Sr isotope excursion across the Precambrian–Cambrian boundary in the Three Gorges area, South China

The Precambrian/Cambrian (PC/C) boundary is one of the most important intervals for the evolution of life. However, the scarcity of well-preserved outcrops across the boundary leaves an obstacle in decoding surface environmental changes and patterns of biological evolution.In south China, strata through the PC/C boundary are almost continuously exposed and contain many fossils, suitable for study of environmental and biological change across the PC/C boundary. We undertook deep drilling at four sites in the Three Gorges area to obtain continuous and fresh samples without surface alteration and oxidation. ⁸⁷Sr/⁸⁶Sr ratios of the fresh carbonate rocks, selected based on microscopic observation and geochemical signatures of Mn/Sr and Rb/Sr ratios, aluminum and silica contents, and δ¹³C and δ¹⁸O values, were measured with multiple collector-inductively coupled plasma–mass spectrometric techniques.The chemostratigraphy of ⁸⁷Sr/⁸⁶Sr ratios of the drilled samples displays a smooth curve and a large positive anomaly just below the PC/C boundary between the upper part of Baimatuo Member of the Dengying Formation and the lower part of the Yanjiahe Formation. The combination of chemostratigraphies of δ¹³C and ⁸⁷Sr/⁸⁶Sr indicates that the ⁸⁷Sr/⁸⁶Sr excursions preceded the δ¹³C negative excursion, and suggests that global regression or formation of the Gondwana supercontinent, possibly together with a high atmospheric pCO₂, caused biological depression.     

Acanthomorph Biostratigraphic Succession of the Ediacaran Doushantuo Formation in the East Yangtze Gorges, South China

Large acanthomorphic acritarchs have been found in chert of the Ediacaran Doushantuo Formation at several localities in South China, including the East Yangtze Gorges of Hubei Province, Weng’an area of Guizhou Province, and elsewhere. However, their potential for biostratigraphic subdivision and correlation of Ediacaran successions is limited by facies control, taphonomic biases, and taxonomic problems. In the Yangtze Gorges, the Doushantuo Formation is generally subdivided into four lithologic members. However, in the Weng’an area, the Doushantuo Formation comprises just a lower and upper part separated by a mid-Doushantuo erosional surface. In the Yangtze Gorges at the Zhangcunping section, the Doushantuo succession is similar to that at Weng’an. So far, the correlation between the Yangtze Gorges and Weng’an area, and elsewhere has been an issue of debate. To resolve the debate, we selected eight sections in the Yangtze Gorges area and systemically sampled chert nodules of the Doushantuo Formation, focusing in particular on the upper Doushantuo Formation. Our data confirm two different assemblages appearing separately in the second and third members, which are separated by a negative δ13C excursion (EN2). The lower assemblage is characterized by Tianzhushania and a diverse suite of large acanthomorphic acritarchs. The upper assemblage is distinguished from the lower assemblage, by (1) absence of Tianzhushania; (2) occurrence of abundant, 100–150 μm, smooth-walled spherical microfossils; (3) occurrence of highly diverse acanthomorphic acritarchs including species extending from the second member and new forms in this member; (4) occurrence of unnamed new forms of protist; and (5) occurrence of the tubular microfossil Sinocyclocylicus guizhouensis. Since the Tianzhushania-dominated assemblage is not present in Australia, it seems that only the upper acanthomorph assemblage is present and thus the lower Doushantuo acanthomorph assemblage is missing in Australia.     

