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Thorium isotopes (228Th, 230Th, 232Th) and applications in reconstructing the Yangtze and Yellow River floods
International Journal of Sediment Research xx (2013) 588-595
Thorium isotopes (228Th, 230Th, 232Th) and applications in reconstructing the Yangtze and Yellow River floods Wei-feng YANG1, Min CHEN1, Xin-xing ZHANG1, Zhi-gang GUO2, Guang-xue LI3, Qiang MA1, Jun-hong YANG1, and Yi-pu Huang1
Abstract In the past decades, the floods of the Yangtze and Yellow River introduced unexpected changes of the ecological community and sedimentary dynamics in the East China Sea (ECS). To reconstruct the flood events in the ECS, 228Th, 230 Th and 232Th have been examined in a sediment core. The specific activities of three thorium isotopes have good positive relations with fine fractions (<63 μm), indicating that Th activity concentrations heavily depend upon the sediment grain size. The size-normalized activities of 228Th, 230Th and 232Th showed significant variations. Coincidences between the higher Th activities and historical floods of the Yangtze and Yellow River demonstrated that size-normalized Th recorded the two rivers’ flood events. The activity ratios of thorium isotopes, i.e. 230Th/232Th and 228Th/232Th, also showed similar patterns to the historical river floods. In three periods (1740s, 1840–1860s and 1930–1960s), characterized by frequent floods, the thorium activity ratios were fairly low and close to the Yangtze and Yellow River estuary sediments, coinciding with the less oceanic 228Th and 230Th contributions during the flooding periods. Accordingly, these results support the size-normalized Th activity and thorium ratios as proxies of the river floods in coastal seas. Key Words: Thorium, Yellow River, Yangtze River, Flood, East China Sea
1 Introduction The Yangtze River, the third longest river in the world, directly discharges 9×1011 m3 of freshwater into the East China Sea (ECS) annually (Li et al., 2007). The Yellow River is the most turbid river on the earth with an annual freshwater flux of 41×109 m3 into the Bohai Sea (Zhang et al., 1995). For more than 700 years prior to 1855, the Yellow River wandered eastward to the south Yellow Sea (YS). After 1855, the Yellow River changed its course, emptying into the Bohai Sea (Yang et al., 2009). Although the Yellow River does not directly inject into the ECS, amounts of sediment carried by the Yellow River have been transported to the ECS through the Yellow Sea Coastal Current either before 1855 (Yang et al., 2009) and after 1855 (Edmond et al., 1985; Fan et al., 2002). The loads of nutrients (i.e. nitrate, silicate), carried by the two world’s large rivers, significantly influence the ecological processes such as the harmful algae blooms (Lin et al., 2005), the primary productivity (Tian et al., 1993; Kim et al., 2009) and the productive fisheries (Li et al., 2007) in the ECS. Thus, the variability of the two rivers’ discharge, usually introduced by the river floods and anthropogenic activities, could result in changes of the nutrient loads to the ECS and finally trigger a lot of unexpected ecosystem perturbations. For example, the first impoundment of the Three-Gorge Dam (TGD) in June 2003 caused noticeable changes in the microbial community structure in the ECS two months later (Jiao et al., 2007). Besides dissolved materials, the Yangtze and Yellow River annually discharge about 1.6×109 tons of fine-grained sediments to the Chinese marginal seas, which represents about 10% of the global sediment discharge (Milliman and syritski, 1992). The variability of the sediment discharge, accompanying the two world’s large river floods, modified the spatial patterns of sedimentation in the ECS (Fan et al., 2002; Xu and Milliman, 2009; Yang et al., 2009). Hence, the two rivers’ flood events, to a varying degree, changed the records of the ecosystem variability in the ECS sediments. Although it is clear that the Yangtze and Yellow River significantly affect the ECS through both nutrients and sediments, discriminating the effects of the two rivers’ floods in the ECS have been still difficult due to the lack of methods for distinguishing the flood events recorded in the ECS sediments (Yang et al., 2009), which preventing us 1
State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, China, E-mail: [email protected] 2 Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China 3 College of Marine Geo-Science, Ocean University of China, Qingdao 266100, China Note: The original manuscript of this paper was received in July 2012. The revised version was received in Feb. 2013. Discussion open until Dec. 2014. - 588 International Journal of Sediment Research, Vol. 28, No. 4, 2013, pp. 588–595
from either reconstructing the influence of the two rivers’ floods on the ECS or predicting the upcoming scenario after a flood event in the near future. Thorium isotopes (228Th, 230Th and 232Th), due to their different sources in the ocean, have been used to track the origins of particle and sediment in marine environments (Roy-Barman et al., 1996; Zhang et al., 2005). 232Th (T1/2=1.39 ×1010 yr), a typical terrestrial radionuclide (Huang et al., 1993), has been widely used as a proxy of the land-derived material (Luo and Ku, 1999, 2004). At a specific site in the ECS, the river flood events might introduce drastic variations of 232Th activities by changing the percentage of terrestrial material in sediments. 228Th (T1/2=1.91 yr) and 230 Th (T1/2=7.52 ×104 yr) have both terrestrial source (Huang et al., 1993) and in situ source generated by their parents of 228Ra and 234U respectively (Zhang et al., 2005). During flood events, the ratios of 228Th or 230Th to 232Th could be modified in that much more terrestrial material than non-flood periods as indicated by plutonium isotopes (Pan et al., 2011). Therefore, thorium isotopes and their activity ratios in the ECS sediments, theoretically, may discriminate the Yangtze and Yellow River flood events. In this study, a sediment core from the ECS, which has been proved to be influenced by the Yangtze and Yellow River (Yang et al., 2009), was used to examine the availability of thorium isotopes, including 228Th, 230Th and 232Th, as proxies of river floods in sediments. 2 Methods 2.1 Sampling site The sampling station (30º50'47"N, 125º51'15"E) is located in the East China Sea (i.e. ECS) with a water depth of 75 m (Fig. 1). This station has been influenced by several important currents, including the Taiwan Warm Current (TWC), the Kuroshio Current (KO), the Yellow Sea Coastal Current (YSCC) and the Yangtze River (Huh and Sn, 1999; Yang et al., 2009). Before 1855, the Old Yellow River (OYR) directly discharged terrestrial sediment to the sampling site every year. In 1855, the OYR shifted its watercourse from Jiangsu Province to Shandong Province (its current passage). After
