A high-resolution chemostratigraphy of post-Marinoan Cap Carbonate using drill core samples in the Three Gorges area, South China

Cap Carbonates overlie the Marinoan Snowball Earth-related glacial diamictite, and possibly record the drastic surface environmental change and biological evolution after the Snowball Earth. We conducted on-land drilling from the Liantuo Formation, through the Nantuo, to the lower Doushantuo Formation in the Three Gorges area of South China to collect fresh, continuous samples in the Three Gorges area. We obtained high-resolution chemostratigraphies of ~13C and 6180 values of carbonates from the topmost part of the Nantuo Formation to the Cap Carbonate, in order to decode the detailed surface environmental change in the shallow marine setting. The δ3C chemostratigraphy possesses some unique characteristics： （1） stable δ13C values as a whole, but ubiquitous low δ13C anomalies through the Cap Carbonate, （2） increase of the δ13C values from -3 to ＋5‰ across the C2/C3 boundary, （3） no δ13C anomaly between the CI and C2 boundary, and （4） presence of an anomalous high δ13C value （＋2.3%0） and a faint positive correlation between δ13C and δ18O values in the C1 unit. Evidence of quite low δ13C anomalies （with a nadir of -41‰）, ubiquitous negative δ13C anomalies through the Cap Carbonate, and a high 613C anomaly accompanied with a faint positive correlation between δ13C and δ18O values in the C1 unit supports decomposition and formation of methane hydrate during Cap Carbonate formation. The drastic increase of δ13C values from the upper C2 to C3 units in- dicates enhancement of primary productivity and organic carbon burial, possibly due to high continental fluxes after the Snowball Earth event, evidenced by high Sr isotope values. The increase is restricted to the proximal side of the inner shelf in South China, and the timing of the increase of δ13C values of carbonates is earlier at Three Gorges area than any other area, suggesting that the enhancement of primary productivity started in the proximal environment because of higher continental influxes. The increase in oxygen contents of seawater due to the enhanced primary productivity possibly resulted in the emergence of multicellular animals soon after Cap Carbonate deposition.

Marinoan glaciation in the Indian subcontinent——Anatomy and global implications

Detailed sedimentological analysis of the Pokaran Boulder Bed,representing the most basal unit of the Neoproterozoic-Lower Cambrian Marwar Supergroup,clearly indicates its glacial origin.The glacial sediments are interpreted as an ice-contact submarine fan deposit.Based on the detrital and inherited zircon population of the Marwar Supergroup sediments and interlayered pyroclastic deposits,a Marinoan cryochron of the Cryogenian Period is envisaged for the Pokaran Boulder Bed.The well-preserved Ediacaran elements in the post-glacial sedimentary succession also support a Cryogenian to Early Cambrian age of the Marwar Supergroup.The glacial deposition at the base of the Marwar Supergroup strengthens the regional correlation between the studied sediments with the Lesser Himalayan Blaini-Krol-Tal sediments and the Haqf Supergroup of Oman.The available zircon ages and paleomagnetic data of the Malani Igneous suite,along with the Marinoan glacial deposits,detrital zircon ages,and Ediacaran fossil elements of the Marwar Supergroup add significant information to the Neoproterozoic Earth history.

Stratigraphic framework and sedimentary evolution during the Cryogenian-Ediacaran transition in northeastern Sichuan Basin,South China

The northeastern Sichuan area in the northern Yangtze margin has unique Ediacaran geological records,especially the Doushantuo Formation(DST),and become a hot research area in recent years.However,the Cryogenian-Ediacaran(C-E)boundary has not been precisely identified,which restricts the in-depth study of geological information during this crucial transitional period and is unfavorable for a systematic and complete understanding of the Yangtze Block and even the global paleogeographic pattern.This study conducted stratigraphy,sedimentology,and chronostratigraphy to establish the stratigraphic framework and sedimentary evolution of the C-E transition strata in northeastern Sichuan.The results showed that the Ediacaran sediments,without the cap dolomite,unconformably overlaid the Cryogenian sediments in the studied area.The Member II of the DST,characterized by 50-160 m of red-green sandstone(approximately equivalent to the original Chengkou“Guanyinya Formation”),directly overlaid the Cryogenian sediments and displayed a 623±2.3 Ma maximum depositional age from the detrital zircon U-Pb dating.Regional stratigraphic correlations indicate that the C-E transition strata in northeastern Sichuan had a consistent lithological association and sedimentary sequence characteristics but differed from the Three Gorges.Typically,the upper Nantuo massive glacial diamictites transition to the icebergs rafted lonestone-bearing mudstones at the top,then change upward to DST barrier coast sandstones.The proposed DST of the northeastern Sichuan Basin was divided into three lithostratigraphic members without the regional Member I cap dolomite:(i)Member II purple-red,gray-green sandstone strata,(ii)MemberⅢblack mudstone strata,and(iii)Member IV P-Mn bearing strata.During the C-E transition,the study area experienced(i)the global deglaciation stage in the terminal Marinoan glaciation and(ii)the filling-leveling up stage with clastic rocks in the early Ediacaran.Overall,the early Ediacaran of northeastern Sichuan succeeded the paleogeographic features of the late Cryogenian.

