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Climate changes are known to have been a key regulator of the biodiversity in Earth's history. However, the dry‐humid degrees and alternating patterns throughout the Phanerozoic remain largely unconstrained. In this study, we report high contents (2.4 ± 3.8 mg N kg−1) and 17O anomalies (11.0 ± 7.4‰) of nitrate (NO3−) in the early Cambrian black shale from South China, likely caused by atmospheric NO3− intrusion under dry climates that followed tectonic uplift. By developing new methods to quantify aridity indices (AI, 0.06 ± 0.08) in combination with observational data with paleoclimate models, we reconstructed the historical AI variations. Our analyses revealed three significant dry‐to‐humid transitions which include Cambrian‐Ordovician to Silurian‐Permian, Permian‐Triassic boundary to middle Triassic‐early Jurassic, and Jurassic‐Paleogene to Neogene. This study quantitatively unlocked the Phanerozoic climate changes in South China, offering important evidence for understanding the co‐evolution of ecological and climatic systems in Earth's history. Plain Language Summary: Climate changes have played a critical role in shaping the biological evolution during the Phanerozoic, but the precise degree of dry‐humid fluctuations and their patterns remain unclear. This study observed high contents and 17O anomalies of nitrate in Cambrian black shale from South China, attributed to atmospheric deposition during dry periods following tectonic uplift. By integrating isotopic and sedimentological data with climate models, we quantified historical aridity and reconstructed its variations in South China. We identified three major dry‐to‐humid transitions in the Phanerozoic. These findings improve our understanding of how climate dynamics interacted with biological and geological changes. Key Points: High 17O anomaly (11.0 ± 7.4‰) of NO3− in Cambrian black shale was not explained by early diagenesis and modern surface processesA new framework using the ∆17O of sedimentary NO3− to quantify the deep‐time aridity was developedThe data suggest three alternations of dry‐humid climates in the Phanerozoic were recorded by sedimentary geochemistry in South China [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• The composition of the parent magmas of the late Ediacaran K-bentonites in South China is trachyandesitic. • The parent magmas of the K-bentonites mainly came from the crust. • The K-bentonite in the top of the Miaohe Member is equivalent to those in the basal Liuchapo Formation. • The termination of the Doushantuo negative carbon isotope excursion and Shuram excursion may be isochronous. • The nutrients brought by volcanic activities play a role in the evolution of late Ediacaran organisms. Several K-bentonite beds have been discovered in the upper part of the Ediacaran system in South China. Existing researches mainly focus on the zircon U-Pb geochronology of the K-bentonites. However, the nature of the primary magmas of the K-bentonites, the tectonic setting of their source volcanoes, the significance of their geochemical correlation, and their potential implications to evolution of late Ediacaran life have not been unraveled. Here, whole-rock and zircon geochemical studies have been carried out on the K-bentonites in the top of the Miaohe Member from the Jiuqunao section, and in the basal Liuchapo Formation and the lower Dengying Formation from the Fanglong section. The major and trace element contents of the K-bentonites and the trace element contents of volcanic zircons in the K-bentonites suggest that the nature of the primary magmas of the K-bentonites from the Fanglong and Jiuqunao sections is trachyandesitic. The oxygen isotope compositions of the volcanic zircons in the K-bentonites suggest that the source magmas of the K-bentonites from the two sections are mainly from the crust. The whole-rock Y-Nb, Ta/Yb-Th/Yb, Yb-Th/Ta, and Ta/Hf-Th/Hf diagrams, and the volcanic zircon Hf-U/Yb, Y-U/Yb, Th/U-Nb/Hf, and Th/Nb-Hf/Th diagrams of the K-bentonites show that the tectonic setting of source volcanoes of the K-bentonites was a convergent environment, which may be the result of the assembly of Gondwana during late Ediacaran. The geochemical results suggest that the K-bentonites in the basal Liuchapo Formation at the Fanglong section are correlative with the K-bentonite in the top of the Miaohe Member at the Jiuqunao section. Therefore, the geochemical correlation of the K-bentonites indicate that the age of 550.5 ± 1.0 Ma for the K-bentonite in the top of the Miaohe Member may be too young to be used as the end time of the Doushantuo negative carbon isotope excursion (DOUNCE), and that the end time of this event should be older than the age 557 ± 3 Ma of the K-bentonite in the lower part of the Dengying Formation at the Fanglong section. Thus, the termination of the DOUNCE and the Shuram carbon isotope excursion (terminated at 567.3 ± 3.0 Ma) may be isochronous. As volcanic ashfall events are known to supply nutrients to the oceans, we hypothesise that the widespread volcanism of the late Ediacaran may have enhanced oceanic productivity and play a role in the evolution of life at this time. [ABSTRACT FROM AUTHOR]