Fig. 1
The sampling site and circulation patterns in the northern Chinese Marginal Seas, modified according to the map of Liu et al. (2007). TWC: Taiwan Warm Current; CJCW: Cangjiang (Yangtze River) Cold Water; YSCC: Yellow Sea Coastal Current; YSWC: Yellow Sea Warm Current
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the relocation, the influence of the Yellow River on the sampling site was significantly weakened. In contrast, the Yangtze River has much greater contribution than before (Yang et al., 2009). At present, the sediment at the studied site mainly consists of the Yellow and Yangtze River sediments (Fan et al., 2002). Due to the variability of 228Th, 230Th and 232 Th resulted from the rivers’ (i.e. the Yellow and Yangtze River) discharge of terrestrial sediments; it is possible to reconstruct the flood events of the two world’s large rivers via thorium isotopes in the ECS. 2.2 Sample collection and analysis of thorium isotopes The sample was collected on July 16, 2006 by a multiple-core sediment sampler. The sediment core was frozen after collection. In the land laboratory, the sediment core was sampled at 1 cm intervals and dried at 60ºC. According to the variability of terrestrial and oceanic organic matter contributions in the sediment, constrained by the į13C, į15N values and C/N ratios in organic matter (Yang et al., 2009), selected samples at different depths were used to analyze thorium isotopes in order to reveal the probable river flood events. Generally, 2–3 grams of dried sediment were quantified and digested with mixed acids (i.e. HClO4-HNO3-HF) after the addition of 229Th as the chemical yield tracer of thorium. Thorium isotopes, including 228Th, 230Th, 229Th and 232Th, were separated according to the traditional methodology (Chen et al., 2003; Zhang et al., 2005). Briefly, the digested sample was dissolved with 9 M HCl. Thorium isotopes were separated from uranium isotopes (i.e. 234U, 235U and 238U) and iron through an anion resin column (Bio-Rad AG1X8) pretreated with 9 M HCl. The effluent containing thorium was evaporated to almost dryness and dissolved in 8 M HNO3. Then, the sample was transferred to a pretreated nitrate-form anion resin column. After purification of thorium by washing the resin column using 8 M HNO3, thorium isotopes were eluted from the resin column with 9 M HCl. The effluent containing thorium was concentrated by evaporation. Finally, thorium isotopes were extracted into TTA-benzene solution and deposited onto a stainless steel disc. The activities of 228Th, 230Th, 229Th and 232Th in the steel disc were counted by an alpha spectrometer (Ortec) at 5.43, 4.687, 4.845 and 4.013 MeV, respectively. The net counts of each radionuclide were more than 400 in order to reach ±1 ı counting error. The background counts of spectrometers for each radionuclide were less than 4 every day. The chemical yields of each thorium nuclide were calculated according to the recoveries of 229Th comparing with the initial added 229Th. The thorium activities in this study were corrected for the backgrounds and chemical yields. For 228Th, the counts of its daughter 224Ra were also subtracted based on the branch ratio of 0.049 (Zhang et al., 2005). The specific activities of thorium isotopes were expressed in dpm g-1. The errors were propagated from one sigma counting uncertainty. 3 Results To illustrate the temporal variability of thorium in the ECS, the age of the sediment, determined by the 210Pb technique, was adopted instead of depth in this study. The dates of sediment prior to 1900 were validated by the consistence between the Yellow River shift in 1855 and the date extrapolated from the 210Pb chronology (Yang et al., 2009). Besides, the periodic cycles of biogenic indexes, such as Baex, Znex and biogenic silica, did not show evident variations in the past 200 years (our unpublished data), also indicating the slight variations of sedimentation rates at the studied site in the East China Sea. Thus, the 210Pb chronology could reflect the events whenever recorded in the studied sediment core. Three thorium nuclides (228Th, 230Th, and 232Th) showed significant variations in the sediment core (Fig. 2). The activities of 228Th ranged between 0.54 dpm g-1 and 1.41 dpm g-1 with the mean of 1.02 dpm g-1 (Table 1). On average, the 228Th activity was lower than either the Yellow River or Yangtze River estuary sediments which were 1.32 dpm g-1 and 1.62 dpm g-1 respectively (Huang et al., 1993). The specific activities of 230Th and 232Th varied from 0.22 dpm g-1 to 0.78 dpm g-1 and from 0.32 dpm g-1 to 1.4 dpm g-1, with the averages of 0.54 dpm g-1 and 0.95 dpm g-1, respectively. Huang et al. (1993) reported the mean activities of 230Th were 1.17 dpm g-1 and 1.04 dpm g-1 for the Yellow and Yangtze River sediments respectively. Our results were much less than the river estuary samples. The 232Th activities in the two river estuary sediments (with the averages of 1.80 dpm g-1 and 1.76 dpm g-1, Huang et al., 1993) were also much higher than the 232Th activity concentrations in the ECS (Table 1). The lower thorium activities in the ECS comparing to the two river estuary sediments were consistent with the less influence of the terrestrial materials on the ECS than the estuary sediments. 4 Discussion 4.1 The activities and temporal patterns of thorium isotopes Comparison with the estuary sediments showed the significant low activities of 228Th, 230Th and 232Th in the ECS sediments. These inconsistencies indicated that the sediments did not completely come from the Yellow and Yangtze River although the two rivers contributed the majority of the sediments as reported by Fan et al. (2002). The higher sediment rates in the Yangtze River estuary, ranging from 1.3–4.7 cm yr-1 (Pan et al., 2011), than the studied site (Yang et al., 2009) also suggested that the ECS shelf received less terrestrial materials than the estuary zone. In situ biogenic particles, to a certain extent, may dilute the specific activities of thorium in the ECS shelf sediments in a manner similar to some metal elements (Yang et al., 2009). In fact, plenty of marine biogenic organic matter in the studied sediments - 590 -
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(Yang et al., 2009) supported the less terrestrial and more marine particulate materials in the ECS than the estuary sediments.
