Sedimentary records off the modern Huanghe (Yellow River) delta and their response to deltaic river channel shifts over the last 200 years

Journal of Asian Earth Sciences 108 (2015) 68–80

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Sedimentary records off the modern Huanghe (Yellow River) delta and their response to deltaic river channel shifts over the last 200 years Xiao Wu a,b, Naishuang Bi a,b, Yutaka Kanai c, Yoshiki Saito c,d, Yong Zhang e, Zuosheng Yang a,b, Dejiang Fan a,b, Houjie Wang a,b,⇑ a

Collage of Marine Geosciences, Ocean University of China, 238 Songling Road, Qingdao 266100, PR China Key Laboratory of Submarine Geosciences and Prospecting Technology, Ministry of Education, China, 238 Songling Road, Qingdao 266100, PR China Geological Survey of Japan (GSJ), National Institute of Advanced Industrial Science and Technology (AIST), Central 7, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan d Department of Natural Environmental Studies, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwanoha 5-1-5, Kashiwa, Chiba 277-8563, Japan e Qingdao Institute of Marine Geology, China Geological Survey (CGS), 62 Fuzhou Road, Qingdao 266071, PR China b c

a r t i c l e

i n f o

Article history: Received 23 July 2014 Received in revised form 12 April 2015 Accepted 14 April 2015 Available online 30 April 2015 Keywords: Huanghe delta Sedimentation rate Grain size Deltaic channel migration

a b s t r a c t The modern Huanghe (Yellow River) delta sedimentary complex has developed since 1855, when the lower river channel migrated northward from the Yellow Sea to the Bohai Sea. As a result, the river-laden sediment accumulated rapidly on the lowlying plain and formed the mega-delta. Here we present high-resolution sedimentary sequences based on 137Cs and 210Pb dating and grain-size parameters of two sediment gravity cores collected near the present river mouth (core A11) and in adjacent Laizhou Bay (core A26). Based on these sedimentary sequences, the different responses of the sediment records to natural and artificial channel shifts are presented. The average sedimentation rates of the two cores A11 (1951–2006) and A26 (1808–2006) were estimated to be 2.25 cm/year and 0.80 cm/year, respectively. The results indicated that the geometry of the delta (e.g. the location of river mouth and changing coastline) and sediment supply from the river played an important role in the sedimentation of the subaqueous delta. The channel shifts in 1976 and 1996 shortened the distances from the river mouth to the location of core A11, resulting in the local accumulations of relatively coarse particles from the river mouth. The sedimentation of core A26 indicated four major channel shifts in 1855, 1904, 1947 and 1976. When the river mouth approached the core location, the accumulated sediment became finer; otherwise, the active resuspension due to strong hydrodynamics resulted in the accumulation of coarser sediment. The sediment records preserved in the two gravity cores illustrated different sedimentary responses to the lower channel shifts of the Huanghe and sediment supply from the river at centennial scales, which is critical to understanding the evolution of the modern Huanghe Delta in the past and to predicting the future trend. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction As the major link between continents and oceans, global rivers connect 87% of the Earth’s land surface to the ocean and annually deliver 20  109 metric tons (Gt) of river-borne sediment to the ocean (Milliman and Syvitski, 1992; Ludwig and Probst, 1998). The sediment delivered by rivers to the ocean mostly accumulates in coastal environments and forms numerous deltas, such as the Huanghe (Yellow River), Changjiang (Yangtze River), Nile, and Mekong river deltas (Wright, 1977; Stanley and Warne, 1993; Saito et al., 2000; Hori et al., 2002; Ta et al., 2002; Chu et al., ⇑ Corresponding author at: College of Marine Geosciences, Ocean University of China, 238 Songling Road, Qingdao 266100, PR China. Tel.: +86 532 66782950. E-mail address: [email protected] (H. Wang). http://dx.doi.org/10.1016/j.jseaes.2015.04.028 1367-9120/Ó 2015 Elsevier Ltd. All rights reserved.

2006). These deltas are both ‘drivers’ and ‘recorders’ of natural and anthropogenic environmental change (Bianchi and Allison, 2009). In recent years, global changes resulting from natural and anthropogenic forces have altered the global river system (Lu and Chen, 2008; Jiang et al., 2012). It is therefore critical to better understanding the response of delta evolution to changes in river systems. The Huanghe (Yellow River) has been well known for frequent channel migrations within its lower reaches and has been ranked the second largest in term of sediment load among the rivers worldwide (Milliman and Meade, 1983). Given the high sediment load and relatively low water discharge, the river sediment is considerably deposited on the riverbed, forming a ‘suspended river’ in the lower reaches of the Huanghe (Wang and Li, 2011). Therefore, there have been few major tributaries joining the main stream,

