Ediacaran Doushantuo Research Articles

A LA-ICP-MS analysis of rare earth elements on phosphatic grains of the Ediacaran Doushantuo phosphorite at Weng'an, South China: implication for depositional conditions and diagenetic processes

AbstractThe Ediacaran Doushantuo Formation at Weng'an, South China hosts well-preserved phosphatized microfossils known as the Weng'an biota. A laser ablation ICP-MS analysis of rare earth element (REE) characteristics of the fossil-bearing black phosphorite unit of the Doushantuo Formation at Weng'an was conducted, with the aim of unravelling the depositional conditions and diagenetic processes during formation of the phosphorites. Spherical phosphatized microfossils and phosphatic clasts were analysed, and the REE data display middle REE (MREE) -enriched shale-normalized REE patterns. The spherical phosphatized microfossils show an increase in total REE contents (∑REE) from core to rim. Negative correlations between ∑REE and the extent of MREE enrichment over the other REE (indicated by LaN/SmN, YbN/SmN) are observed for analysed spots within individual phosphatic grains, which may be due to complex diagenetic history of the phosphatic grains, with fluctuations in REE sources and chemical parameters in a high-energy shallow-water environment being additional factors. The LaN/YbNand LaN/SmNcharacteristics of the phosphatic grains suggest they were mostly influenced by early diagenetic alteration rather than late extensive recrystallization. The negative Ce anomalies in the samples suggest they formed under oxic-bottom-water conditions. Positive Eu anomalies are present in all samples, and are likely to reflect involvement of hydrothermal fluids rather than changes in redox conditions of porewater. Overall this study has major implications for phosphorites as important archives for the study of geochemical proxies, the Ediacaran period and also evolutionary changes.

Carbon isotope gradient of the Ediacaran cap carbonate in the Shennongjia area and its implications for ocean stratification and palaeogeography

The geochemistry and paleogeography of the Doushantuo succession in the Shennongjia area have been insufficiently studied. Here, we report on the carbon and oxygen isotope compositions of Ediacaran Doushantuo cap carbonates from four sections (Longxi, Muyu, Yazikou and Songluo) in the Shennongjia area. A large C-isotopic gradient (~5‰) between the Longxi and Songluo sections, here identified for the first time, is inferred to have been dynamically maintained by photosynthesis in surface waters and anaerobic oxidation of dissolved organic carbon (DOC) in deep waters. Spatial variation in C-isotope chemostratigraphy among the four sections was related to Marinoan post-glacial sea-level elevation changes. At Longxi, a positive δ 13Ccarb shift below the horizon containing tepee-like structure resulted from intense photosynthesis during early regression. A negative δ 13Ccarb shift within the tepee-horizon was triggered by sulfate reduction and freshwater mixing with 13C-depleted dissolved inorganic carbon (DIC) during late regression. A positive δ 13Ccarb excursion in the uppermost part of the cap carbonate was related to enhance primary productivity and organic matter burial during early transgression. At Muyu, the carbon isotopic variation tendency, which is similar to that at Longxi, may have been mildly influenced by the surface water environment. At Songluo, the positive δ 13Ccarb excursion, up to -4‰ in the lower part of the cap carbonate, was probably associated with methanogenesis in deep waters during late transgression to early regression and subsequently disappeared due to decomposition of methane hydrate during late regression. At Yazikou, the consistently stable δ 13Ccarb values around -4‰ indicate that the cap carbonate may have deposited at intermediate water depths. As evidenced by diagnostic sedimentary characteristics of the study sections, the palaeogeographic framework of the Shennongjia area exhibited deepening from SE to NW during the Early Ediacaran Period in the aftermath of Snowball Earth.