湖北峡东地区埃迪卡拉系陡山沱组微体化石的新材料

通过对牛坪剖面和新测的晓峰河剖面燧石结核和条带的切片研究,进一步证实峡东地区陡山沱组下部和上部分别含有不同的微体化石组合。下组合以大型具刺疑源类Tianzhushania为主,包括疑源类Appendisphaera magnifica、Leiosphaeridia tenuissima、Meghystrichosphaeridium chadianensis、Tanarium sp.、Tianzhushania fissura、T.spinosa、T.sp.、Unnamed A;丝状蓝藻Oscillatoriopsis longa、Polytrichoides induviatus、Salome hubeiensis、Siphonophycus kestron、S.robustum;多细胞藻类Sarcinophycus palilloformis、Unnamed multicelluar alga A。上组合以出现大量个体介于50～140μm的呈聚集保存的球状膜壳为特征,包括疑源类Leiosphaeridia tenuissima、Knollisphaeridium maximum、K.sp.、Meghystrichosphaeridium perfectum及Unnamed B、C。本文的两个微体化石组合与Yin等(2011)和Liu等(2013)的组合特征基本一致。     

湖北宜昌樟村坪埃迪卡拉系陡山沱组硅磷质结核中的微体化石

本文报道的微体化石产于湖北宜昌樟村坪万家沟剖面埃迪卡拉纪陡山沱组第10层的硅磷质结核中,化石组合包括疑源类Appendisphaera grandis、Ericiasphaera spjeldnaesii、Knollisphaeridium maxi mum、Leiosphaeridia tenuissi ma、Meghystrichosphaeridium perfectum、Tianzhushania polysiphonia、T.spinosa、T.ornata;丝状蓝藻Oscillatoriopsis obtusa、Polytrichoides induviatus、P.lineatus、Salome hubeiensis、Siphonophycustypicum;多细胞藻类Sarcinophycus palilloformis、Wengania minuta。该微体化石组合面貌与黄陵背斜东、南翼陡山沱组二段硅质结核中以大型具刺疑源类Tianzhushania为特征的组合相同,与贵州瓮安地区陡山沱组上磷块岩下部保存的微体化石组合面貌基本一致。     

Quantitative evaluation of the biostratigraphic distribution of acanthomorphic acritarchs in the Ediacaran Doushantuo Formation in the Yangtze Gorges area, South China

Doushantuo acanthomorphic acritarchs are large morphologically complex organic-walled microfossils broadly constrained between the 635 Ma Nantuo glaciation and the 551 Ma Miaohe Biota. They are potential biostratigraphic tools for subdivision and correlation of the Ediacaran System in South China. However, major variations in sedimentary facies and stratigraphic thickness present challenges in understanding the spatial and temporal distribution of these acritarchs. Further, the distribution of acritarchs in the Doushantuo Formation is associated with the presence of early diagenetic chert and phosphate, implying a certain degree of preservational bias and/or environmental control. The purpose of this paper is to document the stratigraphic distribution of Doushantuo acritarchs and to quantitatively evaluate their biostratigraphic significance and possible taphonomic–environmental biases, based on high-resolution paleontological data from six sections over 100 km in the Yangtze Gorges area, South China.A total of 1082 acritarch fossils were recorded from 84 chert horizons in the six sections of the study area. These chert horizons are not uniformly distributed throughout the Doushantuo Formation, thus the presence/absence of early diagenetic chert does play a role in controlling the distribution of acritarchs. The sampled chert horizons can be grouped into two stratigraphic intervals in the lower and upper Doushantuo Formation, respectively, based on regional stratigraphic correlation. Quantitative analysis shows that the two intervals are distinct taxonomically and largely independent of taphonomic or facies controls. Thus, the two intervals can be regarded as assemblage biozones. The lower biozone (biozone 1) is numerically dominated by Tianzhushania spinosa (n = 587; 68.3%) and Meghystrichosphaeridium magnificum (n = 74; 8.6%), whereas the upper biozone (biozone 2) is dominated by Ericiasphaera rigida (n = 104; 47.1%) and Tianzhushania spinosa (n = 34; 14.1%). The three most common genera, Meghystrichosphaeridium, Tianzhushania, and Ericiasphaera, have been identified in all sections. Correspondence analysis (CA), detrended correspondence analysis (DCA), discriminant analysis (DA), and pairwise comparisons of samples (Spearman rank coefficient and Jaccard–Chao index), all consistently support biostratigraphic zonations. Thus, the distribution of the Doushantuo acanthomorphs is primarily controlled by biostratigraphic position of samples, with facies and taphonomic differences playing a secondary role. Our case study suggests that acanthomorphic acritarchs can offer a viable tool for regional correlation of the Ediacaran System.     