Episode of intense chemical weathering during the termination of the 635 Ma Marinoan glaciation

Subject Code:D02With the support by the National Natural Science Foundation of China and the Thousand Talent Program for Young Outstanding Scientists,a research team led by Prof.Shen Bing(沈冰)at the School of Earth and Space Sciences of Peking University,uncovered chemical weathering during the termination of Marinoan Snowball Earth.This study was published in PNAS(2016,113:14904—14909).

新元古代冰期及其年代

新元古代在全球范围内出现了几期冰期事件,称之为“雪球地球”事件。这种剧烈的环境变化带来此后地球上生命演化的一次飞跃。“雪球地球”事件的核心是全球冰期的同时性,需要同位素地质年代学的证据。新元古代末期两次主要的冰期事件是Marinoan冰期和Sturtian冰期,其中Marinoan冰期结束于635Ma;Sturtian冰期可能发生在710～720Ma,已发表的年龄数据限定它在670Ma之前结束。Marinoan冰期后的Gaskiers冰期发生在580～590Ma。对华南的古城、铁丝坳、长安组、江口组等进行进一步精确定年,将对限定Sturtian冰期持续时间和Cryogenian、南华系的下限年龄具有重要意义。

三峡地区震旦系碳酸盐岩碳氧同位素特征

本文对三峡地区秭归庙河陡山沱组第一段至第三段、宜昌棺材崖陡山沱组第三段上部至灯影组底部以及四溪灯影组石板滩段和白马沱段碳酸盐岩碳氧同位素组成特点的研究结果表明,三峡地区震旦系碳同位素组成与全球新元古代广布的后Marinoan冰期地层δ^(13)C的分布模式基本一致。本文还讨论了地层中碳同位素组成与生物演化和地层层序发展的关系,指出陡山沱组一段白云岩中δ^(13)C的负异常可以作为震旦系底界划分的标志,最晚Varanger冰期在三峡地区由陡山沱组三段上部出现的三级层序界面所替代,震旦系内部陡山沱组一段和三段可能形成于Marinoan冰期之后,伊迪卡拉生物分异发展之前,而陡山沱组四段及上覆灯影峡组则大致与国外伊迪卡拉期地层对比。

长江三峡地区上震旦统稳定同位素异常及地层意义

三峡地区广泛发育的上震旦统 ,及下伏的南沱组和古城组冰碛层 ,有良好的生物地层控制及全球对比意义。新秭归城附近的雾河剖面 ,自南沱组顶部到水井沱组下部出露有利于化学地层学研究的一系列碳酸盐岩沉积。对这一剖面的碳、氧、锶同位素研究发现 :1)碳、锶和氧同位素在陡山沱组顶部到灯影组底部存在明显异常 ;2 )碳同位素在寒武系 -前寒武系界线附近有负异常 ;3 )碳同位素和锶同位素在剖面上的演化具有全球对比意义 ;4)陡山沱组顶部到灯影组底部的碳、锶同位素异常 ,与可能存在末远古纪的冰期 ,即所谓的“后 Marinoan冰期”有关。

西昆仑塔什库尔干马尔洋岩组的厘定及控矿意义

塔什库尔干大型铁矿成矿带位于昆仑造山带西段,原岩恢复表明该区含铁岩系主要为一套双峰式火山岩碎屑岩碳酸盐岩建造组合,岩石普遍遭受绿片岩相变质和多期构造变形。本文报道了在老并一带变安山岩中获得的LA ICP MS锆石U Pb同位素年龄值(603±10)Ma,并结合前人年龄数据,将原划古元古界布伦阔勒岩群的含铁岩系厘定为震旦系寒武系纽芬兰统马尔洋岩组(Z 1m),其内发育典型的双峰式火山岩组合,证实了区内震旦纪早寒武世仍处于拉张伸展背景,变质火山沉积组合特征表明其形成于大陆边缘裂谷构造环境。含铁岩系典型的膏(钡)铁建造很可能为新元古代晚期“Marinoan雪球”冰期后地球系统变化的火山沉积响应。