Fig. 2 The activity variations of thorium isotopes including 228Th, 230Th and 232Th for the past 230 years, as well as grain size of sediment in the East China Sea Table 1 The specific activities of 228Th, 230Th, 232Th and activity ratios in sediments in the East China Sea 228 230 232 Th Th Th 228 230 Year AD Th/232Th Th/232Th -1 (dpm g ) 1981 0.83±0.03 0.69±0.03 1.21±0.05 0.68±0.04 0.56±0.04 1973 1.15±0.03 0.57±0.02 1.21±0.04 0.94±0.04 0.47±0.04 1960 1.37±0.09 0.74±0.07 1.41±0.11 0.98±0.10 0.53±0.12 1939 1.41±0.06 0.63±0.03 1.32±0.06 1.07±0.06 0.48±0.07 1926 1.08±0.05 0.78±0.05 1.19±0.06 0.91±0.06 0.65±0.07 1920 0.97±0.07 0.31±0.04 0.65±0.06 1.50±0.18 0.49±0.23 1910 1.12±0.05 0.59±0.04 0.91±0.05 1.23±0.09 0.65±0.10 1863 1.39±0.09 0.69±0.06 1.27±0.10 1.09±0.11 0.54±0.13 1857 1.01±0.05 0.57±0.04 0.94±0.06 1.07±0.09 0.61±0.11 1836 0.64±0.03 0.34±0.06 0.62±0.09 1.02±0.15 0.55±0.22 1815 0.78±0.04 0.43±0.03 0.47±0.03 1.65±0.13 0.91±0.15 1778 0.54±0.07 0.22±0.05 0.32±0.06 1.69±0.37 0.70±0.46 1763 0.94±0.13 0.38±0.08 0.56±0.11 1.67±0.39 0.67±0.49 1747 1.04±0.03 0.66±0.02 1.21±0.04 0.86±0.03 0.54±0.04
The thorium specific activities showed significant temporal variations (Fig. 2). Because this site was influenced by the drastically varied riverine freshwater (Shi et al., 2004; Yang et al., 2009) and little varied TWC and Kuroshio Currents (Liu et al., 2003), the variability of thorium activities probably resulted from the Yellow and Yangtze River floods. Indeed, stable isotopic (į13C and į15N) and trace elemental signals (Ti, Al, Pb et al.) reflected that the riverine effects were mainly responsible for their variability (Yang et al., 2009). Therefore, the activity variations of thorium isotopes possibly recorded the floods of the Yellow and Yangtze River. However, recent investigations suggested that 230Th activities in the sediments strongly depend on the sediment grain size, and more than half of the total 230Th was combined with the <10 μm fine material (Kretschmer et al., 2010, 2011). Thus, size effects need to be examined to reveal the essential relationships between thorium isotopes and the river floods. 4.2 Influence of sediment grain size on the thorium activities The sediments were operationally separated into sand (>63 μm), silt (4–63 μm) and clay (<4 μm) based on their grain sizes (Hu et al., 2011). The three fractions of sediment showed different variations (Fig. 1). Although the variability of sediment grain size may reflect the river floods theoretically, they are probably ineffective in clearly recording flood events because they are affected by either many physical processes such as topographic and circulation conditions or biogenic particulate matter. In contrast, thorium isotopes are less influenced by these processes, but they may be affected by sediment grain size. Indeed, there were significant inverse correlations between the activities of three thorium isotopes and the sand fractions (Fig. 3). However, the thorium activities showed positive relations with the silt International Journal of Sediment Research, Vol. 28, No. 4, 2013, pp. 588–595
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and clay fractions (i.e. <63 μm). Apparently, the sediment grain size is of great importance in regulating the specific activities of 228Th, 230Th and 232Th in the ECS. The higher specific activities of thorium isotopes in fine fractions were attributed to their larger specific surface area and higher sorption capacities for the particle reactive radionuclides (Zhang et al., 2005).