峡东地区埃迪卡拉系陡山沱组碳同位素特征研究进展与国内外对比

本文重点通过研究峡东地区埃迪卡拉系陡山沱组碳同位素的变化规律,来探讨其全球地层对比意义.峡东地区埃迪卡拉系陡山沱组经历了三次比较显著的碳同位素负漂移(EN1~EN3)和两次正漂移(EP1、EP2).第一次碳同位素负漂移(ENI)发生在南沱冰期之后陡山沱组底部"盖帽碳酸盐岩"层位及其上部的白云岩,与世界其他地区Marinoan冰期之后"盖帽碳酸盐岩"的碳同位素负漂移可进行对比;第二次碳同位素负漂移(EN2)发生在陡山沱组中部;第三次负漂移(EN3)发生在陡山沱组顶部,是该地质历史时期持续时间最长的一期碳同位素负漂移,同澳大利亚、阿曼、纳米比亚、加里福尼亚、西伯利亚、苏格兰等地碳同位素异常均可进行对比.两次正漂移(EP1、EP2)分别位于第二段中部和第三段的下部,分别与陡山沱组所含的两个疑源类组合具有很好的对应关系;而EN3则与陡山沱组上部所含的宏体化石组合(庙河生物群)相对应;表明碳同位素的变化与生物演化具有一定的对应关系.研究表明,利用碳同位素变化特征进行埃迪卡拉纪地层洲际对比是行之有效的手段之一.     

Determination of Sr/Sr mass-dependent isotopic fractionation and radiogenic isotope variation of Sr/Sr in the Neoproterozoic Doushantuo Formation

We measured both mass-dependent isotope fractionation of δ⁸⁸Sr (⁸⁸Sr/⁸⁶Sr) and radiogenic isotopic variation of Sr (⁸⁷Sr/⁸⁶Sr) for the Neoproterozoic Doushantuo Formation that deposited as a cap carbonate immediately above the Marinoan-related Nantuo Tillite. The δ⁸⁸Sr and ⁸⁷Sr/⁸⁶Sr compositions showed three remarkable characteristics: (1) high radiogenic ⁸⁷Sr/⁸⁶Sr values and gradual decrease in the ⁸⁷Sr/⁸⁶Sr ratios, (2) anomalously low δ⁸⁸Sr values at the lower part cap carbonate, and (3) a clear correlation between ⁸⁷Sr/⁸⁶Sr and δ⁸⁸Sr values. These isotopic signatures can be explained by assuming an extreme greenhouse condition after the Marinoan glaciation. Surface seawater, mixed with a large amount of freshwater from continental crusts with high ⁸⁷Sr/⁸⁶Sr and lighter δ⁸⁸Sr ratios, was formed during the extreme global warming after the glacial event. High atmospheric CO₂ content caused sudden precipitation of cap carbonate from the surface seawater with high ⁸⁷Sr/⁸⁶Sr and lighter δ⁸⁸Sr ratios. Subsequently, the mixing of the underlying seawater, with unradiogenic Sr isotope compositions and normal δ⁸⁸Sr ratios, probably caused gradual decrease of the ⁸⁷Sr/⁸⁶Sr ratios of the seawater and deposition of carbonate with normal δ⁸⁸Sr ratios. The combination of ⁸⁷Sr/⁸⁶Sr and δ⁸⁸Sr isotope systematics gives us new insights on the surface evolution after the Snowball Earth.     

中国华南陡山沱组烃源岩的形成机制与分布预测

震旦系(埃迪卡拉系)陡山沱组是成冰系(南华系)极端冰期事件结束后首次沉积的泥岩和碳酸盐岩地层,大规模的海侵活动导致在全球广泛发育富有机质黑色页岩,可能是未来深层油气和页岩气勘探的重要烃源岩层.中国华南陡山沱组烃源岩分布广泛,由于受古构造格局、沉积相带等因素影响,烃源岩非均质性强、平面上分布差异大.本文在前人研究基础上,...     

