新元古代是地质历史上一个关键又特殊的时期，见证了板块构造的剧烈运动、全球性冰川发育、后生多细胞动物的辐射演化、新元古代大氧化事件以及全球古海洋地球化学成分的剧烈波动等一系列变革。构造、气候、环境与生态系统变化反映了大气-陆地-海洋与生态圈的协同作用。华南陆块各个沉积相区均分布新元古界地层，且沉积纪录完整、出露良好，是研究该时期内古海洋性质及生命演化等科学问题的重要窗口。然而，相对于华南陆块埃迪卡拉纪地层学和古生物学研究取得的耀眼成果，古海洋化学性质与大陆圈层具体的时间与空间变化的研究相对薄弱。因此本论文针对新元古代埃迪卡拉纪陡山沱期盖帽碳酸盐岩沉积时期及“DOUNCE/Shuram-Wonoka”时期这两个极端事件选取扬子板块代表性剖面开展地球化学地层学工作，应用多种指标地球化学研究(碳氧同位素、主微量及稀土元素、锶同位素、硼同位素、锂同位素)，试图系统恢复地史上这一重大转折时期的古海洋化学性质变化及大陆风化作用变迁。 本文将华南不同沉积相区无机碳同位素与全球其他地区进行对比，并结合生物化石记录和锆石U-Pb年龄数据，得出盖帽碳酸盐岩沉积初期及中期δ¹³C值表现稳定负值，范围约在-5‰～-3‰之间，沉积末期可降至-22‰，呈现负漂移趋势。这可能与大气CO₂急剧升高，海平面上升及后期叠加甲烷水合物释放有关。“DOUNCE/Shuram-Wonoka”时期δ¹³C值负漂移最低值可达-10.8‰，负值持续长且稳定。这可能与深海巨大有机碳库氧化有关，溶解有机碳通过有氧氧化与硫酸盐还原作用产生大量富¹²C的碳酸氢根离子，通过海水上涌从深部海水带到浅水导致碳同位素负偏。不同沉积相区δ¹³C梯度可能受海水分层与局部沉积环境影响。生物繁盛时期海洋初级生产力提高，有机质埋藏增加，δ¹³C正偏，反之δ¹³C负偏。 锶同位素比值可示踪大陆风化通量及热液活动对海水影响。通过对三峡地区台盆相乡党坪剖面与陕西地区斜坡相司上剖面“DOUNCE/Shuram-Wonoka”时期的锶同位素比值研究发现，锶同位素比值呈现逐渐升高之后降低的趋势(0.7099～0.7080)，与碳同位素负漂移趋势相反，表明大陆风化通量呈现先增加后降低的特征。本文通过动态模拟计算发现乡党坪剖面大陆风化通量降低了49%，司上剖面大陆风化通量降低了79%。大陆风化通量增加导致大量营养物质和陆源阳离子进入海洋，为宏体后生动物的兴起与繁盛提供物质基础和条件。 微量稀土元素可指示古海水氧化还原状态并代表海水氧含量变化。本文重点分析了华南三峡地区三条盖帽碳酸盐岩沉积剖面的Ce异常值，发现Ce在盖帽沉积末期发生富集，形成Ce正异常现象(Ce/Ce*＞1.3)，这可能与锰的氢氧化物还原性溶解有关。结合古地理研究、其他氧化还原指标及REY配分模式分析，本文推测在盖帽碳酸盐岩沉积末期具有锰化楔过渡区，代表锰化亚氧状态，无法达到大型后生动物所需的氧气条件，从而限制了该时期生命演化，直到陡山沱二段浅层海水恢复至氧化阶段(Ce/Ce*＜1.0)才出现大型带刺疑源类生物。 硼同位素可示踪海水pH值变化。本文通过TIMS方法测定δ¹¹B值发现有机质干扰使测量数值比真实值低，因此利用红外光谱、拉曼光谱、热重质谱等方法进一步确定酰胺基官能团是有机质干扰的主要成分。由此在硼同位素化学纯化分离过程中需排除有机质干扰。本文针对华南三峡地区盖帽碳酸盐岩δ¹¹B值研究发现海水经历酸化事件。宜昌三峡地区九龙湾剖面δ¹¹B值(-4.8‰～11.2‰)表明pH值从8.46降低至7.50再回升至8.89；花鸡坡剖面δ¹¹B值(-1.8‰～6.9‰)表明pH值从8.6降低至7.95再回升至8.31。海水酸化事件反映大气二氧化碳含量急剧升高、陆源风化输入增强，海洋中[H⁺]增加。“DOUNCE/Shuram-Wonoka”时期硼同位素数据表明三峡地区乡党坪剖面海水pH值从8.81降低至8.47再回升至9.10，陕西司上剖面pH值从8.59降低至8.22再回升至8.69。表明这一时期大气CO₂含量不同于盖帽碳酸盐岩沉积时期的极端水平，酸化原因可能由大陆风化作用通量输入与深海硫酸盐还原作用失衡导致。 锂同位素可示踪大陆风化作用强度。本文首次测定华南三峡地区盖帽碳酸盐岩δ⁷Li值并采用动态模拟计算发现九龙湾剖面盖帽碳酸盐岩沉积及陡山沱二段时期河流锂同位素值由-0.97‰上升至23‰再回落至7.1 ‰，河流锂输入通量由现代值1.1倍上升至最高237倍再回落至63倍；花鸡坡剖面河流δ⁷Li值由-0.97‰上升至最高23‰再回落至3.5‰，河流锂输入通量由现代值4.59倍上升至最高90.40倍再回落至50倍。这表明盖帽碳酸盐岩沉积时期大陆硅酸盐风化速率急剧升高且化学风化强度逐渐增强，更多次生粘土矿物生成，由一致风化向不一致风化转变。大陆风化通量的增加为盖帽碳酸盐岩沉淀带来大量的阳离子，次生粘土矿物的生成消耗大气CO₂并将更多的[H⁺]带入海洋，这与硼同位素值显示的海洋酸化事件相吻合。“DOUNCE/Shuram-Wonoka”时期动态模拟计算显示事件前后锂同位素值呈现负漂移趋势。河流锂输入通量升高至现代值180倍且河流δ⁷Li值由7.1‰降低至0‰再回升至6.75‰，这表明大陆风化速率极高且化学风化作用强度由弱变强，从更一致风化向不一致风化转变。受Gondwana超大陆聚合造山作用影响，更多新鲜硅酸盐矿物溶解可能导致河流锂同位素降低。锶同位素数据也表明此时期大陆风化通量先升高后降低，与锂同位素模拟计算结果相吻合。硅酸盐风化消耗大气CO₂含量，由此推测这一时期的大气CO₂水平呈现先抑后扬的趋势。 关键词：硼同位素；锂同位素；埃迪卡拉纪；海相碳酸盐岩；海水酸碱度；大陆风化作用；扬子地台

 The Neoproterozoic was a key and unique period in the geologic history. It witnessed the strenuous movement of plate tectonics, the global development of glaciers, the radiation of early metazoans, the Neoproterozoic oxygenation event, and the intense geochemical perturbation in the ancient oceans. The variations and co-evolution in tectonics, climate, environment and ecosystem reflect the connections between the atmosphere-land-ocean and biosphere. The Neoproterozoic strata is distributed in all sedimentary facies of the South China Block. These sedimentary records are preserved as complete and well exposed stratigraphic sequences, which serve as crucial windows for studying the