贵州清镇陡山沱组帽白云岩地球化学特征以及其中藻类化石Beltanelliodes podolicus发现

贵州清镇陡山沱组为浅海白云岩、泥质白云岩,在陡山沱组底部泥质白云岩中产大量的圆盘状炭质化石,个体直径为2～15 mm。由于以前发现类似的化石主要保存在泥岩中,而碳酸盐岩中这类化石只有俄罗斯地台上发现过,从化石保存的结构分析,其应该归属藻类中的Chuarid。通过对陡山沱组帽白云岩的碳同位素分析,其具有典型的帽碳酸盐岩碳同位素负异常的特征,这表明在冰后浅海滨岸泻湖环境,球形藻类已经繁盛。同时,研究发现浅海滨岸泻湖环境冰后碳酸盐沉积是一个钙质(白云质)逐渐增加的渐变过程,而非Hoffman等认为的非常快速沉积。

鄂西走马地区大塘坡组顶部泥岩碎屑锆石LA-ICP-MS U-Pb年龄及其地质意义

地球在新元古代经历了Rodinia超大陆的聚合和解体,并发生了Sturtian和Marinoan两期全球规模的冰川事件。华南地区南华系古城组和南沱组冰碛岩分别对应Sturtian冰期和Marinoan冰期沉积记录,大塘坡组属于其间冰期沉积。对鄂西走马地区大塘坡组顶部泥岩中的碎屑锆石进行了LA-ICP-MS U-Pb定年,锆石具有高Th/U比值(0.47～1.69),并显示明显的振荡环带,为岩浆成因;62颗锆石的62个数据点分析,获得的60个谐和年龄分布在651～2435Ma,主要峰值为～794Ma。最年轻单颗锆石U-Pb年龄651±7.7Ma,结合前人研究认为华南地区Marinoan冰期启动时间应晚于651Ma;碎屑锆石为亲扬子型,可能来自鄂中古陆和扬子北缘;分布最集中的751～851Ma年龄段内～850Ma峰值记录了扬子地台与华夏褶皱带碰撞拼合事件,而～820Ma、～800Ma、～760Ma等峰值则揭示了与Rodinia超大陆解体有关的幕式岩浆活动。

长江三峡地区埃迪卡拉(震旦)系锆石U-Pb新年龄对庙河生物群和马雷诺冰期时限的限定(英文)

首次报道应用高分辨率、高精度离子微探针(SH R IM PⅡ)定年方法,对采自三峡地区黄陵背斜西翼湖北省秭归县九曲脑剖面震旦系灯影组与陡山沱组界线和陡山沱组底部的凝灰岩锆石进行的U -Pb定年研究。分析结果,灯影组与陡山沱组界线的凝灰岩锆石测点形成2组,较年轻的一组由16个测点组成,舍去偏离较大的1个点,给出的206Pb /238U 比值年龄加权平均值为549.9M a±6.1M a(M SW D =1.48),代表该层位的实际年龄并限定了庙河生物群的最小年龄。采自陡山沱组底部的另一样品测点同样分为2组:一组由3个点组成,给出784M a±15M a(M SW D =0.05)的继承年龄值;另一组由15个测点组成,给出206Pb/238U比值年龄加权平均值为628.3M a±5.8M a(M SW D =0.86)。这一定年结果是目前中国震旦(埃迪卡拉)系陡山沱组底部首次获得的SH R IM P锆石U -Pb年龄。采样点紧邻作为震旦系底界的“盖帽碳酸盐岩”之上,基本可以限定震旦系的底界年龄。该结果与国际地层委员会2004年发布的《国际地层表》中埃迪卡拉系(Ediacaran)的底界年龄630M a非常接近,表明中国修订后的震旦系/南华系界线年龄应为630M a左右。

