华南早二叠世冰室向温室转折期海水酸碱度演化与古环境重建

The Early Permian witnessed the transition from the climax of the Late Paleozoic great ice age to the glacial meltdown period. Global climate during this interval developed from icehouse to greenhouse conditions, resulting in a great impact to marine chemistry, climatic characteristics and biodiversity evolution. Variation of seawater pH may influence calcified biomineralization and exert constraints on the living conditions of marine biosphere further. This brings into question the role of seawater pH during the Early Permian icehouse-greenhouse transition, which is exacerbated by a lack of evidence for the exact timing, rate of change and intensity of seawater pH variation. Boron isotope composition of marine carbonate can be a useful tracer of palaeo-seawater pH. Therefore, this study aims to investigate marine carbonate boron isotopes at different sedimentary facies from Guizhou and Guangxi regions, supplemented by biostratigraphic, carbon isotopic, major and trace elemental analyses and Earth system modelling , to reconstruct the spatio-temporal changes of seawater boron cycle and pH variation during the icehouse to greenhouse transition of the Early Permian. In addition, this project attempts to quantitatively evaluate the pH variation and its relationship to carbon and nutrient cycling, and to provide a comprehensive analysis on the linkage between palaeo-environment and biodiversification, which would provide new insights to the interaction between habitable Earth and permissive environmental conditions for life.

早二叠世历经了晚古生代大冰期的巅峰及冰川消融时期，全球气候由冰室向温室效应转折，对海洋化学环境、气候特征和生物多样性演化具有重要影响。气候的改变造成海水酸碱度变化，进而影响生物钙化作用，对生物生存条件产生制约。然而，目前对早二叠世盛冰期至冰融期海水酸碱度变化的确切时间、过程及程度研究仍处于空白阶段。海相碳酸盐岩的硼同位素组成可直接示踪古海水酸碱度变化，因此本项目拟通过对华南黔桂地区下二叠统不同沉积相地层剖面的硼同位素研究，辅以生物地层、主微量元素、碳同位素数据及地球系统模型计算，重建早二叠世晚古生代冰期后冰室向温室效应转折期的海水硼物质循环及酸碱度的时、空变化，定量分析海水pH与碳循环及营养元素供给的耦合关系，进而综合探究环境变化对生物演化的意义，为推动对未来地球环境及生命生存条件的认识提供新的借鉴与启示。

早二叠世晚古生代大冰期开始消融，但海洋化学变化中的气候敏感性背景尚不清楚。本项目通过研究华南乌拉尔统（299-281Ma）碳酸盐岩层序的无机碳氧同位素、稀土微量元素、黄铁矿硫同位素、CAS硫同位素及地球系统模型，探究早二叠世海水氧化还原状态演变。.在中下扬子地台的安徽巢湖平顶山剖面，栖霞组的铈异常表现为弱到正的异常，结合平顶山剖面的富含沥青质的岩相特征，可能是有机质参与的结果。中稀土富集指数表现为大于1，结合钇的正异常及Eu的正异常，可能指示水体是原始海水混合了磷化的铁锰结壳或海底热液输入。黄铁矿硫同位素与CAS硫同位素呈现正相关耦合关系，代表早二叠世黄铁矿埋藏升高，可能指示水体缺氧的还原环境。.在滇黔桂盆地的岜山-铁桥剖面和江南盆地的孔山剖面，Asselian时期的无机碳同位素与CAS硫同位素呈现三段漂移旋回，且正向耦合关系。这种耦合关系表明黄铁矿和有机碳埋藏通量或净埋藏速率增加，可能指示中深水层位广阔陆架或斜坡相位置上缺氧程度增加导致的海水富含34S。当缺氧程度减弱，碳硫循环呈现同步负漂。这种缺氧可能是由于冰期末期周期性的气候非稳态波动而导致的周期性风化输入、营养物质及生物生产力的振荡，从而间接影响了氧化还原的波动。SCION建模显示，这种同位素耦合反映了由天文周期旋回驱动温度及光照率而产生的周期性振荡，这一旋回约为1.2个百万年。空谷时期，碳同位素表现为持续高值，尽管整个扬子地台的CAS硫同位素值表现为可变的，但整体上说明海洋处于低硫酸盐浓度，且气候过渡至相对温暖，大量稳定的风化及营养源输入海洋，使生物分异度呈现上升趋势。