Analysis on the source of organic matter in the wetlands of Diaokou and current river channels in the Yellow River Delta

This paper is the result of environmental geological survey engineering.

Objective Due to changes in the Yellow River channel, sedimentation has formed different lobes in the Yellow River Delta. Studying the source characteristics of organic carbon in different lobes in the Yellow River Delta will help reveal the impact of lobe oscillation on the sedimentary environment of the delta wetland.

Methods Different depth sediment samples were collected from Diaokou old channel wetland and the current river channel wetland in the Yellow River Delta. Samples were analyzed for total organic carbon, organic nitrogen and biomarkers, spatial distribution characteristics of organic matter in different lobes were compared, the significance of carbon source indication were analyzed through molecular parameters to study the reasons for the differences in the organic matter composition of the delta lobes.

Results The current channel wetland exhibits typical deltaic progradation characteristics. The concentrations of TOC in its surface sediments reached as high as (6.83±4.17)‰, with the terrestrial organic carbon contribution rate calculated based on n-alkanes being as high as (45.79±7.79)%. Towards the middle and deep layers, the concentrations of TOC sharply decreased to (1.45±0.51)‰, and the terrestrial contribution dropped to (32.34±3.10)%. The organic matter maturity was low, with an average OEP of 7.63, reflecting the vertical differentiation of organic carbon under continuous river input. By contrast, in the abandoned Diaokou old channel wetland, the concentrations of TOC across different layers were in range of (1.86±0.75)‰~(2.33±0.85)‰, and the n-alkane distribution tended to be uniform. The terrestrial contribution fluctuated between 36% and 42% without significant vertical variation, and the organic matter maturity was relatively high, reflecting the long-term transformation and mixing by marine tides and wave action after abandonment. It is worth noting that fatty acid analysis results indicate a dominant marine source contribution, however, since fatty acids are easily degradable, leading to the attenuation of terrestrial signals, the inference of dominant marine sources based on this is unreliable. In comparison, n-alkanes are more reliable for tracing terrestrial organic matter in deltas.

Conclusions Delta lobe oscillation significantly alters the composition and distribution of sedimentary organic carbon in wetlands. Active lobes form a "terrestrial-over-marine" stratified pattern through continuous fluvial supply, while abandoned lobes tend to be "mixed and homogeneous" due to long-term reworking by marine dynamics. The study emphasizes that using n-alkanes to trace organic matter sources is more reliable than fatty acids in deltaic regions. These findings provide critical scientific basis for accurate assessment and management of blue carbon resources in deltaic wetlands.