The Yellow River Delta adjacent to the Bohai Sea is located in the monsoon region of northern China, which is sensitive to global climate change. The core data of KY-01 borehole in the Yellow River Delta and the published records were used to reconstruct the climate environment. Based on the analysis of carbonate content, magnetic susceptibility, Ostracoda, foraminifera, sporopollenin and AMS 14C dating on the KY-01 borehole sediment core, the evolution of both the climate and sedimentary environment has been discussed. The results show that: 7656–4145 cal.aBP, seawater moved toward the land surface and the climate was generally warm and humid, when there were small-scale extreme climate fluctuations; 4145–2544 cal.aBP, transgression and recession occurred, the climate changed from warm-wet to dry-cold and there were small-scale extreme climatic events; and 2544 cal.aBP–1855 AD, the sea level was relatively low, transgression and retreat alternately occurred, the climate was drier and colder than the previous stage and the warming and cooling alternated. During the middle Holocene, cold-dry events occurred between 5737–5422 and 4451–4081 cal.aBP, which is consistent with both Chinese and worldwide climate records.

  • Reconstruction of Holocene environment in the Yellow River Delta using multi-proxy indices.

  • Two extreme climate events in the study area since Holocene were analyzed.

  • Compared with the Holocene climate and environmental changes in Shandong Province.

Graphical Abstract

Graphical Abstract
Graphical Abstract

Holocene is an important geological period in geology, also known as the post-glacial period. It is also a geological period most affected by human activities and closely related to human evolution and development. The climate change during Holocene reflects the future trend of climate change and has become the focus of PAGES (Burciaga et al. 2019; Chen et al. 2020; Han et al. 2020; Javadinejad et al. 2020). Due to the comprehensive influence of solar radiation, atmospheric circulation, physical and geographical environments in different regions, the climate change in different regions has its own characteristics since Holocene, and there are some differences among regions, so the regional research of Holocene climate change is worthy of attention. The living environment of humans is facing the increasingly dangerous threat of climate change. To predict the future tendency of climate change, it is necessary to study temporal and spatial changes in climate, the driving mechanism of past climate change and the impact of climate change on the terrestrial environment (Gabriela et al. 2019). With the deepening of research theories and content, the research methods and technologies of paleoclimate reconstruction tend to be diversified. In terms of research methods of environmental proxy indicators, Zhu used a variety of proxy indicators to reveal the important role of climate change and natural disasters such as floods and transgressions in cultural evolution (interruption, extinction) since the mid-Holocen (Zhu et al. 2006). Vött combined the comparative and integrated studies of geomorphology, sedimentology, geochemical elements and micropaleontology, and confirmed the cycle of multiple tsunami activities in the Lefkada Strait and adjacent areas of Greece in the middle ages (Vött et al. 2008). In addition, many new dating techniques are also widely used in the reconstruction of the Holocene natural environment, such as AMS 14C dating, 137Cs and 210Pb, and cosmic genesis 10Be, 26Al and 36Cl dating (Guo et al. 2018). In the application of new technical means, the emergence of analysis methods, such as stalagmite record (Huang et al. 2019), Phytolith record (Zhang et al. 2019), bone carbon/hydrogen/oxygen isotope (Ma et al. 2019) and biological mitochondrial DNA (Railsback 2018), provided more perfect ideas for the study of climate and environmental changes.