Fig. 3
Relationships between the activities of three thorium isotopes (i.e. 228Th, 230Th and 232Th) and the sand contents or silt and clay contents in the East China Sea
In order to examine the relations between thorium activities and the river flood events, the grain size effects need to be removed. Traditional method was adopted in our study. The thorium activities were normalized to the sand contents (Fig. 4). By comparison with non-normalized dataset, the temporal patterns of three thorium nuclides did not show obvious differences (Figs. 2 and 4). Hence, the grain size was not the essential cause of the evident temporal variability of 228Th, 230 Th and 232Th. The thorium variations were ascribed to the floods of the Yellow and Yangtze River. Generally, 232Th is a typical terrestrial radionuclide (Huang et al., 1993; Zhang et al., 2005) which has been used as a proxy of terrestrial materials (Luo and Ku, 1999, 2004). Theoretically, floods would transport much more 232Th to the studied site and thus result in higher 232Th activities than non-flood time. Although our samples had a relatively low temporal resolution, the three 232Th peaks (Fig. 4) still indicated that there were more flood events in 1740s, 1860s and 1950s. The differences for the normalized 232Th activities reached up to threefold, showing clear river flooding records. In order to further certify the higher thorium activities denoted the flood events, the historical recorded catastrophic floods of the Yangtze and Yellow River (Shi et al., 2004; Zheng et al., 2005) were provided in Fig. 4. Although the historical floods seemed random, they occurred frequently within several periods, which were coincident with the higher 232Th activities. Similarly, the variations of 228Th and 230Th also showed the flood events. Nevertheless, the variability of the normalized 228 Th and 230Th were less evident as 232Th. These differences might be related to the 228Th and 230Th sources. Unlike 232 Th, parts of 228Th and 230Th came from their parents of 228Ra and 234U in seawater besides riverine input (Zhang et al., 2005). During non-flood periods, production of 228Th and 230Th from their parents would diminish the activity differences between flood and non-flood time. At the same time, it should be noted that floods not only bring plenty of thorium isotopes, but also contribute more large size sediment due to much stronger hydrodynamics. Large particle has relatively low specific activities of thorium (Fig. 3). Thus, the increased fraction of large size sediment would lower the specific activities of thorium. In other words, floods generated a competing mechanism for the Th specific activities. In our study, no corresponding relation was observed between the flood events and the variations of grain size (i.e. sand, silt and clay), indicating that large - 592 -
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particle effects did not reflect the floods. The cause probably was that the grain size of sediment, in the East China Sea, may be not only affected by the flood events but also affected by other processes such as biogenic activities and sediment re-suspension. As a result, the grain size did not directly respond to the flood events. In contrast, thorium isotopes, after normalized to the grain size, have close relations with the floods, indicating that thorium isotopic signals were less influenced by other processes. Therefore, the Th input term introduced by the flood events, overall, was much more than the accompanying grain size effects at the studied site. Finally, the specific activities of thorium isotopes recorded the flood events. Based on these observations, it appears that Th isotopes are better proxies of the floods than the grain size at least in the East China Sea.
Fig. 4
Variations of the size-normalized 228Th, 230Th and 232Th for the past 230 years in the East China Sea. Normalization was conducted based on the correlations between the specific activities of thorium isotopes and the sand contents in sediments. The black and grey bars represent the occurring year of the Yangtze and Yellow River floods, respectively (Shi et al., 2004; Zheng et al., 2005).
4.3 Activity ratios of thorium and the river flood events Besides the thorium activities, theoretically, the activity ratios of thorium isotopes can also record the river flood events. Both 228Th and 230Th have oceanic sources in the ECS although riverine inputs contribute the majority of the two nuclides (Huang et al., 1993), while 232Th exclusively come from the land. Along with the floods, the 230Th/232Th or 228 Th/232Th activity ratios at the studied site would be close to the Yellow and Yangtze River sediments because much more terrestrial material would have been transport to the ECS than non-flood time. By contrast, the 230Th/232Th or 228 Th/232Th activity ratios would be higher in the non-flood periods than the flood time owing to the more oceanic contributions of 230Th and 228Th with less terrestrial material. Thus, low and high 230Th/232Th activity ratios respond to flood and non-flood periods, respectively. For the 230Th/232Th activity ratios, 1740s, 1840–1860s and 1930–1960s had fairly lower values in comparison with other years (Fig. 5). These periods were consistent with the historical flood events of the Yellow and Yangtze River (Shi et al., 2004; Zheng et al., 2005). This match directly supports our hypothesis that thorium activity ratios record the river flood events. In contrast, the ratios during the non-flood periods were much higher, indicating increased contributions of in situ produced 228Th and 230Th without flood influence. Similarly, the 228Th/232Th activity ratios also showed similar temporal patters to 230Th/232Th ratios although they are less explicit, which may be related to the short half-life of 228Th. The consistencies of thorium activity ratios between the estuary sediments and the ECS sediments during the flood periods also provide evidence for the availability of the 230Th/232Th (or 228Th/232Th) ratios as proxies of the river floods. Accompanying the flood events, the average 230Th/232Th (or 228Th/232Th) ratios were very close to the Yellow and Yangtze River estuary sediments which have similar 230Th/232Th (or 228Th/232Th) ratios (Fig. 6). On the contrary, the thorium activity ratios showed significant differences between the non-flood period sediments and the river sediments especially for 228Th/232Th ratios since much more 228Th had been produced than 230Th in seawater (Huang et al., 1993; Zhang et al., 2005). International Journal of Sediment Research, Vol. 28, No. 4, 2013, pp. 588–595
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Fig. 5 Variations of the 228Th/232Th and 230Th/232Th ratios for the past 230 years in the East China Sea
4.4 Implications Based on the same removal mechanisms of 228Th, 230Th and 232Th and their different sources, the thorium isotopes provide a new tool for reconstructing the river’s flood events recorded in the East China Sea. Both the size-normalized specific activities of thorium and activity ratios (230Th/232Th and 228Th/232Th) are efficient to distinguish the flood signals in the coastal sediments. Hence, it is possible to differentiate the usual environmental changes from those related to the river floods in the coastal seas combining the thorium proxies.
