Shuram-Wonoka carbon isotope excursion:Ediacaran revolution in the world ocean's meridional overturning circulation

The late Ediacaran Shuram-Wonoka excursion, with δ^(13)C_(carb) values as low as-12‰(PDB) in marineshelf deposits and spanning up to 10 Myr, is the deepest and most protracted δ^(13)C_(carb) negative anomaly recognised in Earth history. The excursion formed on at least four continents in low(≤32°) palaeolatitudes, and in China is associated with a major phosphogenic event. Global and intrabasinal correlation, magnetostratigraphy, isotope conglomerate tests and further geochemical data are consistent with a primary or syn-depositional origin for the excursion. Continental-margin phosphorites are generated by oceanic upwelling driven by surface winds, and δ^(13)C_(carb) negative anomalies are explicable by oceanic upwelling of 13 C-depleted deep oceanic waters, arguing that a feature common to these exceptional Ediacaran events was unprecedented perturbation of the world ocean. These events occurred during the transition from an alien Proterozoic world marked by low-palaeolatitude glaciation near sea level and strong seasonality to the familiar Phanerozoic Earth with circum-polar glaciation and temperate climate, suggesting that the Shuram-Wonoka excursion is related to this profound change in Earth's climate system. Of various hypotheses for Proterozoic low-palaeolatitude glaciation, only the high obliquity(>54°) hypothesis, which posits secular decrease in obliquity to near the present-day value(23.5°) during the Ediacaran, predicts an unparalleled revolution in the Ediacaran world ocean. The obliquity controls the sense of the world ocean's meridional overturning circulation, which today is driven by the sinking of cold, dense water at the poles and upwelling driven by zonal surface winds.When the decreasing obliquity passed the critical value of 54° during the Ediacaran the meridional temperature gradient reversed, with the equator becoming warmer than the poles and Hadley lowlatitude(<30°-35°) atmospheric zonal circulation reversing. This reversal of the temperature gradient is unique to the Ediacaran Period and caused reversal of the oceanic meridional overturning circulation,with upwelling of anoxic, 13 C-depleted deep oceanic waters producing a deeply negative and protractedδ^(13)C_(carb) signature on late Ediacaran marine-shelf deposits.

Carbon Isotope Excursions and Paleo-Oceanography of the Ordovician–Silurian Boundary Carbonate Rocks from the Xainza Area, Tibet

Objective The Ordovician–Silurian transitional period is a special time when the global paleo-environment changed greatly.It witnessed the first mass extinction as of Phanerozoic period and glaciations that occurred frequently at a large scale in a very short time,which has thus attracted much attention among geoscientists at home and abroad.

Paleoenvironment reconstruction of the Middle Ordovician thick carbonate from western Ordos Basin, China

Reconstructing paleoenvironments has long been considered a vital component for understanding the development and evolution of carbonate reservoirs.The Middle Ordovician Period is considered the archetypical greenhouse interval,and also a critical period in biological evolution.The Middle Darriwilian isotope carbon excursion has been observed in many areas of the world and may be related to the biological explosions caused by decreases in the temperature.The thick carbonate rocks in the fifth member of the Middle Ordovician Majiagou Formation in the Dingbei area of the Ordos Basin were chosen as an example,based on the concentration of major,trace and rare earth elements as well as C,O and Sr isotopic analyses,the paleoenvironment was reconstructed.And its impact on natural gas exploration was analyzed.The results show that the seawater paleotemperature was 29℃,suboxicanoxic paleoredox conditions were observed,and the seawater paleosalinity was high.A large number of plankton in the biological explosion caused a rapid increase in the total organic carbon in carbonate rocks,which provided natural gas as supplemental source rocks.Affected by early meteoric water,the dissolution of gypsum laid the foundation for high-quality reservoirs,and the residual gypsum also further preserved natural gas.This study provides new data for the paleoenvironment and a theoretical basis for further natural gas exploration.

Timing the termination of the Doushantuo negative carbon isotope excursion: evidence from U-Pb ages from the Dengying and Liuchapo formations,South China

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.