During the Holocene, sea-level changes and different rivers interacted to form different types of deltas. For a river delta located on a plain coast, the division sequence not only has significance for sedimentology but also for studying sea-level changes, river evolution, temperature changes, and other environmental changes (Nian et al. 2018). The Yangtze River Delta was characterized by a pattern of incised valleys that form many delta lobes from west to east, as well as different stratigraphic structures that recorded paleoenvironmental information (Mao 2012). The Red River Delta (Zhang et al. 2016) and the Mekong River Delta (Hoang et al. 2016) were formed during the period of sea-level rise in the middle of the Holocene when the seawater transgressed along the Pearl River. With a rapid rise in the sea level and delta accumulation, the coastline, river type and geomorphic type all change correspondingly. Studies had shown that the Yellow River Delta, located in the transitional zone between land and sea, is controlled by land/sea interactions and is extremely sensitive to changes in the environment and climate (Wang et al. 2020; Zhao et al. 2020). Therefore, studying the evolution of paleoclimate and paleoenvironment in the Yellow River Delta has become an important means to study the local, regional and global climate and environmental changes in the Yellow River Delta. According to the existing research, most of the research areas are located in Northwest, Central, Northeast and Southeast China, while there is less research in East China, especially in the coastal land–sea interaction zone of East China. Therefore, the Holocene environment of the Yellow River Delta was reconstructed by using a multi-agent index, and the evolution law of paleoclimate and paleoenvironment in the sedimentary core area of the Yellow River Delta was proposed, the process of environmental change in the study area since 7656 cal.aBP was reconstructed, and then two extreme climate events since Holocene were explored. Combined with the existing paleoclimate data of Shandong Province, a comprehensive comparative analysis has also been carried out in order to provide a reference for exploring the Holocene environmental evolution of Shandong Province.

In this paper, the study of the Yellow River Delta mainly includes sedimentological, chronological, carbonate and magnetic susceptibility analyses in sediments, ostracods and foraminifera analysis in sediments and pollen analysis in sediments. The Holocene environmental evolution, paleoclimate reconstruction and abrupt climate change events in the Yellow River Delta were discussed, At the same time, the Holocene climate and environmental changes and the regional sea level in Shandong Province were compared and analyzed.

Study area

The Yellow River Delta is located at the mouth of the Yellow River on the west bank of the Bohai Sea (117°31′00′′–119°18′00′′ E, 36°55′00′′–38°16′00′′ N) (Figure 1(a)–1(c)). It is an alluvial plain formed by the deposition of a large amount of sediment in the estuary carried by the Yellow River, as it passes through the Loess Plateau. Due to the high soil salinity in the Yellow River Delta, the distribution of forest trees is limited by soil type and topography.

Figure 1

(a) Map showing the location of the Yellow River Delta, (b,c) location of KY-01 borehole and other boreholes in the Yellow River Delta mentioned in the paper. (d–g) are the photo of cores drilled in the Yellow River Delta.

Figure 1

(a) Map showing the location of the Yellow River Delta, (b,c) location of KY-01 borehole and other boreholes in the Yellow River Delta mentioned in the paper. (d–g) are the photo of cores drilled in the Yellow River Delta.

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Coring and sediment core processing

21 m sediment core was drilled at a latitude of 38° 06′ 08′′ N, – a longitude of 118° 40′ 08′′ E and an altitude of 3.026 m, with the number of KY-01. The core was extracted using an XY-100 drilling machine from a hole of 21 m in depth (Figure 1(d)–1(g)). The 100 m drilling machine was selected to ensure the core rate. The sediment core segments were sealed and preserved in PVC core tubes, stored in shock-absorbing boxes and transported back to the laboratory (Figure 2).

Figure 2

Photograph of a 21-m continuous sediment core from KY-01 in the Yellow River Delta.

Figure 2

Photograph of a 21-m continuous sediment core from KY-01 in the Yellow River Delta.

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Radiocarbon dating

In this study, the sedimentary age was mainly determined by the Radiocarbon Dating Laboratory for large organic sediment samples. Choosing peat with high organic carbon content, peat with high organic layer or lacustrine clay as radiocarbon dating samples could more accurately determine the sedimentary age. AMS 14C dating was performed on bulk organic sediment samples at the Beta Analytic Radiocarbon Dating Laboratory. The AMS 14C dates were converted into calendar ages using the program Calib 7.02 based on the INTCAL 13 calibration.

Carbonate content analysis

The carbonate content of the sediment was measured using a GMY-4A carbonate content analyzer. Samples were taken every 10 cm, and a total of 200 samples were obtained. All of the samples were fully ground and sieved through a 0.25-mm sieve and were uniformly weighed at 200 mg. Samples were fully reacted with a 1:3 (V/V) hydrochloric acid solution using a GMY-4A carbonate analyzer to convert the pressure of the generated CO2 gas into an electrical signal, which was used by the computer to automatically calculate the carbonate content of the sample.

