Onset of large-scale terrestrial organic carbon burial driven by Early Devonian changes in vascular plants and environments

Abstract

The Early Devonian witnessed a major phase in the terrestrialization of land by plants. Understanding the implications for organic matter formation and accumulation during this interval is key to understanding global carbon burial. Existing research on the Early Devonian primarily focuses on marine carbonate records that do not permit evaluation of the mechanisms of terrestrial organic carbon burial, particularly in the early stages of plant colonization. Here, we examine facies associations, environmental evolution, and organic carbon burial within the late Pragian aged Posongchong Formation at the Zhichang section in Wenshan, Yunnan Province, China, a formation previously noted for recording the early radiation of land plants in coastal settings. Sedimentological and geochemical data include: (1) Paleosalinity proxies (B* and B/Ga) and sedimentary structures that indicate fluviolacustrine Facies Association (FA A), tidal flat (FA B), and shallow marine (FA C) facies developed in the study area, with two pulses of deepening separated by shallowing; (2) Total Organic Carbon (TOC) and kerogen maceral analyses that reveal an increase in terrestrial organic carbon content derived from terrestrial higher plants and phytoplankton; and (3) Organic matter preserved in coaly and carbonaceous shales that is mainly derived from continental higher plants that grew in a restricted, stable freshwater-brackish lagoon and floodplain environment. The land plant radiation recorded by the Posongchong Formation occurred within a stable coastal plain that provided a suitable setting for the supply and preservation of organic carbon in this early terrestrial ecosystem, in turn influencing the development of global terrestrial carbon burial.