Marine Carbon-Sulfur Biogeochemical Cycles during the Steptoean Positive Carbon Isotope Excursion(SPICE) in the Jiangnan Basin, South China

Global occurrences of Steptoean Positive Carbon Isotope Excursion（SPICE） during Late Cambrian recorded a significant perturbation in marine carbon cycle, and might have had profound impacts on the biological evolution. In previous studies, SPICE has been reported from the Jiangnan slope belt in South China. To evaluate the bathymetric extent of SPICE, we investigate the limestone samples from the upper Qingxi Formation in the Shaijiang Section in the Jiangnan Basin. Our results show the positive excursions for both carbonate carbon（δ^（13）C） and organic carbon（δ^（13）C_（org）） isotopes, as well as the concurrent positive shifts in sulfur isotopes of carbonate associated sulfate（CAS, δ^（34）S_（CAS）） and pyrite（δ^（34）S_（pyrite））, unequivocally indicating the presence of SPICE in the Jiangnan Basin. A 4‰ increase in δ^（13）C_（carb） of the Qingxi limestone implies the increase of the relative flux of organic carbon burial by a factor of two. Concurrent positive excursions in δ^（34）S_（CAS） and δ^（34）S_（pyrite） have been attributed to the enhanced pyrite burial in oceans with extremely low concentration and spatially heterogeneous isotopic composition of seawater sulfate. Here, we propose that the seawater sulfur isotopic heterogeneity can be generated by volatile organic sulfur compound（VOSC, such as methanethiol and dimethyl sulfide） formation in sulfidic continental margins that were widespread during SPICE. Emission of 32S-enriched VOSC in atmosphere, followed by lateral transportation and aerobic oxidation in atmosphere, and precipitation in open oceans result in a net flux of ^（32）S from continental margins to open oceans, elevating δ^（34）S of seawater sulfate in continental margins. A simple box model indicates that about 35% to 75% of seawater sulfate in continental margins needs to be transported to open oceans via VOSC formation.

Significant carbon isotope excursions in the Cambrian and their implications for global correlations

The existing 13C data in the Cambrian from different regions of the world are analyzed here.There are four well-documented carbon isotope excursions with global significance.In ascending order,they are:(1) a large negative excursion,comparable to "BACE" (BAsal Cambrian Carbon isotope Excursion) event,which occurs near the Precambrian-Cambrian boundary with a magnitude of 4‰-10‰ (PDB);(2) the "ZHUCE" (ZHUjiaqing Carbon isotope Excursion) event,a distinct positive excursion (over 5‰) that can be recognized at the Fortunian Stage to Stage 2 transition;(3) another strong negative one,so-called "ROECE" (Redlichiid-Olenellid Extinction Carbon isotope Excursion) event,shifting at the interval between Series 2 and Series 3,peaking at 3‰-5‰ (PDB);(4) the famous Steptoean positive carbon isotope excursion (SPICE),which has been widely identified at the base of Furongian Series,Paibian Stage,with an amplitude about 4‰ (PDB).The four sharp δ 13 C shifts correlate well with coeval paleoceanographic changes and bioevents.Besides,there are some 13 C excursions from a few sections in previous studies,and more data are required to identify whether they are global or regional ones.

Carbon-Isotope Excursions Recorded in the Cambrian System,South China:Implications for Mass Extinctions and Sea-Level Fluctuations

Cambrian carbonates with abundant fossils of agnostoid trilobites deposited on the southern slope （Jiangnan slope belt） of the Yangtze Platform and in the Jiangnan deepwater basin are well exposed in the Wangeun Section of western Hunan, South China, and in the Duibian A Section of western Zhejiang, southeastern China, respectively. To better understand the response of carbonisotope excursions to depositional environment changes, mass extinctions and eustatic events, we collected 530 carbonate samples in fresh roadcut exposures of the two measured sections for analysis of carbon and oxygen isotopic compositions. Data of δ^13C from the Wangcun Section, western Hunan, South China, demonstrate that the Cambrian carbon-isotope profile includes three remarkable positive excursions CPEwc-1, 2, 3 in the Upper Series 2, in the Lower and in the Middle Furongian Series. Three distinctive negative excursions CNEwc,-1, 2, 3 were separately tested in the Lower Terreneuvian Series, Lower Series 3 and in the Upper Furongian Series. Similarly, in the corresponding horizons in the Duibian A Section, Zhejiang Province, southeastern China, three positive excursions CPEdb-1, 2, 3 and three negative excursions CNEdb-1, 2, 3 also have been discovered. We interpret these significant carbon-isotope excursions as being associated with enhanced biogenic prodnctivity, mass extinctions and eustatic events.