Megascopic Symmetrical Metazoans from the Ediacaran Doushantuo Formation in the Northeastern Guizhou, South China

In South China, various megascopic symmetrical metazoan fossils were found in the upper Doushantuo (陡山沱)Formation of the Neoproterozoic Ediacaran.The worm-like fossil is characterized by modern taxological annelid, for many metameres, parapodia, one possible tentale, an alimentary canal, and a dorsal vessel. The triradiate discoidal fossil is belong to Trilobozoa, and the octaradiate discoidal fossil might be Ctenophora. All these fossils indicate that the megascopic metazoans have appeared in the Doushantuoian of Eidacaran and imply that the symmetrical metazoans must have originated at least 550 Ma ago.     

Nitrogen isotope chemostratigraphy of the Ediacaran and Early Cambrian platform sequence at Three Gorges,South China

The appearance of multicellular animals and subsequent radiation during the Ediacaran/Cambrian transition may have significantly changed the oceanic ecosystem. Nitrogen cycling is essential for primary productivity and thus its connection to animal evolution is important for understanding the co-evolution of the Earth's environment and life. Here, we first report on coupled organic carbon and nitrogen isotope chemostratigraphy from the entire Ediacaran to Early Cambrian period by using drill core samples from the Yangtze Platform, South China. The results show that δ¹⁵N_TN values were high (~ + 6‰) until middle Ediacaran, gradually dropping down to − 1‰ at the earliest Cambrian, then rising back to + 4‰ in the end of the Early Cambrian. Organic carbon and nitrogen contents widely varied with a relatively constant C/N ratio in each stratigraphic unit, and do not apparently control the carbon and nitrogen isotopic trends. These observations suggest that the δ¹⁵N_TN and C/N trends mainly reflect secular changes in nitrogen cycling in the Yangtze Platform. Onset of the observed negative N isotope excursion coincided with a global carbon isotope excursion event (Shuram excursion). Before the Shuram event, the high δ¹⁵N probably reflects denitrification in a nitrate-limited oceanic condition. Also, degradation of dissolved and particulate organic matter could be an additional mechanism for the ¹⁵N-enrichment, and may have been significant when the ocean was rich in organic matter. At the time of the Shuram event, both δ¹³C_carb and δ¹⁵N_TN values were dropped probably due to massive re-mineralization of organic matter. This scenario is supported by an anomalously low C/N ratio, implying that enhanced respiration resulted in selective loss of carbon as CO₂ with recycled organic nitrogen. After the Shuram event, the δ¹⁵N value continued to decrease despite that δ¹³C_carb rose back to + 4‰. The continued δ¹⁵N drop appears to have coincided with a decreasing phosphorus content in carbonate. This suggests that ocean oxygenation may have generated a more nitrate-rich condition with respect to phosphorus as a limiting nutrient. Similar to the Shuram event, another negative δ¹³C_carb event in the Canglanpuan stage of the Early Cambrian is also characterized by carbon isotopic decoupling as well as the low C/N ratio. The results strongly support that the two stages of the decoupled negative δ¹³C_carb excursions reflect a disappearance of a large organic carbon pool in the ocean. The two events appear to relate with the appearance of new metazoan taxa with novel feeding strategies, suggesting a link between ocean oxygenation, nutrient cycling and the appearance and adaptation of metazoans. The nitrogen isotope geochemistry is very useful to understand the link between the environmental, ecological and biological evolutions.     

