|Alternative Title||The Late Cretaceous-early Eocene Paleoclimate and Paleoenvironment Change Revealed by Deep Drilling in Jianghan Basin|
|Place of Conferral||兰州|
|Keyword||江汉盆地 早古近纪 蒸发盐 碳氧同位素 PETM|
江汉盆地位于中扬子地区的东北部，是一个典型的内陆断陷盆地，盆地内发育多个次级的凸起和凹陷带。其中江陵凹陷是江汉盆地最大的次级构造单元，面积6500 km2，是在扬子陆块上形成的白垩-新生代断坳型沉积凹陷，发育了近万米河流-湖泊相的陆相碎屑岩沉积体系。钻自江陵凹陷的SKD1井获取了连续完整的晚白垩世-早古近纪陆相地层，由下向上依次为渔洋组、沙市组和新沟咀组。钻孔磁性地层年代学结果显示渔洋组-沙市组和沙市组-新沟咀组分界的磁性地层年代分别为66.0 Ma 和56.3 Ma。依据钻孔沉积物的岩性、沉积组合及沉积构造等特征，可将钻孔湖相沉积划分出滨湖、浅湖、半深湖及深湖4种沉积亚相。通过采样以及室内水溶组分易溶盐离子含量、酸溶组分元素（Mg和Ca）含量、总有机碳（TOC）、矿物和色度的测试分析，揭示钻孔的沉积环境经历了一个完整的演变周期，即从底部的微咸水滨浅湖逐渐向上演变为深水盐湖，最后再次演化为微咸水滨浅湖沉积环境的过程。进一步结合其他钻孔的研究成果，重建了江陵凹陷沉积环境的时空演化过程，很好地记录了晚白垩世-始新世早期凹陷沉降中心逐步向南迁移的过程，同时指示了江汉盆地晚白垩世-始新世早期整体较暖干的气候特点，尤其在66.0~51.9 Ma气候最为暖干。钻孔碳酸盐碳氧同位素和有机碳同位素主要受控于气候的干旱程度，干旱程度增加时，二者增加，反之亦然。晚白垩世-古新世期间（73.5~56.0 Ma）江汉盆地气候总体呈现逐渐变干的趋势，古新世-始新世界线附近（56.0 Ma）出现短暂暖湿气候，之后再次变暖干，沉积厚层蒸发盐，始新世早期51.9~47.6 Ma气候相对湿润。碳酸盐碳氧同位素完整地记录了古新世-始新世极热事件（PETM），并可进行全球对比。其δ13C 值负偏幅度达9.6‰，主要包括快速负偏、缓慢负偏以及逐渐回返三个阶段。极短时间内大量甲烷的快速释放导致全球温度升高，并引发正反馈作用，使得大陆坡上的甲烷气体失稳而被缓慢释放，从而导致δ13C继续缓慢降低，之后地球表层系统对碳的固定导致δ13C逐渐恢复至pre-PETM的状态。工业革命以来的大气CO2增加将对生态环境和气候产生重大影响，全球增温可能触发海底天然气水合物的分解，从而进一步加速全球变暖。沉积学和地球化学等证据均表明，古新世-始新世极热事件（PETM）期间存在一次极为短暂的湿润事件，可能指示季风或类似季风气候的形成，PETM期间极高的大气pCO2可能是季风形成的最关键因素。SKD1钻孔内的蒸发盐是在构造和气候的共同作用下形成的。在古新世-始新世早期整体暖干的气候背景下，膏盐岩首先在江陵凹陷北部沉积，之后随着沉降中心逐渐向南迁移，钻孔附近的下始新统新沟咀组中开始出现膏盐岩沉积，据此推测凹陷南部的次级洼陷可能具有钾盐成矿潜力。此外，干湿波动的季风气候可能促进了新沟咀组中蒸发岩的沉积。上述研究成果为晚白垩世-始新世早期古气候的全球对比，以及气候模型的修正提供数据和理论支持，对预测未来气候变化意义重大。同时探讨了凹陷的成盐及成钾机制，为今后在该地区找钾提供科学依据。
Global Warming has caused a series of ecological and environmental changes and has become a hot topic in recent years. How will the climate evolve in the future has beome an important issue which causes great concern from governments and the public. Studies on the similar climate pattern in the geology history can act as a mirror for the modern climate change, helping to predict future climate. Mid-Cretaceous to Early Paleogene was a typical “greenhouse climate” during the Phanerozoic, espectially the extremely hot event—the Paleocene-Eocene thermal maximum (PETM) that happened at the P/E boundary. The process of PETM is comparable with the increasing atmospheric CO2 since the industrial revolution. Thus, study of the palaeoclimate during Cretaceous-Early Paleogene helps a lot in predicting the future climate change. However, most of the researches on the greenhouse climate during Cretaceous-Early Paleogene are from marine records. Lacking of palaeoclimate records from terrestrial deposits with high resolution has seriously limited the accurate understanding of the evolution on global climate and the coupling effects of sea - land - gas systems.
