Discrimination in magnetic properties of different-sized sediments from the Changjiang and Huanghe Estuaries of China and its implication for provenance of sediment on the shelf

Marine Geology 260 (2009) 121–129

Contents lists available at ScienceDirect

Marine Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r g e o

Discrimination in magnetic properties of different-sized sediments from the Changjiang and Huanghe Estuaries of China and its implication for provenance of sediment on the shelf Yonghong Wang a,⁎, Zhigang Yu b, Guangxue Li a, Takashi Oguchi c, Huijun He b, Huanting Shen d a

Key Laboratory of Submarine Geosciences and Technology, Ministry of Education, College of Marine Geoscience, Ocean University of China, 238 Songling Road, Qingdao, 266100, China College of Chemistry and Chemical Engineering, Ocean University of China, 238 Songling Road, Qingdao, 266100, China c Center for Spatial Information Science, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa 277-8568, Japan d State Key Laboratory of Estuarine and Coastal Research, East China Normal University, 3663 Zhongshan Road (N), Shanghai, 200062, China b

a r t i c l e

i n f o

Article history: Received 28 March 2008 Received in revised form 15 February 2009 Accepted 21 February 2009 Keywords: Sediment Magnetic properties Size fraction Provenance Estuaries

a b s t r a c t Provenance discrimination of marine sediments is an important topic in marine geology. Although the catchments of the Changjiang and Huanghe Rivers are regarded as the main sources of sediments on the shelves of the China Sea, identification of detailed sediment provenance is necessary for understanding sediment transport patterns and the paleoenvironment. Magnetic properties of both bulk sediment samples and different-sized particles from the Changjiang and Huanghe Estuaries are studied to discuss the origin of the sediments. The results show that the sediments are dominated by ferrimagnetic magnetite with a minor content of anti-ferromagnetic minerals such as hematite, and a higher concentration of ferrimagnetic minerals is observed for the Changjiang Estuary sediments. The crystal grains of ferrimagnetic minerals in the Changjiang Estuary are coarser than those in the Huanghe Estuary, whereas superparamagnetic grains significantly contribute to the Huanghe sediments. Magnetic parameters of χ SIRM, SOFT and χARM from different-sized particles present more obvious differences than these parameters from bulk particles, and S− 100 and S− 300 for both bulk samples and different-sized particles show distinct differences between the sediments from the two estuaries. Therefore the latter parameters are more useful to distinguish sediment origins. A comparison of the magnetic properties of sediments in these two estuaries with those on the shelves of the adjacent China Sea also suggests that the analysis of sediment magnetic properties is effective to identify sediment provenances. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Sediment magnetic measurement provides a fast, low-cost and sensitive characterization of sediments and offers an effective way for identifying sediment provenances (Oldfield, 1991; Oldfield and Yu, 1994; Walden et al., 1997; Caitcheon, 1998; Lees, 1999; Dearing, 2000; Duck et al., 2001; Jenkins et al., 2002; Booth et al., 2005; Rotman et al., 2008). The measurements can be applied to different environments. In the marine environment, Duck et al. (2001) and Jenkins et al. (2002) investigate the origins and transport of sediment in the Tay Estuary, Scotland; and Rotman et al. (2008) investigated the sources and sediment transport in Freiston Shore, England. Magnetic measurement was also applied to study sediment properties and provenances on shelves (Gu et al., 2006; Liu et al., 2007a). Determining the sources of sediments on epicontinental shelves is

⁎ Corresponding author. Tel.: +86 532 66782991; fax: +86 532 85711679. E-mail address: [email protected] (Y. Wang). 0025-3227/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2009.02.008

an important issue for deducing sediment transport processes and the function of ocean circulation and alongshore currents. The shelves of the China Sea (including the Yellow Sea and the East China Sea), bordered by China and the Korean Peninsula, together comprise one of the broadest continental shelves in the world, and sediments accumulated on the shelves originally supplied from rivers provide information about sediment transport patterns. The Changjiang (Yangtz River) and Huanghe (Yellow River) annually discharge about 8.8 × 108 and 4.3 × 108 t of suspended sediments into their estuaries respectively, and altogether contribute about 10% of the global fluvial sediment flux to the ocean (Milliman and Meade, 1983), leading to a significant amount of deposits on the shelves. The provenances and fate of sediment on the shelves has attracted attention (Yang et al., 2002a,b; Liu et al., 2003; Yang et al., 2004; Yang and Youn, 2007; Liu et al., 2007a), and many researchers have attempted to identify the sediment provenances of the East China Sea and reconstruct paleoenvironmental changes using sedimentological and mineralogical methods (Lee and Chu, 2001; Yang et al., 2003; Yang and Liu, 2007; Xiang et al., 2008). Liu et al. (2003) inferred that the majority of sediments on the outer shelves of the East China Sea

122

Y. Wang et al. / Marine Geology 260 (2009) 121–129

came from both the erosion of deposits of the old Huanghe River (Fig. 1) and sediment discharge from the Changjiang River. Katayamaa and Watanabe (2003) and Lee et al. (2004) indicated that terrigenous sediments transported into the Okinawa Trough by the currents of the East China Sea are primarily from the Changjiang and Huanghe Rivers. Some other studies have been undertaken to distinguish sediments from the Changjiang and Huanghe Rivers based on detrital mineral assemblages (Sun, 1990) and geochemical compositions (Yang, 1988; Cho et al., 1999; Kim et al., 1999; Yang et al., 2002a,b, 2004; Yang and Youn, 2007). However, consensus has not been reached regarding the provenance of sediments on the shelves because of complex hydrodynamic transport processes and the poor consideration of grain-size effects on sediment properties (Yang et al., 2003). Recently sediment magnetic data for studying the origins of sediments on the shelves of the China Sea were reported (Gu et al., 2006; Liu et al., 2007a); however, magnetic data from possible source areas such as the Changjiang and Huanghe Rivers are often unavailable, although information from source areas is fundamental to discuss the provenance of sediments preserved in the marine environment. Therefore, the present study aims to analyze the magnetic properties of sediments in the Changjiang and Huanghe Estuaries. Both bulk samples and different-sized particles were analyzed because sedimentological data may be strongly affected by particle sizes (Oldfield and Yu, 1994; Clifton et al., 1999; Booth et al., 2005). It is thus necessary to correct such effects to compare samples of different particle sizes. The present study also compares the magnetic properties of the sediments in the two estuaries, and

