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

Abstract

The use of the marine sedimentary magnetic properties, as tracers for changes in precipitation rate and in oceanic water masses transport and exchanges, implies to identify and to characterize the different sources of the detrital fraction. This is of particular importance in closed and/or marginal seas such as the South China Sea. We report on the magnetic properties of sedimentary samples collected in three main Asian rivers draining into the South China Sea: the Pearl, Red, and Mekong Rivers. The geological formations as well as the present climatic conditions are different from one catchment to another. The entire set of performed magnetic analyses (low-field magnetic susceptibility, ARM acquisition and decay, IRM acquisition and decay, back-field acquisition, thermal demagnetization of three-axes IRM, hysteresis parameters, FORC diagrams, and low-temperature magnetic measurements) allow us to identify the magnetic mineralogy and the grain-size distribution when magnetite is dominant. Some degree of variability is observed in each basin, illustrating different parent rocks and degree of weathering. On average it appears that the Pearl River is rich in magnetite along the main stream while the Mekong River is rich in hematite. The Red River is a mixture of the two. Compared to clay mineral assemblages and major element contents previously determined on the same samples, these new findings indicate that the magnetic fraction brings complementary information of great interest for environmental reconstructions based on marine sediments from the South China Sea.

A kinetic model to explain the grain size and organic matter content dependence of magnetic susceptibility in transitional marine environments: A case study in Ria de Muros (NWIberia)

“…In natural geologic materials, magnetic susceptibility is dependent on the concentration, mineralogy, and grain size of the magnetomineralogical assemblage [Evans and Heller, 2003;Liu et al, 2012]. These factors are usually modulated by nonmagnetic characteristics of sediments, such as dilution by a paramagnetic or diamagnetic matrix [Rey et al, 2000;Font et al, 2010;Maher, 2011;Veerasingam et al, 2014] and mineralogical changes [e.g., Watkins and Maher, 2003;Larrasoaña et al, 2008;Kissel et al, 2016]. Magnetic minerals and trace metals also show some dependence, although unclear relationships exist in marine sediments [e.g., Chan et al, 2001;Rubio et al, 2001;Wehland et al, 2002;Hu et al, 2003;Davila et al, 2006;Prajith et al, 2015].…”

Influence of Sea Level Change and Centennial East Asian Monsoon Variations on Northern South China Sea Sediments Over the Past 36 kyr

“…These cores lie to the northeast of PC338 and are mostly supplied by the Pearl and Taiwan Rivers, with stable and relatively high magnetite contents compared to hematite (Kissel et al, ; Ouyang et al, ). For most surficial sediments, Red River S −300 values (0.7–0.9) are lower than those of the Pearl (0.85–1) and Taiwan Rivers (>0.9), which indicates relatively high hematite contents for the Red River (Kissel et al, ; Nguyen et al, ). Therefore, it seems that Red River‐sourced sediment lowered S –300 during the ∼32–15 kyr B.P.…”

“…The magnetic properties of SCS sediments are analyzed in combination with geochemical and grain size data that are sensitive to paleoclimate variations (e.g., Bloemendal et al, 1988;Boulay et al, 2003;Kars et al, 2017;Kissel et al, 2003;Wei et al, 2004). Our results are also compared with magnetic property data for sediments from adjacent rivers and estuaries (Kissel et al, 2016Nguyen et al, 2016;Ouyang et al, 2013Ouyang et al, , 2017.…”

Magnetic Fingerprints of Modern Sediments in the South China Sea Resulting From Source‐to‐Sink Processes

“…The main magnetic minerals observed in the river samples from all around the SCS are magnetite and hematite with some pyrrhotite contribution around Taiwan (Kissel et al, , ). At sea, pyrrhotite is mainly identified in core tops from the Taiwan Strait (Horng & Huh, ; Horng et al, ) and it probably deposits on the shelves together with illite and chlorite characterizing the Taiwanese clay source (Z. Liu et al, ).…”

“…So far, besides the mapping of the low field magnetic susceptibility in particular on the shelves by J. Liu et al () and in the southern basin by Z. Chen et al () for the specific study of hydrocarbon‐bearing zones, only the clay minerals have been analyzed in widely distributed core tops and compared to river clay composition (Z. Liu et al, , and references therein). As shown by the coupled analyses of the clay and magnetic fractions in river sediments, the two proxies carry complementary information about the provenance area (Kissel et al, , ; Z. Liu et al, ).…”