Recent sedimentary record of storms and floods within the estuarine-inner shelf region of the East China Sea

Abstract

Although extreme weather events make a strong impact in shallow marine sedimentary environments, there is still a paucity of past records for the Holocene period. Estuarine-inner shelf mud regions deposited from rivers that transport a large amount of suspended sediment represent an important archive of the Holocene. Two cores (S5-2 and JC07) retrieved from the estuarine-inner shelf regions of the East China Sea provided an opportunity to use sensitive grain size and ²¹⁰Pb dating to reconstruct a history of extreme weather events in the Yangtze River basin. Here, we show that the average sedimentation rates of the two cores, S5-2 (1930–2013) and JC07 (1910–2013), were estimated to be 3.11 and 1.56 cm/yr, respectively. The results indicated that sediment supply played an important role in sedimentation of the estuarine-inner shelf mud region of the East China Sea. Sand content strongly increased in the late 1980s, a result of downstream riverbed erosion of the Yangtze River and submerged deltas. The grain size versus the standard deviation method was used to identify grain-size intervals with the highest variability along a sedimentary sequence. The Yangtze estuary mud area coarse population correlated well with historical literature on Yangtze River floods since AD 1930. Extreme storm events corresponded well with historical literature on the Zhe-Min mud region of the East China Sea. The spectral analyses of the sample core coastal population demonstrated that flood and storm events were consistent with a ~3–8 a periodic change of El Niño Southern Oscillation (ENSO), suggesting that the flood events usually follow ENSO years in the Yangtze River. Consequently, sediment records preserved in the two cores demonstrated different sedimentary responses to Yangtze River floods and storms, which is important to recover centennial scale flood events, to infer extreme precipitation, and to understand climate change in the estuarine-inner shelf of the East China Sea. Nevertheless, more efforts are still needed to simulate paleo-flood and predict future flood events in the context of global warming.

Keywords

estuarine-inner shelf of the ECS grain-size modern flood and storm events sedimentation rate

Introduction

Rivers are important channels that link continents and oceans, annually transporting about 15–20 billion metric tons (Gt) of sediment to the ocean (Milliman and Meade, 1983; Milliman and Syvitski, 1992), of which more than half of global fluvial sediment have been delivered by Asian rivers (Meade, 1996). The continental shelf of the East China Sea (ECS) is one of the widest shelves and river-dominated ocean margins in the world (Guo et al., 2003), receiving a large amount of river-deposited terrigenous sediment on its estuarine-inner shelf region, forming coastal mud areas such as the Yangtze estuary mud area (YEMA) in north and the Zhe-Min coastal mud area in the south. These mud areas are an accumulative deposition center of sediment loads from the Yangtze River to the sea (DeMaster et al., 1985; Hu et al., 2001), and contain a record of natural and anthropogenic environmental changes (Bianchi and Allison, 2009), becoming one of the key areas in the study of the Chinese land–ocean interaction in the coastal zone (Guo et al., 2003). Most previous research in the Yangtze estuary delta and mud belt has mainly focused on shelf sediment transport, evolution, modern sedimentation, and topographic features (Chen and Stanley, 1998; Hori et al., 2002; Liu et al., 2006, 2007; Wang et al., 2007; Yang, 1989). However, less attention has been paid to the response of delta sedimentary records to natural changes and anthropogenic activities in river systems.

The Yangtze River (Changjiang) is the largest river on the Eurasian continent, having a catchment area of 1,800,000 km², originating on the Qinghai-Tibet Plateau and flowing 6300 km eastward to the ECS (Milliman and Farnsworth, 2011). The climate of the Yangtze River is subtropical monsoon, basin-wide precipitation averages ~1100 mm/yr, and 70–80% of annual precipitation occurs in the summer (Xu et al., 2010). Global climate change has the potential to accelerate the hydrological cycle, further influencing the variability in precipitation and resulting in extreme floods in the large river basins (Allan and Soden, 2008; Pall et al., 2011). Historically, the Yangtze River basin has been known for frequent large floods, especially in the middle and lower reaches, which have seriously affected social-economic development and the lives of people (Cai et al., 2001; Zhao, 2000). Therefore, study of the regulation and mechanisms of influence of flood events in the Yangtze River and predicting future floods have become important in the study of global warming in China.