Magnetic susceptibility analysis

The magnetic susceptibility of the sediments was measured indoors using the MS-2B magnetic susceptibility meter produced by Bartington, UK. Samples were taken every 10 cm, and a total of 200 samples were obtained for magnetic susceptibility analysis. After all of the samples were dried and ground thoroughly (without damaging the granularity), the soil samples were compacted and weighed in a 1 cm3 non-magnetic sample box and then tested by the MS-2B magnetic susceptibility meter.

Pollen analysis

Pollen was measured at the Nanjing Institute of Geology and Palaeonotology, Chinese Academy of Sciences. Samples were taken every 30 cm, and a total of 66 samples were obtained for pollen analysis. In accordance with the standard analytical method of the petroleum and natural gas industry of the People's Republic of China, SY/T5245-91, samples were treated with fluoric acid to remove minerals, then oxidized with concentrated hydrochloric acid and finally sieved to concentrate the spore powder into tubes. Identification under the microscope was based on the oil and gas industry standard of the People's Republic of China, SY/T5915-94.

Foraminifera and Ostracoda analysis

Species assemblages of foraminifera and Ostracoda were analyzed for a total of 55 samples, taken from different intervals based on lithological criteria. Due to their varying preferences of particular habitats, these species provide valuable information for reconstructing sedimentary environments. All of the samples were tested at the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences.

Research route

See Figure 3 for the research flow chart.

Figure 3

Research flow chart.

Figure 3

Research flow chart.

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Sedimentological analysis

In this study, the sampling depth is 21 m, but the indexes of carbonate content, magnetic susceptibility, sporopollenin and ostracodes are all analyzed for cores of 0–20 m. According to the lithological characteristics, the KY-01 section was divided into four stratigraphic units from top to bottom (Figure 4). Basal unit 1 (0–1,040 cm) was 1,040 cm thick, the upper layer is silty clay layer and the lower layer is yellow and grayish-yellow silty sand, occasionally with pea roots. Basal unit 2 (1,040–1,410 cm) was 370 cm thick, and it is mainly gray-black and gray coarse silty sand, containing a small amount of shell fragments, without plant roots. Basal unit 3 (1,410–1,640 cm) was 230 cm thick, most of them are interbedded by gray coarse silt and gray-black silty mud, and the shell fragments are evenly distributed as a whole. Basal unit 4 (1,640–2,100 cm) was 460 cm thick, and it is mainly gray silty clay with a thin layer of clayey silty sand, rich in foraminifera and abundant shell debris, such as small oyster debris.

Figure 4

Histogram of KY-01 borehole section in the Yellow River Delta.

Figure 4

Histogram of KY-01 borehole section in the Yellow River Delta.

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Chronological analysis

AMS 14C dating results

During the Holocene, sediments in the Yellow River Delta were continuously uninterrupted. To establish a time series for the Yellow River Delta, two samples selected from depths of 1,850 and 2,085 cm in the KY-01 sediment core were used as test materials for 14C dating (Table 1).

Table 1

14C dating results of the KY-01 sediment core in the Yellow River Delta

Sample no.Depth (cm)MaterialRadiocarbon age (aBP)Cal age (cal.aBP)
KY-01 184 1,845 Organic sediment 4,630 ± 30 5,418 ± 30 
KY-01 208 2,085 Organic sediment 8,000 ± 30 8,884 ± 30 
Sample no.Depth (cm)MaterialRadiocarbon age (aBP)Cal age (cal.aBP)
KY-01 184 1,845 Organic sediment 4,630 ± 30 5,418 ± 30 
KY-01 208 2,085 Organic sediment 8,000 ± 30 8,884 ± 30 

Comparison to other borehole cores in the Yellow River Delta

To analyze the sedimentary environment of the Yellow River Delta and establish an accurate chronological sequence, the core lithology and sedimentary facies of the KY-01 borehole were compared with those of other existing boreholes in the Yellow River Delta. In consulting the relevant literature, it was found that KY-01 pores have been well studied, as well as ZK228 (Zhou et al. 2014), 9602 (Hanebuth et al. 2011), ZK3 (Zhao et al. 2015), HB1 (Liu et al. 2009) and CB20A (Xu et al. 2006) pores in the northern part of the Yellow River Delta. The comparison of sediment cores from different locations is shown in Figure 5.