Fig. 6 Comparisons of the 228Th/232Th or 230Th/232Th ratios between the Yellow River sediment, Yangtze River sediment, and sediments in the East China Sea during the flood and non-flood periods
5 Conclusions 228 Th, 230Th, and 232Th in a sediment core from the East China Sea have been examined to reveal its relations with the sediment grain size, as well as the Yellow and Yangtze River floods. Several exciting conclusions can be drawn. (1) Thorium isotopes (228Th, 230Th, and 232Th) in the ECS sediments have lower activities than the Yellow and Yangtze River estuary sediments, coinciding with the less influence of terrestrial materials on the ECS shelf. (2) The activities of 228Th, 230Th, and 232Th show close relationships with the sediment grain size. Fine fractions, including silt (4–63 μm) and clay (<63 μm) have higher sorption capacities than the coarse sand (>63 μm). (3) The thorium activity ratios, i.e. 230Th/232Th and 228Th/232Th, as well as the size-normalized 228Th, 230Th, and 232Th activities, can be used as proxies of the river flood events in coastal seas. Acknowledgements We would like to thank Professor Helmut Habersack and two anonymous reviewers for their constructive suggestions. This work was jointly supported by a Chinese COMRA Program (No. DY125–13–E-01), the Chinese National Science - 594 -
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Thorium isotopes (228Th, 230Th, 232Th) and applications in reconstructing the Yangtze and Yellow River floods Wei-feng YANG1, Min CHEN1, Xin-xing ZHANG1, Zhi-gang GUO2, Guang-xue LI3, Qiang MA1, Jun-hong YANG1, and Yi-pu Huang1
Abstract In the past decades, the floods of the Yangtze and Yellow River introduced unexpected changes of the ecological community and sedimentary dynamics in the East China Sea (ECS). To reconstruct the flood events in the ECS, 228Th, 230 Th and 232Th have been examined in a sediment core. The specific activities of three thorium isotopes have good positive relations with fine fractions (<63 μm), indicating that Th activity concentrations heavily depend upon the sediment grain size. The size-normalized activities of 228Th, 230Th and 232Th showed significant variations. Coincidences between the higher Th activities and historical floods of the Yangtze and Yellow River demonstrated that size-normalized Th recorded the two rivers’ flood events. The activity ratios of thorium isotopes, i.e. 230Th/232Th and 228Th/232Th, also showed similar patterns to the historical river floods. In three periods (1740s, 1840–1860s and 1930–1960s), characterized by frequent floods, the thorium activity ratios were fairly low and close to the Yangtze and Yellow River estuary sediments, coinciding with the less oceanic 228Th and 230Th contributions during the flooding periods. Accordingly, these results support the size-normalized Th activity and thorium ratios as proxies of the river floods in coastal seas. Key Words: Thorium, Yellow River, Yangtze River, Flood, East China Sea
1 Introduction The Yangtze River, the third longest river in the world, directly discharges 9×1011 m3 of freshwater into the East China Sea (ECS) annually (Li et al., 2007). The Yellow River is the most turbid river on the earth with an annual freshwater flux of 41×109 m3 into the Bohai Sea (Zhang et al., 1995). For more than 700 years prior to 1855, the Yellow River wandered eastward to the south Yellow Sea (YS). After 1855, the Yellow River changed its course, emptying into the Bohai Sea (Yang et al., 2009). Although the Yellow River does not directly inject into the ECS, amounts of sediment carried by the Yellow River have been transported to the ECS through the Yellow Sea Coastal Current either before 1855 (Yang et al., 2009) and after 1855 (Edmond et al., 1985; Fan et al., 2002). The loads of nutrients (i.e. nitrate, silicate), carried by the two world’s large rivers, significantly influence the ecological processes such as the harmful algae blooms (Lin et al., 2005), the primary productivity (Tian et al., 1993; Kim et al., 2009) and the productive fisheries (Li et al., 2007) in the ECS. Thus, the variability of the two rivers’ discharge, usually introduced by the river floods and anthropogenic activities, could result in changes of the nutrient loads to the ECS and finally trigger a lot of unexpected ecosystem perturbations. For example, the first impoundment of the Three-Gorge Dam (TGD) in June 2003 caused noticeable changes in the microbial community structure in the ECS two months later (Jiao et al., 2007). Besides dissolved materials, the Yangtze and Yellow River annually discharge about 1.6×109 tons of fine-grained sediments to the Chinese marginal seas, which represents about 10% of the global sediment discharge (Milliman and syritski, 1992). The variability of the sediment discharge, accompanying the two world’s large river floods, modified the spatial patterns of sedimentation in the ECS (Fan et al., 2002; Xu and Milliman, 2009; Yang et al., 2009). Hence, the two rivers’ flood events, to a varying degree, changed the records of the ecosystem variability in the ECS sediments. Although it is clear that the Yangtze and Yellow River significantly affect the ECS through both nutrients and sediments, discriminating the effects of the two rivers’ floods in the ECS have been still difficult due to the lack of methods for distinguishing the flood events recorded in the ECS sediments (Yang et al., 2009), which preventing us 1
State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, China, E-mail: [email protected] 2 Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China 3 College of Marine Geo-Science, Ocean University of China, Qingdao 266100, China Note: The original manuscript of this paper was received in July 2012. The revised version was received in Feb. 2013. Discussion open until Dec. 2014. - 588 International Journal of Sediment Research, Vol. 28, No. 4, 2013, pp. 588–595
from either reconstructing the influence of the two rivers’ floods on the ECS or predicting the upcoming scenario after a flood event in the near future. Thorium isotopes (228Th, 230Th and 232Th), due to their different sources in the ocean, have been used to track the origins of particle and sediment in marine environments (Roy-Barman et al., 1996; Zhang et al., 2005). 232Th (T1/2=1.39 ×1010 yr), a typical terrestrial radionuclide (Huang et al., 1993), has been widely used as a proxy of the land-derived material (Luo and Ku, 1999, 2004). At a specific site in the ECS, the river flood events might introduce drastic variations of 232Th activities by changing the percentage of terrestrial material in sediments. 228Th (T1/2=1.91 yr) and 230 Th (T1/2=7.52 ×104 yr) have both terrestrial source (Huang et al., 1993) and in situ source generated by their parents of 228Ra and 234U respectively (Zhang et al., 2005). During flood events, the ratios of 228Th or 230Th to 232Th could be modified in that much more terrestrial material than non-flood periods as indicated by plutonium isotopes (Pan et al., 2011). Therefore, thorium isotopes and their activity ratios in the ECS sediments, theoretically, may discriminate the Yangtze and Yellow River flood events. In this study, a sediment core from the ECS, which has been proved to be influenced by the Yangtze and Yellow River (Yang et al., 2009), was used to examine the availability of thorium isotopes, including 228Th, 230Th and 232Th, as proxies of river floods in sediments. 2 Methods 2.1 Sampling site The sampling station (30º50'47"N, 125º51'15"E) is located in the East China Sea (i.e. ECS) with a water depth of 75 m (Fig. 1). This station has been influenced by several important currents, including the Taiwan Warm Current (TWC), the Kuroshio Current (KO), the Yellow Sea Coastal Current (YSCC) and the Yangtze River (Huh and Sn, 1999; Yang et al., 2009). Before 1855, the Old Yellow River (OYR) directly discharged terrestrial sediment to the sampling site every year. In 1855, the OYR shifted its watercourse from Jiangsu Province to Shandong Province (its current passage). After