At present, methods of reconstructing paleo-flood events mainly include archaeology, compilation of historical literature, and investigation of geological records (Bai et al., 2008; Yang and Xie, 1997; Zhu et al., 2005), but vulnerable flood relicts have been subjected to scour erosion by recent precipitation, resulting in discontinuous flood records. Thus, these have been very difficult to study for high-resolution flood records. The Yangtze estuarine and inner shelf mud areas are an important sediment deposit on the continental shelf and have a high sediment rate, a stable sedimentary environment, and contain a continuous sedimentary record, thus providing substantial information. In this study, we collected two sediment cores from different regions (core S5-2 in the Yangtze estuary mud region and core JC07 in the Zhe-Min coastal region of the ECS) and investigated high-resolution sedimentary sequences to reveal different sedimentary responses of these two sedimentary records to floods and storm, with an expectation to be able to rebuild longer time scales of flood and storm sequences of the last 100 years.

Regional setting

The estuarine-inner shelf region of the ECS, as a typical continental margin in the western Pacific, receives massive terrestrial inputs from Asian land runoff (especially Changjiang) and is greatly influenced by the Yangtze River, which is one of the largest fluvial systems in the western Pacific, draining a catchment area of ~1.94×10⁶ km² and accounting for ~20% of the area of the Chinese mainland with 400 million inhabitants (Bianchi and Allison, 2009; Liu et al., 2006; Yang et al., 2006). As the largest Eurasian river and third longest river (6300 km) in the world, the Yangtze River had the fifth largest water discharge (900 km³/yr) and the fourth largest sediment discharge (500 Mt/yr before its decline in the 1970s) in the world (Milliman et al., 2011; Yang et al., 2005a). Although annual sediment load has been reduced because of dam construction, the Yangtze River is still the predominant sediment contributor to the inner shelf of the coastal ECS, with the contribution from several other major local rivers only amounting to ~4% of the Yangtze’s annual sediment load, including the Qiantang River, the Min River, the Ou River, and the Jiao River in the coastal areas of Zhejiang and Fujian with 0.007×10⁹, 0.006×10⁹, 0.003×10⁹, and 0.001×10⁹ t/yr, respectively (Deng et al., 2006; Liu et al., 2007). Most of these fluvial sediments and associated materials are temporarily deposited offshore of the estuary in summer and are later resuspended and transported southward, mainly along the inner shelf by subsequent winter storms (DeMaster et al., 1985; Milliman et al., 1985).

The Yangtze River floods in summer (May–October) when it discharges ~78% of the mean annual sediments, with July seeing the largest flooding and being 21.9% of the total year (Guo et al., 2003; Shen et al., 1983). The Taiwan warm current (TWWC) intensifies and coastal currents weaken because of prevailing southeast monsoons in the summer, leading to huge amounts of suspended sediments of the Yangtze River being trapped and deposited in the subaqueous delta and estuarine region, forming the Changjiang estuary mud area (Guo et al., 2003; Liu et al., 2006). In winter, with the strengthening of northwest monsoons, fine particles of the estuarine area are resuspended, transported southward along the inner shelf by Zhejiang-Fujian coastal current (ZFCC), and constrained to the inner shelf of the ECS because of obstruction of the strong northward TWWC, forming the Zhe-Min coastal mud area (Guo et al., 2003; Liu et al., 2006, 2007; Yang et al., 1992).

Materials and methods

Sample collection

Figure 1 illustrates the location of the two studied sediment cores (S5-2 and JC07) collected on 20 December 2013, from the estuarine-inner shelf regions of the ECS, using the deployment of a stainless steel gravity core sampler. The site of core S5-2 was located at the YEMA in the north with a water depth of 20–30 m; core JC07 was collected at the Zhejiang coastal mud area in the south with a water depth of 40–50 m (Figure 1 and Table 1). The two cores are mainly composed of silt clay. A total of 178 samples were taken at 2-cm intervals throughout these two cores to perform ²¹⁰Pb and grain-size analysis.

Figure 1.

Study area and locations of cores sampled in the estuarine-inner shelf of the ECS. The estuarine-inner shelf mud region of the ECS map was modified after Gao et al. (2015b).

Table 1.

Sampling records of the sediment cores in estuarine-inner shelf regions of the ECS.