Figure 5

Comparison of sediment cores from different locations in the Yellow River Delta: ZK228 (Zhou et al. 2014), 9602 (Hanebuth et al. 2011), ZK3 (Zhao et al. 2015), HB1 (Liu et al. 2009) and CB20A (Xu et al. 2006).

Figure 5

Comparison of sediment cores from different locations in the Yellow River Delta: ZK228 (Zhou et al. 2014), 9602 (Hanebuth et al. 2011), ZK3 (Zhao et al. 2015), HB1 (Liu et al. 2009) and CB20A (Xu et al. 2006).

Close modal

By comparison, we found that the lithologic zonation of the KY-01 section has a high degree of consistency with the surrounding layering. Through comparative analysis of lithology, the age at 1,040 cm of the KY-01 section is determined to be 1855 AD.

Based on the chronology of the lithology and sedimentary facies and the AMS 14C dating results, time series was established for the KY-01 borehole. The deposition rate of the entire profile is roughly divided into three segments, with the highest deposition rate from 0 to 1,040 cm, with an average of 63.4 mm/a; the deposition rate from 1,040 to 1,845 cm slowed down significantly to 1.5 mm/a; and the deposition rate from 1,845 to 2,085 cm decreased from the previous stage, reaching 0.7 mm/a. The deposition rate curve is shown in Figure 6. Through the deposition rate of different stages, we can determine the starting time of the four stages in this paper (Table 2).

Table 2

The starting time of the four stages

Depth (cm)Age
1,040 1855 AD 
1,410 2,544 ± 30 cal.aBP 
1,640 4,145 ± 30 cal.aBP 
2,000 7,656 ± 30 cal.aBP 
Depth (cm)Age
1,040 1855 AD 
1,410 2,544 ± 30 cal.aBP 
1,640 4,145 ± 30 cal.aBP 
2,000 7,656 ± 30 cal.aBP 
Figure 6

Sedimentation rate curve of KY-01 in Yellow River Delta.

Figure 6

Sedimentation rate curve of KY-01 in Yellow River Delta.

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Carbonate analysis in sediments

The carbonate content throughout the KY-01 borehole from the Yellow River Delta varied greatly ranging from 3.7 to 18.8%, with an average of 11.03% (Figure 7), which shows a trend of increasing slowly and then decreasing. The carbonate content ranged between 3.9 and 14.7% from 2,000 to 1,640 cm, with an average value of 11.05%, which shows a trend of first rising, then slightly declining and finally becoming stable; 6.4 and 15.4% from 1,640 to 1,410 cm, with an average value of 11.44%, the carbonate content fluctuates frequently in this stage; 7.4 and 18.8% from 1,410 to 1,040 cm, with an average value of 11.82%, this stage fluctuates violently; and 1,040–0 cm is the sediment deposition of the modern Yellow River Delta, so it will not be studied in this paper.

Figure 7

Variation in carbonate content and magnetic susceptibility in the KY-01 borehole.

Figure 7

Variation in carbonate content and magnetic susceptibility in the KY-01 borehole.

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Magnetic susceptibility analysis in sediments

The fluctuation in high-frequency susceptibility and low-frequency susceptibility was the most significant in the section from 2,000 to 1,640 cm (Figure 7). In this section, the range in high-frequency susceptibility was 63–237 × 10−8 m3/kg, with an average value of 103.71 × 10−8 m3/kg, and the low-frequency susceptibility ranged between 75 and 257 × 10−8 m3/kg, with an average value of 118.71 × 10−8 m3/kg. For the 1,640–1,410 cm section, the range in high-frequency susceptibility was 68–167 × 10−8 m3/kg, with an average value of 109.67 × 10−8 m3/kg, and the low-frequency susceptibility ranged between 72 and 146 × 10−8 m3/kg, with an average value of 118.71 × 10−8 m3/kg. For the 1,410–1,040 cm section, the range in high-frequency susceptibility was 56–156 × 10−8 m3/kg, with an average value of 107.75 × 10−8 m3/kg, and the low-frequency susceptibility ranged between 60 and 174 × 10−8 m3/kg. The 1,040–0 cm section is the sediment deposition of the modern Yellow River Delta, which will not be studied in this paper.

