marine porewater evolved from the Mn reduction zone (Ce anomalies: 1.04 ± 0.26) to the sulphate reduction zone (Ce anomalies: 1.00 ± 0.48) and the Fe reduction zone (Ce anomaly: 1.40). In situ trace and rare earth elemental contents and the transition from a dull to bright fluorescence colour along the fibrous dolomite transects point to the former activity of microbial sulphate reduction and the degradation of organic matter. These two factors likely facilitated the formation of cap dolostone and multiphase dolomite cement crusts in a marine seawater (porewater) environment. Data shown here are significant for those concerned with the palaeoenvironmental archive and the dolomitisation process of Marinoan cap dolostones.

Stagewise collapse of biotic communities and its relations to oxygen depletion along the north margin of Nanpanjiang Basin during the Permian–Triassic transition

The process of marine ecosystem collapse during the Permian–Triassic mass extinction may have varied in different paleogeographic settings and the intensity of oxygen depletion changed with different water depths. It remains largely unknown about the detailed biotic community collapse and its relation to paleo-redox condition in the deep-water basin. At the Kejiao section in the Nanpanjiang Basin of South China, the continuous sedimentation sequences and complete fossil records from the Changhsingian of Permian to the lowest Triassic provide a unique opportunity to study the progressive biotic crisis associated with oxygen deficiency in a deep-water setting. Here, we document high-resolution records of both the biodiversity dataset of different taxa and the redox data from the Kejiao section to achieve a new understanding of ecosystem deterioration associated with changes of redox conditions. The redox evolution revealed by changes in pyrite framboid size and morphology shows a change trend from lower dysoxic to oxygenated, and then to dysoxic interrupted by oxygenated and anoxic conditions in the late Changhsingian, and ending with euxinic conditions in the earliest Triassic. 60 species in 41 genera from radiolarians, bivalves, brachiopods and cephalopods were identified in the section and their occurrences show two pulses of extinction. The first pulse corresponds to the reduction in species richness of planktonic radiolarians and nektonic cephalopods, and the second pulse is documented by the wipe out of benthic brachiopods and remnant nektonic cephalopods and the decline in the species richness of benthic bivalves. This indicates the collapse of plankton and most nekton communities is ahead of the counterpart of benthos in the deepwater setting, supporting what observed in other sections in South China. The plankton and nekton communities inhabited water columns and the benthic community dwelled in the bottom water. Thus, the stagewise collapse of communities might be contributed by the expansion/migration, in particular, the downward expansion of Oxygen Minimum Zone (OMZ) documented by the information from pyrite framboids and the sedimentation.

Geochronological constraints on Cryogenian ice ages: Zircon U Pb ages from a shelf section in South China

Geochronological constraints on Cryogenian ice ages: Zircon U Pb ages from a shelf section in South China

Climate fluctuations during the Ordovician-Silurian transition period in South China: Implications for paleoenvironmental evolution and organic matter enrichment

The Ordovician-Silurian (OS) transition witnessed dramatic tectonic, climatic, marine, and biological coevolution, during which organic-rich black shale was widely deposited and is considered an important source rock. However, the potential links between the differential enrichment pattern of organic matter and climate fluctuations and paleoenvironmental evolution during this period are still unclear. In this paper, the Late Ordovician-early Silurian black shale in a JieLong (JL) section in South China was identified by graptolitic fossils, total organic carbon (TOC) content, X-ray diffraction (XRD), scanning electron microscopy (SEM) and geochemical analysis. The results show that the provenances of sedimentary rocks are all granodiorite-like. According to the variations in the TOC content and geochemical indicators, four layers are delineated. Under warm and humid climatic conditions, sea level rise and unstable seasonal upwelling led to Unit 1 organic matter enrichment controlled by high primary productivity, anoxic conditions and effective smectite production. Unit 2 organic matter burial was controlled by cold, arid and oxic environmental conditions, but primary productivity was still relatively high. After the end of the glacial period, warm and humid climatic conditions and rapid transgression were the main factors affecting the burial of organic matter in Unit 3. Under hot and humid climate conditions, higher terrigenous detrital inputs and unfavourable preservation environments, the dilution of organic matter in Unit 4 was accelerated. The corrected chemical index of alteration (CIAcorr) values show that the study area experienced a climate change of alternating cold and warm conditions in response to the climate fluctuation during the glacial-interglacial period. The CIAcorr value is negatively correlated with the TOC content, while the generation of available clay minerals (smectite) in warm and humid climates is usually consistent with the enhanced burial flux of organic matter, supporting the important role of climate fluctuations in the accumulation of organic matter. Finally, a comparison of the data from the present study with geochemical data from different profiles in South China reveals the role of climatic fluctuations during the OS transition in controlling paleoenvironmental evolution and organic matter enrichment, while such a heterogeneous factor is usually associated with global or regional geological events. Our study highlights this differential enrichment mechanism as a combined response of multiple factors and provides important implications for glacial-interglacial organic matter enrichment mechanisms.

