Magnetic properties of sediments within the water-level fluctuation zone of the Three Gorges Reservoir and their response to provenance and hydrodynamic conditions

Purpose

Iron-bearing magnetic minerals in the sediments of the water-level fluctuation zone (WLFZ) in large reservoirs play critical roles in the burial and release of organic carbon and pollutants. This study aims to reveal the spatial distribution of magnetic minerals across different elevations and reaches within the WLFZ of the Three Gorges Reservoir (TGR), the largest reservoir in the world, and their links to sediment provenance and hydrodynamic conditions. This study will provide a fundamental basis for future investigations in the roles of magnetic minerals in the circulation processes of pollutants and organic carbon within the WLFZ of fluvial-reservoir systems.

Materials and methods

~ 300 sediment samples and nearby side-slope soils were collected from various altitudes within the WLFZ at ten different sites of the TGR. The magnetic properties of these sediments and soils were examined using environmental magnetism. Additionally, previously reported data on magnetic properties of suspended sediments from upstream rivers flowing into the TGR were assembled. Nearby side-slope soils and suspended samples were used to represent magnetic properties of potential sources for the sediments within the WLFZ.

Results and discussion

Our findings reveal that magnetite/maghemite and hematite particles dominate the sediments within the WLFZ of the TGR. Relatively higher concentration of hematite particles in the upper part (> 165 m) of WLFZ mainly originates from or are nearby side-slope soils, whereas higher concentration of magnetite/maghemite particles in the lower part of WLFZ is primarily due to sediment supply from upstream of the Yangtze River. The dominance of coarser magnetite/maghemite within the WLFZ of the upper reach of the TGR, from Jiangjin district to Fuling district, is due to strong water flow velocity. An increase in the concentration of nanosized fine-grained magnetic particles within the WLFZ of the middle and lower reaches of the TGR can be attributed to a combination of reduced water dynamics and intense regional soil erosion of nearby side slopes.

Conclusions

Our findings indicate that the concentration and grain size of magnetic minerals exhibit spatial variations within the WLFZ of reservoirs under the influences of sediment provenance and hydrodynamics. These findings provide fundamental insights on future exploration into the crucial roles of iron-bearing magnetic minerals in the accumulation, migration, and transformation of pollutants and organic carbon within the WLFZ of reservoirs and their response to changes in sediment sources and water dynamics.