computes susceptible magnetic parameters to compare them with those of distant shelf sediments. This study provides a typical example to evaluate whether the magnetism method successfully indicates sediment provenance in an epicontinental sea. 2. Geographical setting The Changjiang and Huanghe Rivers flow from the Tibetan Plateau of western China, and enter the East China Sea and Bohai Sea, respectively (Fig. 1). The Changjiang and the Huanghe are the third and sixth largest river systems in the world in terms of river length (6380 km and 5464 km, respectively). The Changjiang is also among the tenth largest rivers in terms of drainage area (1.8 × 106 km2, 9th) and water discharge (9.2×1011 m3, 5th) (Saito et al., 2001). The Changjiang and Huanghe also have the fourth largest (ca. 5.0×108 t) and the second largest (ca. 1.1×109 t) annual sediment discharge to the oceans (Milliman and Meade, 1983). Nearly half of the Changjiang-derived sediments are deposited near the river mouth area, and the other half is transported into the open sea and then to the northern and southern coasts by littoral currents (Chen et al., 1985). The Changjiang River Basin is located in the Yangtze craton framed by the Mesozoic Yanshanian orogenic belt. The basin is characterized by complex geology with Paleozoic marine and Quaternary fluviolacustrine sedimentary rocks and by various types of soil. The major part of the Changjiang Basin is located in a humid subtropical zone influenced by a typical East Asian monsoon climate. Annual precipitation ranges from b400 mm in the river source area to N2000 mm in the middle to lower

Fig. 1. Locations of sampling sites and schematic geology map of the Changjiang and Huanghe River basins. There are Mesozoic intermediate-acid intrusive and volcanic rocks in the Changjiang basin. Shaded areas (areas 1 and 2) show the collected magnetic data from references. This figure also indicates the oceanographic circulation pattern in these seas. YSCC = Yellow Sea Coastal Current; ECSCC = East China Sea Coastal Current; TWC = Taiwan Warm Current; KC = Kuroshio Current; YSWC = Yellow Sea Warm Current. Magnetic data of core samples from areas 1 and 2 and from core Y179 (Gu et al., 2006; Liu et al., 2007a,b) were examined to discuss the source of the sediments.

Y. Wang et al. / Marine Geology 260 (2009) 121–129

123

3.3. Particle-size separation

Fig. 2. Water elutriation apparatus.

reaches. The mean tidal range near the river mouth is 2.0 to 3.1 m (Shen, 1981). Waves in the river mouth area are mainly governed by local winds. The mean and maximum wave heights at the seaward section of the mouth are 1.0 and 6.2 m, respectively. The Huanghe River Basin is situated on the North China craton. The Loess terrain in the middle reaches is the most prominent source of river sediments. The Huanghe is characterized by relatively low water discharge but very high suspended-sediment load. The Huanghe Basin is located in an arid to semiarid temperate zone with an annual precipitation of 476 mm and an annual evaporation of 700–1800 mm. The Huanghe shifted its river course from Shandong Province to the northern Jiangsu coastal area in 1128, and delivered huge amounts of sediments to the coastal area and the Yellow Sea until the latest shift of the river course in 1855 (Ren and Shi, 1986; Milliman et al., 1987). The Huanghe River delta has a microtidal estuary with a tidal range less than 2 m. Tides and tidal currents around the river mouth are controlled by the amphidromic point of M2 tidal constituent (principal lunar semidiurnal) located offshore in the Huanghe Delta. 3. Materials and methods 3.1. Sample collection

Wet samples of about 30 g each, taken from the 22 samples collected in 2005, were put into 2000 ml beakers and treated with 30% H2O2 to remove organic matter. Then the samples were wetsieved through a 63 µm sieve. Particles coarser than 63 µm were dried at 40 °C and then passed through 63–125, 125–250 and 250– 500 µm sieves. The particles finer than 63 µm were transferred into beakers to extract the b4 µm fraction according to Stokes' law. After repeating this several times, almost all b4 µm fractions in the samples were extracted. The remaining samples in the beakers were transferred into the water elutriation apparatus to obtain particles with different sizes, e.g., 4–8, 8–16, 16–32 and 32–63 µm. The College of Chemistry and Chemical Engineering in the Ocean University of China set up this apparatus based on the operation principle recommended by Walling and Woodward (1993). The effectiveness of this apparatus for the separation of fine-grained sediments has been verified through comparisons with the results of the pipette and Coulter Counter methods (Beavers and Jones, 1966; Muller and Tisue, 1977). The apparatus mainly depends on Stokes' law, and uses sediment chambers with different diameters to obtain different-sized particles (Fig. 2). Finally, 118 different-sized samples were obtained (note that some of the 22 sediment samples did not contain coarse fractions). 3.4. Magnetic measurement The 31 bulk samples and 118 different-size samples were dried at 40 °C, and then ground gently with an agate mortar. We packed the samples into 10 ml styrene pots, and measured magnetic properties including low (0.47 kHz) and high (4.7 kHz) frequency susceptibilities, Anhysteretic Remanent Magnetization (ARM) and Isothermal Remanent Magnetizations (IRMs). The susceptibilities were determined using a Bartington MS2 dual frequency susceptibility meter. ARM was measured in a Molspin AF demagnetizer using a peak AF field of 100 mT imposed with a DC biasing field of 0.04 mT. Both forward (20 and 1000 mT) and backward (−100 and −300 mT) IRMs were induced using a Molspin pulse magnetizer and measured with a Molspin Minispin fluxgate magnetometer. Low field IRM obtained at 20 mT was defined as SOFT. Saturation isothermal remanent magnetizations (SIRMs) for each sample were induced in a 1.6 T steady field. Two reverse fields (−100 and −300 mT) were then applied to evaluate the S ratio parameters: S− 100 = (−IRM− 100 mT) / SIRM, and S− 300 = (−IRM− 300 mT) / SIRM. S− 300 is an indicator of the relative proportion