Global and regional controls on carbon-sulfur isotope cycling during SPICE event in south China

The positive S-isotopic excursion of carbonate-associated sulfate(δ34S_(CAS))is generally in phase with the Steptoean positive carbon isotope excursion(SPICE),which may reflect widespread,global,transient increases in the burial of organic carbon and pyrite sulfate in sediments deposited under large-scale anoxic and sulphidic conditions.However,carbon-sulfur isotope cycling of the global SPICE event,which may be controlled by global and regional events,is still poorly understood,especially in south China.Therefore,theδ13CPDB,δ18OPDBδ34S_(CAS),total carbon(TC),total organic carbon(TOC)and total sulfate(TS)of Cambrian carbonate of Waergang section of Hunan Province were analyzed to unravel global and regional controls on carbon-sulfur cycling during SPICE event in south China.Theδ34S_(CAS)values in the onset and rising limb are not obviously higher than that in the preceding SPICE,meanwhile sulfate(δ34S_(CAS))isotope values increase slightly with increasingδ13CPDB in rising limb and near peak of SPICE(130–160 m).The sulfate(δ34S_(CAS))isotope values gradually decrease from 48.6‰to 18‰in the peak part of SPICE and even increase from 18%to 38.5%in the descending limb of SPICE.The abnormal asynchronous C−S isotope excursion during SPICE event in the south China was mainly controlled by the global events including sea level change and marine sulfate reduction,and it was also influenced by regional events such as enhanced siliciclastic provenance input(sulfate),weathering of a carbonate platform and sedimentary environment.Sedimentary environment and lithology are not the main reason for global SPICE event but influence theδ13CPDB excursion-amplitude of SPICE.Sea level eustacy and carbonate platform weathering probably made a major contribution to theδ13CPDB excursion during the SPICE,in particularly,near peak of SPICE.Besides,the trilobite extinctions,anoxia,organic-matter burial and siliciclastic provenance input also play an important role in the onset,early and late stage of SPICE event.

Negative carbon isotopic excursion on the Lower/Middle Cambrian boundary of Kaili Formation, Taijiang County, Guizhou Province, China: Implications for mass extinction and stratigraphic division and correlation

The stratigraphic division and correlation of the Lower/Middle Cambrian boundary is a global problem that has not yet been perfectly solved up to now. That is because there existed two global biogeographic regions during the period from Early Cambrian to Middle Cambrian. Although much work has been done from the angle of paleontology and great achievements have been acquired in this aspect, no biological assemblage has yet been established for global stratigraphic correlations due to the coexistence of the two global biogeographic regions —— the Atlantic biogeographic region and the Indian-Pacific biogeographic region during the Early-Middle Cambrian. So, to develop and establish other approaches to the stratigraphic division and correlation of the Lower/Middle Cambrian on a global scale is a possible way to solve the puzzling problem. This work systematically studied acritarch fossils from the Early-Middle Cambrian Kaili Formation at Taijiang County, Guizhou Province. The Lower/Middle Cambrian boundary was divided in terms of acritarch fossil assemblage. The divided boundary is generally consistent with what was divided by trilobite and can be correlated with the Lower/Middle Cambrian boundaries divided by acritarch assemblage in Siberia and Europe. On this basis, the Lower/Middle Cambrian boundary is divided in terms of an obvious carbon isotopic excursion on a global scale during the transitional period from Early Cambrian to Middle Cambrian boundaries in Siberia and North America. The method for the stratigraphic division and correlation of the Lower/Middle Cambrian boundary in terms of carbon isotopic oscillations is helpful to solving the global problem on the division and correlation of the Lower/Middle Cambrian boundary. It is also evidenced that the extinction of a lot of trilobites at the end of Early Cambrian is closely related with this event of carbon isotopic excursion.

Quantitative model evaluation of organic carbon oxidation hypotheses for the Ediacaran Shuram carbon isotopic excursion

The largest global carbon-cycle perturbation in Earth history was recorded in the Ediacaran—a persistent negative shift in the global marine dissolved inorganic carbon(DIC) reservoir that lasted for ~25–50 million years, with a nadir of –12‰(i.e.,the Shuram Excursion, or SE). This event is considered to have been a result of full or partial oxidation of a large dissolved organic carbon(DOC) reservoir, which, if correct, provides evidence for massive DOC storage in the Ediacaran ocean owing to an intensive microbial carbon pump(MCP). However, this scenario was recently challenged by new hypotheses that relate the SE to oxidization of recycled continentally derived organic carbon or hydrocarbons from marine seeps. In order to test these competing hypotheses,this paper numerically simulates changes in global carbon cycle fluxes and isotopic compositions during the SE, revealing that:(1) given oxygen levels in the Ediacaran atmosphere-ocean of ≤40% PAL, the recycled continental organic carbon hypothesis and the full oxidation of oceanic DOC reservoir hypothesis are challenged by the atmospheric oxygen availability which would have been depleted in 4 and 6 million years, respectively;(2) the marine-seep hydrocarbon oxidation hypothesis is challenged by the exceedingly large hydrocarbon fluxes required to sustain the SE for >25 Myr; and(3) the heterogeneous(partial) DOC oxidation hypothesis is quantitatively able to account for the SE because the total amount of oxidants needed for partial oxidation(<50%)of the global DOC reservoir could have been met.