Sample	Latitude (°N)	Longitude (°E)	Water depth (m)	Length (cm)	Location
S5-2	30.37	122.76	20–30	186	Yangtze estuary
JC07	28.23	122.15	40–50	160	Zhe-Min coastal

²¹⁰Pb dating

The radioactivity of ²¹⁰Pb in the sediment sample cores was measured using an HPGe Alpha-ray spectrometer at the Ministry of Education Key Laboratory for Coast and Island Development, Nanjing University. The radionuclide ²¹⁰Pb, for which the physical half-life is 22.3 years, has been widely used in marine environments to date aquatic sediments (Andersen et al., 2000; Huh and Su, 1999; Li et al., 2003). The basic methodology of ²¹⁰Pb dating was established by Goldberg (1963). A flow chart of the pretreatment process (basic theory) for the determination of Pb isotopes is illustrated in Figure 2.

Figure 2.

(a) Potential sources of ²¹⁰Pb in sediments and (b) flow chart of analytical procedures for determination of Pb isotopes.

Grain-size analysis

Grain-size composite of the sample cores was measured using a Malvern Mastersizer 2000 laser particle analyzer (Malvern Instruments Ltd, UK), which measures grains in the 0.02–2000 µm range at a size resolution of 0.01ϕ. The measuring error was within 3%. Before the grain-size analyses, all air-dried sediment samples were successively pretreated with H₂O₂ (30%) to remove organic matter and with HCl (10%) to remove carbonates. Other parameters such as sorting coefficient (σ), skewness (Sk), and kurtosis (Kg) were calculated according to Folk and Ward (1957). Particle sizes were categorized into less than 4 µm for clay, 4–63 µm for silt, and larger than 63 µm for sand.

The ‘grain size versus standard deviation’ method was used to identify grain-size intervals with the highest variability along a sediment sequence (Boulay et al., 2002).

Results

Sediment chronology

The cores (S5-2 and JC07) were composed of grayish yellow clayey silt (Figures 4 and 5). Sediment core S5-2 was divided into three sections (0–43, 43–62, and 62–196 cm) according to sediment structure and characteristics. Sediment in the lower section (62–196 cm) was mainly composed of gray clayey silt, with relatively lower sand content, with a sandy silt interlayer at 128 cm, which may have resulted from a storm. Sediment in the 43–62 cm layer was yellowish gray clayey silt with sand content less than 1%, but the clay content showed an increasing trend. Sediment in the upper section (0–43 cm) was mainly composed of grayish clayey silt with a sand content of 10%, with silt content decreasing and sand content increasing upwardly, resulting in courser grain size. In addition, a few black and sand stripes were found irregularly throughout the entire sequence. Sediment core JC07 primarily consisted of grayish yellow clayey silt with a thin interbedding of silty sand and sandy silt, and a few shell fragments in the entire sample sequence.

The profiles of excess ²¹⁰Pb for S5-2 and JC07 are shown in Figure 3. Core S5-2 was collected at the YEMA, and core JC07 was obtained at the Min-Zhe coastal mud area, farther away from the Yangtze estuary than core S5-2. Both sample cores presented a relatively stable sedimentary environment, and excess ²¹⁰Pb activities exhibited an approximately exponential decay trend with depth (Figure 3). Although there were some excess ²¹⁰Pb fluctuations going down the core, it was consistent with typical ²¹⁰Pb profiles in the constant initial concentration (CIC) model (Zaborska et al., 2007). The CIC model was used to calculate sedimentation rates on the basis of excess ²¹⁰Pb activities. The mean sedimentation rate was calculated to be 3.11 and 1.56 cm/yr with a chronology of about 1930–2013 AD and 1910–2013 AD, and a best fit linear regression with a determination coefficient R = 0.76 and R = 0.92 for core S5-2 and JC07, respectively.

Figure 3.

Profiles of excess ²¹⁰Pb activity in sample cores S5-2 and JC07.

Results of grain-size analysis

The grain-size compositions of cores S5-2 and JC07 and related parameters (including sorting coefficients, skewness, kurtosis, and median grain size) are presented in Figures 4 and 5:

1. Sample core S5-2

Figure 4.

Grain size parameter variations of sample core S5-2 (σ – sorting coefficient; Sk – skewness; Kg – kurtosis; Md – median grain size).

Figure 5.