Abstract The Doushantuo negative carbon isotope excursion (DOUNCE) is the largest known marine inorganic carbon isotope anomaly. The origin of this pronounced negative excursion is still an enigmatic issue that attracts geologists. Time constraints on the excursion are the critical information that would provide insight into its genesis. In previous decades, the timing of its termination has been constrained by the widely cited zircon U-Pb age of 550.5 ± 0.8 Ma for the tuff at the top of the Miaohe Member at the Jiuqunao section in the Yangtze Gorges area, South China. However, results of recent studies indicate that the reliability of this time constraint needs to be re-evaluated. Here, a geochronological study was carried out using two K-bentonites from Fanglong in South China. A K-bentonite in the lower Dengying Formation yielded a U-Pb age of 557 ± 3 Ma, while a K-bentonite in the basal Liuchapo Formation yielded an age of 550 ± 3 Ma. Based on regional correlations between the Ediacaran successions in South China, the age (557 ± 3 Ma) for the K-bentonite in the lower Dengying Formation may serve as a second critical timing constraint for the ending of the DOUNCE. Combined with available estimates of the DOUNCE duration, our new data indicate that the DOUNCE has a maximum onset age ∼570 Ma. [ABSTRACT FROM AUTHOR]

The Meishucun section in Yunnan is the stratotype section for stratigraphic correlation of the Lower Cambrian strata across the Yangtze Block. Known for enriched small shelly fossils, it is a prominent section for investigating the Early Cambrian phosphogenic event. Pašava et al. (Econ Geol 105:1047-1056, ) reported anomalously high PGE concentrations in this section, up to 576 × 10 (434 × 10 Pt, 142 × 10 Pd) for the total PGE concentrations of a K-bentonite sampled from the bottom of Bed 5. This finding can illustrate two significant statements: (1) in addition to the attested polymetallic Ni-Mo-PGE ore layer, another potential PGE enrichment layer exists with PGE concentrations up to the mineralization level; and (2) acid volcanics have high PGE contents overturning conventional views. To inspect whether the anomalous PGE concentration is pervasive, we investigated Bed 5 of the Meishucun section systematically, and sampled from a profile with a thickness of 3.5 m. The major and trace element geochemistry indicate the Bed 5 K-bentonite is derived from acid volcanic ash. PGE concentrations were determined repeatedly by isotope dilution-ICP-MS using improved digestion technique (Qi et al., in J Anal At Spectrom 26:1900-1904, ), and were duplicated by fire assay method. The results showed that each sample had total PGE concentrations of less than 0.90 × 10, and Pt + Pd concentrations of no higher than 0.70 × 10. Combined with the petrological and mineralogical features, and trace and rare earth element analyses, it is inferred that no generality of PGE enrichment exists in Bed 5 and that the anomalous PGE concentration is likely due to the nugget effect of volcanic ash modified by currents in a shallow coastal environment. [ABSTRACT FROM AUTHOR]