Recycled carbon degassed from the Emeishan plume as the potential driver for the major end-Guadalupian carbon cycle perturbations

Massive gas emissions(e.g.,CO_(2),CH_(4) and SO_(2))during the formation of large igneous provinces(LIPs)have been suggested as the primary cause of dramatic climatic change and the consequent ecological collapses and biotic crises.Thermogenic carbon of crustal sediments induced by intrusive magmatism throughout the LIPs is considered as the primary trigger for environmental catastrophe including mass extinction,as illustrated in the case of the Emeishan LIP in Southwest China.Herewe evaluate the Emeishan LIP to address the causal link between carbon degassing and environmental crises during the end-Guadalupian of Middle Permian.An assessment of the carbon flux degassed from recycled oceanic crust in the Emeishan plume shows that recycled oceanic crust contributed significantly to the carbon flux.Using evidence fromcarbonate carbon isotopic records at the Gualupian-Lopingian(G-L)boundary stratotype at Penglaitan of South China,our study suggests that carbon degassed from massive recycled components in the Emeishan plume served as a major end-Guadalupian(Middle Permian)carbon isotope excursion.The model based on the Emeishan LIP also offers new insights into the important role of recycled carbon released from other LIPs in climatic change and mass extinctions,as in the cases of the end-Permian Siberian and end-Cretaceous Deccan Traps.Our work highlights that carbon released from subducted slabs is returned to the atmosphere via upwelling mantle plumes,which could drive global climatic change and mass extinction.

2.0 Ga orogenic graphite deposits and associated 13C-enriched meta-carbonate rocks from South China Craton:Implications for global Lomagundi event

The Lomagundi(-Jatuli)event,characterized by extremely high positive global inorganic carbon isotope excursion at about 2.2 billion years ago,is pivotal in investigating the causes and consequences of great oxygenation event,inventory and sequestration of carbon on the Earth’s surface,evolution of life,and more profoundly tectonic control on Earth’s environment.However,the reasons that caused the isotopic excursion are not resolved yet.Herein,we report the discovery of meta-carbonate rocks with distinct positive carbon isotopic excursion from the Paleoproterozoic continental collision zone of the Kongling Complex,South China Craton.The δ^(13)C_(V-PDB) values for meta-carbonate rocks show positive values in the range from+5.5‰to+11.6‰,whereas the δ^(13)C_(V-PDB) values of associated graphite deposits range from-25.8‰to-9.5‰.Zircon U-Pb-Hf isotopes from zircon-bearing meta-carbonate sample yielded weighted average _(207)Pb/_(206)Pb age of 2001.3±9.5 Ma,with correspondingε_(Hf)(t)range from-7.05 to-3.16,comparable to the values of local 2.9–2.6 Ga basement rocks.Geochemical characteristics of meta-carbonate rocks,such as their rare earth element patterns and the trace element parameters of La,Ce,Eu,and Gd anomalies and Y/Ho ratio,suggest that the carbonate deposition took place in passive continental margin in association with large volumes of organic carbon.The extensive graphite deposits from Kongling Complex in South China Craton,their equivalents in the North China Craton and elsewhere across the globe prove that the burial of ^(12)C-enriched organic carbon has eventually resulted in the global enrichment of ^(13)C in the atmospheric CO_(2),which is recorded in the marine carbonate rocks.Isotopic mass balance estimates indicate that more than half of the organic carbon was buried during the oceanic closure.Hence,the observed global shift could be directly related to the continent collision event in greater China,thus resolving the long-standing paradox of the Lomagundi global positive carbon isotope excursion.Moreover,the present results suggest that orogenesis play a significant role in sequestration of carbon into the continental crust.