Grain size parameter variations of sample core JC07 (σ – sorting coefficient; Sk – skewness; Kg – kurtosis; Md – median grain size).

The contents of sand and silt of core S5-2 varied obviously with depth, especially in depths of 0–62 cm, where there was no significant change for clay content (Figure 4). The composition of sand and silt ranged from 0.07% to 22.13% and from 67.66% to 90.76%, with average values of 1.78% and 87.66%, respectively. The median grain size ranged from 6.2 to 15.71 µm (mean: 7.9 µm) and was coarser in the upper (0–62 cm) and finer below 62 cm. The sorting coefficient, skewness, and kurtosis fluctuated significantly in the upper 62 cm, but the variability was small. The sedimentation of core S5-2 can be divided into five stages according to grain-size parameters. The grain-size distribution curves clearly reflected the contents of each grain-size class and directly reflected the grain-size populations (Figure 6a). At the same time, the other grain-size parameters are usually used for sedimentary environment interpretations (Huang et al., 2011). Stage 1 (196–122 cm) was the period of 1930–1962, and there was relatively high silt content and low sand content. The grain-size distribution showed fluctuating change from the bottom up, with median grain size, sorting, and kurtosis mean value of 7.18 µm, 2.78, and 1.02, respectively. The sediment at the depth of 144 cm represented normal sediment. Stage 2 (122–62 cm) corresponded to the period of 1962–1987, and showed that sand content increased (mean grain size was 7.48 µm) with decreasing silt content. The sorting coefficient decreased to 2.71 and kurtosis increased to 1.04, and the sediment grain size of the maximum percentage changed from 8 to 10 µm. Stage 3 (62–44 cm), representing the period of 1987–1995, showed that grain-size curves first increased and then decreased. However, sand content was significantly higher than stage 2. Stage 4 (44–10 cm) contained sand content that significantly increased, and had a relatively coarser median grain size (9.54 µm) with a higher value of sorting (3.41). The grain size of the maximum percentage shifted from 11 to 28 µm. Stage 5 in the upper 10 cm represented the period of 2009–2013, and the sand content increased to 7.19%, while sorting highly fluctuated with a mean value of 3.52, resulting in median grain size increasing to 9.46 µm.

2. Sample core JC07

Figure 6.

Grain-size distribution curves of representative samples at different stages for core S5-2 (a) and core JC07 (b).

The sample core JC07 was collected from the Zhe-Min mud area of the ECS and, compared with core S5-2 that was collected far from the Yangtze estuary, had sand, silt, and clay with relatively low variability from bottom to top (Figure 5). Except for the sandy silt interlayer grain-size fraction showing a relatively high fluctuation, most of the samples displayed low amount of change, which indicated a stable sedimentary environment. Based on grain size and grain parameters, the sample core JC07 could be divided into five sections. The value of grain-size distribution curves for JC07 presented consistent results at different representative depths (Figure 6b). At stage 1 (160–128 cm), corresponding to the period of 1910–1932, there was relatively high sand content, and grain-size curves showed high fluctuations, with a coarser median grain size (7 µm) together with a relatively high value of sorting. Stage 2 (128–98 cm), corresponding to the period of 1932–1950, displayed a slightly decreasing pattern of skewness and kurtosis, resulting in a decreased trend in median grain size. Stage 3 (98–50 cm), from the period of 1950–1980, showed sedimentation coincident with stage 2; however, sediment of grain size showed fine change. Stage 4 (50–10 cm) was characterized by a relatively high silt content and low clay content, with a relatively coarse median grain size and low value of sorting and kurtosis. In stage 5, in the upper 10 cm from the period of 2006–2013, median grain size showed a slight decreasing trend.

