Vegetal Undercurrents—Obscured Riverine Dynamics of Plant Debris.

Much attention has been focused on fine‐grained sediments carried as suspended load in rivers due to their potential to transport, disperse, and preserve organic carbon (OC), while the transfer and fate of OC associated with coarser‐grained sediments in fluvial systems have been less extensively studied. Here, sedimentological, geochemical, and biomolecular characteristics of sediments from river depth profiles reveal distinct hydrodynamic behavior for different pools of OC within the Mackenzie River system. Higher radiocarbon (14C) contents, low N/OC ratios, and elevated plant‐derived biomarker loadings suggest a systematic transport of submerged vascular plant debris above the active riverbed in large channels both upstream of and within the delta. Subzero temperatures hinder OC degradation promoting the accumulation and waterlogging of plant detritus within the watershed. Once entrained into a channel, sustained flow strength and buoyancy prevent plant debris from settling and keep it suspended in the water column above the riverbed. Helical flow motions within meandering river segments concentrate lithogenic and organic debris near the inner river bends forming a sediment‐laden plume. Moving offshore, we observe a lack of discrete, particulate OC in continental shelf sediments, suggesting preferential trapping of coarse debris within deltaic and neritic environments. The delivery of waterlogged plant detritus transport and high sediment loads during the spring flood may reduce oxygen exposure times and microbial decomposition, leading to enhanced sequestration of biospheric OC. Undercurrents enriched in coarse, relatively fresh plant fragments appear to be reoccurring features, highlighting a poorly understood yet significant mechanism operating within the terrestrial carbon cycle. Plain Language Summary: Coarse sediments are generally neglected as pathways and sinks of terrestrial organic carbon (OC) due to their presumed low OC contents and preservation potential. However, sedimentological and geochemical properties of sediments collected from the Mackenzie River reveal the co‐occurrence of coarse‐grained lithogenic particles with substantial quantities of pieces of plant biomass close to the riverbed. We argue that cold conditions in Arctic regions limit organic matter decomposition and favor the accumulation and water saturation of plant debris. These plant fragments will initially sink but are carried along the riverbed due to their shape, which is more elongated and planar than other particles. As rivers meander and navigate bends, water flow can divert coarser particles toward the inner bend, creating sediment‐laden undercurrents. We find that offshore in the Arctic Ocean, discrete plant detritus is apparently less common and we suggest that these particles are trapped within the Mackenzie Delta. The large amount of sediment exported during the spring flood promotes rapid burial of plant debris, likely preventing the decomposition of OC and the release of CO2 to the atmosphere. Our observations reveal a previously hidden mechanism that transports OC‐rich detritus within river systems that needs to be accounted for. [ABSTRACT FROM AUTHOR]