Thirty-one sediment samples from the sea-floor surface (within top 10 cm), including 10 from the Huanghe Estuary (near the river mouth, with water depths of 8–15 m) and 21 from the Changjiang Estuary (within the estuary, with water depths of 10–20 m), were collected in August and September 2004, respectively (Fig. 1) to analyze their magnetic properties. Particle sizes may give a strong influence on the result of sediment analyses. Therefore another 22 surface samples, 14 from the Huanghe Estuary and eight from the Changjiang Estuary, were taken again at the study areas in May and July 2005, respectively (Fig. 1). The samples were separated into eight particle-size fractions (b4, 4–8, 8–16, 16–32, 32–63, 63– 125, 125–250 and 250–500 µm) before measuring magnetic properties. 3.2. Particle-size analysis of bulk samples Particle size of the 53 (31 + 22) bulk samples was measured using a laser particle-size analyzer (Coulter LS-100Q). Samples of 2–3 g were put into 50 ml beakers to which distilled water and 5 ml H2O2 (30%) were added. The beakers were left for one night to remove organic matter. After ultrasonical dispersion, the samples were put into the analyzer in turn to measure the particle size of each sample.

Fig. 3. Particle-size distribution of sediments from the Changjiang and Huanghe Estuaries.

124

Y. Wang et al. / Marine Geology 260 (2009) 121–129

Table 1 Magnetic properties of different particle sizes (in 2005) and bulk samples (in 2004) of sediments from the Changjiang (n = 78) and Huanghe (n = 40) Estuaries (SD = standard deviation).

The Changjiang Estuary

2004

2005

The Huanghe Estuary

2004

2005

Min Max Average Min Max Average Min Max Average Min Max Average

Clay

Silt

Sand

Median

SD

4 32 17 5 29 15 4 36 11 13 38 26

11 69 43 12 71 42 14 76 42 23 77 63

3 84 41 1 81 43 2 82 48 0 64 11

8 160 59 10 193 85 6 98 62 6 77 21

18 110 63 14 148 69 18 54 37 5 40 30

of ferrimagnetic (e.g., magnetite) and anti-ferromagnetic (e.g., hematite and goethite) minerals. S− 100 is a similar ratio parameter but it can be influenced more by the grain size of magnetite (Thompson and Oldfield, 1986). Magnetic susceptibility χ and SIRM generally indicate the concentration of magnetic minerals contained in the sediment. The susceptibility of ARM (χARM) is roughly proportional to the concentration of ferromagnetic grains in the fine-grained stable single domain (SSD) (Maher, 1988). Frequency-dependent susceptibility (χfd, in %) is an indicator of the relative contribution of fine viscous grains at the SSD/ superparamagnetic (SP) border to the total ferromagnetic assemblage. SOFT indicates an approximate concentration of remanence carrying ferrimagnets. Two ratio parameters, χARM / χ and χARM / SIRM, are regarded as grain-size indicators because the higher the values, the finer the grain size of magnetite across the SSD-MD (multi-domain) range (Maher, 1988; Oldfield, 1994). More detailed interpretations and explanations of these magnetic parameters are found in Thompson and Oldfield (1986) and Oldfield and Yu (1994). 4. Results 4.1. Particle sizes Sediment compositions of the samples from the Changjiang Estuary collected in 2004 and 2005, determined using a laser

particle-size analyzer, are 17% clay, 43% silt and 41% sand in 2004, and 15% clay, 41% silt and 44% sand in 2005. Compositions of the samples from the Huanghe Estuary are 11% clay, 42% silt and 48% sand in 2004, and 26% clay, 63% silt and 11% sand in 2005 (Fig. 3). The Changjing sediments are rich in sand, whereas the Huanghe samples are finer especially in 2005. Median particle sizes and standard deviations also reflect that much finer particles exist in the Huanghe sediments collected in 2005 (Table 1). 4.2. Recovery ratios after particle separation The sample recovery ratio after the grain-size separation of the 22 wet samples (20–30 g) was 90% for eight samples, and 70–80% for the other 13 samples except for one very low value (b70%), when water contents in sediments from the two estuaries were assumed to be 20–25%. The loss was mainly due to the removal of organic material and the particle separation process; for example, it was impossible to separate all b4 µm sediments from water. The pipette method was employed for seven selected samples (about one-third of all samples) to examine the detailed separation performance of the water elutriation process. The results show 15% of the samples have recovery ratios above 80% in the size ranges of 8–16, 16–32 and 32–63 μm, and 90% of the samples have recovery ratios above 60% for these ranges. In contrast, only 30% of each of the 4–8 and b4 µm samples has recovery ratios above 60%, but recovery ratios of 80% can be achieved for 80% of the b8 µm samples (combined the 4–8 and b4 µm samples). Therefore, the obtained samples of these size ranges represent well their original characteristics. 4.3. Magnetic mineral concentration The magnetic properties of the 31 bulk samples of about 10 g each were measured (Table 2). χ, SIRM, SOFT, and χARM values for the Changjiang Estuary sediments are 1–2 times as high as those for the Huanghe Estuary sediments. The results of magnetic measurements for the 118 different-sized samples, about 0.5–10 g each, are also presented in Figs. 4 and 5, except seven samples with abnormal magnetic values. Some fractions of these seven samples, less than 0.5 g in weight, have magnetic susceptibility χ 5–10 times higher than that for other heavier fractions. This suggests that pollution during sample preparation significantly altered χ values for fractions of very small quantity, and data for samples with such fractions are unreliable. The magnetic

Table 2 Averaged magnetic properties and T-test results of different particle sizes (in 2005) and bulk samples (in 2004) of sediments from the Changjiang (n = 78) and Huanghe (n = 40) Estuaries (SD = standard deviation).