Jianghan basin is a typical Cretaceous-Cenozoic inland rifted salt lake basin in eastern China, with many secondary bulges and depressions developed inside of the basin. Jiangling depression, located at the west part of the Jianghan basin, is the largest tectonic units, covering an area about 6500 km2. Jiangling depression is a Cretaceous-Paleogene rift-depression basin that was developed on the Yangtze continental block, with nearly ten thousands meters’ river-lake sediments of continental clastic sedimentary system deposited. SKD1 was drilled in the Jiangling depression and has obtained complete Upper Cretaceous to Lower Paleogene sedimentary formations, which are Yuyang Fm., Shashi Fm. and Xingouzui Fm. from bottom to the top, respectively. The boundary age of Yuyang and Shashi Fms., Shashi and Xingouzui Fms. are 66.0 Ma and 56.3 Ma, respectively according to the magnetstratigraphic chronology result.Based on lithology and sedimentary structure investigation, four sedimentary subfacies were found in the core, which are lakeshore, shallow lake, semi-deep and deep lake subfacies. Analysis of the thin sections, water-soluble salt ions, HCl-soluble Mg and Ca contents, TOC, minerals and color index demonstrate a deepening - shallowing cyclicity of environmental evolution，from a shore - shallow brackish lake to a deep brine lake, and finally return to a shore - shallow brackish lake. In addition, through the comparison of researches from other cores, we reconstructed the spatio-temporal evolution of the depositional environment in the Jiangling depression, which records the southward migration of the depocenter of the Jiangling depression from the Late Cretaceous to early Eocene. Meanwhile, it also suggests that the Jianghan Basin experienced rather warm and dry climate during Late Cretaceous-early Eocene, especially from 66.0 Ma to 51.9 Ma.Carbonate C and O isotopes and organic C isotopes are mainly controlled by the climate and the values of all the isotopes will increase when the climate becomes more arid and vice versa. The climate in Jianghan Basin during Late Cretacous-Paleocene (73.5~56.0 Ma) showed a gradually drying trend, and transient warm and humid period existed near the boundary of P/E, after which the climate became warm and dry again with thick evaporates deposited. And the climate gradually became humid during early Eocene from 51.9 Ma to 47.6 Ma.Carbonate carbon istopes in lake sediments of SKD1 completely record the PETM and can be compared with other records around the world, with the CIE (carbon isotope excursion) to be 9.6‰, which mainly includes three stages: a rapid negative excursion, followed by a slowly decreasing trend, and then a gradual positive recovery. The rapid release of large quantities of methane in a very short period of time leads to global warming and a positive feedback that causes the methane gas on the continental slope to destabilize and slowly released, causing the δ13C to continue to decrease slowly, after which the carbon fixation by the the earth's surface system leads to a gradual recovery of δ13C to pre-PETM. The increase in atmospheric CO2 since the industrial revolution will have a major impact on the ecological environment and climate. Global warming may trigger the decomposition of natural gas hydrates on the seabed, thereby further accelerating global warming.Both sedimentary and geochemistry evidences indicate that there was a short humid event during PETM and might be related with the monsoon or monsoon-like climate, which might be caused by the higher pCO2 during PETM.The basin subsidence, coupled with a warm and extremely arid climate during the Paleocene results in the precipitation of evaporites at the north part of the depression, and with the southward migration of the depocenter, evaporates were deposited near SKD1 core in Lower Eocene Xingouzui Fm. This indicates there is a great potential for finding potash resources in the sub-sagged southern Jiangling depression. Moreover, the highly fluctuated monsoon climate might have promoted the deposition of the evaporites in Xingouzui Fm.
Above research achievements not only contribute to the global comparison of the palaeoclimate during Late Cretaceous-early Eocene, but also provide both data and theory support for the amendment of the climate models, helping to predict climate change in the future. In addition, the mechanism of salt formation and potassium formation in the depression was discussed to provide a scientific basis to find potassium in this area in the future.
|滕晓华. 江汉盆地深钻揭示的晚白垩世-始新世早期古气候与古环境变化[D]. 兰州. 兰州大学,2018.|
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