FTIR microspectroscopy of Ediacaran phosphatized microfossils from the Doushantuo Formation,Weng'an,South China

Phosphatized microfossils from ca. 580 Ma from the Doushantuo Formation in the Weng'an region of South China were analyzed by Fourier transform infrared (FTIR) microspectroscopy for their chemical characterization. Two morpho-types of phosphatized embryo-like fossils (Megasphaera and Megaclonophycus) were analyzed, together with algal fossils. Transmission IR spectra of the microfossils have absorption bands of around 2960 cm^− 1 and 2925 cm^− 1, indicating the presence of aliphatic hydrocarbon (anti-symmetric aliphatic CH₃ and aliphatic CH₂), and have an additional band of around 1595 cm^− 1, probably derived from aromatic moieties (aromatic CC). In addition, IR microscopic mapping shows that aliphatic hydrocarbon and aromatics are distributed inside the embryo-like fossils. The embryo-like fossils appear to show three types of CH₃/CH₂ peak height ratios (R_3/2) and aromatic CC/CH₂ peak height ratios (R_CC/2 values): (1) high-R_3/2/low-R_CC/2 type (R_3/2 = ~ 0.2–1.0 and R_CC/2 ~ 0–2), (2) low-R_3/2/medium-R_CC/2 type (R_3/2 = ~ 0.2–0.6 and R_CC/2 = ~ 1–4); and (3) low-R_3/2/high-R_CC/2 type (R_3/2 = ~ 0.2–0.6 and R_CC/2 ~ 1–8). All three types are contained in both Megasphaera and Megaclonophycus. Raman spectra for the carbonaceous matter within the rock sample show a similar degree of thermal alteration, indicating that the organics were embedded in situ prior to thermal maturation. The IR spectroscopic differences might reflect differences in original organic compositions of microorganisms, and/or immediate post-mortem alteration. This suggests that the origins of phosphatized embryo-like fossils are more diverse than was previously recognized on the basis of their morphology. A comparison of R_3/2 and R_CC/2 values in the embryo-like fossils with those of the algal fossils and the extant microorganisms indicates the possible interpretation that some of the embryo-like fossils belong to animal embryo, others are algae, but none of them originate from bacteria.     

峡东埃迪卡拉系陡山沱组稳定碳同位素记录及其年代地层意义

基于峡东秭归青林口、泗溪和宜昌晓峰河埃迪卡拉系下部陡山沱组剖面岩石、层序和精细的碳同位素地层研究,在埃迪卡拉系下部陡山沱组底部、下部、中部、上部和顶部分别识别出5次以灰岩出现或黑色页岩增厚为标志的海侵事件,划分出5个可以进行区域对比的三级层序,自下而上分别命名为BDSS,LDSS,MDSS,UDSS和TDSS。以层序界面为对比标志,峡东埃迪卡拉纪不同古地理部位碳同位素组成变化的对比显示峡东埃迪卡拉系陡山沱组中内部发育5次可以进行区域乃至全球对比的碳同位素负异常,分别是分布在BDSS下部的SN1,LDSS上部的SN2,MDSS上部和顶部的SN3和SN4,以及分布在UDSS上部的SN5。其中SN1和SN4碳同位素负偏离最为明显,最小δ13C分别达到-4‰~-5‰和-9‰~-10‰,分别与Marinoan冰期上覆盖帽白云岩以及Gaskier冰碛沉积上、下地层中的碳同位素组成特点接近。SN4是埃迪卡拉系最晚冰期的产物。与SN1是埃迪卡拉系底界划分对比的标志相对比,SN4的出现应该是埃迪卡拉系内部再划分中另一个不可忽视的界线标志。     

Germanium/silica ratios in diagenetic chert nodules from the Ediacaran Doushantuo Formation,South China