Discussion

Response of sedimentary record to Yangtze channel erosion

Over the past century, sediment loads of many rivers have decreased in the world (Milliman, 1997; Syvitski et al., 2009), including the Nile, Colorado, Ebro, Mississippi, and Yellow Rivers (Blum and Roberts, 2009; Carriquiry et al., 2001; Fanos, 1995; Sánchez-Arcilla et al., 1998; Xu, 2008). In the Yangtze River, the third longest river in the world, sediment load in the Datong measure station fell from ~490 Mt/yr in the 1950s to ~150 Mt/yr after its closure because of the construction of the Three Gorges Dam (Yang et al., 2011). Mainly because of explosive population growth, more than 50,000 water reservoirs were constructed and water and soil conservation projects were implemented within this watershed (Yang et al., 2005b; Zhao et al., 2015). As the largest dam in the world, the Three Gorges Dam has trapped 1.8 Gt of sediments during the first decade after its completion in 2003 (Yang et al., 2014). During 1953–2010, total sediment interception of reservoirs of the Changjiang River amounted to 354 Mt/yr, of which the Three Gorges Dam alone contributed 41.2% (Figure 7a) (Gao et al., 2015a). Thus, the erosion/accretion balance has been destroyed, resulting in erosion occurring in the downstream channel of the Yangtze River (Figure 7b) (Dai et al., 2006; Yang et al., 2011). In the sample core S5-2 taken from Yangtze estuary mud, sand content increased in depths from 62 to 0 cm, corresponding to the period of 1986–2013. These results were similar to sediment load decrease and represented an abrupt time point in the Yangtze River (Yang et al., 2002; Zhao et al., 2015). Therefore, we deduced that downstream riverbed erosion of the Yangtze River occurred in the late 1980s, and the strength of erosion was much weaker. At the same time, in the upper 10 cm of sample core S5-2 from the period of 2009–2013, sand content markedly increased, implying extensive downstream channel erosion. However, it is unusual that sand content did not obviously increase in sample S5-2 from the operation of the Three Gorges Dam in 2003. The downstream erosion induced by the Three Gorges Dam from 2003 to 2013 was much weaker than expected. These results were consistent with Yang et al. (2007). Thus, we conclude that the downstream riverbed has been converted from deposition to erosion in the late 1980s, whereas the operation of the Three Gorges Dam in 2003 has trapped nearly two-thirds of the sediment from upstream and accelerated riverbed erosion, although this change seems much weaker than expected (Yang et al., 2007). The sample core JC07 was located in the Zhe-Min mud area of the ECS and, compared with core S5-2, the sand content exhibited relatively low variability from bottom to top.

Figure 7.

(a) Reservoir storage capacity and average sediment load of Datong station in the Yangtze River (modified after Gao et al., 2015a). (b) Channel deposition/erosion in the Yichang-Datong reach between 1956 and 2009 (modified form Dai et al., 2006, 2014; Yang et al., 2011).

Impacts of changing Yangtze River sediment load and East Asian winter monsoon on the sedimentary record

The Yangtze River average annual water and sediment loads were 900 km³/yr and 500 Mt/yr before its decline in the 1970s, respectively. Channel aggradation and delta progradation trap about 70% of the sediment load, leaving about 150 Mt/yr of sediments transported southward, forming the mud wedge extending along the inner shelf (Milliman et al., 1985). Smaller mainland Qiantang, Ou, and Min Rivers can collectively discharge ~15 Mt/yr (Kao et al., 2003). However, the sediment load has declined dramatically in response to the construction of >50,000 dams since the 1980s in the Yangtze River (Xu et al., 2006, 2007). The dramatic stepwise decreases of the Yangtze sediment load led to a decreasing sediment supply to the coastal ocean (Figure 8d). Consequently, sediment rates did not really change in the estuarine-inner shelf of the ECS and had consistent results with the 2002-year sediment rate (Liu et al., 2006; Su and Huh, 2002). These probably result from a corresponding decrease in coastal salt marsh accretion and erosion in the subaqueous delta front (Yang et al., 2011). Figure 8 shows the correlation of annual sediment discharge with median grain size of sample core S5-2, and indicates a negative relationship. Thus, with sediment load declined, median grain size of core S5-2 became coarse and contained a higher coarse population since the 1980s. The main reason was that riverbed erosion has resulted in large numbers of coarse particles being discharged into the Yangtze estuary and deposited near the YEMA (Yang et al., 2011). In addition, erosion has occurred, resulting in coarse grain-size sediment redistribution in the subaqueous delta.

Figure 8.

Temporal variations of (a) grain size of sample core S5-2, (b) East Asian winter monsoon index, (c) water level of Datong station in the Yangtze River, and (d) sediment load of Datong station.

