Temporal variability in composition and fluxes of Yellow River particulate organic matter

Abstract

Abstract This study examines temporal variations of the abundance and carbon isotopic characteristics of particulate organic carbon (POC) and specific‐source compounds in the context of hydrological variability in the Yellow River. The content and bulk carbon isotopic characteristics ( 13 C and 14 C) of POC were relatively uniform over the hydrologic (seasonal) cycle. We attribute these temporally invariant geochemical characteristics to the dominant contribution of loess material to the suspended particulate matter (SPM). In contrast, molecular‐level signals revealed that hydrologic conditions exert a significant influence on the proportional contributions of petrogenic and especially fresh plant‐derived OC, while pre‐aged soil OC is mobilized via deeper erosion processes (e.g., gully erosion, mudslides) and is independent of hydrodynamics and surface runoff. A coupled biomarker‐isotope mixing model was applied to estimate the time‐varying supply of contemporary/modern biomass, pre‐aged soil, and fossil OC components to Chinese marginal seas from the Yellow River. We found that natural (e.g., precipitation) and human‐induced (e.g., water and sediment regulation) variations in hydrological regime strongly influence the flux with the magnitude of the corresponding annual fluxes of POC ranging between 0.343 ± 0.122 Mt yr −1 and 0.581 ± 0.213 Mt yr −1 , but less strongly infleunce proportions of the different OC constituents. Inter‐annual differences in pre‐aged soil and fossil OC fluxes imply that extreme climate events (e.g., floods) modulate the exhumation and export of old carbon to the ocean, but the OC homogeneity in the pre‐aged mineral soil‐dominated watersheds facilitates robust predictions in terms of OC transport dynamics in the past (sediment cores) and in the future.