Germanium/silica (Ge/Si) ratios of dolostone- and mudstone-hosted chert nodules from the Ediacaran (635–542 Ma) Doushantuo Formation in the Yangtze Gorges area, South China, are reported. These chert nodules typically have a calcite rim, a pyrite rim, and a silica core, the latter sometimes containing disseminated pyrite. The silica core was precipitated by early diagenetic replacement of carbonate and silty/muddy sediments. Two types of chert nodules are identified based on their mineralogy and geochemistry. Type-1 chert nodules are poor in disseminated pyrite in the silica core. They also have low Al, and show a strong positive correlation between Al contents and Ge/Si with a near-zero or negative intercept. In contrast, Type-2 chert nodules contain abundant disseminated pyrite in the silica core and show a weakly positive correlation between Ge/Si ratios and Al contents (with a large positive intercept on the Ge/Si axis). The Ge/Si of Doushantuo nodules are greater than those of Cretaceous deep-sea cherts, suggesting that the Ge/Si ratio of Ediacaran seawater/porewater was greater than the Cretaceous due to the more effective discrimination against Ge by inorganic opal precipitation relative to biogenic opal precipitation. The positive correlation between Ge/Si and Al can be interpreted using a mixing model with a pure chert (characterized by a low Ge/Si ratio) and an Al-rich endmember (characterized by a high Ge/Si ratio). The latter is most likely represented by a clay component, but the model-based estimate of the Ge/Si ratio inferred for the Al-rich (clay) endmember is much higher than that of Phanerozoic clay minerals. These high Ge/Si ratios for the clay endmember may be related to the generally high Ge/Si ratio of Ediacaran seawater, but could also be related to clay–organic matter interactions. Organic matter absorbed to clays could provide an additional source of Ge because certain organic molecules are known to have a high affinity for Ge due to their strong metal ion-chelating properties. The high Ge/Si ratio of the Al-rich endmember in Type-1 chert nodules suggests that Ge in porewaters from which these cherts precipitated may have been dominated by Ge–organic complexes. The low Ge/Si ratio inferred for the Al-rich endmember in Type-2 chert nodules is therefore taken to indicate that Ge was released from organic matter, perhaps due to anaerobic degradation of organic matter (accompanied by the formation of pyrite), and was redistributed between clay–organic endmembers and pure chert, resulting in a non-zero intercept in the Ge/Si vs. Al₂O₃ plots. These observations suggest that a strong terrestrial influence in a restricted sedimentary basin or a high content of dissolved organic carbon in Ediacaran seawater and porewater may have contributed to the dominance of Ge–organic complexes in the Doushantuo basin in the Yangtze Gorges area.     

宜昌地区埃迪卡拉纪陡山沱组硒的富集与地质意义

新元古代一早寒武世过渡时期发生了地质历史上第2次大氧化事件,沉积岩中的氧化还原敏感元素在这一时期出现不同程度的富集。分散元素硒是典型的氧化还原敏感元素,沉积岩中硒的富集可以指示海洋氧化环境的变化。为了讨论这一时期硒的富集与海洋氧化还原状态的关系,本文对宜昌地区陡山沱组第4段黑色页岩中的Se及其他氧化还原敏感元素进行了系统地分析。结果发现,Se含量范围1.0×10~(-6)~58.0×10~(-6),平均含量14.49×10~(-6),远高于Se的地壳丰度(0.05×10~(-6))。Se元素的富集同Co、Ag和V等都有不同程度的相关性。利用Mo-U关系指示了这一时期为缺氧环境。宜昌地区埃迪卡拉纪陡山沱组硒的富集作用与硫化水体波动、生物作用和不稳定的缺氧海洋环境有关,不同剖面的区域因素也造成了富集程度的不同。与前寒武纪的黑色页岩相比,Se含量发生了明显富集,暗示这一时期强烈的大陆风化输入,进一步指示大气的氧化。     

华南黄陵背斜西翼兴山地区乡儿湾剖面埃迪卡拉系陡山沱组微体化石

本文首次报道并描述了华南黄陵背斜西翼湖北宜昌兴山县乡儿湾剖面埃迪卡拉系陡山沱组二段中上部的燧石结核和条带中的疑源类微体化石,经系统鉴定共识别出8属8种,包括Appendisphaera grandis,Crassimembrana multitunica,Leiosphaeridia minutissima,Megasphaera inornata,Schizofusa zangwenlongii,Symphysosphaera basimembrana,Urasphaera nupta,Variomargosphaeridium gracile。该化石组合包含Liu Pengju and Moczyd?owska (2019)所建立的第二生物组合带(Tanarium tuberosum-Schizofusa zangwenlongii组合带)的特征分子Schizofusa zangwenlongii,因此,可尝试性地与第二生物组合带相对比,但未见黄陵背斜东南翼陡山沱组二段的常见属Tianzhushania,据此推测黄陵背斜两翼在古环境上可能有所差异。     