Selenium, arsenic and molybdenum variation and bio-radiation in the Ediacaran-Cambrian interval

The Ediacaran-early Cambrian period is critical for the evolution of multicellular life. However, the distribution and enrichment patterns of biological trace elements and their correlations with the bio-changes are not yet fully understood. Here we present the variation patterns of trace elements in the Yangtze Gorges area, South China. Compared to the upper continental crust, selenium (Se), cadmium (Cd), arsenic (As), and molybdenum (Mo) are the most enriched and varied trace elements in the whole Ediacaran and early Cambrian Formations, the concentrations of which vary from 10−5> to 30.1, 0.02 to 163.1, 5×10−5> to 196.5, and 0.02 to 289.0mg/kg, respectively, with average values of 1.3, 1.0, 14.0, and 8.0mg/kg, average enrichment factors (EF) of 27.0, 10.7, 9.7, and 5.4, and EF variations of 601.2, 1663.6, 131.0, and 192.6, respectively. The significantly enriched (EF>10) strata sequences of Se and As mainly include the lower and upper Ediacaran Doushantuo member II (DST-II), DST-IV, bottom and upper Yanjiahe Formation (Precambrian-Cambrian), and Shuijingtuo Formation (early Cambrian). Mo displays similar significantly enriched strata with Se and As from DST-IV and above. Compared to the Cryogenian Nantuo Formation, Se, As, and Mo are enriched and concentrated in the whole Ediacaran and early Cambrian strata, especially at the early Ediacaran Doushantuo Formation, the diachronous Yanjiahe Formation (Precambrian-Cambrian) and the early Cambrian Shuijingtuo Formation, which are the horizons of the great bio-changes. Moreover, the significantly enriched strata of Se, As, and Mo are also consistent with intervals of great bio-changes.

Carbonaceous biosignatures of diverse chemotrophic microbial communities from chert nodules of the Ediacaran Doushantuo Formation

The Ediacaran Doushantuo Formation (DST) is renowned for exceptionally preserved Precambrian fossils including metazoans. Some of these fossils, particularly microfossils such as multicellular algae and acanthomorphic acritarchs, are preserved in DST chert nodules. To better understand the geomicrobiological processes that contributed to the authigenic formation of DST chert nodules and facilitated exceptional fossil preservation, we analyzed organic matter in these chert nodules and the surrounding matrix (calcareous mudstone) using multiple in-situ techniques: confocal laser Raman spectroscopy, micro-Fourier transform infrared spectroscopy (FTIR), and secondary ion mass spectroscopy (SIMS). We found strong ultrastructural, chemical, and isotopic heterogeneities in the organic matter as indicated by the Raman spectral parameter I-1350/1600 ranging from 0.49 to 0.88, the infrared spectral index R3/2 from 0.12 to 0.90, and an estimated δ13Corg-SIMS range of 44‰ (V-PDB). These micron-scale heterogeneities imply that the organic matter preserved in the DST chert nodules is derived from different carbonaceous sources in a diverse microbial ecosystem, including eukaryotic and/or prokaryotic photoautotrophs, as well as chemotrophs involved in the fermentation and probably anaerobic oxidation of organic remains. Thus, the microbial ecosystems in Ediacaran ocean waters and sediments were more complex than previously thought, and these microbial processes controlled dynamic micro-environments in DST sediments where chert nodules were formed and fossils were mineralized. The results also show that variations in the relative abundances, activities, and interactions of co-existing microorganisms in DST sediments may have modulated δ13Corg shifts, causing local decoupling between δ13Corg and δ13Ccarb as measured in bulk samples.

Integrated carbon, sulfur, and nitrogen isotope chemostratigraphy of the Ediacaran Lantian Formation in South China: Spatial gradient, ocean redox oscillation, and fossil distribution.

The Ediacaran Doushantuo Formation in South China is a prime target for geobiological investigation because it offers opportunities to integrate chemostratigraphic and paleobiological data. Previous studies were mostly focused on successions in shallow-water shelf facies, but data from deep-water successions are needed to fully understand basinal redox structures. Here, we report δ13 Ccarb , δ13 Corg , δ34 Spyr , δ34 SCAS , and δ15 Nsed data from a drill core of the fossiliferous Lantian Formation, which is a deep-water equivalent of the Doushantuo Formation. Our data confirm a large (>10‰) spatial gradient in δ13 Ccarb in the lower Doushantuo/Lantian formations, but this gradient is probably due to the greater sensitivity of carbonate-poor deep-water sediments to isotopic mixing with 13 C-depleted carbonate cements. A pronounced negative δ13 Ccarb excursion (EN3) in the upper Doushantuo/Lantian formations, however, is spatially consistent and may be an equivalent of the Shuram excursion. δ34 Spyr is more negative in deeper-water facies than in shallow-water facies, particularly in the lower Doushantuo/Lantian formations, and this spatial pattern is interpreted as evidence for ocean redox stratification: Pyrite precipitated in euxinic deep waters has lower δ34 Spyr than that formed within shallow-water sediments. The Lantian Formation was probably deposited in oscillating oxic and euxinic conditions. Euxinic black shales have higher TOC and TN contents, but lower δ34 Spyr and δ15 Nsed values. In euxinic environments, pyrite was predominantly formed in the water column and organic nitrogen was predominantly derived from nitrogen fixation or NH4+ assimilation because of quantitative denitrification, resulting in lower δ34 Spyr and δ15 Nsed values. Benthic macroalgae and putative animals occur exclusively in euxinic black shales. If preserved in situ, these organisms must have lived in brief oxic episodes punctuating largely euxinic intervals, only to be decimated and preserved when the local environment switched back to euxinia again. Thus, taphonomy and ecology were the primary factors controlling the stratigraphic distribution of macrofossils in the Lantian Formation.