χ(10− 8 m3 kg− 1) χarm(10− 8 m3 kg− 1) SIRM (10− 6 Am2 kg− 1) χARM/χ χARM/SIRM (10− 5m A− 1) SIRM/χ (kA m− 1) χfd(%) SOFT (10− 6 Am2 kg− 1) S− 100(%) S− 300(%)

Sizes

The Changjiang Estuary

(µm)

Min

Max

Mean ± SD

Min

The Huanghe Estuary Max

Mean ± SD

T-test

Sized Bulk Sized Bulk Sized Bulk Sized Bulk Sized Bulk Sized Bulk Sized Bulk Sized Bulk Sized Bulk Sized Bulk

57 59 187 105 8336 6531 3.2 1.1 17 14 9 8 1.1 0.2 871 985 81.7 77.0 92.7 93.2

147 140 636 467 42546 23504 5.3 4.5 37 28 19.3 20.0 3.4 4.5 4582 2323 88.7 86.4 98.1 96.4

93 ± 28 98 ± 23 383 ± 136 218 ± 94 11586 ± 3917 13969 ± 4981 4.2 ± 1.1 2.3 ± 1.0 26 ± 7 19 ± 4 15.4 ± 2.7 12.3 ± 3.4 1.6 ± 1.5 1.7 ± 1.1 1828 ± 468 1631 ± 388 85.7 ± 2.0 82.7 ± 2.7 96.3 ± 2.0 94.5 ± 0.9

39 32 215 69 4580 4935 4.1 1.0 36 8 12 13 3.8 0.6 702 639 78.5 71.9 90.9 89.9

69 147 389 161 7495 18986 5.7 2.1 64 14 15 16 7.0 1.3 1180 2430 84.8 76.4 93.6 94.7

53 ± 10 82 ± 46 303 ± 57 108 ± 33 6122 ± 1074 11044 ± 5650 5.1 ± 0.6 1.5 ± 0.5 48 ± 9 10 ± 3 12.7 ± 1.6 14.3 ± 1.2 4.3 ± 1.2 1.0 ± 0.2 1001 ± 167 1535 ± 728 81.5 ± 1.8 74.4 ± 1.2 92.1 ± 2.0 92.3 ± 1.4

P = 0.0001, t = 7.3686 P = 0.1899, t = 1.3446 P = 0.1399, t = 1.4884 P = 0.0691, t = 1.8931 P = 0.0001, t = 8.7350 P = 0.6345, t = 0.4987 P = 0.0133, t = 2.5214 P = 0.8623, t = 0.1749 P = 0.0068, t = 2.7665 P = 0.7567, t = 0.3421 P = 0.0017, t = 3.2290 P = 0.2078, t = 1.3245 P = 0.0020, t = 3.1347 P = 0.0634, t = 1.9332 P = 0.0020, t = 3.1797 P = 0.3097, t = 1.0732 P = 0.0001, t = 4.6101 P = 0.0001, t = 7.4392 P = 0.0001, t = 3.1797 P = 0.0001, t = 5.1847

Y. Wang et al. / Marine Geology 260 (2009) 121–129

125

Fig. 4. Comparison of average sediment magnetic parameter values between the Changjiang and Huanghe Estuaries. Grey bars and open bars indicate samples in the Changjiang and Huanghe Estuaries, respectively. Black dots and white circles express the standard deviations of data in the Changjiang and Huanghe Estuaries, respectively.

parameters χ, SIRM, SOFT, and χARM are related to the concentration of ferrimagnetic minerals such as magnetite. Most χ values are larger than 7.0 × 10− 7 m3 kg− 1 for sediments from the Changjiang Estuary but lower for those from the Huanghe Estuary (Fig. 4). Higher χ values for both bulk samples and different-sized particles reflect higher concentrations of ferrimagnetic minerals in the Changjiang Estuary, especially for coarse particles. Average χ values for the sediments from the Changjiang Estuary are about 1–2 times those of the Huanghe Estuary for the fine particles (b16 µm), and 2–5 times for coarse particles (N16 µm) (Table 2, Fig. 4). T-tests show that differences of average χ values for different-sized particles between these two estuaries are statistically significant (T (= 7.39) ≥ T (df) 0.01 (= 2.91), and P = 0.0001), where T is the calculated T value, T (df) is the theoretical value of T for a degree of freedom (df) and P is probability), but not significant for the bulk samples (T (=1.35) NT (df) 0.05 (=0.64), P = 0.1899) (Table 2).

The average SIRM, SOFT and χARM values for the Changjiang Estuary sediments are about 1–3 times those for the Huanghe Estuary sediments (Figs. 4 and 5, Table 2). The value differences are greater for different-sized particles than for the bulk samples, especially for the coarse grains. For example, most SIRM values for different-sized particles are above 10,000 × 10− 6 Am2 kg− 1 in the Changjiang Estuary and below 6500 × 10− 6 Am2 kg− 1 in the Huanghe Estuary, but the bulk samples do not show obvious differences (Table 2, Fig. 5). T-tests also show that differences of average SIRM, SOFT and χARM values for different-sized particles between these two estuaries are statistically significant, but not significant for the bulk samples (Table 2). χARM and χARM /χ values for different-sized particles of the two estuaries are also different and they clearly decrease with increasing grain size (Fig. 4). Standard deviation of the magnetic parameter values of the Changjiang Estuary is higher than those of the Huanghe Estuary. The particle size of 125–250 µm was not

126

Y. Wang et al. / Marine Geology 260 (2009) 121–129

Fig. 5. Differences in magnetic parameters between the Changjiang (black circle) and Huanghe (open circle) Estuaries for bulk samples (n = 29, a–d) and for different-sized particles (n = 113, e–h) Panel g also presents that SOFT has a apparent positive correlation with SIRM values for sediment from the Changjiang Estuary (R2 = 0.7) that those from the Huanghe Estuary (R2 = 0.5).

included in the above comparisons because it was not found in the Huanghe Estuary sediments. 4.4. Magnetic granulometry The frequency-dependent susceptibility (χfd) values from the majority of the bulk samples are close to or below 3%, indicating that SP magnetic grains may not contribute significantly to the magnetic component (Maher,1986). The χfd values of the finer fractions (b32 µm) are higher (N3%) than those of the coarser fractions (N32 µm, χfd b3%) for both estuaries. χfd values of the fine particle size (b32 µm) are also higher (5–10%) for the Huanghe Estuary sediments than for the Changjiang Estuary sediments (2–5%), showing that SP grains give certain contributions to the magnetic component of the fine fractions in the Huanghe Estuary sediments. T-tests indicate that there are obvious differences of average χfd values for different-sized particles between these two estuaries, and their statistical difference is stronger than that of the bulk samples (Table 2). Higher values of the χARM/χ and χARM/SIRM ratios reflect more SSD grains, while low ratio values indicate more MD or SP grains (Banerjee