黔东北伊迪卡拉纪陡山沱组的宏体藻类

宏体生物广泛地生活于扬子地区伊迪卡拉纪陡山沱期的海洋中,它们为后生动物的快速发展和演化奠定了一个崭新的环境和一个新生态链的基础.产自于黔东北伊迪卡拉系陡山沱组的瓮会生物群生活环境应在贫氧的、透光性良好的浅海低能环境,以宏体藻类为主,共描述15属18种(其中包括3个新属、5个新种、2个修定属和4个修定种)和1个未命名藻以及不明藻类固着器,另包含有宏体后生动物,可能的后生动物以及遗迹化石.瓮会生物群不但含有鄂西庙河生物群和皖南蓝田植物群的分子,也有南澳大利亚Ediacara生物群和俄罗斯WhiteSea生物群的分子,而且具有自已的特征而有别于其他伊迪卡拉期的生物群.其宏体藻类形态变化较大,组织器官和功能已出现明显的分化,并与高等植物之间存在一定的演化关系.宏体藻类多为直立固着于海底,提高了光合作用的效率,增加了水体中的含氧量.同时,宏体藻类作为生态系统中的初级生产者成为一种新的食物,改变了后生动物的食谱.因此,陡山沱期的宏体植物不仅改善了环境,为依赖一定氧而生活、生长和繁殖的后生动物给予了必要的支持,而且作为生态系统中的必要基础,为宏体后生动物的快速发展和演化提供了一个新的食物源.另外,宏体生物群的繁盛不仅提高了有机质的生产率,而且促进了富有机碳的沉积和保存.     

The anomalous Ca cycle in the Ediacaran ocean: Evidence from Ca isotopes preserved in carbonates in the Three Gorges area,South China

The Ediacaran–Cambrian was one of the most important periods for the evolution of life. Recent studies have provided detailed chemostratigraphies that decipher the linkages between ambient surface environmental changes and biological evolution. The occurrence of skeletal fossils in Ediacaran rocks suggests that the possible onset of Ca-biomineralization started in the latest Neoproterozoic. Molecular clocks also predict the emergence of animals containing Ca-carbonate spicules in the Neoproterozoic. Therefore, it is important to estimate the transition of the Ca cycle in seawater. Ancient Ca cycles in the oceans are estimated from the calcium isotopic compositions of carbonate rocks. However, the lack of continuous Ca isotopic data in the Ediacaran leaves the Ca cycle unresolved.The almost continuously exposed, Ediacaran and Cambrian strata in South China mainly comprise carbonate rocks with subordinate black shales and sandstones, which contain many fossils, suitable for the study of environmental and biological changes. We conducted drilling from the Liantuo, via the Nantuo, Doushantuo, Dengying and Yanjiahe to the Shuijingtuo formation at four sites in the Three Gorges area in order to obtain continuous, fresh samples without surface alteration and oxidation. We analyzed ⁴⁴Ca/⁴²Ca ratios in carbonate rocks with a multiple collector-inductively coupled plasma-mass spectrometer (MC-ICP-MS) screened for diagenetic alteration.The resultant δ^44/42Ca ratios exhibit a smooth curve. The values of δ^44/42Ca are anomalously high relative to those in the Phanerozoic, and they gradually decrease towards the end of the Ediacaran. Our new Ca isotope chemostratigraphy suggests that a different Ca cycle had existed during the Ediacaran. The long-term, high δ^44/42Ca is not simply explicable by changes in temperature, differences in carbonate mineralogy, or changes in the input/output fluxes of Ca to the ocean. We propose three possible explanations for the observed high δ^44/42Ca ratios in the Ediacaran: high δ^44/42Ca ratios of Ca inputs, potential undiscovered sinks of Ca with low δ^44/42Ca values, and negligible isotope fractionation between carbonate and seawater; we consider that one or all of these contributed to the high δ^44/42Ca ratios. Of these, we prefer the small isotope fractionation, which implies that the Ca concentration in seawater was initially low in the early Ediacaran and increased throughout that period.     

