Transport of Particulate Organic Carbon in The Huanghe: Insights from Lateral And Vertical Heterogeneity in a River Cross-section

The Huanghe (Yellow River), one of the largest turbid river systems in the world, has long been recognized as a major contributor of suspended particulate matter (SPM) to the ocean. However, over the last few decades, the SPM export flux of the Huanghe has decreased over 90% due to the high management, impacting the global export of particulate organic carbon (POC). To better constrain sources and modes of transport of POC beyond the previously investigated transportation of POC near the channel surface, SPM samples were for the first time collected over a whole channel cross-section in the lower Huanghe. Riverine SPM samples were analyzed for particle size and major element contents, as well as for POC content and dual carbon isotopes (13C and 14C). The results show clear vertical and lateral heterogeneity of SPM physical and chemical characteristics within the river cross section, with for example finer SPM carrying more POC with higher 14C activity near the surface and the right bank. Notably, we discuss how bank erosion in the alluvial plain is likely to generate lateral heterogeneity in POC composition. The Huanghe POC is millennial-aged (4,020 ± 500 radiocarbon years), dominated by organic carbon (OC) from the biosphere, while the lithospheric fraction reaches up to ca. 33%. The mobilization of aged and refractory OC from deeper soil horizons of the loess-paleosol sequence through erosion in the Chinese Loess Plateau is an important mechanism contributing to fluvial POC in the Huanghe drainage basin. The involvement of this OC fraction has significance for the regional and global carbon cycles, especially regarding its final fate in the estuary. Altogether, this study sheds light on the mechanism of fluvial transfer of POC and corresponding impacts on the carbon cycle in large river systems strongly perturbed by anthropogenic activities.