Uncovering the spatial heterogeneity of Ediacaran carbon cycling.

Records of the Ediacaran carbon cycle (635-541 million years ago) include the Shuram excursion (SE), the largest negative carbonate carbon isotope excursion in Earth history (down to -12‰). The nature of this excursion remains enigmatic given the difficulties of interpreting a perceived extreme global decrease in the δ13 C of seawater dissolved inorganic carbon. Here, we present carbonate and organic carbon isotope (δ13 Ccarb and δ13 Corg ) records from the Ediacaran Doushantuo Formation along a proximal-to-distal transect across the Yangtze Platform of South China as a test of the spatial variation of the SE. Contrary to expectations, our results show that the magnitude and morphology of this excursion and its relationship with coexisting δ13 Corg are highly heterogeneous across the platform. Integrated geochemical, mineralogical, petrographic, and stratigraphic evidence indicates that the SE is a primary marine signature. Data compilations demonstrate that the SE was also accompanied globally by parallel negative shifts of δ34 S of carbonate-associated sulfate (CAS) and increased 87 Sr/86 Sr ratio and coastal CAS concentration, suggesting elevated continental weathering and coastal marine sulfate concentration during the SE. In light of these observations, we propose a heterogeneous oxidation model to explain the high spatial heterogeneity of the SE and coexisting δ13 Corg records of the Doushantuo, with likely relevance to the SE in other regions. In this model, we infer continued marine redox stratification through the SE but with increased availability of oxidants (e.g., O2 and sulfate) limited to marginal near-surface marine environments. Oxidation of limited spatiotemporal extent provides a mechanism to drive heterogeneous oxidation of subsurface reduced carbon mostly in shelf areas. Regardless of the mechanism driving the SE, future models must consider the evidence for spatial heterogeneity in δ13 C presented in this study.

Silicified glendonites in the Ediacaran Doushantuo Formation (South China) and their potential paleoclimatic implications

The Ediacaran Period is punctuated by the ca. 580 Ma Gaskiers glaciation in Newfoundland. However, paleoclimatic data are scarce in Ediacaran successions in South China, where abundant geochemical and paleobiological data are shaping current understanding of Ediacaran evolutionary and environmental history. Here, we report the occurrence of silicified glendonites in the Ediacaran Doushantuo Formation deposited in an inner-shelf environment on the South China block. Petrographic evidence suggests that these silicified glendonites are pseudomorphs after syndepositional or early authigenic ikaites formed at near-freezing temperatures. The glendonite-bearing stratigraphic interval is characterized by positive δ13C values. It predates both the negative δ13C excursion EN3 (widely believed to be an equivalent of the Shuram negative excursion) and excursion EN2. Although alternative interpretations may be possible, these glendonites may be related to and correlated with the Gaskiers glaciation. If confirmed, this correlation suggests that the Shuram event postdates the Gaskiers glaciation, thus having important implications for Ediacaran climate changes, carbon cycles, and biological evolution.

