Climate forcing of terrestrial carbon sink during the Middle Jurassic greenhouse climate: Chronostratigraphic analysis of the Yan’an Formation, Ordos Basin, North China

Abstract

Exploring the relationship between coal deposits as an important terrestrial carbon sink and orbital forcing of climate is critical for understanding the global carbon cycle and climate change. The Jurassic greenhouse period, characterized by extensive coal reserves widely distributed in the mid-latitude terrestrial basins, marks a significant coal-forming interval in Earth’s history. However, understanding of the processes that controlled the formation and distribution of coal at this time is inadequate. The Yan’an Formation of the Ordos Basin in north central China is among the largest and most extensively studied Jurassic coal reservoirs of the world. Here we establish a high-resolution age framework for the Yan’an Formation derived from integrated, high-precision U-Pb zircon geochronology using chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) on interstratified ash beds and cyclostratigraphy based on centimeter-scale magnetic susceptibility. Accordingly, the main coal-forming interval of the Yan’an Formation spanned ca. 174.0 Ma to <171.7 Ma, which coincided with the onset of the Middle Jurassic. The spectral analyses of the Yan’an Formation coal seams demonstrate a strong correlation to minima in the 405 k.y. orbital eccentricity cycles, suggesting a strong climate control on lake level fluctuations and clastic sediment input. Finally, we explore the cyclicity of a large set of published marine carbon isotope data from western Tethys and its phase relationship to cyclic coal deposition in the Ordos Basin. Our resutls underscore the role of terrestrial organic carbon burial in the global carbon cycle during the Middle Jurassic.