Palaeoenvironment and palaeoclimate during the late Carboniferous–early Permian in northern China from carbon and nitrogen isotopes of coals

Abstract

Coupling carbon and nitrogen isotopes and petrography of coals and related intra-seam carbonaceous mudstone partings from basins in northern China provides insight into regional palaeoenvironments and palaeoclimate during the late Carboniferous–early Permian. The carbon isotopic composition (δ13Ccoal, VPDB) of coal samples from the Taiyuan and Shanxi formations of Qinshui and North China-Bohaiwan basins ranges from −25.3‰ to −22.7‰, with an average of −23.7‰. The average δ13Ccoal value is −23.6‰ in the late Carboniferous, −23.4‰ in the early Permian and −23.5‰ in the mid–early Permian. By contrast, equivalent early Permian coals in the southern North China-Bohaiwan Basin to the east were found to be significantly more negative at −25.2‰, likely as a function of regional aridity changes. Related δ15N in coal seams ranges from +2.3‰ to +4.7‰, with an average of +3.7‰. Within the thick, economically important #15coal seam of the Qinshui Basin, δ15Ncoal is significantly more negative than δ15N in mudstone partings (avg. +7.1‰), implying that the coals underwent a stronger degree of microbial degradation during peat formation. δ15N of mudstone partings varies stratigraphically, with significantly more positive δ15N (+8.1‰, +8.3‰) for the lower partings and lower δ15N (+4.8‰) for the upper parting. This may reflect varying degrees of microbial activity, but could reflect higher thermal maturity in the upper part of the seam as indicated in the coal rank profile.δ13Ccoal in this region was used to calculate the carbon isotopic composition of atmospheric CO2(δ13Ca). Calculated δ13Ca ranges from −6.0‰ to −3.4‰, with an average of −4.5‰, which is more positive than the δ13Ca of modern atmospheric CO2(−8.5‰, Graven et al., 2017). Two δ13Ca excursions are noted: a positive one (magnitude 3.7‰) in the late Carboniferous and a tentative negative excursion (magnitude 6.0‰) in the mid–early Permian. The positive shift coincided with a sea-level transgression in the Qinshui Basin. The rise in sea level may have led to the burial of more terrestrial plant debris that decreased photosynthesis and enriched atmospheric CO2in 13C. More data are needed to understand mid-early Permian variation in δ13C, as this could reflect a regional aridity or humidity effect rather than a global signal.