29 / 2025-03-27 09:53:56

Influence of plant evolution on the coal formation during the Devonian-Carboniferous period

Devonian-Carboniferous,Plant evolution,coal accumulation,terrestrial carbon burial,global carbon cycle,land surface environmental changes

Theme 3: Earth-Life Co-evolution Through Time > Session 3c: Life-Mineral Interactions during the Plant Terrestrialization

The emergence of land plants was one of the most important biological evolutionary events of the geohistoric period and was the beginning of the evolution of terrestrial habitability. The evolution of land plants from the Devonian to the Carboniferous can be divided into five stages: the clade explosion (early Devonian Lochkovian-Emsian stages/Phase I), the first appearance of forests (middle Devonian Eifelian-late Devonian Frasnian stages/Phase II), the first appearance of pteridosperms (seed ferns) (late Devonian Famennian-early Carboniferous Tournaisian stages/Phase III), the development of the pre-Cathaysian flora (early Carboniferous Visean-late Carboniferous Bashkirian stages/Phase IV), and the development of the Cathaysian flora (Moscovian-Gzhelian stages/Phase V). Coal accumulation is the process by which plant remains accumulate in large quantities in a wetland environment under suitable paleobotanical, paleoclimatic, paleogeographical, and paleotectonic conditions, ultimately forming coal deposits. It is one of the major pathways for the burial of atmospheric CO2 on land, and is an essential part of the global carbon cycle. Currently, the direct and potential impact of plant terrestrialization on coal accumulation remains to be further understood and explored. In this paper, we review the relationship between the evolutionary characteristics of coal-forming plants at different evolution phases and global coal accumulation in Devonian and Carboniferous. The results show that: in phase I, terrestrial plants were still dominated by early vascular plants (e.g., lycopods and trimerophytes), but their body size increased, and they evolved organs such as roots and primitive small leaves, with an increase in the content of vascular tissues. Coal-bearing sediments formed during phases I are scattered sporadically along equatorial tropical coasts and in shallow marine environments, and consist primarily of saprobic coal formed by low-grade algae. In phase II, terrestrial plants (e.g., trimerophytes, lycopods, and archeaopteris) began to evolve tree-like habits, with the emergence of secondary xylem and concentrated root systems, leading to the formation of the earliest forests on Earth. In the tropical coastal plains near the equator, sporadically mineable lignitic coal dominated by lipid group (~80%) began to form from higher plants. From phase III to V, plant structure generally evolved towards larger individuals, deeper root systems, and more lignified tissues, with habitat gradually expanding to higher and drier areas. In phase III, terrestrial plants mainly included lycopods, arthrophyta, progymnosperms, and pteridosperms, of which the emergence of deep root systems (e.g., arthrophyta) and seed habits (e.g., pteridosperms) marked the beginning of plants occupying more arid environments (e.g., drylands). In phase IV, lycopods, arthrophyta, and pteridosperms had further developed, and tree-like habits became prevalent across various groups. In phase V, in addition to arborescent lycopods, arthrophyta (sphenopsids), filicinae, and pteridosperms, cordaitopsids emerged and became the dominant coal-forming gymnosperm. Similar to phase II, phase III to V also primarily concentrated humic coal dominated by vitrinite and inertinite in low-latitude equatorial regions. However, the distribution range and thickness of coal seams gradually increased, and their minability evolved from sporadic to local to regional. The abundant observational data presented above shows that the establishment and flourishing of vegetation on land not only directly participated in coal accumulation, and in the formation of coal deposits of various distribution ranges and material compositions as coal-forming materials, but also indirectly and irreversibly promoted the development of surface environments conducive to coal accumulation, making land an equally important reservoir of carbon as the ocean. Specifically, 1) organic acids derived from plants facilitate the chemical weathering of parent rocks, generating plant nutrients (e.g., phosphorus, potassium) and fine-grained sedimentary materials (e.g., clay). The abundance of these nutrients enhances plant productivity and promotes the plant colonization on land, while the fine-grained sediments form clay aquitards within the basin, contributing to the formation of larger areas of coal-enriched swamp wetlands. 2) The increase in vegetation cover on land surfaces (from lowlands to highlands, and from wetlands to drylands) reduces and inhibits the rates of physical weathering and terrigenous clastic production in the source areas, as well as the intensity of surface runoff and the drainage efficiency within the basin. This, in turn, weakens the interference of inorganic terrigenous clastic material during the process of coal accumulation, facilitating the formation of high-quality coal resources such as low-ash coal. 3) The differentiation of roots, stems, and leaves, together with the elongation of the stem, enhances the efficiency of photosynthesis and primary productivity of the plant, thereby facilitating the development of thick coal seams.