The stratigraphic complexity of the middle Ediacaran carbon isotopic record in the Yangtze Gorges area, South China, and its implications for the age and chemostratigraphic significance of the Shuram excursion

The middle Ediacaran Shuram excursion represents the most pronounced negative carbon isotopic shift in Earth history, and has been considered as evidence for a profound disturbance to the global carbon cycle and proposed as a key chemostratigraphic marker for Ediacaran stratigraphic subdivision and global correlation. Previous study has revealed a pronounced negative δ13C shift (EN3) in the upper Doushantuo Formation of South China, which has been interpreted as an equivalent of the Shuram excursion. Detailed δ13C investigation of multiple sections of the Ediacaran Doushantuo Formation around the Huangling Anticline, western Hubei Province, South China, indicates that the δ13C variation in the upper Doushantuo Formation is more complex than previously reported. In the western region of the Huangling anticline, a moderately positive δ13C excursion (here termed the Diaoyapo positive excursion) bisects two strongly negative δ13C excursions (here termed the Jiuqunao negative excursion and the younger Miaohe negative excursion), with a ∼551Ma ash bed capping the Miaohe negative excursion. In the central region, there is only one negative δ13C excursion, whereas in the eastern region, the negative δ13C excursion is absent. The variable stratigraphic expression of these δ13C excursions is most parsimoniously interpreted as reflecting local facies variation, diagenesis, and/or a cryptic unconformity. The more complete but more complex δ13C chemostratigraphic record in the western region of Huangling Anticline implies that, depending on how this record is correlated with the Shuram excursion, the latter may be significantly older than 551Ma if it does represent a global chemostratigraphic marker.

Pyrite morphology and episodic euxinia of the Ediacaran Doushantuo Formation in South China

The Ediacaran Doushantuo Formation with well-preserved fossil record in South China provides a rare window for our understanding of biological evolution, global carbon cycle, and oceanic redox states. Prominent negative δ13C anomalies (i.e., the Shuram excursion) in Ediacaran successions worldwide fundamentally challenge the traditional models of isotopic mass balance. Additionally, conflicting opinions of both oxic and anoxic conditions have been proposed for the deep waters during this period. Here, we present a detailed study of pyrite morphology and carbonate carbon isotope data documented from drill core samples at Songtao County, northeastern Guizhou. Framboid aggregates are the dominant pyrite form in black shale and they can transfer to euhedral crystals through continuous growth of the constituent microcrystals. A positive correlation between microcrystal sizes (d) and framboid diameters (D) is observed, while the different D/d ratios of framboids in argillaceous dolostone and black shale reflect different substrate availability. Electron microprobe analyses reveal no consistent compositional patterns between framboidal and euhedral pyrites. Framboid size distributions of the investigated drill core, in combination with previously published redox data from the intra-shelf Jiulongwan section, shelf margin Zhongling section, and lower slope Wuhe section, suggest that three episodes of marine euxinia have been established throughout the deposition of the Doushantuo Formation. The time lag between the uppermost euxinic interval and the Shuram excursion may arise from the depression of sulfate reduction maintained by other oxidants.

A petrographic and geochemical study of carbonate and silica phases from the Ediacaran Doushantuo Formation in the Three Gorges area of South China: Implications for diagenetic conditions

The carbonate of the Ediacaran Doushantuo Formation contains various silica phases in the Three Gorges area of South China. To better understand the diagenetic conditions under which these silica phases formed, we carried out a petrographic and geochemical study of the silica phases and the coadjacent carbonate phases at the Xiaofenghe section (XFH), in the Three Gorges area of South China. Five types of silica phases can be distinguished based on petrographic characteristics: S1 and S2 are composed of microcrystalline quartz, in chert nodules and disseminated in the host dolostone, respectively. S3 is composed of the megaquartz, while S4 and S5 are microcrystalline quartz and fibrous silica, respectively, both replacing calcite cement. Six distinct types of carbonate can be recognized, including the host matrix carbonate (C1), sparry calcite nodules (C2), calcite around chert nodules (C3), dispersed calcite cement within chert nodules (C4), fine micropeloidal calcite cements (C5) and calcite veins (C6). Sedimentary structures suggest that formation of the S1 through S5 silica phases and C2 through C5 carbonate phases occurred during early burial diagenesis, and therefore their formation recorded early diagenetic conditions.Assuming that Ediacaran surface seawater temperature ranged from 28 to 39°C, surface seawater δ18O values can be calculated based on the oxygen isotopic composition of the host dolostone matrix (C1), which range from −6 to 0‰, −4 to 0‰, and −2 to 0‰, for the D2, D3 and D4 subunits of the Doushantuo Formation, respectively. Based on the δ18O values of chert nodules and beds (mainly phase S1), diagenetic temperatures can be constrained to between 27 and 60°C. Under such low diagenetic temperatures, the primary geochemical signatures of the host dolostone, such as REE+Y patterns, δ13CCarb and δ34SPy values, are likely to be preserved.Those silica phases without evidence of contamination of continental silicate detritus show characteristic REE+Y patterns, with minor light REE depletions, positive La, Eu and Gd anomalies, negative Ce anomalies and slightly super-chondritic Y/Ho ratios, implying that the source of silica may be derived from seawater. Negative Ce anomalies in the silica phases in D2 through D4 subunits indicate that the bottom seawater or early diagenetic porewater was oxic at the time of silica precipitation.