华南南华纪南沱冰期海洋环境的沉积地球化学记录——来自黔东部南华系南沱组白云岩碳氧同位素和微量元素的证据

新元古代"雪球地球"事件代表地球经历的极端气候条件,对其后的大气和海洋氧化、生物地球化学循环和真核生物的演化都产生了深远的影响。然而同冰期化学沉积岩的缺乏,严重制约了对冰期海洋环境的了解。笔者等在贵州松桃地区一钻孔岩芯南华系南沱组中采集到一套同冰期白云岩,为南沱冰期海洋环境研究提供了理想材料。岩芯中白云岩层位于南沱组下部,夹持于两套杂砾岩、粉砂质泥岩组合中间,厚度1.61 m。偏光显微镜和扫描电镜观察表明岩性为微晶白云岩。采用Delta V Advantage稳定同位素质谱仪分析白云岩的碳、氧同位素,测定结果为:δ^18OV-PDB在-16.97‰^-8.37‰之间,δ^13CV-PDB在-9.68‰^-8.42‰之间,与冰期前后碳同位素组成对比,δ^13CV-PDB具显著低负值特征。电感耦合等离子体质谱仪(ICP-MS)微量元素分析结果表明:白云岩样品中铁、锰含量很高,铁含量的平均值约为92867×10^-6,锰含量的平均值约为10644×10^-6;Mn/Sr值较大,平均值约为26.89。综合碳、氧同位素的相关性及微量元素特征分析表明,南沱组同冰期白云岩碳同位素组成代表了原始的沉积记录。研究结果表明:①南沱组白云岩的出现反映了南沱冰期海洋中存在着开放的水体,这些开放水体为此时真核生物提供了重要的栖息场所。②南沱冰期海洋处于富铁缺氧的环境,白云岩中碳同位素低负值主要是甲烷厌氧氧化的结果。③同位素质量平衡计算表明,冰期海洋中较高比例的有机质埋藏和极其有限的大气-海洋气体交换导致了大气中氧浓度的升高。

Linking sedimentary provenance,climate and tectonics in the Neoproterozoic SeridóBelt,Borborema Province(NE Brazil)

The SeridóBelt,at the extreme northeast of Brazil,is one of the several Neoproterozoic mobile belts related to the Brasiliano/Pan-African Orogeny in Western Gondwana.The lithostratigraphy of the SeridóBelt comprises basal quartzites and metaconglomerates(Equador Formation)overlain by hornblende-bearing paragneisses,calc-silicate rocks and marbles(Jucurutu Formation)and biotiteschists(SeridóFormation).We investigated the sedimentary provenance of these rocks based on an integration of compiled whole-rock geochemistry,Sm-Nd isotopes,U-Pb detrital zircon ages and a new set of whole-rock trace elements analyses.Equador Formation quartzites have major and trace elements patterns that resemble cratonic sandstones,with a typical felsic provenance consistent with the isotopic composition of plutonic Archean to Paleoproterozoic basement rocks.Geochemical proxies suggest that these rocks were deposited in fresh waters under anoxic conditions and moderate to intense weathering conditions(CIA=58–79).A shift in provenance led to an increase in intermediate-mafic sources during the deposition of the Jucurutu and Seridóformations.Trace elements from rock types in these units are nearly identical,yet they differ on the contents of some high field-strength elements(e.g.,Sc,V and Ni),which are higher in the SeridóFormation than average pelites.Geochemical proxies show similar depositional environments,with brackish to marine conditions for the Jucurutu and Seridóformations under anoxic to euxinic conditions.Weathering conditions are consistently lower in the Jucurutu Formation(CIA=58–66)in comparison with the SeridóFormation(CIA=60–88).Compiled U-Pb data suggest maximum depositional ages of 633±11 Ma and 633±6 Ma for Jucurutu and Seridóformations,respectively,suggesting deposition following the ca.645–635 Ma Marinoan glaciation.We compared the geochemistry of the SeridóFormation schists with currently exposed basement rocks and showed a strong affinity with Neoproterozoic arc-related intermediate rocks,which is in agreement with their slightly radiogenic signature(averageε_(Nd)(630 Ma)=–1.68).When comparing with other pelitic sequences on the western Borborema Province,most of them fit the same sources and weathering conditions of the SeridóFormation schists and Jucurutu Formation paragneisses.We propose that the SeridóBelt represents an intracontinental rift basin(Equador Formation)that was overlain by fine-grained magmatic arc-related sediments(Seridóand Jucurutu formations)during the post-Marinoan deglaciation and sea level rise.Similar pelitic deposits in western Borborema Province suggest a large marine-influenced continental depositional system that was mostly sourced from magmatic arcs of the West Gondwana Orogen.

Discussion on the Neoproterozoic glaciations in the South China Block and their related paleolatitudes

The Kaigas, Sturtian, Marinoan, and Gaskiers glaciations are widely recognized in Neoproterozoic. However, in the South China Block only the Jiangkou (Sturtian) and Nantuo (Marinoan) are symbolized by sedimentary records. The Kaigas, recorded by isotopic and chemical proxies, exhibited likely the nature of cold paleoclimate with local mountain glaciation. The correlation of the Doushantuo Formation with the Gaskiers is indicated by the carbon isotope excursion and the dated age from the interval, however the South China Block was then under non-glacial weather. With no paleomagnetic data, the position of the South China Block during the Sturtian glaciation cannot be determined. The paleolatitudes of the South China Block during the Kaigas and Nantuo glaciations are intermediate, even though the Nantuo was once rendered erratically deduced equatorial. In fact, the paleolatitudes of the South China Block during the Neoproterozoic glaciations are all likely at about 30°―40°.