X. Wu et al. / Journal of Asian Earth Sciences 108 (2015) 68–80

forcing the lower reaches to meander for the past several thousand years (Chen and Luo, 2000). The deltaic river channel of the Huanghe has changed more than 50 times, forming the modern Huanghe delta following the northward shift of the Huanghe river mouth from the Yellow Sea coast in North Jiangsu Province to the Bohai Sea in 1855 (Pang and Si, 1979; Xue, 1993). These migrations of the lower channel and their effects on the sedimentary environments of the modern Huanghe delta and the adjacent Bohai Sea have been of broad interests to scientists and public. Saito et al. (2000) presented the Holocene evolution of the Huanghe delta based on analysis of sediment cores collected from the Huanghe subaerial delta and concluded that the delta progradation is dominantly controlled by the channel shifts of the lower reaches of the Huanghe. Changes in the sedimentary environment of the Bohai Bay and the mud areas in the central basin of Bohai Sea are also highly correlated with channel shifts of the Huanghe over the last 100 years (e.g. Hu et al., 2011; Qiao et al., 2011). The sediment supply from the Huanghe was cut off due to the relocation of the Huanghe river mouth in 1976, and locally resuspended sediment became a primary source to sediment dispersal in the Bohai Bay. As a result, there was a significant increase in the proportion of sand in surface sediment in the Bohai Bay (Hu et al., 2011). However, most previous studies based on sediment cores collected from the subaerial delta or in the central Bohai Sea and Bohai Bay only identified the channel shift in 1976 (Hu et al., 2011; Qiao et al., 2011), while the effects of other channel shifts since 1855 on the evolution of the modern Huanghe delta still remain unclear. The Huanghe river mouth area is an estuarine depocenter due to the weak hydrodynamics and a high riverine sediment supply (Shi et al., 2003). As a result, the sediment records off the Huanghe river mouth may contain substantial information on changes of the river system. The general circulation pattern of the Bohai Sea involves the North Shandong Coastal Current, which originates from the west coast of Bohai Bay and flows along the coast of the Huanghe delta and the shore of Laizhou Bay in winter and summer (Fang et al., 2000; Fig. 1). The transport of sediment from the river mouth to the Bohai Strait that connects the Bohai Sea and Yellow Sea (Fig. 1) is mainly driven by the North Shandong Coastal Current through Laizhou Bay. Therefore, the sedimentary records in Laizhou Bay, which is located on the main pathway of the Huanghe sediment from the Bohai Sea to the Yellow Sea, could be more sensitive to relocations of the Huanghe river mouth compared with those in the center of the Bohai Sea or Bohai Bay. In this study, we collected two sediment cores from different regions (core A11 at the present river mouth and core A26 in the adjacent Laizhou Bay) and investigated the high-resolution sedimentary sequences to reveal the different sedimentary responses of these two sedimentary records to natural and anthropogenic channel shifts at centennial scale.

2. Regional setting The Bohai Sea, a semi-enclosed epicontinental sea, is connected to the Northern Yellow Sea via Bohai Strait. The area of the Bohai Sea is 77,000 km2 and the average water depth is 18 m (Hu et al., 2011). Several large to median-sized rivers enter into the Bohai Sea, including the Huanghe, Luanhe and Haihe (Fig. 1). These totally deliver 89 km3 of fresh water annually into the Bohai Sea (Sündermann and Feng, 2004). There are three major bays within the Bohai Sea: Bohai Bay to the west, Liaodong Bay to the north, and Laizhou Bay to the south (Fig. 1). The Laizhou Bay with water depth <15 m, covers approximately ten percent of the total area of the Bohai Sea. The surface sediment is mainly composed by silt and becomes coarser from the Huanghe river mouth toward the Bohai strait (Jiang and Wang, 2005). Near the

69

Longkou shoal, the surface sediment is mainly coarse silt and fine sand (Jiang et al., 2004). The modern Huanghe delta, located west of the Bohai Sea, has developed due to the rapid deposition of a large amount of sediment delivered by frequent shifts of the deltaic river channel since 1855, when the Huanghe reentered into the Bohai Sea. The tidal regime of the Huanghe delta is dominated by an irregular semi-diurnal tide with tidal range of 0.6–0.8 m in average at the river mouth area but increasing both southwards and northwards to 1.5–2.0 m in the Laizhou Bay and Bohai Bay. The isobath-parallel tidal currents flow southward during the flood tide and northward during the ebb tide at an average speed of 0.5–1.0 m/s. The waves in the Huanghe river mouth are mostly driven by winds in the Bohai Sea and thus have strong seasonal variability. The dominant northerly waves in wintertime are much stronger than the prevailing southerly waves in summertime because of much longer fetch for winter monsoon (Wang et al., 2014). 3. Materials and methods 3.1. Sampling Two sediment cores were collected using a stainless steel gravity core sampler deployed by the R/V ‘Dong Fang Hong 2’ in 2006. The site of core A11 was located directly at the present river mouth with a water depth of 14 m, whereas core A26 was collected northeast to Laizhou Bay with a water depth of 18 m (Table 1 and Fig. 1). Cores A11 and A26 were 124 cm and 157.5 cm in length, respectively. No sediments were lost or distorted in the upper layers during the processes of sampling and storage. Sediment core A11 can be divided into 3 parts (0–41, 41–57, 57–124 cm) on the basis of sediment characteristics. Sediments in the upper 41 cm mainly consisted of grayish yellow clayey silt sediments with sand content less than 1.8%. In addition, a few black stripes were found occasionally in the sequence. Sediment in the 41–57 cm layer was yellowish gray clayey silt with relatively higher sand content. Grey clayey silt was observed at 57–124 cm in depth with 7% sand content. Sediment core A26 was divided into two parts based on sediment characteristics. The lower part from 79 to 157.5 cm consisted of grey clayey silt sediments. Some mollusk shell fragments were found at 127, 143 and 148 cm, respectively. The overlying upper part mainly consisted of grayish yellow clayey silt and is further divided into three parts (0–36, 36–58, 58–79 cm) based on the sediment characteristics. Sediment in the upper 36 cm mainly consisted of grayish yellow clayey silt sediments with a black stripe occurred at 4–6 cm. Grayish yellow clayey silt was found at 36–58 cm. Sediment in the 58–79 cm layer was yellowish gray silt with two shell fragments at depths of 60 and 70 cm. Sediment core A11 was sampled every 0.5 cm, whereas core A26 was sampled every 1 cm for grain-size analysis in laboratory. 3.2.

