Net primary productivity and its control of the Middle Jurassic peatlands: An example from the southern Junggar coalfield

The Jurassic is an important period of global coal formation, including the development of several large coalfields in central Asia and northern China. Individual seams within these peatlands represent sustained periods of terrestrial carbon accumulation and a key environmental indicator attributed to this record is the rate of carbon accumulation. Determining the rate of carbon accumulation requires a measure of time contained within the coal and this study aimed at determining the rate via the identification of Milankovitch orbital cycles using spectral analysis. Spectral analyses of geophysical data from two thick coal seams, No. 43 (35.9 m) and No. 3 (13.2 m), of the Middle Jurassic of the southern Junggar coalfield were conducted to identify significant signals of variations in ash content. The results showed that the variations in ash content of the coal showed spatial cycles at 0.2, 0.7 and 1.1 m^?1, which were interpreted to represent 123 ka (eccentricity), 37.1 ka (obliquity), and 21.2 ka (precession) orbital periodicities, respectively. Using this timeframe, the depositional time of the No. 43 and No. 3 coal seams were calculated to be 876–970 and 322–357 ka, respectively. In combination with an understanding of carbon loss during coalification, the carbon accumulation rates of these Middle Jurassic peatlands were calculated to be 58.6–64.9 and 60.3–66.8 g C m^?2 a^?1 for the No. 43 and No. 3 coal seams, respectively. Given that the net primary productivity (NPP) was 4.3 times the value of the carbon accumulation in a mid-latitude region of 40°–45°N, an NPP of 251.8–279.1 and 259.1–287.1 g C m^?2 a^?1 was calculated for the No. 43 and No. 3 coal seams, respectively. In the context of the same paleolatitude (40°–45°N) and peat type, the NPP values of the Middle Jurassic strata in the study area were higher than those of the peatlands of the Holocene and Permian, and were similar to the NPP values of Early Cretaceous peatlands. Considering the NPP of a peatland is predominantly controlled by atmospheric CO₂ and O₂ levels and temperature, the lower content of CO₂ and an excessive O₂ level in the temporal atmosphere would lead to a decrease in peatland NPP. Therefore, it is inferred that the CO₂ level during the Middle Jurassic was higher than that of the icehouse Permian and Holocene periods, and it was similar to the CO₂ level of the greenhouse Cretaceous period. The results are consistent with the global CO₂ variation curve of Berner. In conclusion, Milankovitch orbital cycles calculated from geophysical logs can be used to infer the NPP of temporal peatlands during different geological periods, based on which the deep-time paleoclimates can be analyzed.