Fig. 5

Sketch showing procedures of the peatland productivity analysis. f1, f2, f3 represent the main average frequencies (cycles⋅m^− 1) of the spectral analysis. t1, t2, t3 represent Milankovitch periodicities of 123 kyr (eccentricity), 35.6 kyr (obliquity), and 21.2 kyr (precession). T1, T2, T3 represent different cycle length (m). V_coal represents rate of coal deposition (m⋅kyr ^− 1). L represents thickness of coal seam (m). T_coal represents period of coal deposition (kyr). ρ is the measured dry bulk density of coal (g⋅cm^− 3). X₁ is the measured average carbon concentration of the coal. X₂ is carbon loss percentage during the coalification. V_{C in coal} represents the long-term average carbon accumulation rate for the coal (g⋅C⋅m^− 2⋅a^− 1). V_{C in peatland} represents the long-term average carbon accumulation rate for the initial peatland (g⋅C⋅m^− 2⋅a^− 1). X₃ is the quantitative relation between the Holocene net primary productivity (NPP) level (g⋅C⋅m^− 2⋅a^− 1) and the long-term average carbon accumulation rate for the peatland