Organic geochemical evidence of past changes in hydro- and sediment-dynamic processes at river mouths: A case study of Holocene sedimentary records in the Changjiang River delta, China

Estuarine fronts are the boundaries between different water masses at river mouths and play important roles in trapping suspended particles. Investigating the response of estuarine fronts to climate and sea-level changes during the Holocene could elucidate the evolution of complex hydro- and sediment-dynamic processes at river mouths. To assess the estuarine frontal dynamics of the Changjiang (Yangtze) River mouth, we examined organic geochemical proxies in surficial sediments to investigate the spatial distribution of terrestrial- and marine-sourced organic carbon in recent decades. To explore the Holocene migration of estuarine fronts, we analysed organic carbon compositions in seven age-constrained sediment cores collected across the onshore and offshore delta. The suspended sediment front was identified as a major boundary for trapping terrestrial organic carbon, and the landward limit of the salt wedge was found to constrain the landward spread of marine-sourced organic carbon. The temporal and spatial distribution of organic carbon compositions in the sediment cores indicated a rapid landward retreat of the estuarine fronts between ~10.0 and 6.0 ka (cal. kyr BP) in response to sea-level rise; this coincided with the retreat of the entire river mouth system. Despite significant deltaic progradation, the estuarine fronts slowly migrated seaward after ~6.0 ka in response to a weakened summer monsoon and a resultant decline in freshwater discharge or the lateral spreading of the delta. Accelerated deltaic progradation induced by both intensified human impacts and infilling of the palaeo-incised Changjiang valley resulted in the accelerated seaward migration of the estuarine fronts over the past ~1500 years. This interdisciplinary study strengthens our understanding of the links between hydro-sediment dynamic processes and sedimentary records to further elucidate the complex systemic behaviour at tide-dominated river mouths.