The East Asian monsoon (EAM) plays a significant role in the global climate system (Yasunari and Seki, 1992) and greatly impacts and controls the climate of China (Zhang and Lin, 1992). The East Asian winter monsoon (EAWM) is the active factor in the East Asian winter–summer monsoon system (Ding et al., 1995). The environmentally sensitive grain-size component in coastal muddy sediments of China has been widely used to reconstruct the variation in the EAWM intensity (Hu et al., 2012; Xiang et al., 2006; Xiao et al., 2006). However, these studies focused on a millennial time scale, and the monsoon intensity from different sediment cores is inconsistent and contradictory (Zhou et al., 2014). Therefore, we chose the median grain-size distribution in sediment core S5-2 from the ECS inner shelf as a high-resolution proxy, and compared our results with historical record data to discuss variation of the EAWM during the last 100 years (Figure 8).

Coarser grain sizes are usually used to explain strong EAWM in the inner mud area of the ECS by sediments in the YEMA and were characterized as ‘deposited in summer and transported in winter’. Xiang et al. (2006) indicated that the sensitive population of 10.5–65.5 µm in grain size shows good correlation with the EAWM. Xiao et al. (2006) showed that the environmentally sensitive component of sediment grain size 45 µm has a relationship with temperature decline of historical periods in China by studying the inner shelf of the ECS, and indicated that these were the result of strengthening EAM activities. Thus, in our study, the 26.30 µm coarse population of sample core S5-2 could be selected for comparison with the EAWM index from 1930 to 2013. We found that the coarse population correlated well with the EAWM index, which was not consistent with previous work (Yang et al., 2007). Hence, the environmentally sensitive grain size could be used to construct and interpret the intensity of the winter monsoon in the inner shelf mud area of ECS.

Identification of modern flood and storm events

Grain-size analysis is an important method to identify and determine modern and ancient flood and storm events. The grain-size-sensitive compositions derived from the standard deviation method seem to be more sensitive than median grain to changes in sediment source and hydrodynamics (Sun et al., 2003). Thus, it can be concluded that the high power environment event could be taken as an effective indicator to the sedimentary environment (Fan et al., 2011; Wright, 1977). There have been many methods used to extract grain-size populations of sediments such as Weibull distribution (Sun et al., 2004), end-member modeling (Prins et al., 2000), and grain size versus standard deviation (Sun et al., 2003; Xiao et al., 2005). The standard deviation method allows a clear identification of grain-size intervals with the highest variability along a sedimentary sequence (Wu et al., 2015) and can be calculated for sediments based on different grain-size classes. Thus, in our study, we used the grain size versus standard deviation method to identify the grain-size intervals with the highest variability along a sedimentary sequence.

The standard deviations versus grain-size classes are displayed in Figures 9 and 10. Two main peaks in core S5-2 were observed for the <11.48 µm fine population and the >11.48 µm coarse population (Figure 9a). The fine population (<11.48 µm) represented the typical sediment particle size range of the sedimentary environment in the YEMA, whereas the coarse population indicated the more sensitive population of the sedimentary environment. According to the theory of marine sedimentary dynamics, the acceleration of the river flow velocity will transport a large number of the coarse particle size sediment deposited in the estuary. Thus, we selected the >11.48 µm coarse population for identifying the highest variability along a sedimentary sequence. The sensitive population and grain size of sediment core S5-2 showed several changes, which may provide a record of floods of the Yangtze River (Figure 9b). Core S5-2 was located in the YEMA, and when Yangtze River floods occurred, much coarse particle size from the river was rapidly deposited.

Figure 9.

(a) Decomposition of the grain-size distributions based on standard deviation method, and (b) vertical profiles of fine and coarse populations as well as median grain size of sediment particles for core S5-2.

Figure 10.

(a) Decomposition of the grain-size distributions based on the standard deviation method, and (b) vertical profiles of fine and coarse populations as well as median grain size of sediment particles for core JC07.

The standard deviation versus grain-size classes of core JC07 are displayed in Figure 10. Two peaks were observed, one at 3.32 and the second at 17.38 µm, representing the mode size of each grain-size component. Since 1980, the sedimentology of core JC07 was dominated by coarse sediment as shown by a high content of sand, perhaps coinciding with lower channel erosion and stepwise decreases in sediment load from the Yangtze River to the sea.