210

Pb and

137

Cs dating

The radionuclides 210Pb (half-life: 22.3 years) and 137Cs (half-life: 30.7 years) have been widely used in marine environments to date aquatic sediments (Huh and Su, 1999; Andersen et al., 2000; Li et al., 2003). Measurements of 210Pb and 137Cs activities of the samples were conducted at the Geological Survey of Japan, AIST, using an EG&G Ortec gamma spectrometer (Kanai, 2009; Kanai and Saito, 2011). After air-drying and pulverization, the powdered sample was sealed in a container for one month to attain radioactive equilibrium. The 137Cs contents were determined at 662 keV, whereas the total 210Pb and 214Pb activities were measured at 46.5 keV and 352 keV, respectively. The excess 210Pb

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X. Wu et al. / Journal of Asian Earth Sciences 108 (2015) 68–80

41°N

119 E

118 E

120 E

Location of sediment core

121 E

122 E

Circulation in wintertime Circulation in summertime

20m

g Bay

Liaodon

Water depth

m

40°N L

Lua

nR

Lia

ay

m

Yellow Sea

Bo

20m 38°N

P

la

tra it

Bohai Sea

ai B

iS

Boh

m

ha

m

39°N

ng

su

m

i

Ha

iver

er Riv

o od

in en

A11 A26

iver

wR Yello

Laizhou Bay

Longkou

Shandong Peninsula 37°N

118°E

120°E

119°E

121°E

122°E

Fig. 1. Study area and locations of gravity cores sampled in the Bohai Sea. The general patterns of seasonal circulation system were modified after Guan (1994) and Fang et al. (2000). The contour interval of water depth is 5 m.

4. Results

Table 1 Sampling records of the sediment cores A11 and A26. Sample

Latitude (N)

Longitude (E)

Water depth (m)

A11 A26

37°51.60 37°40.80

119°21.00 119°49.20

14.0 18.0

was calculated by subtracting activity.

214

Pb activity from total

210

Pb

3.3. Analysis of sediment grain size Air-dried and disaggregated sediment samples were pretreated with 30% H2O2 to decompose the organic matters and 1 mol/L HCl to remove carbonates. The grain sizes of the sediment samples were measured using a Mastersizer 2000 instrument (Malvern Incorporation) after dispersion and homogenization by ultra-sonic vibration. The sorting coefficient (r), skewness (Sk) and kurtosis (kg) were calculated as follows (Folk and Ward, 1957):

r¼

/84  /16 /95  /5 þ 4 6:6

ð1Þ

Sk ¼

  1 /84 þ /16  2/50 /95 þ /5  2/50 þ 2 /84  /16 /95  /5

ð2Þ

kg ¼

ð/95  /5 Þ 2:44ð/75  /25 Þ

ð3Þ

where /5 ; /16 ; /25 ; /50 ; /75 ; /84 and /95 are the particle sizes (in phi) of the different frequencies. The particle sizes were categorized into three fractions: sand (>63 lm), silt (63–4 lm), and clay (<4 lm).

4.1. Results of dating 4.1.1. Chronological framework of core A11 The excess 210Pb profile (activity versus depth) of core A11 presented valuable information regarding sediment accumulation. Generally, a downward decline due to the decay of 210Pb indicates that the sediment accumulated at a relatively constant rate in a stable sedimentary environment. In an unstable sedimentary environment, the excess 210Pb profiles exhibit segmented patterns as a result of the changes in sediment supply, grain size, hydrodynamic-induced mixing and biological perturbation (Li, 1993). Core A11 was collected at the present river mouth (Fig. 1), where strong hydrodynamics dominated the accumulation process. The excess 210Pb profile of core A11 illustrated relatively low values and did not show a clear decrease trend downward (Fig. 2A), and an inverse trend of excess 210Pb activity was observed between the depths of 61 cm and 82 cm. Therefore, the 210Pb data seemed to be unsuitable for estimating the sedimentation rate of core A11. The 137Cs isotope is fallout-derived and began to be observed in sediments in 1954 due to nuclear weapons testing (Fuller et al., 1999). Therefore, the deepest appearance of 137Cs is an effective marker of the year 1954. Two peaks in the 137Cs profile corresponding to 1963 and 1986 are also typically observed (Palinkas and Nittrouer, 2007). In core A11, 137Cs was firstly detected at a maximum depth of 117 cm, which corresponded to the year of 1954 (Fig. 2A). The two peaks of the 137Cs profile at the depths of 92 cm and 26 cm corresponded to the years of 1963 and 1986, respectively. Consequently, core A11 could be divided into three stages in terms of the sedimentation rate based on the 137Cs profile. The first stage (92–124 cm) corresponded to the period from 1951

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X. Wu et al. / Journal of Asian Earth Sciences 108 (2015) 68–80

(A) Core A11 Excess 210Pb activity (Bq/g) 0.0001 0

0.001

0.01

137Cs

0.1 0

0.004

(Bq/g)

0.008

0.012

0.016 0 10

10

20

1986

30

30

40

40

50

50

60

60

70

70

Depth (cm)

Depth (cm)

20

80

80

1963

90

90 100

100

110

110

1954 120

120

130

130

(B) Core A26 Excess 210Pb activity (Bq/g) 0.001

0.01

137Cs

0.1 0

0.004

(Bq/g)

0.008

0.012

0.016 0

0

el

1986 10

nce

lev

10

20

con

fide

20

%

30

40

lev

el

40

1954

nce

50

50

Depth (cm)

1963

60

70 80

90

%

60

con

fide

Depth (cm)

90

30

70

Detection limit

Y=-23.25*Ln(X)-75.28 N=18 R2 =0.78

80 90

90 Fig. 2. Profiles of excess

210

Pb activity and

to 1963 and had an average sedimentation rate of 2.78 cm/a (the sedimentation rate below 117 cm was regarded as being equal to the rate of 92–117 cm). The second (26–92 cm) and third stages (0–26 cm) corresponded to the periods of 1963–1986 and 1986– 2006, respectively, and had average sedimentation rates of 2.87 cm/a and 1.30 cm/a, respectively. Overall, the sediment in core A11 was deposited during the past 60 years with an average sedimentation rate of 2.25 cm/a. 4.1.2. Chronological framework of core A26 Core A26 was obtained northeast to Laizhou Bay, farther away from the present river mouth than core A11, presenting a relatively stable sedimentary environment with an average tidal current of

137

Cs in cores A11 (A) and A26 (B).