geochemical properties of paleo-oceans and evolution of life during this period. However, in contrast to the dazzling achievements of the Ediacaran stratigraphy, palaeontology in the South China Block, the paleo-ocean chemistry and its interaction with other temporal and spatial changes in the Earth surface systems are still poorly understood. This study targets on the two extreme events ("DOUNCE/Shuram-Wonoka" and cap carbonate deposition) and provides detailed chemostratigraphy at type sections of the South China. Various geochemical indicators (carbon and oxygen isotopes, trace and rare earth elements, strontium isotopes, boron isotopes and lithium isotopes) are applied to systematically restore the variations in paleo-ocean chemistry and continental weathering evolution during this major transition. This study integrates inorganic carbon isotope profiles from different sedimentary facies in South China and elsewhere using a consistent age model by biostratigraphy and zircon U-Pb absolute ages. The results show negative δ¹³C values but remain stable at -5 ‰～-3 ‰ during the early-middle stages of early Ediacaran cap carbonate deposition. This is followed by a sudden decrease to -22 ‰ at topmost cap carbonates, suggesting a negative drift in δ¹³C trend. The negative δ¹³C swing may be related to a sharp rise in atmospheric CO₂ or/and sea level rise of/and late-stage release of methane hydrate during the deposition of cap carbonate. During the mid-Ediacaran "DOUNCE-Shuram-Wonoka" negative carbon isotope excursion, δ¹³C values decrease to as low as -10.8 ‰ and sustain at negative values through most of the mid-Ediacaran. This may be related to the oxidation of a considerable dissolved organic carbon pool in the coeval ocean. The utilisation of dissolved organic carbon via aerobic oxidation and microbial sulfate reduction produces a large flux of ions to the deep ocean. The δ¹³C of shallow seawater is then negatively biased by surging ¹²C-rich bicarbonate from the deep ocean. The δ¹³C gradient from different sedimentary facies may also be affected by water-column stratification and variations in regional depositional environment. On the other hand, enhanced marine primary productivity would increase the rate of organic carbon burial, and result in the positive excursion in seawater δ¹³C. By the contrast, negative excursion in seawater δ¹³C may be related to biological extinction. The seawater ⁸⁷Sr/⁸⁶Sr traces the imbalance of continental weathering flux and hydrothermal activity on oceanic strontium cycle. This study investigates the variations in ⁸⁷Sr/⁸⁶Sr through the "DOUNCE-Shuram-Wonoka" at the intra-shelf facies Xiangdangping section and slope facies Sishang section. The ⁸⁷Sr/⁸⁶Sr trend shows a gradual increase to 0.7099, followed by a decrease to 0.7080, and is negatively correlated to pairδ¹³C trend, indicating the positive excursion of continental influx. Dynamic simulation of isotope mass-balance model suggests that a decrease of continental weathering flux of 49 % for the Xiangdangping section and 79 % for