Astronomical Cyclonic Constraints of Carboniferous-Permian Organic Matter Enrichment in North China

Key words: 
• astronomical theory /
• paleo-sedimentary environmental conditions /
• clostratigraphy /
• organic matter-rich sedimentary system /
• land-ocean transition phase shale

Abstract: Abstract: [Objective] Climate change caused by astronomical orbital parameters is closely related to biological turnover and sedimentary evolution of the ocean and lakes, which in turn affects the organic matter enrichment in the strata. The late Paleozoic coal-bearing rock system in North China is dominated by coal, mudstone, siltstone, sandstone and tuff observed in a number of depositional cycles in the vertical direction. It is also an important system for current oil and gas exploration and development. The aim of this study is to systematically reveal the influence of the astronomical orbital cycle on organic matter enrichment in this coal-bearing system. [Methods] Natural gamma logging sequences were obtained for four wells (sampling distance 0.05 m) and continuous mineral elements for one well (sampling distance 1 m) in western North China. Astronomical cyclotron time series data and elemental geochemical analysis of typical samples were used to clarify the pattern of organic matter enrichment in stratigraphic cyclotrons at different scales. [Results and Discussions] (1) Six 1.2 Myr ultra-long obliquity cycles and eighteen 405 kyr long eccentricity cycles are recognizable in the Carboniferous–Permian Benxi, Taiyuan and Shanxi Formations. (2) Using Mg/Ca, SiO2/Al2O3, Fe/Mn and V/(V+Ni) ratios as paleoclimate and paleoredox proxies, six long-period variations and 18 medium-period variations were identified, all corresponding with ultra-long obliquities and long eccentricities. (3) Comparative analyses of paleoenvironmental restoration and organic carbon content during the depositional period of the Benxi–Shanxi Formations show that paleoclimate evolution and organic matter enrichment are basically synchronized and controlled by the astronomical orbital cycle, and that during periods of increasing ultra-long obliquity or long eccentricity the climate is warm and humid and the reducing nature of bodies of water is enhanced, which promotes organic matter enrichment. [Conclusions] The above results reveal Carboniferous–Permian climate change in North China influenced by the astronomical orbital cycle and the development of a constrained organic matter-rich stratigraphy, and find that the paleoclimate change during the same period was obviously constrained by the long eccentricity. The findings can be used as a reference for the study of global climate evolution and organic matter enrichment mechanism in the Permian.