Meroblastic cleavage identifies some Ediacaran Doushantuo (China) embryo-like fossils as metazoans

Abstract Ediacaran Doushantuo (Formation) embryo-like fossils (EDEFs, ca. 600 Ma) from South China display cellular and sub-cellular structures and provide a unique window on the early evolution of multicellular eukaryotes. But there have been widely disparate interpretations of these fossils. Here we report new fossil embryo-like forms from the Doushantuo phosphorite that exhibit a meroblastic cleavage pattern. Our results from high-resolution propagation phase contrast–synchrotron radiation X-ray microtomography (PPC-SRµCT) demonstrate that these fossils preserve features directly comparable to those of modern meroblastic animal embryos that utilize discoidal cleavage. Given that discoidal-type meroblastic cleavage occurs only in metazoans, the phylogenetic positions of these fossils probably fall into the animal branch of the holozoan tree. Meroblastic as well as holoblastic cleavage forms were thus present by ca. 600 Ma, substantiating the conclusion derived from molecular clock estimates that a variety of metazoan lineages had evolved by the mid-Ediacaran after the termination of the Marinoan glaciation, if not earlier.

New observations of Ambient Inclusion Trails (AITs) and pyrite framboids in the Ediacaran Doushantuo Formation, South China

Ambient Inclusion Trails (AITs) are intriguing microtubular structures that commonly occur in association with pyrite in Precambrian organic-rich cherts and phosphorites. They are thought to be created by the migration of pyrite or other crystal grains through a lithified substrate driven by pressure solution from the in situ thermal decomposition of organic matter. New phosphorite samples of the Ediacaran Doushantuo Formation (South China) contain abundant AITs exhibiting diverse morphotypes, which may be distinguished from filamentous microfossils and endolithic microborings with a suite of morphological criteria based on optical microscopy and scanning electron microscopy (SEM). Black shales of the Baizhu section contains abundant pyrite framboids whose size distributions reveal significant temporal variations of redox conditions in shallow marine waters that probably promoted the formation of the Doushantuo phosphorites.AITs in the phosphorites are categorized into three types and further into five subtypes (I-a, I-b, II-a, II-b, and III) based on their morphologies and observed or interpreted associations with various kinds of terminal pyrite crystals. Among these, subtype II-a, single striated microtubes 2–10μm wide, are interpreted to have resulted from migration of intact pyrite framboids. Those of subtype II-b, dense clusters of outward radiating microtubes with consistent widths and inward-facing cuspate ridges, likely have formed by explosive disintegration and propulsion of pyrite framboids due to highly concentrated carbon dioxide gas during the oxidation of organic matter. During early diagenesis, formation of euhedral and framboidal pyrites involve a suite of biogeochemical and physical processes including non-biological oxidation of organic matter and reduction of sulfate in the presence of ferrous iron. Following the burial of pyrites, further oxidative degradation of organic matter produced abundant CO2 gas, which drives the pyrites to move through the solid, but not yet fully lithified phosphatic gel composing granules. This model explains the formation of previously reported but unexplained star-burst type AITs and it may be tested by experimental studies.Our new observations provide evidence for the widespread occurrence of AITs in the Doushantuo phosphorites and urge careful petrographic examinations and differentiation between AITs and morphologically similar biogenic microstructures.