0.2–1.0 m/s. The 210Pb profile of core A26 showed a stable downward decrease in the activity of excess 210Pb, and a best-fit linear regression with a determination coefficient R2 = 0.78 was found within 90% confidence level (Fig. 2B), which yields an estimate of average sedimentation rate of 0.72 cm/a. 137Cs was firstly detected at a maximum depth of 53 cm (Fig. 2B). Two peaks in the 137Cs profile were identified that corresponded to the years of 1963 at depth of 37 cm and 1986 at depth of 9 cm, respectively (Fig. 2B). Therefore, the combined data of excess 210Pb and 137Cs were used to calculate the varying sedimentation rates of core A26. We applied more reliable 137Cs age model to the upper part of the core after 1954 and excess 210Pb age model to the middle to lower part of the core before 1954. Consequently, core A26 could be divided

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X. Wu et al. / Journal of Asian Earth Sciences 108 (2015) 68–80

into three stages in terms of the sedimentation rate based on the profiles of excess 210Pb activity and 137Cs: (1) 157.5–53.0 cm accumulated from 1808 to 1954; (2) 53.0–9.0 cm accumulated from 1954 to 1986; and (3) 9.0–0.0 cm accumulated from 1986 to 2006. The corresponding average sedimentation rates of the 3 stages were 0.72 cm/a, 1.38 cm/a and 0.45 cm/a, respectively. The chronological data of core A26 suggested an accumulation within 200 years with an average sedimentation rate of 0.80 cm/a.

A26 (Fig. 5B) at different representative depths. Stage 1 (124– 117 cm), corresponding to the period of 1951–1954, was characterized by a relatively high sand content (21.36%), low clay content (12.27%) and coarser median grain size (32 lm) together with a relatively higher value of sorting (1.67) and a higher value of kurtosis (1.11). Stage 2 (117–54 cm) indicated the sedimentation from 1954 to 1976 with decreasing sand content (5.26%) and increasing clay content (22.38%). The sorting coefficient increased to 1.75 and the kurtosis decreased to 0.98. As a result, the median grain size at this stage decreased to 12 lm correspondingly. The sediment at the depth of 81.5 cm presented a uni-modal distribution with maximum volume content at grain size of 12 lm (Fig. 5A). At stage 3 (at a depth of 54–40 cm) the sand content evidently increased to 7.85% together with decreasing clay content (15.53%). The sorting of this stage highly fluctuated with a mean value of 1.50, and the kurtosis increased to 1.20, resulting in increase in median grain size to 21 lm. The grain size of maximum percentage in representative sediment (45.0 cm in depth) changed from 12 lm to 48 lm (Fig. 5A). Stage 4 (40–28.5 cm), corresponding to the period of 1981–1985, showed highly smooth characteristics of all grain size parameters. There are almost no fluctuations of skewness (0.06– 0.10), kurtosis (1.02–1.05), sand content (0.30–1.34) and median grain size (6.7–7.7 lm). Compared with those at stage 3, the sand content decreased by 7.12% and clay content increased by 14.75% from 15.53% to 30.28%, as indicated by a finer pattern of median grain size (from 21 to 7 lm) and the maximum-percentage grain size shifting from 48 lm to 9 lm (Fig. 5A). At stage 5 (the upper 28.5 cm in depth with the period of 1985–2006) the sand content increased to 2.36% with clay content decreased to 23.31%. The median grain size of this stage was 11 lm with an abrupt increase (9 lm) at a depth of 13 cm that corresponded to the year of 1996, where the maximum-percentage grain size was 24 lm (Figs. 3 and 5A).

4.2. Results of grain-size analysis 4.2.1. Sedimentary sequence of core A11 Figs. 3 and 4 present the top-to-bottom grain-size compositions of core A11 and A26 and related parameters such as sorting coefficients (r), skewness (Sk), kurtosis (kg) and median grain size (Md). It seems that contents of sand and clay of core A11 varied significantly with depth, whereas there was no evident change for content of silt (Fig. 3). The sand and clay contents ranged from 0% to 35.41% (mean: 5.45%) and from 3.41% to 34.49% (mean: 21.96%), respectively. The median grain size ranged from 6 to 48 lm (mean: 14 lm). The profiles of the sorting coefficient, the skewness and the kurtosis fluctuated significantly with values of 0.97–1.90 (mean: 1.66), 0.02–0.49 (mean: 0.17) and 0.83–1.62 (mean: 1.03), respectively. According to the grain-size parameters the sedimentation of core A11 can be divided into five stages with evident differences in grain size and sorting coefficient. Besides the grain-size parameters, the grain size distribution curves can intuitively express the contents of each grain-size class and directly reflect the grain-size populations (uni-modal, bimodal or multi-modal shape). Thus the grain size distribution curves are usually used in the interpretations of sedimentary environment (e.g. Huang et al., 2011). Fig. 5 shows grain size distribution curves for core A11 (Fig. 5A) and core

Content (%) 0

0

20

40

60

80

Sk 0.8 1.2 1.6 2 -0.4 0

Kg

0.4 0.8 0.8 1.2

Md (µm) 1.6

0

25

50 2005

Stage 5

10

2000 1995

20

1990 1985

30 Stage 4 40

1980

Depth (cm)

50 1975 60 70

Age (a)

Stage 3

1970

80

Stage 2 1965

90 1960

100 110

1955 120

Stage 1 0

20

40

Sand

60

8 0 100 Silt

Clay

1951

Core A11

Fig. 3. Grain-size parameters of core A11 (r – sorting coefficient; Sk – skewness; kg – kurtosis; Md – median grain size).

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X. Wu et al. / Journal of Asian Earth Sciences 108 (2015) 68–80

content (%) 0

0

20

40

60

Sk 80 100 1.6

2.4

Md(µm) 0

25

50 2000 1990

Stage 6

10

Kg

0 0.25 0.5 0.4 0.8 1.2 1.6

1980

20 30

Stage 5

1970

Stage 4

1960

40 50

Depth (cm)

1940 1930

Stage 3

70

1920

80

Age (a)

1950 60

1910 90

1900 Stage 2

1890

100

1880

110

1870

120

1860

130

1850 1840

Stage 1

140

1830

150 160

1820 1810

0

20

40

60

Sand

80 100 Silt

Core A26

Clay

Fig. 4. Grain-size parameters of core A26 (r – sorting coefficient; Sk – skewness; kg – kurtosis; Md – median grain size).

