Depositional environments and historical contamination as a framework to reconstruct fluvial sedimentary evolution

Area of Science:

• Environmental Science
• Geology
• River Ecology

Background:

• Sediment cores are vital archives of hydrosystem activities but complex to interpret in fluvial settings due to variable sedimentation.
• Understanding depositional environments is crucial for accurate interpretation of sediment core records.
• Fluvial systems present unique challenges for sediment core analysis due to factors affecting record quality and continuity.

Purpose of the Study:

• To investigate sedimentation condition variability across diverse fluvial depositional environments.
• To assess the suitability of geophysical methods combined with sediment cores for fluvial corridor analysis.
• To develop and validate a multi-proxy dating methodology for recent fluvial sediments.

Main Methods:

• Comparative analysis of four depositional environments (floodplain, secondary channels, reservoir) in the Rhône River.
• Sediment core retrieval and characterization (grain-size, Total Organic Carbon, organic contaminants).
• Application of geophysical methods (Ground Penetrating Radar) and robust age-depth modeling using radiometric dating (¹³⁷Cs, ²¹⁰Pb_ex) and Persistent Organic Pollutants (POPs).

Main Results:

• Each depositional environment yielded distinct sedimentation records, including varying contamination levels and trends.
• A significant shift in predominant Persistent Organic Pollutants (POPs) from polychlorinated biphenyls (PCBs) to polybrominated diphenyl ethers (PBDEs) was observed around the 1970s.
• Grain-size analysis identified two types of legacy sediments linked to historical river engineering interventions.

Conclusions:

• The combination of geophysical surveys and sediment core analysis is a powerful approach for reconstructing fluvial corridor evolution.
• Interpreting fluvial sediment cores requires a thorough understanding of the specific depositional environment.
• A multi-proxy dating approach, incorporating POPs trends, is essential for overcoming challenges in dating recent fluvial sediments.