et al., 1981; King et al., 1982). χARM/SIRM values b7.0 × 10− 4 m A− 1 indicates the dominance of grains coarser than 0.07 mm (Oldfield, 1994). χARM/SIRM values for the finer fraction (N8 µm) of the Huanghe Estuary sediments are higher than 7.0 × 10− 4 m A− 1, suggesting abundant fine magnetic grains in the sediments, which is consistent with the observed χfd values. χARM/χ values for the sediments of the two estuaries are less than 10. Most χARM/SIRM values are below 30 × 10− 5 m A− 1 for the Changjiang Estuary and above 50 × 10− 5 m A− 1 for the fine sediment of the Huanghe Estuary (Table 2, Figs. 4 and 5), showing more MD and PSD in the sediment of the Changjiang Estuary. T-test results for χARM/χ and χARM/SIRM from the two estuaries show that statistically significant differences only exist in different-sized particles (Table 2). Their peaks occur in the finer grain sizes b4 and 4–8 µm, suggesting the enrichment of SP/SD grains in clay and fine silt as inferred by Oldfield and Yu (1994). 4.5. Magnetic mineral types S− 300 values indicate the relative proportion of ferrimagnetic (e.g., magnetite) and anti-ferromagnetic (e.g., hematite and goethite) minerals and S− 100 is a similar ratio parameter (Thompson and

Y. Wang et al. / Marine Geology 260 (2009) 121–129

Oldfield, 1986). S− 300 values of all samples from the Changjiang Estuary range between 94.2% and 98.0% and S− 100 values are above 83% (Table 2, Figs. 4 and 5), suggesting ferrimagnetic mineralogy dominated by low-coercivity minerals such as magnetite and maghaemite. Most S− 300 values of all samples from the Huanghe Estuary are below 94% and S− 100 values are below 83%, showing lower proportion of ferrimagnetic minerals in sediments from the Huanghe Estuary than those from the Changjiang Estuary. T-test results for S− 300 and S− 100 values of the two estuaries show that there are statistically significant differences for both bulk and different-sized particles (Table 2). SOFT provides approximate remanence carried in ferrimagnetic minerals. It shows a more apparent positive correlation with SIRM values for sediments from the Changjiang Estuary (R2 = 0.7) than those from the Huanghe Estuary (R2 = 0.5) (Fig. 5) because of larger ranges of SOFT and SIRM, which are consistent with the dominance of ferrimagnetic minerals in the former. The SIRM/χ values (b20 KA m− 1) do not reflect greigite formation. 5. Discussion 5.1. Factors influencing magnetic properties There are significant differences between the magnetic properties of sediment from the two estuaries according to the present study. T-tests also confirm this difference, especially for the particle-sized samples. Here we discuss reasons for these differences. 5.1.1. Sources of magnetic minerals Features of magnetic minerals in the sediment affect its magnetic properties. Because sediments in an estuarine area come from the whole catchment, geology and other environmental factors of the catchments affect features of magnetic minerals in estuarine sediments. The ecosystem, weathering conditions and lithology are significantly different between the Changjing and Huanghe basins (Yang, 1988; Yang et al., 2004). The drainage basin area of the Changjiang doubles that of the Huanghe, and the Changjiang basin possesses many tributaries with a variety of bedrock types (Fig. 1). Magnetic minerals in the Changjiang Estuary largely come from Mesozoic intermediate-acid intrusive and volcanic rocks, which contact metasomatic iron deposits of the Mesozoic Yanshan phase in the middle reach of the river (Yang et al., 2004). In contrast, the main provenance of sediments in the Huanghe Estuary is loess in the Chinese Loess Plateau (Fig. 1). Therefore, magnetic minerals are more abundant in the sediments of the Changjiang Estuary than those of the Huanghe Estuary. Standard deviation values of the Changjiang Estuary sediments are higher than those of the Huanghe Estuary sediments (Fig. 4), implying that variable rock types in the Changjiang basin may cause the divergence of magnetic data. Sediment geochemical compositions with distinct FeO and Fe2O3 indicate that iron contents are higher in the Changjiang Estuary than in the Huanghe Estuary (Yang et al., 2002a,b). Magnetite and hematite

127

are ~ 7.4% and ~ 14.2% in sediments of the Changjiang and are ~2.3% and ~ 10.5% in sediments of the Huanghe Estuaries, respectively (Wang et al., 1997). These observations are consistent with the higher magnetic mineral concentrations in the Changjiang Estuary. 5.1.2. Particle size Particle sizes also have a significant influence on the magnetic properties of sediment. This study shows distinct relationships between particle sizes and some magnetic parameters, as indicated by Oldfield (1994), Clifton et al. (1999) and Zhang et al. (2001). For example, χARM, χARM/SIRM, χARM/χ and χfd show significant correlations with the percentage of finer fractions, and the χARM, χARM/SIRM and χARM/χ ratios decrease with increasing grain size (Figs. 4 and 5). Some other magnetic parameters are also influenced by sediment particle sizes. For instance, χARM/χ of the sediments taken in the Huanghe Estuary in 2005 is two-fold of that in 2004, which is consistent with the higher clay component in the samples collected in 2005 (Table 2). After the particle separation process, impacts of particle size on the magnetic properties are reduced to the minimum, and differences in the magnetic properties between the sediment of the two estuaries mainly reflect other factors such as sediment provenance. Different particle sizes have different contributions to the bulk magnetic parameters. Although there is no obvious relationship between χ and the percentage or quantity of different-sized sediments, high χ values tend to be found in finer particle sizes for different-sized fractions of the samples of the Huanghe Estuary (Fig. 6). Similar results are also found in the samples from the Changjiang Estuary. Therefore, finer particle sizes in sediments give a significant contribution to χ values because magnetic minerals are mainly in the fine composition of sediments. 5.2. Choice of magnetic indicators and sediment fractions Magnetic analyses of different-sized particles are often needed due to the non-uniform distribution of particle-size classes, which affects the magnetic composition of sediment samples. A popular approach is to carry out analyses on a specifically separated particle-size fraction. Most analyses of organic micropollutants and heavy metals are carried out in such a manner (e.g. b16 µm, Klamer et al., 1990; b20 µm, Christiansen et al., 2002; b63 μm, Klamer et al., 1990 and b150 μm, Jones and Turki, 1997) or by analyzing more than two fractions with different sizes, e.g. six fractions (Caitcheon, 1998). The effect of particle size reflects the fact that finer sediments possess larger specific surface areas and thus higher cation exchange capacities (Booth et al., 2005). Although the separation of sediment fractions requires additional time-consuming laboratory work, it leads to more reliable and advantageous analyses than analyzing bulk samples (Booth et al., 2005). Our study has confirmed that the differences in magnetic parameters of different-sized particles between estuaries are more distinct than those in bulk samples (Fig. 5 and Table 2). These observations suggest that particle-sized magnetic parameters indicate the provenance of sediments collected at a location far from these two estuaries.