Fig. 1
The sampling site and circulation patterns in the northern Chinese Marginal Seas, modified according to the map of Liu et al. (2007). TWC: Taiwan Warm Current; CJCW: Cangjiang (Yangtze River) Cold Water; YSCC: Yellow Sea Coastal Current; YSWC: Yellow Sea Warm Current
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the relocation, the influence of the Yellow River on the sampling site was significantly weakened. In contrast, the Yangtze River has much greater contribution than before (Yang et al., 2009). At present, the sediment at the studied site mainly consists of the Yellow and Yangtze River sediments (Fan et al., 2002). Due to the variability of 228Th, 230Th and 232 Th resulted from the rivers’ (i.e. the Yellow and Yangtze River) discharge of terrestrial sediments; it is possible to reconstruct the flood events of the two world’s large rivers via thorium isotopes in the ECS. 2.2 Sample collection and analysis of thorium isotopes The sample was collected on July 16, 2006 by a multiple-core sediment sampler. The sediment core was frozen after collection. In the land laboratory, the sediment core was sampled at 1 cm intervals and dried at 60ºC. According to the variability of terrestrial and oceanic organic matter contributions in the sediment, constrained by the į13C, į15N values and C/N ratios in organic matter (Yang et al., 2009), selected samples at different depths were used to analyze thorium isotopes in order to reveal the probable river flood events. Generally, 2–3 grams of dried sediment were quantified and digested with mixed acids (i.e. HClO4-HNO3-HF) after the addition of 229Th as the chemical yield tracer of thorium. Thorium isotopes, including 228Th, 230Th, 229Th and 232Th, were separated according to the traditional methodology (Chen et al., 2003; Zhang et al., 2005). Briefly, the digested sample was dissolved with 9 M HCl. Thorium isotopes were separated from uranium isotopes (i.e. 234U, 235U and 238U) and iron through an anion resin column (Bio-Rad AG1X8) pretreated with 9 M HCl. The effluent containing thorium was evaporated to almost dryness and dissolved in 8 M HNO3. Then, the sample was transferred to a pretreated nitrate-form anion resin column. After purification of thorium by washing the resin column using 8 M HNO3, thorium isotopes were eluted from the resin column with 9 M HCl. The effluent containing thorium was concentrated by evaporation. Finally, thorium isotopes were extracted into TTA-benzene solution and deposited onto a stainless steel disc. The activities of 228Th, 230Th, 229Th and 232Th in the steel disc were counted by an alpha spectrometer (Ortec) at 5.43, 4.687, 4.845 and 4.013 MeV, respectively. The net counts of each radionuclide were more than 400 in order to reach ±1 ı counting error. The background counts of spectrometers for each radionuclide were less than 4 every day. The chemical yields of each thorium nuclide were calculated according to the recoveries of 229Th comparing with the initial added 229Th. The thorium activities in this study were corrected for the backgrounds and chemical yields. For 228Th, the counts of its daughter 224Ra were also subtracted based on the branch ratio of 0.049 (Zhang et al., 2005). The specific activities of thorium isotopes were expressed in dpm g-1. The errors were propagated from one sigma counting uncertainty. 3 Results To illustrate the temporal variability of thorium in the ECS, the age of the sediment, determined by the 210Pb technique, was adopted instead of depth in this study. The dates of sediment prior to 1900 were validated by the consistence between the Yellow River shift in 1855 and the date extrapolated from the 210Pb chronology (Yang et al., 2009). Besides, the periodic cycles of biogenic indexes, such as Baex, Znex and biogenic silica, did not show evident variations in the past 200 years (our unpublished data), also indicating the slight variations of sedimentation rates at the studied site in the East China Sea. Thus, the 210Pb chronology could reflect the events whenever recorded in the studied sediment core. Three thorium nuclides (228Th, 230Th, and 232Th) showed significant variations in the sediment core (Fig. 2). The activities of 228Th ranged between 0.54 dpm g-1 and 1.41 dpm g-1 with the mean of 1.02 dpm g-1 (Table 1). On average, the 228Th activity was lower than either the Yellow River or Yangtze River estuary sediments which were 1.32 dpm g-1 and 1.62 dpm g-1 respectively (Huang et al., 1993). The specific activities of 230Th and 232Th varied from 0.22 dpm g-1 to 0.78 dpm g-1 and from 0.32 dpm g-1 to 1.4 dpm g-1, with the averages of 0.54 dpm g-1 and 0.95 dpm g-1, respectively. Huang et al. (1993) reported the mean activities of 230Th were 1.17 dpm g-1 and 1.04 dpm g-1 for the Yellow and Yangtze River sediments respectively. Our results were much less than the river estuary samples. The 232Th activities in the two river estuary sediments (with the averages of 1.80 dpm g-1 and 1.76 dpm g-1, Huang et al., 1993) were also much higher than the 232Th activity concentrations in the ECS (Table 1). The lower thorium activities in the ECS comparing to the two river estuary sediments were consistent with the less influence of the terrestrial materials on the ECS than the estuary sediments. 4 Discussion 4.1 The activities and temporal patterns of thorium isotopes Comparison with the estuary sediments showed the significant low activities of 228Th, 230Th and 232Th in the ECS sediments. These inconsistencies indicated that the sediments did not completely come from the Yellow and Yangtze River although the two rivers contributed the majority of the sediments as reported by Fan et al. (2002). The higher sediment rates in the Yangtze River estuary, ranging from 1.3–4.7 cm yr-1 (Pan et al., 2011), than the studied site (Yang et al., 2009) also suggested that the ECS shelf received less terrestrial materials than the estuary zone. In situ biogenic particles, to a certain extent, may dilute the specific activities of thorium in the ECS shelf sediments in a manner similar to some metal elements (Yang et al., 2009). In fact, plenty of marine biogenic organic matter in the studied sediments - 590 -
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(Yang et al., 2009) supported the less terrestrial and more marine particulate materials in the ECS than the estuary sediments.
