Pliensbachian (Early Jurassic) deep-time peatland evolution in Northwest China driven by climate change

Abstract

As peat deposits represent significant terrestrial carbon sinks, the processes of peatland initiation, evolution and termination are important components of the global carbon cycle. Research on recent peatlands leaves many questions unanswered concerning peatland evolution and the driving mechanisms for changes in peat forming environments in deep-time. Using a combination of sedimentology, coal petrology and palynology, this study investigates coal seams #B, #C and #D from the Pliensbachian (Early Jurassic) Qaidam Basin in China to elucidate the evolution of mire types and vegetation in their precursor peats, and to evaluate paleoenvironmental conditions during peat formation. Based on ash yield, total sulfur contents and coal maceral proxies, coals #B and #C record rheotrophic swamp forest mires that gradually transitioned to ombrotrophic bog forest mire conditions. Coal #D has a more complex evolution, initially forming under the ombrotrophic bog forest mire conditions that characterized coal #C before transitioning to a rheotrophic mire with initially fen and then wet swamp forest conditions. Finally, coal #D records a reversion to ombrotrophic mire conditions dominated by bog forest. Palynological assemblages and the ratios of hygrophytic (H) to xerophytic (X) plants reveal a remarkable change in peat-forming vegetation and paleoclimate. Coals #B and #C are dominated by woody gymnosperms, while deposition of the precursor peats of coal #D gradually evolved into a mix of woody and herbaceous plants. This floristic transition coincided with intensified climatic oscillations, establishing cyclic dry-wet conditions during the later stages of coal #D deposition. Our results reveal that during the Pliensbachian in the Qaidam Basin, climate forcing on peatland environments was manifested primarily through changes in precipitation and water availability brought on by intensification of seasonality. Peatland evolution was a multi-phase process of changing mire types and an overall vegetation succession from woody to herbaceous and woody plants.