Fig. 6. χ values for different particle sizes of seven selected sediments in the Huanghe Estuary.

128

Y. Wang et al. / Marine Geology 260 (2009) 121–129

Table 3 Magnetic parameters of the Changjiang and Huanghe Estuaries and the outer continental shelf of the East China Sea (“–” means no values). Areas Area 1

Core Average B459 B439

Area 2

Average ST 9 Y179

Changjiang Estuary Huanghe Estuary

Location

D50

Χ

SIRM

SIRM/χ

S− 100

S− 300

(cm)

(µm)

(10− 8 m3 kg− 1)

(10− 6 Am 2kg− 1)

(kA m− 1)

(%)

(%)

0–50 50–120 58 68 93 103 0–50 0.5 1.5 0–2 2–50 Sized Bulk Sized Bulk

33.7 10.7 – – – – 60 – – – – 85 59 21 62

38 25 105 81 30 28 53 224 133 65 40 93 98 53 82

5229 3705 8632 8007 13,142 12,231 4270 10,836 7479 10,700 7000 11,586 13,969 6122 11,044

14.6 15.1 8.2 9.9 43.7 42.6 8.6 4.8 5.6 23 17 15.2 12.6 12.7 14.3

85.6 77 89.7 89.3 71.7 70.9 86.4 89.9 89.7 – – 85.7 82.7 81.5 74.4

– – – – – – – – – 94 95 96.3 94.5 92.1 92.3

Rotman et al. (2008) discussed the provenance of sediments on the Freiston Shore, UK, by analyzing and comparing surface mineral magnetic properties (χfd, SOFT and SIRM). In our study area, however, values of χ decrease when sediments are collected far from the estuaries due to the hydrodynamic transport and sorting, and mixing with sediments from other sources (Table 3). The values also decrease with increasing depth in a core (Table 3), probably due to postdepositional diagenesis. T-tests show that χfd, SOFT and SIRM for the bulk samples from the Changjiang and Huanghe Estuaries do not indicate significant differences (Table 2). In contrast, magnetic parameters of S− 100 or S− 300 indicate significant differences between the two estuaries according to T-tests, for both bulk and different-sized sediments. Gogorza et al. (2004) found that ARM− 100 mT (S− 100 =(−IRM− 100 mT)/SIRM) is the best parameter for identifying the sediment in the Lake Escondido (South Argentina). This may reflect that SIRM and ARM− 100 are obtained when samples were remagnetized in a “saturating” field of 1 or 1.6 T and a reverse field (−100 and −300 mT), and thus other influences on S− 100 or S− 300 can be reduced. Therefore, S− 100 or S− 300 effectively indicates depositional environments. S− 100 and S− 300 of 32–64 µm and 64–125 µm particles show obvious differences between the two estuaries (Fig. 4). However, our study also indicates that magnetic measurements may be inaccurate if the quantity of sediment fraction is less than 0.5 g. Therefore, it is necessary to decide an appropriate particle fraction for magnetic analyses based on the amounts of available samples. The use of a combined fraction such as b63 µm can also be effective. Such an approach has also been applied to organic material, pollutants and heavy metals (Araujo et al., 1988; Klamer et al., 1990). 5.3. Origin of shelf sediments Sediment provenances can be identified by comparing sediment magnetic properties of source areas and sink areas (Walden et al., 1997; Duck et al., 2001; Jenkins et al., 2002; Rotman et al., 2008). Here, magnetic data for areas 1 and 2 and Y179 on the shelf of the East China Sea (Fig. 1; Gu et al., 2006; Liu et al., 2007a) are reexamined to discuss the provenance of the shelf sediments. Although most sediments from the Changjiang and Huanghe catchments may be trapped in their estuaries or deposited on adjacent continental shelves (Milliman et al., 1985; Qin,1994; Liu et al., 2007b), Katayamaa and Watanabe (2003) and Lee et al. (2004) indicated that sediments from the two catchments can be transported into the Okinawa Trough by the currents of the East China Sea. It is found that S− 100 and S− 300 values for the sediment of the Changjiang Estuary are larger than 82% and 93%, respectively, whereas almost all values for the Huanghe Estuary sediments are smaller

Sources Gu et al. (2006)

Liu et al. (2007a,b) This study

(Table 3). According to this observation, core data indicate that shelf sediments in the East China Sea mainly come from the Changjiang Estuary and the Huanghe Estuary (Table 3). The cyclonic eddy in the south of Cheju Island, Korea, may be responsible for this process (Liu et al., 2003). In contrast, several small rivers draining the Korea Peninsula contribute only small amounts of suspended sediments to the East China Sea, and they hardly affect sediments on the continental shelves (Schubel et al., 1984; Yang et al., 2003). To summarize, our study has demonstrated a possibility of long-distance correlation of marine sediments using magnetic parameters. However, magnetic data for Asian shelf sediments are still very limited, and more data are needed for in-depth assessment and more complete understanding. 6. Conclusions This study has provided a typical example of sediment magnetic measurement that allows the characterization of marine sediments and identification of sediment provenances in both estuaries and shelves. Magnetic properties of sediment from bulk and fractionated sediment samples in the Changjiang and Huanghe Estuaries show the dominance of magnetite and a minor content of hematite, and the Changjiang Estuary sediments have a higher magnetic concentration. Ferromagnetic mineral grains in the Changjiang Estuary are coarser than those in the Huanghe Estuary. SP grains significantly contribute to the fine fractions (b16 µm) of the Huanghe Estuary sediments. These results are also found to be fundamental to understand the source of shelf sediments in the East China Sea. This study has also indicated that variable geological backgrounds of catchments and particle sizes of sediments should be considered when magnetic minerals of marine sediments are analyzed. In this study, magnetic parameters of sediments after particle separation more clearly indicate differences between the Changjiang and Huanghe sediments. More complex geology in the Changjiang catchment partly accounts for different magnetic properties between the two estuaries. Appropriate magnetic parameters should also be used to discuss sediment provenances. According to our study, S− 100 and S− 300 are effective for both bulk and different-sized sediments. Acknowledgements This study is funded by the National Natural Science Foundation of China (Grant No. 40406015, 40576042), the “973” project (Grant No. 2005CB422304). We appreciate constructive reviews by Prof. J.T. Wells, Prof. N. Fagel and an anonymous reviewer. We also acknowledge useful suggestions from Prof. W.G. Zhang and Prof. J. Liu.