Volume content (%)

14

(A) Core A11

12 10 81.5 cm (Stage 2) 45.0 cm (Stage 3) 30.5 cm (Stage 4) 13.5 cm (Stage 5)

8 6 4 2 0

0.1

1

10

100

1000

100

1000

Grain size (µm) 10

Volume content (%)

(B) Core A26 130 cm (Stage 1) 91 cm (Stage 2) 62 cm (Stage 3) 49 cm (Stage 4) 24 cm (Stage 5) 18 cm (Stage 6)

8 6 4 2 0

0.1

1

10

Grain size (µm) Fig. 5. Grain-size distribution curves of representative samples at different stages for core A11 (A) and core A26 (B).

4.2.2. Sedimentary sequence of core A26 Compared with the grain-size parameters of core A11, the grain sizes of core A26 exhibited relatively low variability from top to bottom (Fig. 4), which indicated a relatively stable environment

for local sedimentation. The median grain size of the sediment varied from 8 lm to 40 lm, with the sand content and clay content fluctuating within the ranges of 0.26–33.70% (mean: 12.04%) and 5.61–24.74% (mean: 13.68%), respectively. Based on the median grain size and grain-size parameters, the sedimentation of the core A26 could be divided into 6 stages. Stage 1 (157.5–125 cm in depth), corresponding to the period 1808–1855, was characterized by a relatively high sand content (11.09%), low clay content (12.13%) and relatively coarser median grain size (21 lm), with a higher value of sorting (1.63), kurtosis (1.01) and a maximum volume content at grain size of 57 lm (Fig. 5B). At stage 2 (124–83 cm in depth), the sedimentation from 1855 to 1912 was evidently characterized by decreasing sand content (9.71%) and increasing clay content (up to 15.77%), with a slightly decreasing pattern of skewness and kurtosis, leading to the median grain size decreasing to 18 lm. And the grain size distribution curve correspondingly changed to a bimodal pattern. Grain size distribution of the sediment (91 cm in depth) shows a primary mode at grain size of 34–57 lm, with a secondary mode at grain size of 6–8 lm (Fig. 5B). Stage 3 with the upper limit at 58 cm in depth represented the sedimentation during the period of 1912–1947, as indicated by the increasing content of sand (16.10%) and decreasing content of silt (69.89%). At this stage the content of clay kept stable, whereas the median grain size increased to approximately 26 lm, with relatively higher values of sorting (1.86) and skewness (0.32). The content of coarse subpopulation increased sharply, while the fine subpopulation was almost absent at a depth of 62 cm (Fig. 5B). The median grain size decreased to 15 lm between the depths of 58 and 37 cm with a lower value of skewness (0.11) (Stage 4, during the period of 1947–1964) because the sand content decreased sharply to 7.95% along with clay content increased to 16.29%. The coarse subpopulation decreased by 3.6% with an evident increase of fine subpopulation at 49 cm in depth (Fig. 5B). At

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Stage 5 (between 37–21 cm in depth with a period of 1964–1976) the sand content increased to 14.65% and clay content decreased to 11.83%, with a relatively higher value of skewness (0.31) and kurtosis (1.07), resulting that median grain size increased correspondingly to 27 lm. Compared with the representative sediment of stage 4, the sample at a depth of 24 cm showed a decreasing pattern in fine subpopulation and an increasing pattern in coarse subpopulation. The maximum-percentage grain size for the coarser subpopulation increased from 41 lm (49 cm in depth) to 48 lm (Fig. 5B). Different from the upward coarsening pattern of stage 5, median grain size of stage 6 (the upper 21 cm with a period of 1976–2006) showed a slightly decreasing trend, especially at depth of 21–14 cm (during the period of 1976–1982). The median grain size changed from 27 lm (21 cm in depth) to 13 lm (14 cm in depth), with 4% decreasing of sand content and 5% increasing of clay content. After that, the grain-size parameters seemed to fluctuate significantly upward above the depth of 14 cm, which perhaps indicated an unstable sedimentary environment (Fig. 4).

sediment core A11 was located) in the summer season due to the trapping effect of tidal shear front and the relatively weak hydrodynamics near the river mouth (Wang et al., 2007a; Bi et al., 2010). As a result, a minor fraction of the river sediment, mostly the fine clay fraction, is transported by the coastal currents over long distance and deposited across or along the shore in summertime (Jiang and Wang, 2005). The climate in the Bohai Sea is strongly controlled by the East Asian monsoon that has strong seasonal variability. The hydrodynamics in the Bohai Sea in winter are much stronger than those in summer due to the strengthening monsoonal climate in wintertime (Wang et al., 2014). Consequently, the sediment resuspension in the coastal region became extremely active in winter season, resulting in much more sediment being kept suspension in the water column (Yang et al., 2011). Therefore, the modern Huanghe delta coastal area acts as a sink for Huanghe sediment in summer but transited to be a sediment source in winter. The resuspended sediment is transported southeastward along the coast of Laizhou Bay by the monsoon-enhanced coastal currents passing through the location of the sediment core A26. Therefore, the sediment of core A11 mainly included material delivered by the Huanghe during the summer season, whereas the sediment in core A26 seemed to be a combination of multiple sources including the fine fraction of the suspended sediment dispersed from the river mouth, the resuspended sediment from the subaqueous delta in winter, and locally resuspended sediment from the Longkou Shoal. However, the grain size of the surface sediment became coarser between the Huanghe river mouth and the northeast of Laizhou Bay (Jiang and Wang, 2005). This pattern indicates that the sediment from the Huanghe (either the fine fraction in summer or the resuspended sediment from Huanghe subaqueous delta in wintertime) was finer than the locally resuspended sediment near the location of core A26.