Ostracoda and foraminifera analysis in sediments

The Ostracoda and foraminifera pore analysis in the KY-01 sediment core from the Yellow River Delta, as well as statistical results on microflora, is presented below. The 0–1,040 cm section is the sediment deposit of the Yellow River since 1855AD, which will not be studied in this paper; 1,040–2,000 cm is the shallow sea sediment (Figure 8).

Figure 8

Analysis of microfossils in the Yellow River Delta section.

Figure 8

Analysis of microfossils in the Yellow River Delta section.

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For the 2,000–1,640 cm section, from 1,655 to 1,895 cm, yellow-brown clay and gray-black clay were interbedded, and from bottom to top, the appearance of the micro-organisms decreased. In the section of 1,685–1,775 cm, the microsomal diversity (species number) was obvious with Spiroloculina communis Cushman et Todd, Quinqueloculina akneriana rotunda, Q. argunica and Triloculina pentagonalis observed. Vitreous shell microsomes were mainly Ammonia beccarii vars. and Nonion glabrum. In the gray–black clay sample obtained at 1745 cm, an unidentified Scaphopoda Dentaliumwas of Laevidentalidae gen. et sp (Figure 9) was observed as well as a shell and large individual foraminifera and Ostracoda (Figure 10). An obvious decrease in the number of microflora was observed in the section of 1,805–1,895 cm, which included Q. akneriana rotunda, N. glabrum Ho, A. beccarii vars. and the wetland species Pseudogyroidina sinensis. Ostracods consisted of Sinocytheridea impressa and Cytherepteron miurense.

Figure 9

Microbodies with larger and smaller proportions in the sample (1,745 cm).

Figure 9

Microbodies with larger and smaller proportions in the sample (1,745 cm).

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Figure 10

Laevidentalidae gen. et sp. (1,745 cm).

Figure 10

Laevidentalidae gen. et sp. (1,745 cm).

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As for 1,640–1,410 cm, at 1,415 cm, the presence of peat-containing matter increased; however, only one Q. argunica and one A. beccarii were found in brown clay containing a small amount of peat. At 1,445 cm and 1,505 cm, one N. glabrum and one Q. akneriana rotunda were found. After washing, the residues of brown clay from 1,565 cm were all mica fragments, and no micro-organisms were found.

As for 1,410–1,040 cm, at 1,055 cm, fourteen Q. akneriana rotunda and four Q. argunica are found; at 1,115 cm, twenty-five Q. akneriana rotunda are found again; and at 1,205 cm, one A. beccarii vars. are found.

Pollen analysis in sediment

Based on the fossil analysis, borehole KY-01 can be divided into the following five stages from bottom to top, 1,040–0 cm is the sediment deposition of modern Yellow River Delta, which will not be studied in this paper (Figure 11).

Figure 11

Sporopollenin fossil percentage chart for borehole KY-01 from the Yellow River Delta, China.

Figure 11

Sporopollenin fossil percentage chart for borehole KY-01 from the Yellow River Delta, China.

Close modal

As for 2,000–1,640 cm, the pollen of woody plants was dominant, with an average content of 43.66%, and the pinus pollen of Pinaceae with an average content of 38.28%, the other Pinaceae pollens such as Picea (0.35%), Abies (0.11%) and Tsuga (0.07%) were sporadic. The average pollen content of herbaceous plants was 26.27%, mainly composed of pollen from Artemisia (7.7%) and Chenopodiaceae (11.82%) of the family Chenopodiaceae, and Gramineae (2.4%) and Cyperaceae (1.76%) of the family Poaceae in small amounts. At this stage, many types of algal fossils were observed with an average content of 25.5%, of which Gonyaulax catenella was the main alga, followed by Peridinioids. G. catenella algae were dominated by Spiniferites (7.08%) and Achomosphaera (6.61%). The spore content of ferns was low, the average content was 4.55%, mainly Selaginella sinensis, the average content was 3.85%, Polypodiaceae, riciopteris, Osmundaceae and Lycopodium of the family sinophyllaceae were less than 1%.