Fig. 2 The activity variations of thorium isotopes including 228Th, 230Th and 232Th for the past 230 years, as well as grain size of sediment in the East China Sea Table 1 The specific activities of 228Th, 230Th, 232Th and activity ratios in sediments in the East China Sea 228 230 232 Th Th Th 228 230 Year AD Th/232Th Th/232Th -1 (dpm g ) 1981 0.83±0.03 0.69±0.03 1.21±0.05 0.68±0.04 0.56±0.04 1973 1.15±0.03 0.57±0.02 1.21±0.04 0.94±0.04 0.47±0.04 1960 1.37±0.09 0.74±0.07 1.41±0.11 0.98±0.10 0.53±0.12 1939 1.41±0.06 0.63±0.03 1.32±0.06 1.07±0.06 0.48±0.07 1926 1.08±0.05 0.78±0.05 1.19±0.06 0.91±0.06 0.65±0.07 1920 0.97±0.07 0.31±0.04 0.65±0.06 1.50±0.18 0.49±0.23 1910 1.12±0.05 0.59±0.04 0.91±0.05 1.23±0.09 0.65±0.10 1863 1.39±0.09 0.69±0.06 1.27±0.10 1.09±0.11 0.54±0.13 1857 1.01±0.05 0.57±0.04 0.94±0.06 1.07±0.09 0.61±0.11 1836 0.64±0.03 0.34±0.06 0.62±0.09 1.02±0.15 0.55±0.22 1815 0.78±0.04 0.43±0.03 0.47±0.03 1.65±0.13 0.91±0.15 1778 0.54±0.07 0.22±0.05 0.32±0.06 1.69±0.37 0.70±0.46 1763 0.94±0.13 0.38±0.08 0.56±0.11 1.67±0.39 0.67±0.49 1747 1.04±0.03 0.66±0.02 1.21±0.04 0.86±0.03 0.54±0.04
The thorium specific activities showed significant temporal variations (Fig. 2). Because this site was influenced by the drastically varied riverine freshwater (Shi et al., 2004; Yang et al., 2009) and little varied TWC and Kuroshio Currents (Liu et al., 2003), the variability of thorium activities probably resulted from the Yellow and Yangtze River floods. Indeed, stable isotopic (į13C and į15N) and trace elemental signals (Ti, Al, Pb et al.) reflected that the riverine effects were mainly responsible for their variability (Yang et al., 2009). Therefore, the activity variations of thorium isotopes possibly recorded the floods of the Yellow and Yangtze River. However, recent investigations suggested that 230Th activities in the sediments strongly depend on the sediment grain size, and more than half of the total 230Th was combined with the <10 μm fine material (Kretschmer et al., 2010, 2011). Thus, size effects need to be examined to reveal the essential relationships between thorium isotopes and the river floods. 4.2 Influence of sediment grain size on the thorium activities The sediments were operationally separated into sand (>63 μm), silt (4–63 μm) and clay (<4 μm) based on their grain sizes (Hu et al., 2011). The three fractions of sediment showed different variations (Fig. 1). Although the variability of sediment grain size may reflect the river floods theoretically, they are probably ineffective in clearly recording flood events because they are affected by either many physical processes such as topographic and circulation conditions or biogenic particulate matter. In contrast, thorium isotopes are less influenced by these processes, but they may be affected by sediment grain size. Indeed, there were significant inverse correlations between the activities of three thorium isotopes and the sand fractions (Fig. 3). However, the thorium activities showed positive relations with the silt International Journal of Sediment Research, Vol. 28, No. 4, 2013, pp. 588–595
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and clay fractions (i.e. <63 μm). Apparently, the sediment grain size is of great importance in regulating the specific activities of 228Th, 230Th and 232Th in the ECS. The higher specific activities of thorium isotopes in fine fractions were attributed to their larger specific surface area and higher sorption capacities for the particle reactive radionuclides (Zhang et al., 2005).
Fig. 3
Relationships between the activities of three thorium isotopes (i.e. 228Th, 230Th and 232Th) and the sand contents or silt and clay contents in the East China Sea
In order to examine the relations between thorium activities and the river flood events, the grain size effects need to be removed. Traditional method was adopted in our study. The thorium activities were normalized to the sand contents (Fig. 4). By comparison with non-normalized dataset, the temporal patterns of three thorium nuclides did not show obvious differences (Figs. 2 and 4). Hence, the grain size was not the essential cause of the evident temporal variability of 228Th, 230 Th and 232Th. The thorium variations were ascribed to the floods of the Yellow and Yangtze River. Generally, 232Th is a typical terrestrial radionuclide (Huang et al., 1993; Zhang et al., 2005) which has been used as a proxy of terrestrial materials (Luo and Ku, 1999, 2004). Theoretically, floods would transport much more 232Th to the studied site and thus result in higher 232Th activities than non-flood time. Although our samples had a relatively low temporal resolution, the three 232Th peaks (Fig. 4) still indicated that there were more flood events in 1740s, 1860s and 1950s. The differences for the normalized 232Th activities reached up to threefold, showing clear river flooding records. In order to further certify the higher thorium activities denoted the flood events, the historical recorded catastrophic floods of the Yangtze and Yellow River (Shi et al., 2004; Zheng et al., 2005) were provided in Fig. 4. Although the historical floods seemed random, they occurred frequently within several periods, which were coincident with the higher 232Th activities. Similarly, the variations of 228Th and 230Th also showed the flood events. Nevertheless, the variability of the normalized 228 Th and 230Th were less evident as 232Th. These differences might be related to the 228Th and 230Th sources. Unlike 232 Th, parts of 228Th and 230Th came from their parents of 228Ra and 234U in seawater besides riverine input (Zhang et al., 2005). During non-flood periods, production of 228Th and 230Th from their parents would diminish the activity differences between flood and non-flood time. At the same time, it should be noted that floods not only bring plenty of thorium isotopes, but also contribute more large size sediment due to much stronger hydrodynamics. Large particle has relatively low specific activities of thorium (Fig. 3). Thus, the increased fraction of large size sediment would lower the specific activities of thorium. In other words, floods generated a competing mechanism for the Th specific activities. In our study, no corresponding relation was observed between the flood events and the variations of grain size (i.e. sand, silt and clay), indicating that large - 592 -
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particle effects did not reflect the floods. The cause probably was that the grain size of sediment, in the East China Sea, may be not only affected by the flood events but also affected by other processes such as biogenic activities and sediment re-suspension. As a result, the grain size did not directly respond to the flood events. In contrast, thorium isotopes, after normalized to the grain size, have close relations with the floods, indicating that thorium isotopic signals were less influenced by other processes. Therefore, the Th input term introduced by the flood events, overall, was much more than the accompanying grain size effects at the studied site. Finally, the specific activities of thorium isotopes recorded the flood events. Based on these observations, it appears that Th isotopes are better proxies of the floods than the grain size at least in the East China Sea.
