Spatial and seasonal variability in grain size, magnetic susceptibility, and organic elemental geochemistry of channel-bed sediments from the Mekong Delta, Vietnam: Implications for hydro-sedimentary dynamic processes

Abstract

Understanding the relationship between depositional processes and their products in the fluvial–marine transition zone in tide-dominated depositional systems is fundamental to improving environmental and stratigraphic interpretations of long-term sedimentary records. The distributary channels of the Mekong Delta represent a typical example of a complex sedimentary environment characterized by strong fluvial and tidal interactions, and seasonal variability in hydro-sedimentary dynamic processes, particularly the migration of a salt wedge front linked to seasonal fluctuations in freshwater discharge. For this study, we analyzed grain size, magnetic susceptibility, and organic elemental geochemistry of channel-bed surficial sediments collected in 2015 from all distributary channels during the dry season and from the Mekong–Co Chien River during the flood season. This paper reports spatial and seasonal differences in the characteristics of channel-bed surficial sediments, which are closely linked to sediment sources and depositional processes during the dry and flood seasons.Distributary channels are characterized by coarse-grained sediments in the upper reaches and fine-grained sediments in the lower reaches, in both dry and flood seasons, reflecting the difference between the upstream fluvial-dominated environment and the downstream tide-dominated environment. Flood season samples from the Co Chien River show higher magnetic mineral contents and a terrestrial source of organic carbon compared with dry season samples, indicating sourcing from the drainage basin and trapping of suspended materials in the distributary channel due to the barrier effect of the saltwater wedge at the river mouth. Compared with the flood season, during the dry season the mud content was lower in the downstream reach close to the river mouth and higher in the upstream reach of the Co Chien River, and the content of magnetic minerals was lower in both reaches, indicating dissolution of magnetic minerals during early diagenesis. Furthermore, the lower C/N ratios (<10) and slightly enriched δ13C values of the dry season samples from the Co Chien River imply an additional contribution of organic carbon from a marine source. These various features imply a landward import of mud into the Co Chien River induced by stronger salinity intrusion during the dry season. Along the downstream tract of the Bassac River, two mud-dominated reaches were identified during the dry season. The magnetic mineral contents, C/N ratios and δ13C values indicate that the mud-dominated reach near the Bassac River mouth is also supplied from resuspended sediments by estuarine circulation and tidal pumping, whereas the mud in the reach that is transitional from fluvial to tide dominance has a riverine supply. The results suggest that the Mekong distributary channels are important sinks for suspended sediments during both the flood and dry seasons, controlled by a sediment trapping mechanism provided by either estuarine processes or the morphological configuration of the channels, both of which are controlled by fluvial–tidal dynamics. These depositional processes may be compensating for the incision of channels induced by sand mining in recent decades.