As for 1,640–1,410 cm, a total of eight samples were found in this stage. Herbaceous pollen was dominant, with an average content of 56.3%. The pollen contents of Artemisia and Chenopodiaceae were 28.9 and 16.6%, respectively. Gramineae (5.63%), other Compositae (2.10%) and Cyperaceae (1.06%) were also common. The pollen content of woody plants was slightly lower, with an average content of 18.65% consisting mainly of Pinus genus. The spore content of pteridophytes was very low, with an average content of 2.39%. The average content of algae was 12.6%, which was still dominated by G. catenella. The contents of Spiniferites and Achomosphaera were relatively high, with average contents of 6.04 and 4.44, respectively. There were few freshwater algae, and the genus Pediastrum and the genus Concentrics were occasionally found. The inner membrane of microsome foraminifera was rarely seen.

As for 1,410–1,040 cm, a total of 12 samples were found in this stage. Herbaceous pollen was dominant, with an average content of 69.5%. Artemisia and Chenopodiaceae were abundant, with average contents of 33.77 and 22.43%, respectively. The average pollen content of woody plants was 16.4%, consisting mainly of Pinus with an average content of 13.6%. The average spore content of pteridophytes was 5.6% dominated by S. sinensis with an average content of 4.1%. The average content of algae was generally low (3.68%).

Environmental evolution and paleoclimate reconstruction in the Yellow River Delta during the Holocene

Based on the collective results of the carbonate, magnetic susceptibility, Ostracoda, foraminifera and pollen analyses in sediments from the KY-01 borehole in the Yellow River Delta, the environmental evolution of the Yellow River Delta from 7656 cal.aBP to 1855AD can be divided into three stages (Figure 12).

Figure 12

Climatic stages in the Yellow River Delta since the Holocene.

Figure 12

Climatic stages in the Yellow River Delta since the Holocene.

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Stage A (2,000–1,640 cm, 7656–4145 cal.aBP)

Overall, carbonate content first increased and then stabilized, with the pollen of woody plants (43.66%) dominating in pollen fossils. The average content of algal fossils was 25.5%. In addition to spores and algae, a large number of microsomal foraminifer inner membranes were also found at this stage. The high abundance of microsomal foraminifer inner membranes reflect a shallow coastal environment suitable for foraminifer development, and the abundance of dinoflagellate and a large number of marine foraminifer inner membranes indicate that the well section was marine sediment. In addition, pollen analysis for the period 4964–4281 cal.aBP showed that during this period woody plants accounted for the largest proportion, the proportion of algal plants gradually increased, the largest number of foraminifera was noted and the number of marine foraminifera increased rapidly. In summary, during this stage, there was marine deposition, the climate overall was warm-humid representing the Holocene warm period, but there were some events of extreme climate fluctuations.

Stage B (1,640–1,410 cm, 4145–2544 cal.aBP)

During this stage, carbonate content fluctuated more frequently, and there were two more obvious content mutations: they were 4.2 kaBP in 4451–4081 cal.abp and 4.2 kaBP in 4451–4081 cal.abp, the magnetic susceptibility fluctuated more intensely than in the previous stage and showed a trend of decreasing fluctuations, the pollen of woody plants and algal plants was significantly reduced and freshwater algae appeared in small amounts. In this stage, the content and diversity of algae were lower than in the previous stage, but Spiniferites and Achomosphaera were still the dominant species, while the number of Polychaetes and micro-foraminifer endomembranes was greatly reduced. Therefore, it is speculated that transgression declined during this stage, with high salinity seawater gradually receding. In addition, in the early period of this stage, the pollen of woody plants and algae decreased significantly, and the number of micro-foraminifer inner membranes decreased significantly, indicating that there were some extreme cooling events during this period. In summary, during this stage, transgression declined, the depositional environment was coastal beach, the climate changed from warm-wet to dry-cold and there were small-scale extreme climate events.