the Sishang section is required to generate the decreasing seawater ⁸⁷Sr/⁸⁶Sr trend. Enhanced continental weathering flux would have resulted in high nutrients and terrestrial cations input flux into the oceans, providing permissive conditions for the emergence and prosperity of macroscopic metazoans. Rare earth elements pattern can reflect the redox state of ancient seawater and the oxygenation state of the ocean. This study analyses Ce anomaly of the three cap carbonate sequences in the Three Gorges area of South China. The results show that Ce is enriched at the top part of cap carbonate, exhibiting a significant positive Ce anomaly (Ce/Ce*>1.3), which may be related to reductive dissolution of manganese hydroxides. By combining with paleogeography study, other redox proxies and REY pattern analysis, this study reveals a manganese wedge transition zone developed in the water-column. This evidence suggests that manganous sub-oxic condition dominate at the end of the cap carbonate deposition, which cannot meet the oxygen conditions required for large metazoans. This limits the emergence of Doushantuo-Pertatatatka acritarchs during this period until the shallow seawater in the Memebr Ⅱ of the Doushantuo Formation evolves to the oxidation stage (Ce/Ce*<1.0). Boron isotopes can trace the variations in ancient seawater pH through time. This study uses TIMS method to measure carbonate δ¹¹B, the results suggest that interference from organic matter would alter the true δ¹¹B values to lower values. The amide functional group is further identified as the main component of organic matter interference by infrared spectroscopy, Raman spectroscopy and thermogravimetric mass spectrometry. Therefore, this study suggests that organic matter interference should be excluded before the chemical separation and separation process of boron isotopes. This study also investigates the variations in δ¹¹B values of the cap carbonate in the Three Gorges area of South China, and the results reveal an oceanic acidification event during the cap carbonate deposition. δ¹¹B values (-4.8 ‰～11.2 ‰) of the Jiulongwan section indicated seawater pH decrease from 8.46 to 7.50 and then rise back to 8.89, whereas seawater pH of Huajipo section (δ¹¹B values from -1.8 ‰ to 6.9 ‰) decreased from 8.60 to 7.95 and then returned back to 8.31. This oceanic acidification event during the cap carbonate deposition demonstrates a sharp increase in atmospheric carbon dioxide content, an increase in terrestrial weathering, and an increase in [H⁺] in the early Ediacaran ocean. The boron isotope data shows that seawater pH at the Xiangfangping section decreased from 8.81 to 8.47 and then rose back to 9.10. At the Sishang section, seawater pH decreased from 8.59 to 8.22 and then rose back to 8.69. These results suggest that the atmospheric CO₂ level during the "DOUNCE/Shuram-Wonoka" period is different from the extreme level during the deposition of cap carbonate. The "DOUNCE/Shuram-Wonoka" acidification event may be caused by the imbalance of continental weathering