Net primary productivity of paleo-peatlands linked to deep-time glacial periods in the late Carboniferous and early Permian icehouse interval

Abstract

Peatlands, an important organic carbon reservoir, play a crucial role in the global carbon cycle. The carbon accumulation of peatlands, reflected by net primary productivity (NPP), can have an impact on global carbon cycling and climate change. The late Carboniferous - early Permian is an icehouse period, during which numerous thick coal beds were accumulated in the North China Block (NCB) located within a low-latitude area, providing an opportunity for studying the carbon cycling under the glacial and interglacial climates. In this study, spectral analysis was performed on the natural gamma-ray (GR) logs of the Benxi, Taiyuan, and Shanxi formations of the late Carboniferous to early Permian in a borehole section located within the Ordos Basin in western NCB. Cyclic signals related to astronomical orbital parameters were identified, including long eccentricity (∼405 kyr), short eccentricity (∼125 kyr and ∼ 95 kyr), and obliquity (∼35.5 kyr). A floating astronomical time scale was established by using the long eccentricity signal, and this time scale was further used to constrain the durations of the accumulation of coal-forming paleo-peatlands. The paleo-peatland for the C₈₊₉ coal seam (9 m thick) of the Taiyuan Formation lasted approximately 203 kyr, and the paleo-peatland for the C5 coal seam (4 m thick) of the Shanxi Formation lasted approximately 46 kyr. Using this timeframe and an estimation of carbon loss during coalification, the carbon accumulation rates of the late Carboniferous - early Permian low-latitude peatlands are calculated to be 104.7 ± 14.9 g·C·m⁻²·a⁻¹for the C₈₊₉ coal seam and 192.6 ± 11.6 g·C·m⁻²·a⁻¹for the C₅ coal seam. The NPP of the paleo-peatlands, which deducts a part of the carbon loss caused by the loss of CO₂ and CH₄, can be calculated from the carbon accumulation rates. The calculated average NPP of the paleo-peatlands for the C₈₊₉ seam was 199 ± 28 g·C·m⁻²·a⁻¹, and that of the C₅ seam was 366 ± 22 g·C·m⁻²·a⁻¹. In combination with the absolute time scale calibrated by high-precision UPb dates from Palougou section in western NCB, the depositional time of the investigated strata was constrained to be from 300.1 ± 0.5 Ma to 294.3 ± 0.5 Ma. The coal seams of the late Carboniferous to early Permian in the NCB correspond to an interglacial interval around ∼298 Ma. The peatland with a lower NPP corresponds to the warming stage and the peatland with a higher NPP corresponds to the cooling stage. This implies that a lower NPP of paleo-peatland tends to be less efficient in carbon storage, and could not reduce the atmospheric CO₂ substantially. In contrast, a higher NPP of paleo-peatland tends to accelerate carbon fixation, leading to temperature decrease and the termination of interglacial interval in early Permian. The results of this study could provide insights into the relationship between the development of paleo-peatlands and the record of the paleoclimates during the same period, and could be helpful for the prediction of future climate change.