5. Discussion 5.1. Sediment sources of the two sediment cores The average annual water and sediment discharges of the Huanghe are 49 km3/a and 1.1 Gt/a, respectively, accounting for more than 50% of the water discharge and 90% of the river-borne sediment delivered to the Bohai Sea (Ren and Shi, 1986; Wang et al., 2014). The dispersal pattern of the massive Huanghe sediment in the coastal region is mainly controlled by tidal shear fronts and tidal currents (Wang et al., 2007a; Bi et al., 2010). A majority of the Huanghe suspended sediment is rapidly deposited in the subaqueous delta region less than 15 km from the river mouth (where

Bohai Sea

4

4

7

4 4

38o00'N

7

6

4 1

4

6

1 2

9 6

Yuwa

6

3

8

Ninghai 37o30'N

5

Lijin

Huanghe Delta

1 1855-1889

2 1889-1897

3 1897-1904

4 1904-1929 1934-1938 6 1947-1964 8 1976-1996

5 1929-1934 7 1964-1976

37o00'N 118o00'E

Laizhou Bay

5

9 1996-present

118o30'E

119o00'E

Fig. 6. Historical channel shifts of the lower Huanghe since 1855 (modified after Xue, 1993).

X. Wu et al. / Journal of Asian Earth Sciences 108 (2015) 68–80

5.2. Sedimentary records indicative to the river channel shifts Since the Huanghe river course migrated northward from the Yellow Sea to reenter into the Bohai Sea in 1855, the lower river channel has shifted more than 50 times due to rapid channel siltation and resultant instability of river channel, with major shifts occurring approximately every 10 years (Pang and Si, 1979) (Figs. 6 and 7). The distance from the river mouth to the locations

75

of core A11 and core 26 has changed due to frequent channel shifts, which resulted in unstable sedimentation rates and varying grain size (see Figs. 3 and 4). The grain-size composition within the sedimentation has been believed to be an effective indicator to the sedimentary environment (Wright, 1977; Friedman and Sanders, 1978; Fan et al., 2011). The populations derived from standard deviation method or principle component analysis seem to be more sensitive than

Fig. 7. Morphological changes of the Huanghe Delta based on LANDSAT images from 1976 to 1997. The LANDSAT images are available at http://glovis.usgs.gov/. Dashed lines indicate the contours of water depth with an interval of 5 m.

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the index of median grain size to changes in both the sediment source and hydrodynamics (Sun et al., 2003). The standard deviation method allows a clear identification of grain size intervals with the highest variability along a sedimentary sequence. In many cases, the standard deviation method was selected to partition the sensitive populations of the sediment from the grain-size distribution (e.g. Fan et al., 2011; Huang et al., 2011). The standard deviations could be calculated for sediment samples based on different grain size classes measured by Malvern Mastersizer 2000. The plot of standard deviation values vs. grain size classes shows several peaks, and each population of these size classes may represent a population of grains with the highest variability through time (Figs. 8 and 9). According to the diagram of the standard deviation of the particle diameters, two populations of core A11 were identified, i.e., <17 lm (fine population) and >17 lm (coarse population) (Fig. 8A). The sensitive populations and grain-size parameters of sediment core A11 changed significantly, which might represent a record of the channel shifts of the Huanghe lower river course since 1954. The location of core A11 was far away from the river

mouth when the Huanghe flowed northward into Bohai Bay during 1954–1976 (Fig. 7A), as the coarser sediment rapidly deposited off the river mouth and the finer suspended sediment was transported along the deltaic coast. As a result the sediment at the depth of 81.5 cm (representative sample at stage 2) presented a uni-modal shape frequency curve with a crest grain size value of 12 lm (Fig. 5A). Correspondingly, accumulation of fine-grained sediment (12 lm in median grain size) was identified at depths of 117–54 cm in core A11 (Fig. 3). After the lower channel shift in 1976, the river mouth migrated southeastward to approach the location of core A11 (Fig. 7B). Consequently, the rapid deposition of coarser sediment contributed much to the sedimentation of core A11, resulting in a maximum-percentage particle size of 48 lm (Fig. 5A) and a coarser layer (54–40 cm in depth) identified in core A11 (Fig. 3). Nevertheless, the grain-size composition varied significantly within this coarser layer, which suggests that the local sedimentary environment was unstable due to the absence of levee and bank constraining on meandering lower channel during this short period (Fig. 7B and C). Along with the channelization since

(A) Standard deviation

3

Core A11

2

Coarse population

Fine population

1

17µm

0

0.1

1

10

100

1000

(B) Fine population (%) Coarse population (%)

Md (µm)

10000

Size (µm)

0

20 40 60 80 100 0

20 40 60 80 100 1

0 10

10

100 2005

Stage 5

2000 1995

20

1990 1985

30

Stage 4 40

Depth (cm)

50 1975

Age (a)

1980

Stage 3

60 70 80

1970

Stage 2 1965

90 1960

100 110

1955 120

Stage 1 Fig. 8. (A) Decomposition of the grain-size distributions based on standard deviation method and (B) vertical profiles of fine and coarse populations, and median grain size of sediment particles for core A11.