The Lower Cambrian on the Yangtze Platform in South China (internationally equivalent to the Nerreneuvian and 2nd series of the Cambrian) is valuable for understanding the early evolution of life, the global biogeochemical circles, and the major changes of the ocean. In particular, a precise radiometric calibration of the Lower Cambrian of this region is a critical realm in its multidisciplinary studies. A SHRIMP U-Pb geochronology study of the K-bentonite in the topmost Laobao Formation at the Pingyin section, Guizhou, South China yielded an age of 536±5 Ma (MSWD = 0.75), suggesting that the K-bentonite here can be correlated with the intensely studied K-bentonite within the middle Zhongyicun Member (Bed 5) of the Zhujiaqing Formation at the Meishucun section in Yunnan. Such a regional correlation result reveals that the Bed 5 K-bentonite also extends into deeper-water environments rather than being deposited merely in the shallow platform interior of the Yangtze Platform. The age of the K-bentonite at the Pingyin section also implies that its overlying polymetallic Ni-Mo layer should be younger than 536±5 Ma. Hence the previous placement of the Precambrian/Cambrian boundary at this layer is inappropriate. Combined with the results of stratigraphic correlations, it is suggested that the K-bentonites in the middle Zhongyicun Member of the Zhujiaqing Formation (or the equivalents) and the base of the Shiyantou Formation (or its equivalents), together with the polymetallic Ni-Mo layer, serve as three important marker beds. Their self-consistent radiometric ages have established an improved geochronologic framework for the Lower Cambrian in South China. [ABSTRACT FROM AUTHOR]

A SHRIMP U-Pb zircon geochronology study of the tuft at the bottom of the Lower Cambrian Niutitang Formation at Songlin, Zunyi, South China yielded a mean 206Pb/238U age of 51 8±5 Ma. It is significantly younger than the Re-Os ages of 537-542 Ma for the overlying polymetallic Ni-Mo-PGE-rich layer in the basal Niutitang Formation at Songlin, suggesting that the Re-Os ages might represent that of the original magma chamber for the Ni-Mo-PGE-rich layer rather than the formation age of the strata. The intra-basinal stratigraphic correlation also implies that the absolute age of the Chengjiang biota should be younger than 518±5 Ma. Our new result, together with the SHRIMP U-Pb zircon age for the K-bentonite in the Zhongyicun member of the Zhujiaqing Formation at the Meishucun section, .Yunnan Province, provides a temporal constraint for the Lower Cambrian of the Yangtze Platform in South China. [ABSTRACT FROM AUTHOR]

• Source magma of K-bentonite in Doushantuo cap dolostone at Jiuqunao is rhyodacitic. • Source magma of K-bentonite in Doushantuo cap dolostone at Heyu is pantelleritic. • Two episode K-bentonites in Doushantuo cap dolostone have been identified. • K-bentonites have potential significance to the genesis of Marinoan cap dolostone. A study of the K-bentonites in the Doushantuo cap dolostones on the Yangtze Platform in South China would facilitate understanding the origin of the Marinoan cap dolostones and testing the snowball Earth hypothesis. To date, sporadic work has been conducted on the whole-rock geochemistry of the K-bentonites from the platform and slope of the Yangtze Platform, to preliminarily understand the genesis of the K-bentonites. However, a study of the whole-rock geochemistry of the K-bentonites from the basin and on zircon geochemistry of the K-bentonites from the entire Yangtze Platform has not been carried out. Therefore, the nature of the primary magmas, source and tectonic setting of the source volcanoes, and the episodes of source volcanic eruptions of the K-bentonites are unknown, which hinders unraveling the possible link between the sedimentary model of the Marinoan cap dolostone and the K-bentonites. Here, we perform a systematic study of the whole-rock and zircon geochemistry of the K-bentonites. The major and trace element concentrations of the K-bentonites and trace element contents in their volcanic zircons suggest that the primary magmas of the K-bentonites from the Jiuqunao and Heyu sections are different in nature. The same geochemical results indicate that the source volcanoes of all the K-bentonites may occur in the extensional setting on the northern margin of the Yangtze Block. Hf isotopic compositions of the volcanic zircons in the K-bentonites imply that the source magmas of the K-bentonites are mainly mantle-derived, and may have mixed with the crustal materials from the northern margin of the Yangtze Block. Based on the difference in whole-rock and zircon geochemistry of the K-bentonites from Heyu and Jiuqunao, two episodes of the source volcanic eruptions of the K-bentonites could be identified. The K-bentonite at Heyu most probably represents the record of the old episode, while that at Jiuqunao represents the record of the young one. The here proposed dual episodic deposition of K-bentonites within the Doushantuo cap carbonates holds potential significance to constrain 1) the duration of the cap carbonate deposition and 2) their genesis in potential slushball or snowball Earth environments. [ABSTRACT FROM AUTHOR]

