Full length articleTemporal and spatial variations in magnetic properties of suspended particular matter in the Yangtze River drainage and their implications
Introduction
By serving as a major pathway for erosion products, large river systems are key components in the study of source-to-sink processes, which is one of the key research objectives of the inter-MARGINS Program. Yangtze River, the largest and longest river in China, connects the Asian continent with the Pacific Ocean. It transports 9 × 1011 m3 of water and 4.7 × 108 t of sediments to the Chinese marginal seas (Milliman and Meade, 1983). The enormous quantity of erosional products from the basin plays a dominant role in regulating the deposition and sedimentary evolution of the marginal sea system (Clift et al., 2002, Clift, 2006).
Geochemical and mineralogical methods have played an important role in provenance investigations of river sediments (Yang et al., 2006a, Yang et al., 2007, Yang et al., 2009, Wang et al., 2007, Luo et al., 2012, He et al., 2013). Over these years, environmental magnetism became a fast growing avenue for provenance identification due to its fast, low-cost and nondestructive advantages (Yu and Oldfield, 1989, Watkins and Maher, 2003, Hatfield and Maher, 2008, Wang et al., 2010, Zhang et al., 2012). Therefore, this approach was applied to sediments of the Yangtze River to trace sources and indicate pollution (Zhang et al., 2008a, Zhang et al., 2008b, Wang et al., 2009, Li et al., 2011a).
However, most of the previous studies have a limited spatial and temporal coverage of samples analyzed. They either focus on estuarine or on specific basins within the drainage and sampling was never conducted over more than two seasons. Seasonal discharge of the Yangtze River is controlled by the Asian monsoon which can be up to an order of a magnitude higher in the monsoon season than that in the non-monsoon season (The Changjiang Water Resources Commission. See http://www.cjw.gov.cn). Precise calculations of annual fluxes and accurate understanding of the processes that control the properties of sediments in rivers can only be achieved by time series investigation, which at least covers a complete year (Kirchner and Neal, 2013, Luo et al., 2014, Voss et al., 2014). An additional limitation is introduced by sampling techniques. Most of the samples used in previous studies were taken from river beds, alluvial plains or water surface. Few data are available for the magnetic properties of suspended particulate matter (SPM) of the Yangtze River, although it accounts for about 98% of the sediment load (Yang et al., 2002). SPM is comparable with the contemporaneous source-to-sink process of rivers, making it more suitable for provenance studies. Besides, no previous work has investigated the influence of dams on source indicators. Anthropogenic influences along the Yangtze River drainage, such as the construction of the Three Gorges Dam (TGD) - the largest dam in the world - has affected the source to sink pattern of the sediment by trapping sediments within the river and activating channel erosion.
Comprehensive knowledge about spatial and temporal variations of magnetic properties of SPM in the Yangtze River is currently data-limited, which might lead to biased conclusions in provenance and environmental studies. Here, we present the first systematic data set of magnetic properties of SPM from the Yangtze River to understand the variations of magnetic properties at different scales and their implications for provenance. Two sets of sample are collected: (1) a geographic transect along the river from headwater to estuary, and (2) a time series of samples collected from different depth at the Datong hydrological station between June 2010 and June 2011 (Luo et al., 2012, Luo et al., 2014). This detailed and systematic study creates a framework which is essential for offering secure source indications and will provide a better understanding about the evolution of the Yangtze River and the monsoon system.
Section snippets
Yangtze River setting
The Yangtze River originates from the Qinghai–Tibetan Plateau and extends over a length of 6300 km from its headwaters to its estuary in the East China Sea. It has a catchment area of 1.8 × 106 km2. Traditionally, the Yangtze River is divided into the upper, middle and lower reaches (Fig. 1). The upper reaches of the Yangtze River end at Yichang, and can be further divided into Jinsha Jiang and Chuan Jiang. The Jinsha Jiang stretches out over a distance of 3300 km and ends at Yibin by the confluence
Sample collection
SPM samples were collected in the Yangtze River basin between June 2010 and September 2011 (Fig. 2; Table 1). Temporal variation was examined by taking biweekly and monthly SPM at the Datong Hydrological Station from June 2010 to June 2011. Datong is the last hydrological station situated in the lower mainstream that is not affected by tidal influences. During the sampling campaign, the water discharge was measured at Datong, varied from 62,800 m3/s in summer to 13,000 m3/s in winter (Fig. 2c).
Results and discussion
The process of erosion, transportation and deposition of the sediment within the catchment is complicated. Provenance and grain size of sediment as well as weathering conditions of different reaches are thought to be the main controlling factors on the magnetic properties of sediments in river drainage (Maher, 1988, Roberts et al., 2011).
Conclusions
Magnetic characteristics of SPM samples from the Yangtze River are investigated with respect to spatial-, temporal- and depth variations. The results demonstrate that:
- (1)
Magnetic minerals in the suspended particulate matter are dominated by magnetite with an additional contribution of hematite. SPM from the upper reaches have a higher concentration of magnetic minerals with coarser grain size, while sediments from the middle-lower reaches have lower concentration and finer grain size. Spatial
Acknowledgements
Many thanks to Yoshiaki Suzuki and Keita Saito (The University of Tokyo) as well as associate Professor Irino (Hokkaido University) and Professor Yang SY (Tongji University) for their help during the sampling campaigns. This work was financially supported by National Basic Research Program of China under Grant No. 2015CB953804 and Chinese National Natural Science Foundation (NSFC) (Grant Nos. 41472146 and 41104043). This study was jointly supported by Grant for Scientific Research from the
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