华南埃迪卡拉(震旦)系陡山沱组生物地层学研究的新进展

本文重点介绍华南地区埃迪卡拉(震旦)系陡山沱组生物地层学研究的新进展:(1)贵州江口陡山沱组上部黑色页岩中发现保存完美的八辐射螺旋结构的埃迪卡拉型动物化石--八臂仙母虫,首次确立陡山沱组上部庙河生物群层位有埃迪卡拉型动物化石的存在,为该层位与澳大利亚典型埃迪卡拉动物群层位对比提供了重要证据.(2)陡山沱组第二段首次发现有饰天柱山藻,确立了陡山沱组中下部出现的以Tianzhushania属为特征的具刺疑源类下组合带,为同期地层的对比提供了新依据.(3)在峡东地区陡山沱组第三段发现以Tanarium属为特征的具刺疑源类上组合带,它们与断面呈圆形和方形的管状化石共生,层位稳定,分布广泛.这些新古生物的发现,不仅进一步完善了我国埃迪卡拉(震旦)系陡山沱组的生物地层序列,而且为解决全球埃迪卡拉系划分和对比提供了可靠的古生物依据.     

扬子板块埃迪卡拉系(震旦系)陡山沱组层序地层划分与对比

在对扬子板块埃迪卡拉系陡山沱组不同相区的25条代表性剖面野外研究基础上,通过沉积岩石学和岩相序列特征的系统分析,认为陡山沱组沉积时期曾发生3次二级海平面升降事件。依据3个海平面升降转换面,可识别出3个层序底界面:(1)陡山沱组底部与下伏南华系南沱组及其同期层位的冰碛杂砾岩之间的界面;(2)在浅水沉积区陡山沱组中部和上部分别出现喀斯特侵蚀面;(3)在深水沉积区相应层序界面为岩相结构转换面。依据火山灰锆石U-Pb同位素年龄,可将陡山沱组层序地层划分为2个半二级层序或超层序(SS1,SS2和SS3-TST),其中SS1时限为35Ma(635~600Ma),SS2时限为35Ma(600~565Ma),SS3-TST时限为14Ma(565~551Ma)。陡山沱组底部广泛发育的盖帽白云岩底和3个层序内的最大海泛面可以作为4个相对等时面,结合事件沉积标志层,可建立扬子板块陡山沱组从浅水沉积区至深水沉积区等时性二级层序地层划分对比格架。研究结果表明,三峡地区陡山沱组四段式划分方案不适用于整个扬子板块内陡山沱组的区域地层划分和对比。因而,建议扬子板块陡山沱组应该以二级层序地层为基础,结合化学地层和生物地层进行综合划分和对比。陡山沱组新的地层划分对比格架为研究陡山沱组古地理演变和编制该时期高精度的岩相古地理图奠定了基础。     

华南埃迪卡拉纪陡山沱组物源风化对全球性成磷事件的启示

华南埃迪卡拉纪陡山沱组沉积于原始地球-生命系统向现代地球-生命系统过渡的关键时期,该套地层记录了地质历史时期一次大规模成磷事件,但关于该次成磷事件中磷元素的来源仍存在诸多争议。浅海区磷元素主要来源于由河流搬运的陆表含磷矿物风化产物及上升流作用所带来的其他海域含磷水体,这两种来源可以通过全岩碎屑元素(Al、Fe、Th等)和稀土元素的含量及其分布特征进行区分。本文通过对上扬子地区乡党坪剖面和四斗坪剖面的陡山沱组主、微量元素特征的综合分析,恢复了该地区的物源体系及其风化作用过程。较高的P₂O₅-Al₂O₃、P₂O₅-Fe₂O₃相关性,表明该时期磷来源于陆地风化而非上升流;较低的Th/Co、La/Sc、Th/Cr等比值,TiO₂-Zr相关性以及Th-Hf-Co、La-Th-Sc三角图分布关系,表明研究区内陡山沱组的物源体系主要为玄武岩;较高的化学蚀变指数(CIA)和较低的Sr/Cu值,表明物源区当时...