Methane-derived authigenic carbonate from the lower Doushantuo Formation of South China: Implications for seawater sulfate concentration and global carbon cycle in the early Ediacaran ocean

Authigenic carbonate associated with anaerobic oxidation of methane (AOM), usually via microbial sulfate reduction (MSR) or ferric iron reduction, is generally characterized by extremely low δ13C values (<−30‰, VPDB). This has been used as one of the major diagnostic features for the recognition of hydrocarbon seep carbonate in the geological past. Previous reports on Precambrian authigenic carbonates are rare, limiting our understanding of the effects of their deposition on the Earth's carbon isotopic mass balance. In this study, mainly based on petrographic features and pronounced negative δ13C values as low as −38.1‰, we discovered authigenic calcite cement immediately above the cap dolostone in the basal Ediacaran Doushantuo Formation in the Jiulongwan section, Yangtze Gorges area, South China. Our observations not only provide direct evidence for the involvement of AOM during carbonate precipitation in the early Ediacaran (~635Ma), but also suggest that the seawater sulfate concentrations in the early Ediacaran may have been higher than previously thought.

Ocean oxidation during the deposition of basal Ediacaran Doushantuo cap carbonates in the Yangtze Platform, South China

Precipitation of cap carbonate lithologies is a key feature of Cryogenian global glaciations. Negative carbonate carbon isotopic compositions (δ13Ccarb) of these cap carbonates have been variably interpreted as massive drawdown of atmospheric CO2 via extensive continental chemical weathering, methane oxidation, or postglacial upwelling. Each of these interpretations argues a non-steady state of carbon cycle in the aftermath of Marinoan global glaciation. To further explore the postglacial marine carbon cycle, we measured δ13Ccarb of cap carbonates from six localities in the Yangtze Platform, South China. The studied cap carbonates were deposited in a variety of sedimentary environments, ranging from the open shelf, slope, to basin facies. Cap carbonates deposited in different environments show distinct stratigraphic trends of δ13Ccarb. In the open shelf, δ13Ccarb profile of the Songlin section remains almost constant (−3 to −4‰), while the δ13Ccarb of the Jiulongwan section records a negative excursion, decreasing from −3.5‰ to −7‰. δ13Ccarb of cap carbonates deposited in the slope environment does not show stable stratigraphic trend. In the basin environment, δ13Ccarb demonstrates a sharp decline in the middle part of cap carbonates to the nadir value of ∼−11‰. The negative δ13Ccarb excursion is best interpreted in terms of oxidation of dissolved organic carbon (DOC), thus recording a pulse of ocean oxidation during cap carbonate precipitation. Clearly absence of negative δ13Ccarb excursion in all slope and most open shelf sections may imply that such oxidation event was not ubiquitous in the Yangtze Platform. We speculate that the renewed thermohaline circulation during deglaciation brought oxic surface water into ocean interior, which oxidized the basin environment of the Yangtze Platform. However, the deglacial thermohaline circulation was not strong enough to cause complete oxidation of the ocean. The sporadic oxidation in the open shelf, on the other hand, might result from the terrestrial influx of oxidant from postglacial continental weathering. Our study suggests that ocean oxidation, though sporadic, might have occurred during cap carbonate precipitation, and predated the first appearance of putative animal embryos.

Constraining ribbon rock dolomitization by Mg isotopes: Implications for the ‘dolomite problem’

Modern dolomite formation is conditional and restricted in specific environmental and geochemical conditions, accordingly cannot be used as the analog of ancient dolostones, which normally have platform-wide distribution. The key to understand ancient dolostone formation is to find a dolomitization process that is independent of physical/geochemical environments. It is well-known that ribbon rock (one type of calcareous rhythmites, sensu stricto referring to the alternating limestone and marlstone layers or the limestone-marl alternations) that are deposited in nearly all marine environments throughout the Earth's history, are exclusively, though normally partially, dolomitized. Thus, understanding ribbon rock dolomitization may provide valuable insight into the ‘dolomite problem’. In this study, we measured Mg isotopic compositions (δ26Mg) of ribbon rock samples collected from the Ediacaran Doushantuo Formation in South China. The marlstone layers contain higher dolomite contents and are characterized by higher δ26Mg (ranging from −2.68‰ to −1.84‰) than the limestone layers (varying between −3.21‰ and −2.53‰). A numerical model calculation indicates that clay minerals cannot provide enough Mg for dolomitization, arguing against the traditional interpretation that dolomitization is resulted from diagenetic Mg release from clays. Instead, contemporaneous seawater might provide sufficient Mg for dolomitization, because both Mg isotopes and dolomite content can be simulated by the Diffusion-Advection-Reaction model. Contemporaneous seawater dolomitization requires kinetic barrier be overcome at shallow depth of sediments. We propose the ribbon rock dolomitization is attributed to differential diagenesis of the clay-rich and clay-poor sediment layers, in which Ca2+-Mg2+ exchange elevates porewater Mg/Ca in clay-rich layers.