Y. Wang et al. / Marine Geology 260 (2009) 121–129

References Araujo, M., Fatima, D., Bernard, P.C., van Grieken, R.E., 1988. Heavy metal contamination in sediments from the Belgian coast and Scheldt Estuary. Mar. Pollut. Bull. 19, 269–273. Banerjee, S.K., King, J., Marvin, J., 1981. A rapid method for magnetic granulometry with applications to environmental studies. Geophys. Res. Lett. 8, 333–336. Beavers, A.H., Jones, R.L., 1966. Elutriation for fractionating silt. Soil Sci. Soc. Am. Proc. 30, 253–263. Booth, C.A., Walden, J., Neal, A., Smith, J.P., 2005. Use of mineral magnetic concentration data as a particle size proxy: a case study using marine, estuarine and fluvial sediments in the Carmarthen Bay area, South Wales, U.K. Sci. Total Environ. 347, 241–253. Caitcheon, G.G., 1998. The significance of various sediment magnetic mineral fractions for tracing sediment sources in Killimicat Creek. Catena 32, 131–142. Chen, J.Y., Zhu, H.F., Dong, Y.F., Sun, J.M., 1985. Development of the Changjiang Estuary and its submerged delta. Cont. Shelf Res. 4, 47–56. Cho, Y.G., Lee, C.B., Choi, M.S., 1999. Geochemistry of surface sediments off the southern and western coasts of Korea. Mar. Geol. 159, 111–129. Christiansen, C., Bartholdy, J., Kunzendorf, H., 2002. Effects of morphological changes on metal accumulation in a salt marsh sediment of the Skallingen Peninsula, Denmark. Wetlands Ecol. Manag. 10, 11–23. Clifton, J., McDonald, P., Plater, A., 1999. Deviation of a grain size proxy to aid the modeling and prediction of radionuclide activity in salt marshes and mud flats of the Eastern Irish Sea. Estuar. Coast. Mar. Sci. 48, 511–518. Dearing, J.A., 2000. Natural magnetic tracers in fluvial geomorphology. In: Foster, I.D.L. (Ed.), Tracers in Geomorphology. John Wiley and Sons, London, pp. 57–82. Duck, R.W., Rowan, J.S., Jenkins, P.A., Youngs, I., 2001. A multi-method study of bedload provenance and transport pathways in an estuarine channel. Phys. Chem. Earth (B) 26 (9), 747–752. Gogorza, C.S.G., Lirio, J.M., Nuñez, H., Chaparro, M., Bertorello, H.R., Sinito, A.M., 2004. Paleointensity studies on Holocene–Pleistocene sediments from lake Escondido, Argentina. Phys. Earth. Planet Inter. 145, 219–238. Gu, J.W., Wang, Z.H., Li, Y.H., Duan, L.Y., 2006. Lithologic and magnetic characteristics and origin of shallow bore sediments on outer continental shelf of East China Sea. J. Palaeogeogr. 8 (5), 269–276 (in Chinese, with English abstract). Jenkins, P., Duck, R., Rowan, J., Walden, J., 2002. Fingerprinting of bed sediment in the Tay Estuary, Scotland: an environmental magnetism approach. Hydrol. Earth Syst. Sci. 6 (6), 1007–1016. Jones, B., Turki, A., 1997. Distribution and speciation of heavy metals in surficial sediments from the Tees Estuary, North-east England. Mar. Pollut. Bull. 34, 68–79. Katayamaa, H., Watanabe, Y., 2003. The Huanghe and Changjiang contribution to seasonal variability in terrigenous particulate load to the Okinawa Trough. DeepSea Res. II 50, 475–485. Klamer, J.C., Hegeman, W.J.M., Smedes, F., 1990. Comparison of grain size correction procedures for organic micropollutants and heavy metals in marine sediments. Hydrobiologia 208, 213–220. Kim, G.B., Yang, H.S., Church, T.M., 1999. Geochemistry of alkaline earth elements (Mg, Ca, Sr, Ba) in the surface sediments of the Yellow Sea. Chem. Geol. 153, 1–10. King, J.W., Banerjee, S.K., Marvin, J., Ozdemir, O.A., 1982. A comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials: some results from lake sediments. Earth Planet. Sci. Lett. 59, 404–419. Lee, H.J., Chu, Y.S., 2001. Origin of inner-shelf mud deposit in the southeastern Yellow Sea: Huksan Mud Belt. J. Sediment. Res. 71, 144–154. Lee, S.Y., Huh, C.A., Su, C.C., You, C.F., 2004. Sedimentation in the Southern Okinawa Trough: enhanced particle scavenging and teleconnection between the Equatorial Pacific and western Pacific margins. Deep-Sea Res., Part 1, Oceanogr. Res. Pap. 51, 1769–1780. Lees, J., 1999. Evaluating magnetic parameters for use in source identification, classification and modelling for natural and environmental materials. In: Walden, J., Oldfield, F., Smith, J.P. (Eds.), Environmental Magnetism: A Practical Guide. Quaternary Research Association, Cambridge, pp. 113–138. Liu, J., Zhu, R.X., Li, G.X., 2003. Rock magnetic properties of the fine-grained sediment on the outer shelf of the East China Sea: implication for provenance. Mar. Geol. 193, 195–206. Liu, J., Zhu, R.X., Li, T.G., Li, A.C., Li, J., 2007a. A sediment-magnetic signature of the midHolocene paleoenvironmental change in the central Okinawa Trough. Mar. Geol. 239, 19–31. Liu, J.P., Xu, K.H., Li, A.C., Milliman, J.D., Velozzi, D.M., Xiao, S.B., Yang, Z.S., 2007b. Flux and fate of Yangtze River sediment delivered to the East China Sea. Geomorphology 85, 208–224. Maher, B.A., 1986. Characterization of soil by magnetic measurements. Phys. Earth Planet Inter. 42, 76–92. Maher, B.A., 1988. Magnetic properties of some synthetic sub-micromagnetites. Geophys. J. 94, 83–96.