Reconstruction of modern flood and storm events in the last 100 years

Floods, droughts, and typhoons are frequent natural disasters in China, with floods inflicting considerable economic and human-life losses (Zhang et al., 2002). In recent decades, Yangtze River basin flooding has shown higher changes in intensity with the present global warming (Zhang et al., 2005). IPCC AR5 (2013) reported that the global mean temperature has increased since the late 19th century, and the first decade of the 21st century has been the warmest; we have found seven floods occurred during that period. Historical flood records showed that about 19 floods occurred in the warm 20th century during the past 200 years (Changjiang Water Resources Commission (Ministry of Water Resources, China) (CWRC), 2000). When floods occurred in the Yangtze River, increasing runoff and flow velocity transported large amounts of terrigenous weathered material into the sea, resulting in an increase in sedimentary coarse grain content. At the same time, previous research has detailed flood record information of the Yangtze River basin from the past 200 years (Shi et al., 2004). Thus, combining sedimentary records and historical literature can confirm the record of accurate flood events in the sedimentary sequence and reconstruct modern flood events in the recent 100 years. Xiao and Li (2005) indicated that the grain sizes of 19 and 130 µm reflect changes of coastal currents and storm currents in the ECS, respectively. For the mud area in the Yangtze estuary, based on grain size versus standard deviation analysis, the population of 26.30 µm was sensitive to flood events.

The vertical variation of the coarse-sensitive population and median grain size for sample core S5-2 is shown in Figure 9, which had correlated with historical literature on the Yangtze River floods since AD 1930 (Shi et al., 2004). We found that sand content, grain size, and flood events obviously increased since the 1990s, mainly because of the fact that the global hydrological cycle was accelerated with the global climate change in recent decades, resulting in increased occurrences of extreme precipitation events (Allen and Ingram, 2002). Global warming will lead to changes in spatial and temporal distributions of regional water resources and global hydrological cycles. Flood season (June, July, and August) precipitation increased in the Yangtze River basin; summer precipitation of the middle and lower stream were the highest of the recent 120 years. Figure 9 shows >26.30 coarse-sensitive population sedimentary years were 1931, 1937, 1949, 1954, 1970, 1980, 1991, 1996–1998, and 2010, which corresponded with historical literature (e.g. 1931, 1936, 1949, 1954,1969, 1980, 1991, 1995, 1998, and 2010). However, we found that two peaks were not recorded in the sedimentary record in 1960 and 2006 because of a catastrophic storm event occurring in the corresponding year.

Core JC07, located in the inner shelf mud of the ECS, was mainly derived from suspended sediments from the Yangtze River transported southward by the winter coastal current (Sun et al., 2000). Thus, there were fewer extreme flood events recorded in the sample core, such as 1921, 1954, and 1998, while others were not recognized in the sedimentary record. However, core JC07 near coastal Zhe-Min was impacted by typhoons, so extreme storm events in the sedimentary core corresponded well with historical literature (Figure 10). Based on grain size versus standard deviation analysis, the population of 17.38 µm was used to reflect storm events in the inner mud of the ECS. The number of landing typhoons and affecting typhoons from 1949 to 2006 corresponded well with the sedimentary record (e.g. 1949, 1956, 1962, 1974, 1989, 2000, and 2004–2006). Therefore, the large flood events in the Yangtze estuary mud region have been recorded and can be used to recover flood events in recent 100 years in the Yangtze River, find extreme precipitation events, and provide corresponding evidence for climate change.

Relationship between floods and atmospheric circulation

At present, the correlation of flood events and climate change has been widely studied and a consistent consensus has been achieved. Flood events have a close relationship with El Niño Southern Oscillation (ENSO) on a short time scale, and ENSO generated flood events through extreme precipitation (Jiang et al., 2006; Yu et al., 2009). Spectral analysis has been widely used to detect periodic change derived from stratigraphy, which helped to better understand the physical process of the sedimentary record in a time series (Xiao et al., 2006). The coarse population of sample time series was analyzed using the REDFIT35 (Schulz and Mudelsee, 2002), and the spectral analyses of the coarse population record are shown in Figure 11. Core S5-2 showed statistically significant periodicities centered on 11.2, 7, 5.6, 3.8, 2.7, and 2.3 a (Figure 11a). Figure 11b illustrates the spectral analysis results of core JC07, which revealed 38.5a, 14.28a, 9a, 5.2a, 4.5a, and 3.6a as being periodic. These were consistent with the ~3–8 a periodicity of ENSO (Peng et al., 2003), suggesting that flood events of the Yangtze River were likely linked to low latitude ocean-atmospheric activity. The 11 a periodic of sample core S5-2 could possibly be associated with sunspot activity. At the same time, a correlation was detected between flood events in the Yangtze River and the sea surface temperature (SST) (e.g. the SST index main cycles were 5.67, 3.78, 5.23, and 10.46 a) (Jiang et al., 2006). Spectral analysis further illustrated that the flood events usually follow ENSO years in the Yangtze River.

