Identification of Milankovitch Cycles and Calculation of Net Primary Productivity of Paleo-peatlands using Geophysical Logs of Coal Seams

Individual coal seams formed in paleo-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.This study aimed to determine this rate via the identification of Milankovitch orbital cycles in the coals.The geophysical log is an ideal paleoclimate proxy and has been widely used in the study of sedimentary records using spectral analysis.Spectral analyses of geophysical log from thick coal seams can be used to identify the Milankovitch cycles and to calculate the period of the coal deposition.By considering the carbon loss during coalification,the long-term average carbon accumulation rate and net primary productivity(NPP)of paleo-peatlands in coal seams can be obtained.This review paper presents the procedures of analysis,assessment of results and interpretation of geophysical logs in determining the NPP of paleo-peatlands.

SHRIMP zircon U-Pb ages from coal beds across the Permian-Triassic boundary, eastern Yunnan, southwestern China

The first SHRIMP zircon U e Pb ages from coal beds close to the end-Permian mass extinction are reported from the Ccoal seam in the Yantang Mine in Laibin Town, Xuanwei County, eastern Yunnan Province.Zircons were extracted from kaolinite claystone layers, defined as tonsteins(volcanic ash deposits), in the subseam Band Bof the coal seam C.The U-Pb ages are 252.0 ± 2.3 Ma and 250.3 ± 2.1 Ma for the sub-seam Band B, respectively. Within analytical uncertainties, these U-Pb ages include the time period of the onset of the mass extinction at 251.941 ± 0.037 Ma, which was obtained from the marine Meishan section in Zhejiang Province, ~1600 km away from the Yantang Mine. These new ages represent not only the first and closest ages to the PTB mass extinction in terrestrial coal beds, but also ages from the nearest site to the Emeishan volcanoes investigated so far. Therefore these new data provide the most accurate stratigraphic horizon of terrestrial facies of the end-Permian extinction in South China. The Emeishan volcanoes were likely the source of volcanic ash in the coal seams at the Xuanwei County and broader areas in South China. Furthermore, the minerals and geochemistry characteristics of the Ccoal seam also implied the influences of contemporaneous volcanic activities.

Using geophysical logs to identify Milankovitch cycles and to calculate net primary productivity(NPP) of the Late Permian coals,western Guizhou,China

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 CO_2 and O_2. This relationship may lead to the use of coals as an indicator for the concentration of these gases(CO_2 and O_2) in the contemporaneous atmosphere encompassing the long records of coal deposition.

Lacustrine basin evolution and coal accumulation of the Middle Jurassic in the Saishiteng coalfield, northern Qaidam Basin, China

Based on an extensive borehole survey of the Middle Jurassic coat-bearing sequences in the Saishiteng coalfield, northern Qaidam Basin （NQB）, a total of 20 rock types and 5 sedimentary facies were identified, including braided river, meandering river, braided delta, meandering river delta, and lacustrine facies. The distribution of rock types and sedimentary facies contributed to the reconstruction of three periods＇ sedimentary facies maps of the Middle Jurassic in the Saishiteng coalfield, namely, the Dameigou age, the early Shimengou age and the late Shimengou age. That also provided the basis for the development of a three-stage depositional model of the Middle Jurassic in the NQB, indicating the lacustrine basin of the NQB in the Dameigou age and early Shimengou age were corresponding to an overfill basin, and that in the late Shimengou age was related to a balanced-fill basin. The analysis of the stability and structure of coat seams based on sedimentary facies maps showed that the preferred coal-forming facies in the Saishiteng coalfield were inter-delta bay and interdistributary bay of lower delta plain in the Dameigou age. In particular, the swamps that developed on the subaqueous pataeohigh favored the development of thick coat seams. Thus, ruinable coal seams may also be found along the Pingtai pataeohigh in the western part of the Saishiteng coalfield.