Environmental magnetic records of PETM in the southwestern Sichuan Basin

Abstract

Collision between the Indian and Eurasian plates produced concomitant uplift of the Tibetan Plateau and its basin-ridge geomorphological systems. As the world's largest and highest plateau, Tibetan Plateau surface relief has significant dynamic and thermal effects on atmospheric circulation and on regional and global climate. Large numbers of Cenozoic sedimentary basins that developed concomitantly in and around the Tibetan Plateau are attributed to intracontinental deformation and Tibetan Plateau uplift due to India-Eurasia collision, such as the Sichuan Basin which is located in the eastern margin of the Tibetan Plateau. During the Cenozoic, the Sichuan Basin has received thick sediments which provide us an excellent opportunity to reconstruct the tectonic and climatic evolution of the southeastern Tibetan Plateau areas. In this study, we present results from a long sedimentary sequence (thick of 742 m) at Shiyang section (102°50'E, 30°0'N) from the southwestern Sichuan Basin. Magnetostratigraphic sequences were constructed for Shiyang section which dates the Mingshan Formation to ~65-42 Ma. Mass-specific low field magnetic susceptibility (χ), anhysteretic remanent magnetization (ARM) and isothermal remanent magnetization (IRM) sequences of 761 bulk samples were measured at the Paleomagnetism Laboratory of the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. Thus, χarm and S-ratio can be calculated. These environmental magnetism sequences display that there is a distinct peak in the stratigraphic interval 340-305 m. Eight samples were selected for detailed rock magnetism including temperature-dependent susceptibility (κ-T) curves and hysteresis loops. Rock magnetism results display that samples from the 340-305 m interval are dominated magnetically by magnetite and maghemite particles. However, magnetic minerals are dominated by hematite particles for samples outside of the 340-305 m interval. Inorganic carbon isotope results of the selected 13 samples display that there exist some distinct negative shifts of carbon isotope values in the 340-305 m interval. The maximum negative shift can reach-10‰. Combining results of geochronology, environmental magnetism and carbon isotope of Cenozoic deposits from the Shiyang section, we can define that the PETM (Paleogene-Eocene Thermal Maximum) palaeoclimatic event was recorded in the 340-305 m interval at Shiyang section. Discovery of the PETM in the Sichuan Basin enriches the PETM records in the world's terrestrial sediments and provides a new view to reveal its processes and mechanism. The various curves of high-resolution environmental magnetism in the 340-305 m interval are comparable to results of the reported carbon isotope from other terrestrial and marine sediments, such as the Nanyang Basin in China, the Bighorn Basin in USA, and the ODP 690B core in Antarctica Ocean. This indicates that environmental magnetism sequences in this study can be used as reliable palaeoclimatic proxies to unveil the PETM. After learning more geoscience records and the paleoenvironment conditions they reveal in different areas in the world during the PETM, we suppose that the peak values of the environmental magnetism sequences in this study were attributed to increased palaeoprecipitation during the PETM period. Firstly, increased precipitation led to chemical weathering enhancement and facilitated the decomposition of silicate minerals and separation of Fe2+ which could quickly transfer into ferromagnetic minerals. Secondly, increased precipitation also contributed to enhanced hydrodynamic force of rivers. This accelerated the depositional processes of the clastic materials and shortened the oxidization time of magnetic debris. It may be another reason for the increased content of ferromagnetic minerals in the stratigraphic interval of PETM. Therefore, changes in environmental magnetic curves could be linked with the detailed process of the PETM.