Figure 11.

Frequency analysis of sample core (a) S5-2 and (b) JC07 median grain-size record for the entire samples.

Conclusion

Sedimentary records from two gravity sample cores collected from the present estuarine-inner shelf mud regions of the ECS (core S5-2 and core JC07) show different responses to modern flood and storm events since 1910. The mean sedimentation rate was calculated to be 3.11 and 1.56 cm/yr with the chronology of about AD 1930–2013 and AD 1910–2013 for cores S5-2 and JC07, respectively. Furthermore, the sediment rate of the YEMA was clearly higher than that of the inner shelf mud region of the ECS, which were consistent with 2002-year sediment rates. These probably resulted from a corresponding decrease in coastal salt marsh accretion and erosion in the subaqueous delta front. Sample core S5-2 was taken from Yangtze estuary mud and the sand content showed increases in depths of 62–0 cm. We deduced that the downstream riverbed erosion of the Yangtze River occurred in the late 1980s. Sample core JC07 showed a similar trend, but the grain size was smaller than that of S5-2. We used the grain size versus standard deviation method to identify grain-size intervals with the highest variability along a sedimentary sequence. We found that for core S5-2, the >11.48 µm coarse population correlated well with historical literature in the Yangtze River floods since AD 1930. However, we found that two peaks were not recorded in the sedimentary record in 1960 and 2006 because of the fact that a catastrophic storm event occurred in the corresponding year. There were fewer extreme flood events recorded in sample core JC07 – such as 1921, 1954, and 1998 – while others were not recognized in the sedimentary record. However, sample core JC07 collected near the Zhe-Min coast was mainly impacted by typhoons, so extreme storm events corresponded well with historical literature. At the same time, we found that the grain-size coarse population of YEMA was a good proxy of the EAWM and becomes magnified when the EAWM strengthens. The spectral analyses of the sample core S5-2 coarse population record showed statistically significant periodicities centered on 11.2, 7, 5.6, 3.8, 2.7, and 2.3 a, and JC07 periodicities were centered on 38.5, 14.28, 9, 5.2, 4.5, and 3.6 a, which were consistent with ~3–8 a periodic change in ENSO, implying that flood events usually follow ENSO years in the Yangtze River. Consequently, the sediment records preserved in the two gravity cores illustrated different responses to the Yangtze River floods and storms during the last 100 years, which is important in recovering centennial scale flood events and inferring extreme precipitation, as well as understanding climate change in the estuarine-inner shelf of the ECS. Nevertheless, more efforts are still needed to simulate paleo-flood and predict future flood events in the context of global change.

Footnotes

Acknowledgements

The authors thank Haijuan Chao and Rui Yu for their help with sediment chronology analysis. The authors also thank Dr Fabienne Marret and two anonymous reviewers for their comments, critical reviews, and suggestions.

Funding

This study was supported by the National Natural Science Foundation of China (973 Program) (No. 2013CB956503), the National Basic Scientific Talent Training Foundation of China (No. J1103408), and Natural Science Foundation of China (No. 41471431).

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Recent sedimentary record of storms and floods within the estuarine-inner shelf region of the East China Sea

Abstract

Keywords

Introduction

Regional setting

Materials and methods

Sample collection

210Pb dating

Grain-size analysis

Results

Sediment chronology

Results of grain-size analysis

Discussion

Response of sedimentary record to Yangtze channel erosion

Impacts of changing Yangtze River sediment load and East Asian winter monsoon on the sedimentary record

Identification of modern flood and storm events

Reconstruction of modern flood and storm events in the last 100 years

Relationship between floods and atmospheric circulation

Conclusion

Footnotes

Acknowledgements

Funding

References

²¹⁰Pb dating