早侏罗世大洋缺氧事件在塔里木盆地的响应

早侏罗世Toarcian期大洋缺氧事件(简称“T-OAE”),被认为与Karoo-Ferrar大火成岩省爆发以及与甲烷气水合物有关的温室气体迅速释放导致全球变暖密切相关。该事件在特提斯洋区有较详细研究,但在陆相生态系统中的环境变化和沉积响应报道较少。本文报道了塔里木盆地北缘库车河地区的有机碳同位素地层数据,“东剖面”和“西剖面”中δ13C曲线总体的正偏移趋势被多幕次地快速负偏移打断,表明地表碳循环受到了巨大扰动。事件层内δ13C值的突然负向波动暗示了大气碳库在短时间内有相对轻同位素特征的碳注入,并呈现不稳定的、阶段性注入的特点,推测可能与增温引发的正反馈有关。沉积相突变与孢粉种类的衰减表明该区沉积环境发生了巨大变化;碎屑白云岩和紫红色泥岩的出现和湿生孢粉的衰减以及中生孢粉的出现,均表明在T-OAE期间塔里木盆地北缘气候转为干热。该研究报道了首个来自陆相沉积盆地边缘的T-OAE记录,对于深入理解二氧化碳浓度显著升高背景下引起的陆地系统响应具有借鉴意义。

下侏罗统辛涅缪尔阶-普林斯巴阶界线地质事件研究进展

早侏罗世经历了赫塘期、辛涅缪尔期、普林斯巴期和土阿辛期,其中土阿辛期大洋缺氧事件(T-OAE)被广泛研究。但在辛涅缪尔阶-普林斯巴阶界线处,δ^(13)C_(org)及δ^(13)C_(carb)曲线均发生负偏,局部伴随黑色页岩沉积。其在英国、法国、意大利、葡萄牙以及中国等地区都有记录,碳同位素的变化伴随着环境、气候的变化,研究显示该事件可能是全球性的,被称为“S-P界线事件”(SPBE)。此次地质事件与T-OAE事件类似,也显示了一次大洋缺氧事件,其发生时有轻碳同位素被排入了大气系统中,但是轻碳同位素的来源尚不明确,部分学者认为是大火成岩省喷发时排放的轻碳物质注入了海洋-大气系统所致。目前对该事件的研究主要集中于欧洲海相地层,其对陆地生态系统的影响还知之甚少。我国四川盆地下侏罗统湖相沉积发育良好,下侏罗统自流井组东岳庙段沉积有与大安寨段(T-OAE发生层段)类似的黑色页岩及介壳灰岩,其有机碳同位素负偏,显示了S-P界线事件的影响,其与T-OAE类似也对四川盆地早侏罗世古湖泊产生了影响。

A record of Miocene carbon excursions in the South China Sea

High-resolution δ13C records are presented for the Miocene benthic foraminifers Cibici doides wuellerstorfi and C. kullenbergi (24-5 Ma) and the planktonic foraminifer Globigerinoides sacculifer (18-5 Ma) from ODP Site 1148A (18° 50.17′N, 116° 33.93′E, water depth 3308.3 m),northern South China Sea. The general pattern of parallel benthic and planktonic δ13C shows a decrease trend of δ13C values from the early-middle Miocene to the middle-late Miocene. Two dis tinct δ13C positive excursions at 23.1-22.2 and 17.3-13.6 Ma, and two negative excursions at 10.2-9.4 and 6.9-6.2 Ma have been recognized. All these events are cosmopolitan, providing the good data for the stratigraphic correlation of the South China Sea with the global oceans as well as for studying the changes of the global carbon reservoir and its corresponding climate.

西藏海相三叠系—侏罗系界线及晚三叠世生物绝灭事件研究

1998—2004年间在西藏聂拉木、林周、墨竹工卡和洛扎等地进行的三叠系/侏罗系界线地层研究表明,西藏隆子县和洛扎县尚未发现具有连续菊石层序的三叠系/侏罗系界线地层剖面。拉萨以北地区广泛分布的火山岩时代确定为晚三叠世—早侏罗世早期,这有助于解释晚三叠世末期生物绝灭与晚三叠世岩浆侵入和大规模的火山活动有关,但T/J界线尚待精确限定。聂拉木县格米格剖面是特提斯地区唯一未曾“压缩”的三叠系/侏罗系界线地层剖面,具有瑞替阶MARSHI菊石带、赫塘阶Tibeticum、Callyphyllum和Pleuro-notum菊石带。格米格剖面三叠系/侏罗系界线碳稳定同位素曲线记录了晚三叠世瑞替阶末期(Marshi菊石带)突然的负偏移,它很有可能和晚三叠世末期的由超级温室效应所产生的生物绝灭事件相对应。