Sheet-crack – primarily filled with chalcedony, quartz and calcite – is a type of significant sedimentary structure in almost all documented cap carbonates in platform-to-slope settings, which sharply overlie the global Marinoan (∼635 Ma) glacial tillite. The Jiulongwan section, located in the Yangtze Gorges areas, South China, is significant for the first discovery of extremely 13 C-depleted (δ 13 C down to −41‰) calcite cements in sheet-crack structure (Jiang et al., 2003a), which provides direct evidence for the methane seep hypothesis (Kennedy et al., 2001). Focusing on the calcite cements in the same section, Bristow et al. (2011) proposed a hydrothermal origin based on high temperatures (378 °C) determined by carbonate clumped isotope thermometry (CCIT). In this study, we provide evidence for the hydrothermal model using fluid inclusion technology (FIT) to quartz cements in the Jiulongwan section. The widespread sheet-crack structures in South China have uniform mineral paragenetic assemblages, which apparently reflect three stages of a hydrothermal fluid activity, including dolomitization at the early stage, silicification comprising chalcedony and quartz at the middle stage, and calcitization characterized by carbonaceous calcite filling preexisting voids at the late stage. Primary aqueous fluid inclusions from quartz crystals have homogenization temperatures of approximately 160–220 °C (mean, 192 °C, n = 31), while the salinity histogram contains two peaks at low salinity (6.3–8.3 wt.% NaCl equiv.) and high salinity (18.0–20.8 wt.% NaCl equiv.), reflecting precipitation caused by mixing of high- and low-salinity fluids. A modified and detailed hydrothermal model is proposed indicating that the sheet-crack structure resulted from successive thermal fluid activity after karstic dissolution due to postglacial isostatic rebound. This model is compatible with the unified sequence of glacio-eustatic events after the termination of Marinoan glaciation (Zhou et al., 2010) and the temperature results determined by CCIT (Bristow et al., 2011). [ABSTRACT FROM AUTHOR]

[Display omitted] • 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. [ABSTRACT FROM AUTHOR]

The Wuchuan–Sihui–Shaoguan (WSS) exhalative sedimentary pyrite belt in the southwestern part of the Qinzhou–Hangzhou (Qin–Hang) belt is the most important sulfur industry base in China. However, a wide range of metallogenetic ages spanning from Ediacaran to Devonian has been reported in the literature. This age range does not support the idea that the typical character of "coeval mineralization" in an exhalative sedimentary mineralization belt in China and worldwide. Therefore, the precise determination of mineralization ages of representative deposits is necessary to provide guides for exploration and metallogenetic models. The Dajiangping pyrite deposit is a typical example of this kind of deposits and is also the largest deposit with a proven reserve of 210 Mt. This deposit was thought to have formed in Ediacaran or Devonian. In this study, 2–3 layers of 10–25 cm thick 2M1-type microcrystalline muscovite slate abruptly embedded in the No. IV massive orebody of the deposit has been identified to be low-grade metamorphic K-bentonite. A Concordia zircon LA–ICP–MS U–Pb age of 432.5 ± 1.3 Ma (mean standard weighted deviation of concordance and equivalence = 1.2; N = 11) has been yielded for the low-grade metamorphic K-bentonite. This age is distinctly different from the Rb–Sr isochron age of 630.1 ± 7.3 Ma for siliceous rock at the top of the No. III banded orebody and the Re–Os isochron age of 389 ± 62 Ma for pyrites from a laminated orebody. Instead, it is close to the intercept age (429 Ma) of the youngest detrital zircons from sandstone interlayers of the No. III banded orebody. The Concordia age is also coincident with those of the Late Caledonian (400–460 Ma) magmatism-metamorphism events which are widely distributed in Cathaysia Block. Particularly, it agrees well with that of the Early Silurian extensional volcanism (434–444 Ma) which have been revealed in the Dabaoshan, Siqian–Hekou, and Nanjing volcanic basins in northern Guangdong Province and southern Jiangxi Province. Hence, the dating result in this study confirms that the sedimentary time of the ore-host Daganshan Formation is Early Silurian, and implies that the mineralization age of the Dajiangping pyrite deposit should also be Early Silurian. In combination with the Early Silurian age of Shezui pyrite deposit and the Dabaoshan volcanic basin along the WSS pyrite belt, it could be inferred that the WSS pyrite belt provides a record of the northern expanding of Qinzhou–Fangcheng trough in Early Silurian and that the exhalative pyrite mineralization was triggered by the post-collisional extension of the margin of Cathaysia Block after the intracontinental collision between Cathaysia Block and Yangtze Block during Late Caledonian stage. [ABSTRACT FROM AUTHOR]

