Abstract
Milankovitch periodicities of 123 kyr (eccentricity), 35.6 kyr (obliquity), and 21.2 kyr (precession) were identified in geophysical logs of three Late Permian coals: 17#, 18#, and 17 + 18#, from the Songhe mining area in western Guizhou Province. Based on the astronomic temporal framework, the periods of deposition of the 17# (5.6 m), 18# (6.4 m), and 17 + 18# (5.4 m) coals were constrained to 140.8–119.8 kyr, 160–136.2 kyr, and 135–114.9 kyr, respectively. The overall depositional period of the 18# coal of 160–136.2 kyr was further subdivided using the wavelet analysis method, into short and precise periods corresponding to the Milankovitch periodicities. It includes one eccentricity periodicity (123 kyr), three obliquity periodicities (35.6 kyr), and five precession periodicities (21.2 kyr). Different thicknesses of the subdivided coal sections, equivalent to the same time span of deposition, indicate different rates of coal deposition, i.e., thicker sections imply higher rates while the thinner sections represent lower rates. The combination of the measured average carbon concentration with the density of the coals gave rise to long-term average values of carbon accumulation rates for the Late Permian coals, in the range of 42.4–50.6 g⋅C⋅m− 2⋅a− 1. This range corresponds to the long-term average carbon accumulation rates for the initial peat in the range of 60.6–72.3 g⋅C⋅m− 2⋅a− 1. Based on the known quantitative relation between net primary productivity (NPP) values and long-term average carbon accumulation rates for the Holocene tropical peatlands, the range of NPP values for the Late Permian tropical peatlands was estimated as 242.4–433.8 g⋅C⋅m− 2⋅a− 1.A comparison of existing information about peatland NPP levels of various ages and latitudes indicated that when conditions of high rain and high humidity prevail in the palaeo-peatland at given latitude, the NPP rates will vary with changes in atmospheric concentration of CO2 and O2. This relationship may lead to the use of coals as an indicator for the concentration of these gases (CO2 and O2) in the contemporaneous atmosphere encompassing the long records of coal deposition.
Highlights
Peatlands are globally important terrestrial carbon pools and vital components of soil-atmosphere carbon exchange processes (Vitt 1994; Page et al 2011; Large and Marshall 2014)
It is reasonable to conclude that these frequencies in coal seams could represent orbital cycles, because the information related to the Milankovitch cycles has been identified
The depositional periods of 17# (5.6 m), 18# (6.4 m), and 17 + 18# (5.4 m) coal seams were calculated as 140.8–119.8 kyr, 160–136.2 kyr, and 135–114.9 kyr, respectively
Summary
Peatlands are globally important terrestrial carbon pools and vital components of soil-atmosphere carbon exchange processes (Vitt 1994; Page et al 2011; Large and Marshall 2014). The organic carbon productivity-decay balance plays a significant role in global carbon cycles, and is critical to climatic variation (Yu et al 2014). This indicates that the understanding of the long-term behavior of peatland carbon reservoirs is increasingly significant. Most peatland carbon is contained within subarctic, boreal, and postglacial peat deposits, which are mostly less than 10 kyr old. These features limit much of our understanding of the characteristics and evolution of high- to mid-latitude and pre-Quaternary peatlands (Kremenetski et al 2003; Yu 2011). Coal deposits can be used to investigate the evolution of older peatlands (Large and Marshall 2014)
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