西藏南部上白垩统高分辨率全岩碳同位素地层学

西藏南部上白垩统半远洋沉积高分辨率碳、氧同位素实验结果显示δ13C值在Cenomanian末期正偏并形成正偏“高原”,从Turonian期开始总体呈现长期持续负偏,到Campanian早期负偏达到最低值。这种长期变化格局与世界有关地区的碳同位素偏移和同期全球海侵—海退旋回型式匹配性甚好,印证了晚白垩世δ13C值长期偏移趋势可作为大区域乃至全球海平面变化的指针。短期δ13C值波动方面,Cenomanian末期、Turonian中—晚期、Turoni-an—Santonian界线时期、Campanian期都存在与世界相关地区的可比性,但Coniacian—Santonian期全球碳同位素偏移型式存在较大差异。

塔里木盆地蓬莱坝剖面寒武系第二统-第三统界线处碳同位素负异常及其地质意义

塔里木盆地柯坪地区蓬莱坝剖面是研究寒武系地层的典型剖面,而迄今未有第二统-第三统界线处碳同位素演化研究。本研究对该剖面第二统吾松格尔组-第三统沙依里克组过渡区碳酸盐岩进行系统取样,分析了18件样品的碳、氧同位素组成。结果表明,δ13C(V-PDB)和δ18O(V-PDB)值分别分布于-4.2‰～0.8‰之间和-7.4‰～-3.8‰之间,二者之间相关性不明显,并且δ13C值在第二统与第三统界线处呈现明显的负漂移,可达-4.2‰。因此,δ13C值负漂移未受成岩作用影响,真实反映了原始海水碳同位素组成特征,可与北美、西伯利亚、华南以及华北地区的ROECE地质事件对比,具有全球意义。鉴于ROECE(Redlichiid-Olenellid Extinction Carbonisotope Excursion)事件约发生在509Ma,所以本研究为区域地层研究提供了一个较为可靠的绝对年龄数据,弥补了传统年代学研究方面的不足。此外,这是ROECE事件在塔里木盆地的首次报道,对寒武系地层划分以及全球对比具有重要科学意义。

自生碳酸盐岩与底栖有孔虫碳同位素特征对多幕次甲烷事件的耦合响应——以IODP 311航次1328和1329站位为例

甲烷渗漏活动及其甲烷厌氧氧化(AOM)在自生碳酸盐岩沉淀的同时,也通过影响孔隙水溶解无机碳(DIC)进而影响着周围环境中底栖有孔虫,以往的文章鲜有报道二者的耦合响应。研究开展了综合大洋钻探计划IODP 311航次两个钻孔(1328和1329)中获得的自生碳酸盐岩和底栖有孔虫(Uvigerina peregrina)同位素研究,发现晚更新世以来多个层位获得的自生碳酸盐岩和底栖有孔虫的稳定碳同位素变化趋势均呈现一致的负偏碳同位素特征,但是,同层位的自生碳酸盐岩碳同位素负偏程度要比底栖有孔虫大一个数量级。自生碳酸盐岩与底栖有孔虫碳同位素变化趋势的一致性表明二者对于甲烷渗漏作用有较好的共同响应。AOM作用对孔隙水中溶解无机碳(DIC)的影响可在重碳酸氢根通过局部环境的过饱和沉淀自生碳酸盐岩的同时,也能部分参与底栖有孔虫的成壳,两者在成因方面是耦合的。综合结合自生碳酸盐岩和底栖有孔虫的碳同位素特征可以避免单一载体易受后期成岩改造影响而掩盖甲烷渗漏活动的识别。

贵州瓮安陡山沱组剖面碳同位素生物地球化学研究

在对扬子地台贵州瓮安陡山沱组剖面干酪根碳同位素组成进行分析后发现其变化显示了一定的规律性。碳同位素规律变化反映了最初的沉积信息,特别是有机质埋藏量的变化。获得的陡山沱期沉积岩中碳同位素组成研究结果初步反映了震旦-寒武交变期沉积环境的面貌;碳同位素组成变化规律可以用于反映扬子地台沉积层序和世界同期地层全球碳循环的对比和参考。为扬子区陡山沱期环境变化和生物演化提供了证据。δ13Cker值在-32.3‰和-24.5‰(PDB)间漂移与δ13Ccarb值在-4.9‰和3.6‰(PDB)间的变化,反映了当时海水的碳同位素组成。无机和有机碳同位素组成之差的Δδorg-carb值沿剖面不断变化,是由于沉积有机质中有促进发酵功能的细菌数量比例变化所致。