Magnetic properties of sediments of the Red River: Effect of sorting on the source‐to‐sink pathway and its implications for environmental reconstruction

Abstract

We conducted a mineral magnetic study of river bank and subaqueous delta sediments from the Red River, in order to examine the role of sedimentary sorting on the variation of sedimentary magnetic properties from source to sink. The magnetic mineralogy mainly consists of magnetite and hematite. Bulk sediment particle‐size variations have a strong influence on magnetic properties, with the frequently used magnetic parameters χfd%, χARM, χARM/χ, and χARM/SIRM exhibiting positive correlations with the <4 µm fraction, while S‐ratios are negatively correlated with this fraction. Compared with river bank sediments and shallow shoreface (<5 m water depth) sediments, sediments from the deeper (>5 m water depth) part of the subaqueous delta have lower χ and SIRM values, a finer ferrimagnetic grain‐size and higher proportions of hematite, consistent with selective loss of coarse ferrimagnetic grains on the source‐to‐sink pathway. We suggest that variations in magnetic properties in response to particle‐size compositions and therefore depositional environment changes should be carefully addressed when magnetic proxies such as χARM/SIRM are used in the study of coastal and marine environmental changes (e.g., sea‐level change). In such cases, the combined use of magnetic properties and geochemical indicators, such as Al/Ti ratio, may provide better results for paleoenvironmental reconstruction.

Magnetic minerals in three Asian rivers draining into the South China Sea: Pearl, Red, and Mekong Rivers

“…Analyses of different proxies including magnetic ones have been conducted off the river deltas, in the SCS. Recently, magnetic grain size of samples from the subaqueous delta of the Red River has been compared to grain size of river samples [ Nguyen et al ., ]. Only one exhaustive study combining major and trace elements, clay mineralogy and assemblages, and Nd, Sr isotopes has been so far conducted in sediments deposited along the streams and at the river mouths of the three rivers [ Liu et al ., ].…”

Fingerprinting Sediment Transport in River‐Dominated Margins Using Combined Mineral Magnetic and Radionuclide Methods

“…Among the magnetic parameters, the χ ARM /SIRM ratio has been found to increase with increasing distance from the source on the transport pathway (e.g., Dong et al, ; Nguyen et al, ). This trend reflects the fact that ferrimagnetic minerals are sensitive to hydrodynamic sorting, as fine‐grained particles can be transported farther away than coarse‐grained particles.…”

“…Therefore, the use of radionuclides as tracers of sediment transport requires a detailed examination of the mechanism (e.g., time scales of processes and constraining boundary conditions) and its suitability, although this method has been utilized to determine timescales of modern sedimentation in estuarine and coastal settings based on certain assumptions, such as radionuclides, is irreversibly absorbed onto particles and transported with particles without any desorption or fractionation (Feng et al, ; Huang et al, ; McKee et al, ). Magnetic minerals (mainly iron oxides such as magnetite) are ubiquitous in sediments and are sensitive to their sources of origin and variations in sorting due to particle size, density, and shape along the transport pathway (e.g., Li et al, ; Nguyen et al, ). Therefore, magnetic minerals have been widely used to trace sediment transport processes in different environmental settings (Evans & Heller, ; Hatfield et al, ; Kim et al, ; Thompson & Oldfield, ).…”

“…To the best of our knowledge, no studies have yet focused on the combined use of magnetic properties and radionuclides as tracers of sediment transport, although each of these methods has been utilized individually to analyze sediment transport around the world (e.g., Du et al, 2010; Ge et al, ; Horng & Huh, ; Nguyen et al, ; Sommerfield et al, ; Wang et al, ; Zhang et al, ). In this study, we have chosen the East China Sea (ECS), which is a typical river‐dominated marginal sea of the Pacific Ocean (Liu et al, ; Mckee et al, ), as a case study area to demonstrate the advantages of the combined use of radionuclides and magnetic methods.…”

Tracing Sediment Erosion in the Yangtze River Subaqueous Delta Using Magnetic Methods

“…Sediments in Groups I, II, and III have markedly different magnetic mineral concentrations and mineralogical compositions, which may be the result of particle size variations in response to hydrodynamic variations and sorting (Dong et al, ; Nguyen et al, ). The bimodal particle size distribution of Group III sediments is consistent with the fact that this group is located in the transitional area between fine‐grained prodelta deposits and coarse relict sands on the continental shelf (Luo et al, ).…”