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the 1980s, the geometry of the Huanghe delta became steady and therefore led to a relatively stable sedimentary environment and caused finer and smooth patterns of grain size parameters during the period of 1981–1985 (Figs. 3 and 5A). As the active delta lobe prograded southeastward, the distance from the location of core A11 to the river mouth gradually increased (Figs. 7D and E). The longer distance between the river mouth and the location of core A11 caused less coarser sediment contributed to the core A11 (Figs. 3 and 7C–E). In 1996, the lower channel was artificially shifted northeastward, and the river mouth approached the location of core A11 again (Fig. 7F), which corresponded to an increase event in coarser fraction and the median grain size at the depth of 13.5 cm (Fig. 8B). The relocation of the Huanghe river channel caused that the maximum-percentage grain size changed from 9 lm (at 30.5 cm in depth) to 24 lm (at 13.5 cm in depth) (Fig. 5A). The sedimentation and grain-size composition of core A11 presented the impact of the Huanghe channel shifts on regional sedimentary environment since 1954. As for core A26, the critical grain size of 29 lm derived from standard deviation method allowed for the division of coarse and

Standard deviation

2.5

fine populations (Fig. 9A). Prior to 1855, when the Huanghe entered the Yellow Sea, the accumulation of core A26 was dominated by coarser sediment as indicated by high content of sand and well-sorted sediment within the base layer (Stage 1 in Figs. 4 and 5B), suggesting a highly energetic environment with active sediment resuspension. When the river reentered into the Bohai Sea after 1855, the dispersal of river-laden sediment contributed much to the sedimentation of core A26, as indicated by the channel migrations from 1855 to 1904 (Fig. 6), and the grain size distribution curve correspondingly changed to a bimodal pattern and the fine population increased to 69.3% (Figs. 5B and 9B). As a result, a fine-grained layer with increasing clay content was formed at depth of 125–83 cm during the period of 1855–1912 (Fig. 4). When the Huanghe flowed northward into southern Bohai Bay during 1904–1929 (see Fig. 6), less riverine sediment was contributed to the sedimentation of core A26 and the local resuspension became a dominant process, which induced an evident increase in median grain size and a decrease of fine population (Figs. 4, 5B and 9B). Although the Huanghe transitorily flowed into the Laizhou Bay during 1929–1938, there was no clear

Core A26

(A)

2

Coarse population

Fine population

1.5 1

29 µm

0.5 0 0.1

1

10

100

1000

10000

Size (µm)

(B)

Fine population (%) 0

20 40 60 80 100

Coarse population (%) 0

20 40 60 80 100 1

0 10

10

100

2000 1990

Stage 6 1980

20 30

Md (µm)

Stage 5

1970

40 1960 50

Stage 4

Depth (cm)

70

1940

Stage 3

1930

Age (a)

1950 60

1920

80

1910 90 100

1900

Stage 2

1890 1880

110

1870

120

1860

130

1850 1840

140 150 160

Stage 1

1830 1820 1810

Fig. 9. (A) Decomposition of the grain-size distributions based on standard deviation method; and (B) vertical profiles of fine and coarse populations, and median grain size of sediment particles for core A26.

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response of sedimentation to the channel shift perhaps because the lower river meandered with unchannelized flow and thus dumped the river sediment mostly on the subaerial delta (Wang and Li, 2011). During the period of 1938–1947, the Huanghe flowed southward into the Yellow Sea and the river had no direct contribution to the sedimentation of core A26. The coarser layer between 83 cm and 58 cm recorded the impacts of channel migrations on the coastal sedimentary environment (Figs. 4 and 9B). From 1947 to 1964, the Huanghe returned to the Laizhou Bay (Fig. 6) and thus directly contributed river sediment to core A26, as indicated by the slight decreases in both sand fraction and median grain size (Fig. 4). The curve of grain size distribution also changed evidently together with fine population increasing to 73.4% (Figs. 4, 5B and 9B). In 1964 the river channel shifted northward to the Diaokou course (Fig. 6), as sediment resuspension became a dominant process at location of core A26, leading to continuous increases in both sand fraction and median grain size within the layer between 37 cm and 21 cm (Figs. 4 and 9B). During the same period (1965–1976) the median grain size in records of core A11 (layer between 88 cm to 54 cm) was found to be 10 lm in average (Fig. 3), much finer than that in the corresponding layer in core A26 (27 lm) (Fig. 9B), which suggested that the alongshore transport of fine-grained river sediment from the river mouth contributed much to the sedimentation of core A11 rather than core A26. After the lower channel shifted southeastward again in 1976 following the present Qingshuigou course (Fig. 7A), the river

sediment was transported to the location of core A26, leading to a decrease in median grain size from 1976 to 1982 (21 cm–14 cm). However, compared with sedimentation formed during the period of 1855–1912, the grain-size parameters fluctuated largely in the upper 40 cm, and the median grain size tended to be coarser (25 lm versus 18 lm), perhaps coinciding with the lower channel shifts (Fig. 7) as well as the stepwise decrease in sediment load from the Huanghe to the sea since the 1950s (Fig. 10A; Wang et al., 2007b, 2010). As we discussed above, the sedimentation at core A11 were mainly resulted from the accumulation of the Huanghe sediment mostly during the summer season, whereas the sedimentation at core A26 was ascribed to a combined accumulation of multiple sediment sources including the sediment discharged from the Huanghe and the locally resuspended sediment. These sediments from different sources eventually mixed with each other in different proportions depending upon the river channel shift and the changing sedimentary environment, which led to an evident bimodal distribution of grain size of core A26, largely different from the uni-modal pattern identified at core A11 (Fig. 5). Therefore, the sediments at core A26 could be effectively sorted by using standard deviation method, and the varying abundances of fine and coarse populations could be effectively indicative of sedimentary environment changes caused by river channel shifts. In contrast, the sediments accumulated at core A11 were directly supplied by the Huanghe with a uni-modal distribution of grain size (Fig. 5A).

(A) sediment load 2.5

0.83 Gt/a (1968-1986)

1.5

0.40 Gt/a (1986-1999)

1.0

Longyangxia Reservoir (1986)

Sediment load (Gt/a)

2.0

Median grain size (µm)

40

(B) Core A26

30

Liujiaxia Reservoir (1968)

0

Sanmenxia Reservoir (1960)

0.5

Xiaolangdi Reservoir (1999)

1.24 Gt/a (1950-1968)

0.15 Gt/a (1999-2010)

27 µm

20

16 µm

10

0 1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

Year Fig. 10. Time series (1950–2006) of the annual sediment load from the Huanghe to the sea (A), illustrating that the stepwise decrease in river sediment load is closely associated with the construction of large reservoirs in the river basin; and the annual median grain size identified from the sedimentary sequence of core A26 (B). The data of annual sediment load at station Lijin are accessible from the Chinese River Sediment Bulletin complied by the Ministry of Water Resources of the People’s Republic of China (http://www.mwr.gov.cn/zwzc/hygb/zghlnsgb/).