Elevated sedimentation of clastic matter in pelagic Panthalassa during the early Olenekian

AbstractThe end‐Permian mass extinction is thought to have greatly altered biogeochemical cycles. The absence of chert and dominance of claystone in low‐latitude pelagic deep‐sea sedimentary sequences of Early Triassic Panthalassa (the deep‐sea chert gap) has been believed to record radiolarian die‐off and consequent decline in biogenic silica production. However, recent studies showed that the upper portion of the deep‐sea chert gap has sedimentation rates higher than bedded chert, meaning that increased clastic inputs, rather than decreased biogenic silica inputs, resulted in the anomalous lithology. In this study, we focus on the Akkamori‐2 section, which preserves a rare sedimentary sequence spanning a large part of the lower portion of the claystone of the deep‐sea chert gap. We obtained conodont fossils that allow correlation with sections in South China that have numerous dated tuffs. By projecting the dates of the tuffs to our measured sections, we show that sedimentation rates of the lower portion of the deep‐sea chert gap is also higher than bedded chert. Hence, most of the deep‐sea chert gap was formed under increased clastic inputs, which likely records disturbance in the terrestrial landscape, probably aridification and/or increased seasonality in arid areas, that lead to elevated dust flux to the pelagic ocean. On the other hand, the idea that the deep‐sea chert gap records lingering effects of the mass extinction event on radiolarians cannot explain the high sedimentation rates of the deep‐sea chert gap. This previously favored scenario needs to be reconsidered, taking into account the burial efficiency of biogenic silica in the Early Triassic ocean, and also effects of increased clay deposition on preservation of radiolarians.

Multiple S-isotope constraints on environmental changes during the Serpukhovian mass extinction

The Serpukhovian (mid-Carboniferous) mass extinction has been ranked fifth among the major Phanerozoic biodiversity crises, based on its ecological impact. Global cooling and environmental deterioration driven by the late Paleozoic ice age (LPIA) have been invoked as potential drivers of the Serpukhovian extinction; however, the underlying killing mechanisms remain poorly understood. Here we present multiple S-isotopic records (δ34S and Δ33S) of pyrites from the Naqing section in South China, a potential Global Stratotype Section and Point (GSSP) for the base of the Serpukhovian Stage. Our results reveal the distinctive S-isotopic characteristics of negative Δ33S and δ34S values, implying changes in redox conditions in the mid-Carboniferous oceans. The mixing signal of S-isotopic compositions suggests the encroachment of deep anoxic water onto the shallow shelves. The recurrence of the S-isotope mixing signal during the Serpukhovian extinction suggests the instability of oceanic redox conditions and that the episodic incursion of toxic anoxic water may have contributed to the Serpukhovian extinction.

Mercury isotope evidence for a non-volcanic origin of Hg spikes at the Ordovician-Silurian boundary, South China

Ordovician-Silurian transition (OST) sections of South China contain extremely high mercury (Hg) concentrations (>1000 ppb) of uncertain provenance. The main hypotheses concerning their origin are: (1) contemporaneous elevated seawater Hg concentrations (e.g., due to volcanogenic inputs) combined with normal Hg-uptake processes by marine sediments, and (2) normal seawater Hg concentrations combined with enhanced Hg uptake by marine sediments (e.g., due to strong adsorption by sulfides). Here, we investigate Hg isotopes, which are a promising tool to track the sources of Hg in sediments, in OST strata of the Jiaoye drillcore from the Yangtze Platform of South China. Black shales and pyritic beds exhibit mass-independent fractionations of odd Hg isotopes (odd-MIF, i.e., Δ199Hg) of +0.13±0.05‰ and +0.13±0.03‰, respectively. These values are similar to those of modern and ancient marine sediments, supporting a seawater source of Hg in the study units and providing evidence against a volcanic source. We infer that the extreme Hg enrichment of the OST beds was due to elevated rates of Hg uptake from seawater through adsorption to pyrite. Local environmental conditions (e.g., intense euxinia and microbial sulfate reduction) played a dominant role in Hg enrichment of OST strata in South China.

Expanded deepwater euxinia recorded in the Ediacaran–Cambrian boundary interval in South China