129

Milliman, J.D., Meade, R.H., 1983. World-wide delivery of river sediment to the oceans. J. Geol. 91, 1–21. Milliman, J.D., Shen, H.T., Yang, Z.S., Meade, R.H., 1985. Transport and deposition of river sediment in the Changjiang estuary and adjacent continental shelf. Cont. Shelf Res. 4, 37–45. Milliman, J.D., Qin, Y.S., Ren, M.E., Saito, Y., 1987. Man's influence on the erosion and transport of sediment by Asian rivers: the Yellow River (Huanghe) example. J. Geol. 95, 751–762. Muller, R.N., Tisue, G.T., 1977. Preparative-scale size fractionation of soils and sediments and an application to studies of plutonium geochemistry. Soil Sci. 124, 191–198. Oldfield, F., 1991. Environmental magnetism – a personal perspective. Quat. Sci. Res. 10, 73–85. Oldfield, F., 1994. Toward the discrimination of fine-grained ferrimagnets by magnetic measurements in lake and near shore marine sediments. J. Geophys. Res. (B) 99, 9045–9050. Oldfield, F., Yu, L., 1994. The influence of particle size variations on the magnetic properties of sediments from the north-eastern Irish Sea. Sedimentology 41, 1093–1108. Qin, Y.S., 1994. Sedimentation in northern China Seas. In: Zhou, D., Liang, Y.B., Zeng, C.K. (Eds.), Oceanology of China Seas, vol. 2. Kluwer Academic Publishers, Dordrecht, pp. 395–406. Ren, M.E., Shi, T.L., 1986. Sediment discharge of the Yellow River (China) and its effect on the sedimentation of the Bohai and the Yellow Sea. Cont. Shelf Res. 6, 785–810. Rotman, R., Naylor, L., McDonnell, R., MacNiocaill, C., 2008. Sediment transport on the Freiston Shore managed realignment site: an investigation using environmental magnetism. Geomorphology 100, 241–255. Saito, Y., Yang, Z.S., Hori, K., 2001. The Huanghe Yellow River and Changjiang Yangtze River deltas: a review on their characteristics, evolution and sediment discharge during the Holocene. Geomorphology 41, 219–231. Schubel, J.R., Shen, H.T., Park, M.J.,1984. A comparison of some characteristic sedimentation processes of estuaries entering the Yellow Sea. In: Park, Y.A., Pilkey, O.H., Kim, S.W. (Eds.), Marine Geology and Physical Processes of the Yellow Sea, pp. 286–308. Shen, H.T., 1981. The tidal wave propagation and its effect on the channel evolution in the Changjiang Estuary. Proceeding of Geomorphologic Symposium of The Geographical Society of China in 1977. Science Press, Beijing, pp. 53–61 (in Chinese). Sun, B.Y., 1990. Detrital mineral assemblages in the Huanghe, Changjiang and Zhujiang River delta sediments. Mar. Geol. Quat. Geol. 10 (3), 23–34 (in Chinese, with English abstract). Thompson, R., Oldfield, F., 1986. Environmental Magnetism. Allen and Unwin. 227 pp. Walden, J., Slattery, M.C., Burt, T.P., 1997. Use of mineral magnetic measurements to fingerprint suspended sediment sources: approaches and techniques for data analysis. J. Hydrol. 202, 353–372. Walling, D.E., Woodward, J.C., 1993. Use of a field-based water elutriation system for monitoring the in-situ particle size characteristic of fluvial suspended sediment. Water Res. 27 (9), 1413–1421. Wang, L.C., Chen, X.L., Chu, T.Q., 1997. A contrast analysis on the loads character of the Changjiang river and the Yellow river. Geogr. Res. 16 (4), 71–79 (in Chinese, with English abstract). Xiang, R., Yang, Z.S., Saito, Y., Fan, D.J., Chen, M.H., Guo, Z.G., Chen, Z., 2008. Paleoenvironmental changes during the last 8400 years in the southern Yellow Sea: benthic foraminiferal and stable isotopic evidence. Mar. Micropaleontol. 679 (1–2), 104–119. Yang, Z.S., 1988. Mineralogical assemblages and chemical characteristics of clays from sediments of the Huanghe, Changjiang, Zhujiang Rives and their relationship to the climate environment in their sediment source areas. Oceanol. Limnol. Sin. 19 (4), 336–346 (in Chinese, with English abstract). Yang, Z.S., Liu, J.P., 2007. A unique Yellow River-derived distal subaqueous delta in the Yellow Sea. Mar. Geol. 240, 169–176. Yang, S.Y., Youn, J.S., 2007. Geochemical compositions and provenance discrimination of the central south Yellow Sea sediments. Mar. Geol. 243 1–4, 229–241. Yang, S.Y., Li, C.X., Jung, H.S., Lee, H.J., 2002a. Discrimination of geochemical compositions between the Changjiang and the Huanghe sediments and its application for the identification of sediment source in the Jiangsu coastal plain, China. Mar. Geol. 186, 229–241. Yang, S.Y., Jung, H.S., Choi, H.S., Li, C.X., 2002b. The rare earth element compositions of the Changjiang (Yangtze) and Huanghe (Yellow) river sediments. Earth Planet Sci. Lett. 201, 407–419. Yang, S.Y., Jung, H.S., Lim, D.I., Li, C.X., 2003. A review on the provenance discrimination of the Yellow Sea sediments. Earth Sci. Rev. 63 (1–2), 93–120. Yang, S.Y., Jung, H.S., Li, C.X., 2004. Two unique weathering regimes in the Changjiang and drainage basins: geochemical evidence from river. Sediment. Geol. 164, 19–34. Zhang, W.G., Yu, L.Z., Hutchinson, S.M., 2001. Diagenesis of magnetic minerals in the intertidal sediments of the Yangtze Estuary, China and its environmental significance. Sci. Total Environ. 266, 169–175.