Global abrupt climate change from Marinoan snowball Earth to greenhouse Earth, recorded as cap carbonate overlain on diamictite, had shed the first light on Cambrian bio-radiation. The most documented cap carbonate sections are typical with comprehensive δ13C negative values and ubiquitous sedimentary structures, such as tepee-like, sheet-crack etc., which are associated with successive glacial eustatic variation caused by isostatic rebound in shallow-water facies. Here we report a deep-water basinal cap carbonate section with strong negative δ13C values in the southern margin of the Qinling Orogen, Heyu, Chengkou County, Chongqing in China, which consists of massive dolostone with abundant carbonaceous laminae. However, it lacks the sedimentary structure as mentioned above and is overlain by thin-bedded silicious shales and cherts. A K-bentonite bed was discovered within the base of cap carbonates, about 0.7 m above the top of the Marinoan diamictite. Magmatic zircons that were separated from the K-bentonite bed yield a SIMS concordia U-Pb age of 634.1 ± 1.9 Ma (1σ, MSWDCE = 0.31, ProbabilityCE = 1.000, n = 20). The age is in good agreement with previously reported TIMS U-Pb ages for the termination of Marinoan glaciation and provides a geochronological constraint for the Ediacaran successions in the Qinling Orogen. [ABSTRACT FROM AUTHOR]

• A karstic surface atop basal Ediacaran Doushantuo cap dolostone in South China. • Abundant speleothems in sheet-cavities of Doushantuo cap dolostone in South China. • An active soil-microbial ecosystem facilitated Ediacaran speleothem formation. Cryogenian snowball Earth glaciations may have had disastrous impacts on the biosphere, particularly the terrestrial ecosystem. However, how the terrestrial ecosystem responded to and recovered from these glaciations remains poorly understood. Speleothems offer important insights into terrestrial life because their formation is critically dependent on soil CO 2 derived from microbial respiration. Here we report the wide distribution of miniature paleo-speleothems from the ∼ 635 Ma Doushantuo cap dolostone in South China and assess their implications for the recovery of terrestrial life after the terminal Cryogenian Marinoan glaciation. The ∼ 3-m-thick cap dolostone was deposited during the initial transgression following deglaciation but subsequently experienced subaerial exposure due to post-glacial rebound, which resulted in the development of extensive sheet-cavities in the cap dolostone and a widespread karstic surface atop the cap dolostone. The sheet-cavities were filled with multiple generations of minerals, including isopachous dolomite interpreted to have formed in the phreatic zone, speleothems consisting of fibrous calcite interpreted to have formed in the vadose zone, as well as later phases (i.e., isopachous radial chalcedony, crystalline quartz, and blocky calcite). The Doushantuo speleothems are millimeter-to-centimeter in size and include gravitational speleothems (stalactites and stalagmites) and non-gravitational speleothems (helictites, moonmilk, botryoids, and flat crusts). Some of them were secondarily silicified by hydrothermal fluids before a renewed transgression in which ∼ 632 Ma shales overlying the cap dolostone were deposited. The wide distribution of Doushantuo speleothems and the preservation of microfossils in these speleothems indicate the presence of an active soil-microbial ecosystem in the earliest Ediacaran Period and the rapid recovery of terrestrial life after the Marinoan snowball Earth. [ABSTRACT FROM AUTHOR]