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

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 (delta C-13(coal), VPDB) of coal samples from the Taiyuan and Shanxi formations of Qinshui and North China-Bohaiwan basins ranges from -25.3 parts per thousand to -22.7 parts per thousand, with an average of -23.7 parts per thousand. The average delta C-13(coal) value is -23.6 parts per thousand in the late Carboniferous, -23.4 parts per thousand in the early Permian and -23.5 parts per thousand 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 parts per thousand, likely as a function of regional aridity changes. Related delta N-15 in coal seams ranges from +2.3 parts per thousand to +4.7 parts per thousand, with an average of +3.7 parts per thousand. Within the thick, economically important #15coal seam of the Qinshui Basin, delta N-15(coal) is significantly more negative than delta N-15 in mudstone partings (avg. +7.1 parts per thousand), implying that the coals underwent a stronger degree of microbial degradation during peat formation. delta N-15 of mudstone partings varies stratigraphically, with significantly more positive delta N-15 (+8.1 parts per thousand, +8.3 parts per thousand) for the lower partings and lower delta N-15 (+4.8 parts per thousand) 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. delta C-13(coal) in this region was used to calculate the carbon isotopic composition of atmospheric CO2(delta C-13(a)). Calculated delta C-13(a) ranges from -6.0 parts per thousand to -3.4 parts per thousand, with an average of -4.5 parts per thousand, which is more positive than the delta C-13(a) of modern atmospheric CO2(-8.5 parts per thousand, Graven et al., 2017). Two delta C-13(a) excursions are noted: a positive one (magnitude 3.7 parts per thousand) in the late Carboniferous and a tentative negative excursion (magnitude 6.0 parts per thousand) 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 CO(2)in C-13. More data are needed to understand mid-early Permian variation in delta C-13, as this could reflect a regional aridity or humidity effect rather than a global signal.