赣东北地区晚白垩世圭峰群古土壤特征与古气候研究

Paleosols were sensitive to paleoclimate changes. At present, the study of paleoenvironment and paleoclimate based on paleosols is one of scientific frontiers in sedimentology. The Late Cretaceous continental basins are characterized by red-colored siliciclastic rocks in northeastern Jiangxi Province, where typical Danxia landforms such as the Longhushan World Geopark occur. In our field geological investigation, abundant paleosols have been found in the reddish rock succession of the Guifeng Group, which provides a good opportunity for understanding paleoclimatic conditions. The Late Cretaceous paleoclimate will be quantitatively established by analysis of paleosol outcrop characteristics, microscopic morphology, element geochemistry and C-O isotopes of pedogenic carbonates in the four main sections. Therefore, this project is designed to investigate the distribution pattern, paleosol profiles and pedotypes, and to obtain quantitative paleoclimatic parameters including MAP, MAT and pCO2. In combination with previous biostratigraphic, isotopic data and U-Pb dating of this work, the paleoclimate changes (Coniacian-Maastrichtian stages) will be untangled for the formation of paleosols in the Guifeng Group. At last, the paleoclimate results of this research need to be compared with other areas (e.g. the Songliao Basin in northeastern China), so that the paleosol characteristics and response to paleoclimate trends during Late Cretaceous in the arid mid-latitudes of eastern Asia can be summarized.

古土壤是古气候变化的灵敏记录者，根据古土壤进行古环境和古气候恢复是当前沉积学领域的研究热点之一。赣东北地区晚白垩世断陷盆地以陆相红色碎屑沉积为特征，发育以龙虎山世界地质公园为代表的丹霞地貌。野外地质调查发现该区圭峰群中古土壤发育，是进行古土壤和古气候研究的良好场所。本申请项目拟对4条主干剖面开展工作，以圭峰群红层中的古土壤为研究对象，通过宏观露头和镜下微观组构观察、矿物组成、元素地球化学和成土碳酸盐稳定同位素分析，建立古土壤剖面，划分成土类型，查明该区古土壤类型和分布，取得反映古气候的定量参数MAP、MAT和pCO2。结合前人古生物地层和同位素测年资料及本项目火山岩夹层U-Pb定年结果，建立圭峰群古土壤形成时期（Coniacian-Maastrichtian期）的古气候变化曲线，进而与东亚其它地区（如松辽盆地）进行对比分析，总结晚白垩世东亚中纬度干旱地区古土壤特征及其对古气候变化的响应。

华南地区白垩纪陆相沉积发育，它们不仅蕴含着丰富的古气候信息，同时也是众多著名旅游景区丹霞地貌的主要成景地层。古土壤是气候敏感型沉积之一，但是这套地层中以古土壤为载体进行古气候的研究还比较少。通过野外剖面测制、显微镜观察、粒度分析等，对江西永丰-崇仁盆地和信江盆地晚白垩世圭峰群及广丰盆地周田组沉积相进行研究，然后对其中的古土壤进行了重点研究。古土壤主要是粘土红壤和钙质土壤，其识别标志包括土壤发生层次、成壤碳酸盐岩、遗迹化石、杂色晕斑、泥裂构造等。样品的化学蚀变指数（CIA）主要为60～80，化学风化程度较低，指示干燥的沉积环境。Cenomanian早期至Turonian期（96～90 Ma）大气CO2浓度在482～1207 ppmv 之间，Campanian晚期（约75 Ma）大气CO2浓度为782～1420 ppmv。圭峰群红层沉积变化可能主要受控于古气候条件，粗粒沉积反映了较为湿润的古气候，细粒沉积则指示古气候向干旱转变。塘边组粒径总体较细，以风成砂岩夹古土壤为特征，可能反映了Campanian期东亚中低纬度广泛的干旱化。另外，该项目研究还为龙虎山等世界地质公园、世界自然遗产地“中国丹霞”的科普教育和研学旅游提供了材料。