Phosphogenesis associated with the Shuram Excursion: Petrographic and geochemical observations from the Ediacaran Doushantuo Formation of South China

The Ediacaran Period witnessed one of the largest phosphogenic events in Earth's history. Coincidently, some phosphorite deposits in South China are associated with the largest-known carbon isotope negative excursion (i.e., Shuram Excursion), suggesting an intimate coupling of the biogeochemical carbon and phosphorous cycles. However, the geomicrobiological linkage between these anomalies remain poorly understood. In this study, we investigated the phosphorite samples from the uppermost Doushantuo Formation in South China. Carbon isotope compositions of authigenic calcite cements and nodules in the phosphorites are as low as –34‰ (VPDB). Petrographic and geochemical investigations indicate that the 13C-depleted carbonates likely formed as the result of microbial sulfate and iron reduction that released phosphorous from iron oxyhydroxide, concentrating phosphorous in pore waters, and thereby promoting phosphate mineralization. The timing of this event appears to coincide with enhanced sulfate delivery to seawater through continental weathering. The basin-scale distribution of Doushantuo phosphorites suggests a redox control on the availability of iron oxyhydroxide and the recycling of pore water phosphorous. Both inner and outer shelf regions were likely characterized by an oxic water column, and were the main loci for phosphogenesis; on the contrary, intra-shelf and slope regions, which are lean in phosphorite, were subjected to euxinic or ferruginous water column conditions. The intimate coupling between Ediacaran phosphogenesis and the Shuram Excursion suggests strong links among seawater redox conditions, C-S-P-Fe cycling, and fossil phosphatization. Increased microbial sulfate reduction driven by enhanced sulfate reservoir in the Ediacaran ocean may have played an essential role on these biogeochemical events.

Depositional environment and micropore characteristics of the Ediacaran Doushantuo Formation black shale in Western Hubei, China

The black shale of the lower Sinian (Neoproterozoic) Doushantuo Formation, located in the Yangtze area (Hubei, China), has a great potential to contain shale gas. The main depositional environments of the region included tidal flats, shelf lagoons, shelf margins, and slopes. While black shale formed from deposits in a shelf lagoon setting, the sediments from the tidal flats were dolomitic. Several factors affect the sediments deposited in these environments, including sediment surface energy (i.e., quiescent or turbulent), salinity, and the presence of either oxic or reducing conditions. The depositional environments of segments B and D of the Doushantuo Formation in the Jiulongwan section were significantly different; specifically, segment D was anoxic to euxinic. As a result, the total organic carbon (TOC) in the segment D shale is higher than that in segment B. This study also used field emission scanning electron microscopy to show that pores in organic matter are the main gas storage space and that the organic matter pores in segment D have diameters less than 100 nm. The increasing of the organic matter content increases organic pore and porosity of the reservoir. TOC is not only the main controlling internal factors of the micropores of the shale in Doushantuo Formation but also provides the important storage space for shale gas. The quiet and hypoxia environment of the black shale in Doushantuo Formation is the main factor controlling the enrichment of organic matter, and the enrichment of organic matter is beneficial to improve the physical property of the shale reservoir. Therefore, black shale formed in the quiet and anoxic depositional environment has better reservoir physical property.

Dendroid multicellular thallophytes preserved in a Neoproterozoic black phosphorite in southern China