• The Ediacaran-Cambrian boundary interval at Pingyin in South China records a deepwater euxinia. • Expanded deepwater euxinia occurred in the Nanhua Basin during the Ediacaran-Cambrian transition. • The expanded deepwater euxinia probably resulted in the extinction of Ediacaran fauna. Global anoxia of shallow seawater during the Ediacaran–Cambrian (EC) transition has been interpreted as being caused by the upwelling of H 2 S-bearing deep seawater. This hypothesis implies the possible widespread occurrence of H 2 S-rich deep seawater during the EC transition. However, due to a lack of detailed investigation of environmental information recorded across the EC boundary (ECB) at deep-water facies sections, the occurrence of widespread H 2 S-rich deep seawater in paleo-basins during the EC transition is uncertain. The Pingyin section in South China is a deep-water-facies section for which the position of the ECB has been constrained by high-precision zircon chemical abrasion–isotope dilution–thermal ionization mass spectrometry U–Pb ages. Therefore, this section is a favourable site for obtaining information about deep seawater during the EC transition. In this paper, we present redox sensitive element (RSE) concentrations and Fe speciation of cherts from this section to study the redox condition of deep seawater recorded in the ECB at Pingyin. The results show dramatic differences of redox proxies in the cherts in the ECB from those underlying ones. The enrichment factors of U, V and Mo of the underlying samples are 2.08–6.89, 0.82–1.52 and 3.36–7.32, much lower than those of cherts in the ECB which are 16.52, 3.65 and 48.60, respectively. The underlying samples have Th/U ratios of 0.69–2.29, much higher than the cherts in the ECB with Th/U ratios of <0.30. The V/(V + Ni) ratios of most of the samples below the ECB are <0.84, whereas those of the samples at the ECB are >0.84. The Ce/Ce* values of the chert samples show an upward-decreasing trend below the ECB, but change to an increasing tendency immediately above the boundary. Almost all of the chert samples from the topmost Liuchapo Formation have Fe HR /Fe T ratios of >0.38. The Fe PY /Fe HR ratios of the samples underlying the ECB are lower than 0.80, whereas those of the samples around the ECB are higher than 0.80. These various indicators show that the ECB at Pingyin records an ancient deep-water redox turnover from anoxic ferruginous to sulphidic conditions. Combining the new findings with previous data, we conclude that the ECB in South China records the extensification of euxinia in deep water of the investigated paleo-basin. This event, which may have been global in extent, could have had a profound influence on early marine life and was likely a major cause of Ediacaran faunal extinction.

A reassessment on the timing and potential drivers of the major seawater 87Sr/86Sr drop in the Ordovician Period: New evidence from conodonts in China

The major drop in marine 87Sr/86Sr records across the Middle-Late Ordovician boundary (Darriwilian-Sandbian) was one of the most dramatic perturbations of the global Sr cycle in Phanerozoic. However, the precise timing of this isotopic event has not been well constrained, partially making it difficult to further clarify the possible drivers of this isotopic event. For this study, we present multi-continental conodont apatite-based Sr-isotope datasets from three well-studied Ordovician carbonate successions in China, i.e., the Huanghuachang section in South China, the Kalpin Shuinichang-Dawangou and the Nanyigou sections in Tarim Basin, northwestern China. Our new 87Sr/86Sr records generally accord with global seawater 87Sr/86Sr secular variation curve and are supposed to be a reflection of coeval primary seawater signatures. Constrained with detailed conodont biostratigraphic and high-resolution δ13C chemostratigraphic framework, the onset of the sharp decline in 87Sr/86Sr ratios was placed within the Histiodella holodentata zone-Eoplacognathus suecicus zone, and approximately synchronous with the initiation of the Middle Darriwilian Carbon Isotope Excursion (MDICE). Numerical modeling reveals that this major 87Sr/86Sr drop could have been generated by a sustainable increase of oceanic hydrothermal (mantle-derived) Sr flux and/or a progressive decrease of continental-derived Sr flux. Multi-proxy analyses further demonstrate that the impact of enhanced oceanic hydrothermal activities accompanying with the accelerating opening of the Rheic and Paleo-Tethys oceans, may have played a central role in driving the major 87Sr/86Sr drop during the Middle-Late Ordovician transition.

Paleo-Environmental Variation and Its Control on the Organic Matter Accumulation in Black Shale of the Permian Gufeng Formation in the Lower Yangtze Area, South China

Mechanism of the organic matter (OM) accumulation in the Middle Permian Gufeng Formation shale in South China is lack of constraints, which restricts the source rock evaluations and shale gas explorations. To decipher the OM accumulation of the Gufeng Formation, geological and geochemical results related to paleo-environmental variations are presented from the shelf Putaoling section in South China. The OM accumulation in the Gufeng Formation is vertically heterogeneous, shown by a medium total organic carbon (TOC) content (2.3%) in the lower member and a high TOC content (9.6%) in the upper member. The organic-rich shales of the Gufeng Formation are deposited in a complex paleo-environment with restrained water conditions, a warm and humid paleoclimate, a relatively strong chemical weathering, significant hydrothermal activities, a high primary productivity, fluctuating redox conditions, and a relatively high sedimentary rate. Compared to the lower member deposited under anoxic conditions, the upper member is formed in a dominantly euxinic environment with higher productivities. The seawater deoxygenation and the upward-increasing productivity jointly lead to the vertical heterogeneity of the OM accumulation in the Gufeng Formation. Thus, an ‘integrated model’ for the OM accumulation in the Gufeng Formation is established, and which adds to our knowledge that no a single factor or model can explain the OM accumulation in all sedimentary environments.