Cheng Guodongye, Yuguang Diao Shaobo (Institute of Marine Geology, Ministry of Geology and Mineral Resources) Abstract: Based on the 210Pb profile data of drill cores in the eastern Yellow River Delta, after standardization to eliminate the influence of 210Pb concentration changes caused by different particle sizes, the hole's

The 210Pb profile can be divided into 11 segments.

Each segment decays exponentially, and the 210Pb radioactivity ratio increases inversely between segments, reflecting the dilution of 210Pb due to redeposition.

From this, the sedimentation and redeposition history of the area can be analyzed.

Keywords: 210Pb profile, standardization, redeposition, dilution. Incidental deposition of 210Pb has been used in the study of the Yellow River Delta for many years.

In 1987, the 210Pb profile of the drill core that passed through the delta sedimentary layer was used to determine the ages of the superimposed lobes and to obtain the 210Pb background value in the Yellow River estuary.

In 1992, the correlation between 210Pb adsorption concentration and particle size was studied, and different particle size parts in the sample were normalized to the 10φ particle size to standardize the 210Pb profile, thus eliminating the influence of 210Pb concentration changes caused by different particle sizes.

Restore the original characteristics of the exponential decay of the 210Pb profile.

These research results lay the foundation for studying the redeposition of the Yellow River Delta.

This paper uses full core cores from Hole 91C2 in the eastern Yellow River Delta.

The system measures the 210Pb value of each layer.

Combined with the historical data on the evolution of the Yellow River Delta, the redeposition of the Yellow River Delta was analyzed from the sedimentary characteristics and 210Pb profile characteristics.

1. Characteristics of Yellow River Delta Sedimentation The modern Yellow River Delta is a young delta that grew and developed after the Yellow River was diverted into the Bohai Sea in 1855. It has only a history of 139 years, which happens to be the effective range of 210Pb dating.

The Yellow River is a sandy river.

The average annual sediment transport volume is about 1 billion tons, and the runoff volume is about 40 billion m3.

The annual average sand content is as high as 25kg/m3.

The highest sand content in history reached 222kg/m3 (September 7, 1973).

The high sandy water body of the Yellow River causes many characteristics of the Yellow River delta sedimentation: (1) Frequent swings of the river channel. A large amount of sediment carried by the Yellow River accumulates rapidly in the mouth section, forming a sand bar at the mouth of the river.

This raises the bottom of the bed and blocks the flow of water, forcing the diversion to be rerouted.

Major diversions in the Yellow River Delta occurred nine times in 139 years, with an average of one diversion every 13 years (Figure 1).

(2) The multi-lobed superimposed Yellow River distributary forms an accumulation body at the mouth of the river during its flow, called a delta lobe, which lies horizontally in the shape of a kidney at the mouth of the sea.

The maximum width of the accumulation reaches 50km. The middle part is composed of very fine sand and coarse silt sand, and the two sides are silt sand and clayey silt sand.

Due to the large lateral span of the leaf lobes, the sides of the leaf lobes in different stages overlap.

Composite sedimentary sequences of 2 to 3 lobes at different stages can be seen in the vertical section.

(3) Paroxysmal sedimentation The Yellow Sea basin is located in a monsoon climate controlled area with four distinct seasons and concentrated rainfall. The runoff in July, August and September every year is shown in Figure 1. The migration of distributary channels in the Yellow River Delta and its corresponding leaflet distribution map (cited in

Zi Cheng Guodong, 1991) Fig.l Modern Yellow River Deltaic lobes. Dashed line is the 1st subdeltaic (1855～1934) ancient channels and lobes. Dark line is the 2nd ones (1934 to present). The dashed line is the 1st subdeltaic (1934 to present).

The paleo-channels and tongue-shaped accumulations from 1855 to 1934); the solid lines represent the paleo-channels and tongue-like accumulations of the second sub-delta (1934 to present).

The formation years of each lobe are: ①1855.6～1889.3; ②1889.3～1897.5; ③1897.5～1904.6; ④1904.6～1926.6; ⑤1926.6～1929.8; ⑥1929.8～1934.8; ⑦1934.8～1 953.7;

⑧1953.7～1964.1; ⑨1964.1～1976.5; ⑩1976.5～now accounts for 65% of the whole year, and the sediment transport volume accounts for 85%.

Therefore, during the flood period, the Yellow River has a large flow rate and high velocity, scouring the river bed. Since the bottom sediments of the Yellow River are mainly composed of silt, which has the same particle size as the suspended sediment in the Yellow River (Figure 2), the bottom sediments are easy to resuspend.

The scour depth can reach 2 to 4m (Figure 3).

On the contrary, when the flood period transitions to the dry period, a large amount of sediment is deposited.

Years of topographic survey of river sections show that the sedimentation of the Yellow River is episodic and has the characteristics of floods, erosion and siltation.

However, the situation outside the estuary is the opposite. A large amount of sediment brought by the flood period accumulates in the estuary. Under the action of wind and waves, especially storm surges, the estuary area is eroded, and the sediment is transported to both sides of the delta and the front delta. This is true during the dry season.

It is more obvious in strong wind season.