Palaeoclimate reconstruction from biomarker geochemistry and stable isotopes of n-alkanes from Carboniferous and Early Permian humic coals and limnic sediments in western and eastern Europe

摘要

The type of organic matter (OM) in European Carboniferous and Permian swamp and lake sediments from the Carboniferous and Permian was determined using organic petrography, Rock-Eval data and biomarker distributions. Coals deposited in swamps contain humic OM formed under oxic conditions. Boghead coals and black shales deposited in lakes contain a mixture of algal and humic OM formed under reducing conditions. Diterpanes and previous palaeobotanic studies constrain the species of plants living near the lacustrine shore or in the swamp during deposition, allowing the palaeoclimate to be inferred. During the Carboniferous, the climate was not always tropical wet, as some periods of dryness are evident from the sedimentology, palaeobotany and organic geochemistry. During the Permian, the climate was not always tropical dry as some periods of wetness associated with the monsoons are recorded (Roscher, M., Schneider, J.W., 2006. Permocarboniferous climate: Early Pennsylvanian to Late Permian climate development of central Europe in a regional and global context. In: Lucas, S.G., Cassini, G., Schneider, J.W. (Eds.), Non-Marine Permian Chronology and Correlation, vol. 265. The Geological Society of London, pp. 95-136). The appearance of xerophyte plants from the Stephanian was also recorded by way of aromatic hydrocarbons, retene for gymnosperms and arborane/fernane for cordaites and probably seed ferns. Cycles of wetness and dryness for Europe during the Carboniferous and Permian are proposed on the basis of comparison of aliphatic and aromatic hydrocarbons.δD values provided information on the palaeotemperature of the air in the swamps and water in lakes, as well as palaeoclimate. Values of ca. -100‰ seen in the Carboniferous coals and Permian limnic deposits are indicative of a tropical climate, in contrast to a glacial or temperate climate, where the values are ca. -200‰. The value for Carboniferous coals could result from high evapotranspiration of plants living under a wet tropical climate vs. a temperate climate. During the Early Permian the climate was dry tropical. For such samples, the δD values for the n-alkanes derived from lacustrine algae were depleted in D relative to the values for the n-alkanes derived from terrestrial higher plants, attributed to the higher evapotranspiration on land than evaporation from the lake. Alternatively, the xerophytic plants that drifted into the lake via a river could have grown during a dry phase and the autochthonous algae bloomed during a wet phase.