X. Wu et al. / Journal of Asian Earth Sciences 108 (2015) 68–80

The standard deviation method statistically allows the differentiation of fine and coarse populations of the sediments at core A11, but the results of which does not match the uni-modal distributions of grain size (Fig. 5). Therefore, the relative abundances of fine and coarse populations at core A11 might not be an effective indicator to the changing sedimentary environment. 5.3. Distinct decreases in the sediment loads Natural and anthropogenic forcing has caused distinct stepwise decreases in the sediment loads from the Huanghe to the sea since the 1950s (Fig. 10A; Wang et al., 2007b, 2012). Human activities have played a dominant role and are estimated to account for 70% of the decrease in river sediment load (e.g. Wang et al., 2006; Yu et al., 2013a, 2013b). The completion of the Liujiaxia reservoir in 1968 marked the first step of decrease in sediment load, causing the river sediment load decreased from 1.24 Gt/a (1950–1968) to 0.83 Gt/a (1969–1985). Another evident decrease was found in 1986 when the Longyangxia reservoir began operating, with the sediment load decreased further to 0.40 Gt/a (1986– 1998). After the completion of the Xiaolangdi reservoir in 1999, sediment load delivered from the Huanghe decreased to 0.15 Gt/a (1999–2010) (Fig. 10A). The evident stepwise decreases of the Huanghe sediment load led to a decreasing sediment supply to coastal ocean. Consequently, lower sedimentation rates of 1.30 cm/a at core A11 and 0.45 cm/a at core A26 occurred during 1986–2006, which were less than 50% of those prior to 1986. Besides the deceasing sedimentation rates, the decrease in sediment supply also led to a decreasing contribution of fine-grained sediment to core A26, as indicated by the generally increased median grain size (Fig. 10B). Together with stepwise decreases of the Huanghe sediment load, the median grain size of core A26 also presented a stepwise increase from 16 lm (before 1968) to 27 lm (from 1968 to 2006) (Fig. 10B). It is noteworthy that the median grain size of core A26 decreased from 36 lm to 13 lm during the period of 1976–1981 when the river sediment decreased (Fig. 10A), which represented a response to the shift of lower channel to the present Qingshuigou course in 1976. The river mouth approached the location of core A26 again (Figs. 6 and 7A), resulting in more fine-grained sediment accumulated at core 26. As the sediment load from the Huanghe decreased further from 0.83 Gt/a (1969–1985) to 0.40 Gt/a (1986–1998), the contribution of riverine sediment to the sedimentation of core A26 became less and the local resuspension became a dominant process again, which induced an evident increase in median grain size (Figs. 9B and 10B).

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6. Conclusions Sedimentary records from two short gravity cores collected at the present Huanghe river mouth (core A11) and in adjacent Laizhou Bay (core A26) presented different responses to lower channel shifts since 1855. We found that the geometry of the delta (e.g. location of river mouth and the changing coastline) and the sediment load delivered by the river played an important role in the sedimentation of the subaqueous delta. The channel shifts in 1976 and 1996 shortened the distances from the river mouth to the location of core A11, resulting in the local accumulations of relatively coarse particles from the river mouth. During the period of 1954–1976 when the river mouth was located north of the delta, the physical sorting over long distance led to accumulation of fine-gained sediment at core A11. The sedimentation of core A26 seemed to be more complex than that of core A11, as impacted by the multiple sediment sources and highly energetic sedimentary environment. The variations of grain-size parameters illustrated four major channel shifts in 1855, 1904, 1947 and 1976. When the river mouth approached the core location, the sediment became finer; otherwise, the active resuspension due to strong hydrodynamics resulted in the accumulation of coarser sediment. The stepwise decreases in river sediment load to the sea since the 1950s has changed the deltaic sedimentary environment. The sedimentation rates of core A11 and A26 during 1986–2006 decreased to 1.30 cm/a and 0.45 cm/a respectively, approximately 50% of those prior to 1986. The grain size of core A26 increased correspondingly due to inadequate river sediment supply. The sediment records preserved in the two gravity cores illustrated different responses to the lower channel shifts of the Huanghe and sediment supply from the river during the last 200 years, which is critical to understanding the evolution of the modern Huanghe Delta in the past and to predicting the future trend. Nevertheless, more efforts are still needed to reveal how the deltaic sedimentary environment was impacted by the river system given the strong interactions between anthropogenic and natural forces in the context of global change.

Acknowledgements We appreciated the editor and the anonymous reviewers for their constructive comments that improved the quality of original manuscript. This work was supported by the National Science Foundation of China (NSFC, Nos. 41376096, 41476069 and 41376079), National Fundamental Research Program, MOST of China (No. 2010CB951202), and by JSPS G8 Research Councils Initiative of Japan (DELTAS).

5.4. Impacts of changing deltaic geometry on the sedimentary process The sedimentary sequences identified from the two gravity cores on the subaqueous Huanghe Delta presented different responses to the historical shifts of lower river channel. The near field core (A11) preserved coarse-grained sedimentations as the river mouth approached the core location, as a result of rapid deposition of river sediment by plume dispersal. In contrast, sediment accumulated at the far field core (A26) was finer due to the physical sorting over long distance between the river mouth and the core location. Therefore the deltaic geometry including the changing location of river mouth and the coastline that constrained the coastal currents and sediment transport seemed to be of primary significance to the sedimentations of the subaqueous delta, in addition to the fluctuating sediment delivery from the river. Understanding the complex interactions between delta morphological changes and sedimentary processes would be critical to predicting the future evolution of the delta.

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