Riverine woodlands as a dynamic source of the marine sedimentary carbon sink

Abstract Land plants account for a significant proportion of the organic matter found in marine sediments. However, due to methodological limitations, the specific ecosystems and taxa contributing to this carbon sink have largely remained unknown. By leveraging ancient DNA data from marine sediment cores, this perspective paper identifies riverine woodlands as key sources of marine sedimentary organic matter. These highly productive ecosystems grow directly along the land-coastal-ocean carbon transport pathway that links the short-term biological with the long-term geological carbon cycle. Furthermore, they are characterized by taxa that are resistant to decay, such as willows in mid-to-high northern latitudes and mangroves in tropical regions. We hypothesize that at the end of the last glaciation, a major negative feedback loop was established, where warmer temperatures caused glacial melt and increased runoff, promoting riverine vegetation growth. This, in turn, enhanced carbon transport into marine sediments, reduced atmospheric CO2 levels, and ultimately contributed to a cooling effect. The efficiency of organic matter burial was further amplified by the increased mineral supply and high sedimentation rates resulting from the mobilization of widespread unconsolidated glacial sediments. Pollen records and other data suggest that human land use led to the widespread disturbance of riverine woodlands globally in the last few hundred years, likely disrupting this feedback loop under current warming conditions. A comprehensive understanding of the interactions between climate, land use, riverine woodland loss, and marine carbon sinks is urgently needed to guide conservation strategies that aim to enhance this natural carbon capture and storage mechanism.