Du, W., Wang, X.L., Komiya, T., Zhao, R. & Wang, Y., April 2016. Dendroid multicellular thallophytes preserved in a Neoproterozoic black phosphorite in southern China. Alcheringa 40, xxx–xxx. ISSN 0311-5518.A new form of dendroid multicellular thallophyte is documented in the Ediacaran Doushantuo phosphorite at Weng’an, Guizhou Province, southern China. The dendroid thallophytes have variable forms, possibly owing to heteromorphic variation. Many lateral branches extend from the upper portions of the main axes; the lateral branches bear terminal vegetative vesicles, reproductive vesicles, monosporangium-like discoidal vesicles and urn-shaped pseudoparenchymatous structures. The vegetative vesicles give rise to clavate pseudoparenchymatous structures, characterized by differentiation of the thallus medulla/cortex, which might represent an early stage of thallus development. An oogamous conceptacle arising from one carpogonial vesicle forms a highly specialized goblet-shaped conceptacle. The discovery of the new dendroid multicellular thallophytes provides not only the first fossil-based evidence of the morphological complexity and tissue differentiation in the Precambrian organisms but also insights into the life cycle of the Precambrian red algae.Wei Du* [duwei@ea.c.u-tokyo.ac.jp], School of Earth Sciences and Resources, China University of Geosciences, Beijing, PR China; Xun Lian Wang [wxl@cugb.edu.cn], School of Earth Sciences and Resources, China University of Geosciences, Beijing, 100083, PR China; Tsuyoshi Komiya [komiya@ea.c.u-tokyo.ac.jp], Graduate School of Arts and Sciences, University of Tokyo, Tokyo 153-8902, Japan. Ran Zhao [zhao_ran13@eps.s.u-tokyo.ac.jp], Department of Earth and Planetary Science, University of Tokyo, Tokyo 113-0033, Japan. Yue Wang [gzyuewang@126.com], School of Resources and Environments, Guizhou University, Guiyang, 550025, PR China. *Also affiliated with Graduate School of Arts and Sciences, University of Tokyo, Tokyo 153-8902, Japan.

Rare earth element geochemistry of phosphatic rocks in Neoproterozoic Ediacaran Doushantuo Formation in Hushan Section from the Yangtze Gorges Area, South China

Phosphatic rocks are widely distributed in Neoproterozoic Ediacaran Doushantuo Formation in Yangtze Gorges Area, South China. In this study, rare earth element geochemistry of eight phosphatic rock samples from the Hushan Section has been studied. All the samples display typical hat-shaped REE patterns, moderate negative Ce anomalies (Ce/Ce*=0.55 to 0.67), slightly positive Eu anomalies (Eu/Eu*=1.05 to 1.22) and low Y/Ho ratios (38.2±5.6). The hat-shaped REE patterns indicate diagenetic alteration of the primary REE signatures, which coincides with detrital siliciclastic sources of REE based on the Y/Ho ratios. The degree of Negative Ce anomalies and positive Eu anomalies may have recorded the redox features of diagenetic fluids, suggesting an anoxic environment during the phosphogenesis processes in Neoproterozoic Ediacaran Doushantuo Formation, South China. The geochemical comparison between the Lower Phosphorite Layer and Upper Phosphorite Layer at Hushan indicates a greater degree of diagenesis occurred in the Upper Layer than the Lower one. Besides the terrigenous sources of REE, organic materials could have also played a role on the REE characteristics.

The long-ranging macroalga Grypania spiralis from the Ediacaran Doushantuo Formation, Guizhou, South China

Wang, Y., Wang, Y. & Du, W., February 2016. The long-ranging macroalga Grypania spiralis from the Ediacaran Doushantuo Formation, Guizhou, South China. Alcheringa 40, xxx–xxx. ISSN 0311-5518Grypania spiralis (Walcott) Walter et al., a macroalga previously reported in pre-Ediacaran successions, has been collected, together with abundant macrofossils (i.e., the Wenghui biota), from black shales of the upper Doushantuo Formation (ca 593 to 551 Ma) in northeastern Guizhou, South China. Morphologically, G. spiralis represents a carbonaceous ribbon with a continuum of forms from coiled to nearly straight. Its helicoid main body might have been suspended in the water column for photosynthesis with one end anchored or nestled into soft sediments. Grypania possessed morphological stability, and its habit endowed great competitiveness for sunlight. Remarkably, it did not change significantly in size or morphology over more than 1200 Myrs.Ye Wang [gboywangye@126.com], School of Earth Sciences and Resources, PR China University of Geosciences, Beijing 100083, PR China; Yue Wang [gzyuewang@126.com] (corresponding author), School of Resources and Environments, Guizhou University, Guiyang 550003, PR China; Wei Du [duwei@ea.c.u-tokyo.ac.jp], Department of Earth Science and Astronomy, The University of Tokyo, Tokyo 153-8902, Japan.