Fig. 4
Variations of the size-normalized 228Th, 230Th and 232Th for the past 230 years in the East China Sea. Normalization was conducted based on the correlations between the specific activities of thorium isotopes and the sand contents in sediments. The black and grey bars represent the occurring year of the Yangtze and Yellow River floods, respectively (Shi et al., 2004; Zheng et al., 2005).
4.3 Activity ratios of thorium and the river flood events Besides the thorium activities, theoretically, the activity ratios of thorium isotopes can also record the river flood events. Both 228Th and 230Th have oceanic sources in the ECS although riverine inputs contribute the majority of the two nuclides (Huang et al., 1993), while 232Th exclusively come from the land. Along with the floods, the 230Th/232Th or 228 Th/232Th activity ratios at the studied site would be close to the Yellow and Yangtze River sediments because much more terrestrial material would have been transport to the ECS than non-flood time. By contrast, the 230Th/232Th or 228 Th/232Th activity ratios would be higher in the non-flood periods than the flood time owing to the more oceanic contributions of 230Th and 228Th with less terrestrial material. Thus, low and high 230Th/232Th activity ratios respond to flood and non-flood periods, respectively. For the 230Th/232Th activity ratios, 1740s, 1840–1860s and 1930–1960s had fairly lower values in comparison with other years (Fig. 5). These periods were consistent with the historical flood events of the Yellow and Yangtze River (Shi et al., 2004; Zheng et al., 2005). This match directly supports our hypothesis that thorium activity ratios record the river flood events. In contrast, the ratios during the non-flood periods were much higher, indicating increased contributions of in situ produced 228Th and 230Th without flood influence. Similarly, the 228Th/232Th activity ratios also showed similar temporal patters to 230Th/232Th ratios although they are less explicit, which may be related to the short half-life of 228Th. The consistencies of thorium activity ratios between the estuary sediments and the ECS sediments during the flood periods also provide evidence for the availability of the 230Th/232Th (or 228Th/232Th) ratios as proxies of the river floods. Accompanying the flood events, the average 230Th/232Th (or 228Th/232Th) ratios were very close to the Yellow and Yangtze River estuary sediments which have similar 230Th/232Th (or 228Th/232Th) ratios (Fig. 6). On the contrary, the thorium activity ratios showed significant differences between the non-flood period sediments and the river sediments especially for 228Th/232Th ratios since much more 228Th had been produced than 230Th in seawater (Huang et al., 1993; Zhang et al., 2005). International Journal of Sediment Research, Vol. 28, No. 4, 2013, pp. 588–595
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Fig. 5 Variations of the 228Th/232Th and 230Th/232Th ratios for the past 230 years in the East China Sea
4.4 Implications Based on the same removal mechanisms of 228Th, 230Th and 232Th and their different sources, the thorium isotopes provide a new tool for reconstructing the river’s flood events recorded in the East China Sea. Both the size-normalized specific activities of thorium and activity ratios (230Th/232Th and 228Th/232Th) are efficient to distinguish the flood signals in the coastal sediments. Hence, it is possible to differentiate the usual environmental changes from those related to the river floods in the coastal seas combining the thorium proxies.
Fig. 6 Comparisons of the 228Th/232Th or 230Th/232Th ratios between the Yellow River sediment, Yangtze River sediment, and sediments in the East China Sea during the flood and non-flood periods
5 Conclusions 228 Th, 230Th, and 232Th in a sediment core from the East China Sea have been examined to reveal its relations with the sediment grain size, as well as the Yellow and Yangtze River floods. Several exciting conclusions can be drawn. (1) Thorium isotopes (228Th, 230Th, and 232Th) in the ECS sediments have lower activities than the Yellow and Yangtze River estuary sediments, coinciding with the less influence of terrestrial materials on the ECS shelf. (2) The activities of 228Th, 230Th, and 232Th show close relationships with the sediment grain size. Fine fractions, including silt (4–63 μm) and clay (<63 μm) have higher sorption capacities than the coarse sand (>63 μm). (3) The thorium activity ratios, i.e. 230Th/232Th and 228Th/232Th, as well as the size-normalized 228Th, 230Th, and 232Th activities, can be used as proxies of the river flood events in coastal seas. Acknowledgements We would like to thank Professor Helmut Habersack and two anonymous reviewers for their constructive suggestions. This work was jointly supported by a Chinese COMRA Program (No. DY125–13–E-01), the Chinese National Science - 594 -
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