flux and deep-sea sulfate reduction. Seawater lithium isotopes can trace the rate of continental weathering. This study for the first time investigates the variations in seawater δ⁷Li values of Doushantuo Formation in the Three Gorges area of South China. Carbonate δ⁷Li results and mass-balance model simulations suggest that riverine δ⁷Li increase from -0.97 ‰ to 23 ‰ during the cap carbonate deposition and then fell back 7.1 ‰ by the Memeber Ⅱ of Doushantuo Formation at Jiulongwan section, revealing an increase of riverine input flux from 1.1 times to 237 times and then decreased to 63 times of modern level. At Huajipo section, riverine δ⁷Li increase from -0.97 ‰ to 23 ‰ during the cap carbonate deposition and then fell back 3.5 ‰ by the Memeber Ⅱ of Doushantuo Formation at Jiulongwan section, revealing an increase of riverine input flux from 4.59 times to 90.40 times and then decreased to 50 times of modem level. These results demonstrate that the continental silicate weathering rate sharply increased and the chemical weathering intensity gradually increased during the deposition of cap carbonate. This is accompanied by elevated secondary minerals formation and transition from congruent weathering to incongruent weathering regimes. The increase in continental weathering flux would have brought large amount of cations to facilitate the cap carbonate precipitation. Enhanced formation of secondary clay minerals would have consumed atmospheric CO₂ and brought more [H⁺] into the ocean, which is consistent with the contemporary ocean acidification event suggested by the boron isotope record. During the event of "DOUNCE/Shuram-Wonoka" negative carbon isotope excursion, carbonate δ⁷Li results and mass-balance model simulations shows negative shifts of δ⁷Li before and after the event. Riverine δ⁷Li would have decreased from 7.1 ‰ to 0 ‰ then rose back to 6.75 ‰, revealing an increase of riverine input flux to 180 times of modem level. These evidences suggest that the continental weathering rate was extremely high and the chemical weathering intensity gradually increase, and is accompanied by the transition from congruent weathering to incongruent weathering regimes. As a result of building up fo Gondwana supercontinent, elevated primary dissolution of freshly exposed silicate minerals may lead to a decrease in the riverine δ⁷Li. On the other hand, the ⁸⁷Sr/⁸⁶Sr record also suggests an initial increase in continental weathering flux followed by a decrease to initial level during the event of "DOUNCE/Shuram-Wonoka", which is consistent with the model simulation results from δ⁷Li data. High [H⁺] input to the ocean via weathering process of clay minerals would have resulted in the oceanic acidification event. This pulse of decrease of continental chemical weathering intensity consumes atmospheric carbon dioxide content, indicating the negative excursion of atmospheric carbon dioxide content level during this period. Keywords: Boron isotopes; Lithium isotopes; Ediacaran; Marine carbonates; Seawater pH; Yangtze platform