Stage C (1,410–1,040 cm, 2544 cal.aBP–1855 AD)

The fluctuation in carbonate content was more pronounced than in the previous stage, the magnetic susceptibility showed a decreasing trend initially and then increased and the proportion of pollen from woody plants and algae was still low. Pollen from herbaceous plants, such as Artemisia and Chenopodiaceae suited to saline-alkali soil, was dominant and more abundant than in the previous stage, which indicates that overall the climate was drier and colder than in the previous stage. Large areas of flat land or saline-alkali land were formed during this period, and the distribution of freshwater in some areas was greater than in the previous stage. It was thus speculated that transgression and regression alternately occurred during this stage, and the sea level was relatively low. In conclusion, the sea level was relatively low, transgression and regression alternately occurred, the climate was drier and colder, the temperature alternately increased and decreased and fluctuations in climate were more intense.

Abrupt climate change event

A comprehensive analysis of the KY-01 borehole magnetic susceptibility, carbonates, spores and microbiological indicators revealed that the climate in the Yellow River Delta began to change around 5737–5422 and 4451–4081 cal.aBP, with significant changes as the temperature decreased and the environment deteriorated.

Cooling events during 4451–4081 cal.aBP

From 4451 to 4081 cal.aBP, the pollen of woody plants and algal plants in the KY-01 sediment core decreased significantly, while that of herbaceous plants increased rapidly to a dominant proportion, consisting mainly of pollen from Artemisia and Chenopodiaceae, and the number of micro-foraminifera decreased significantly. All lines of evidence indicated that the temperature decreased, the precipitation decreased and the climate became dry and cold during this time. The 4.2 kaBP event is a climate cooling event that occurred during the warm period of the middle Holocene. Records from the middle and low latitudes and subtropical regions around the world, such as Asia, Europe, Africa and North America, indicate a similar start time, duration and strength. This incident has also been commonly recorded in China (Figure 13). In Central China, Zhu studied the Dajiu Lake peat profile in Shennongjia and found that the δ18O value around 4200 cal.aBP showed a rapid positive deviation of 1% over about 200 years (Zhu et al. 2006). In the Qinghai-Tibet Plateau (Wu et al. 2006), evidence from Cuoe Lake sediments shows that the Rb/Sr ratio of trace elements reflecting the degree of weathering peaked at 4300–4100 cal.aBP, revealing that the carbon isotope of effective precipitation was reduced. In Southwest China (Wang et al. 2010), the content of δ18O in Chongqing Xinyadong during the period 4400–4100 cal.aBP increased by about 1.61% within a time scale of 200 years. In North China (Jin & Liu 2002), the δ13C value of peat increased significantly from 4800 to 4200 cal.aBP in Taishizhuang, Hebei Province. By comparison, we find that the abrupt climate event in the Yellow River Delta during the period 4451–4081 cal.aBP is strongly consistent with the cold events of 4.2 kaBP, and the time differences between regions may be related to the different proxy indexes and dating methods selected for the reconstruction of the paleoclimate, or to differences in landform, geographical environment or climatic conditions.

Figure 13

Typical records of the 4.2 kaBP cold event in the monsoon area of China (gray bars indicate dry-cold period). (a) The pollen content of woody plants in the KY-01 sediment core from the Yellow River Delta; (b) the δ18O record of Xinyadong stalagmites in Southwest China (Wang et al. 2010); (c) the δ13C record of peat in Taishizhuang, North China (Jin & Liu 2002); (d) the Rb/Sr record of Cuoe Lake sediments in the Tibetan Plateau, Northwest China (Wu et al. 2006) and (e) the δ18O record of Shennongjia stalagmites, Central China (Zhu et al. 2006).

Figure 13

Typical records of the 4.2 kaBP cold event in the monsoon area of China (gray bars indicate dry-cold period). (a) The pollen content of woody plants in the KY-01 sediment core from the Yellow River Delta; (b) the δ18O record of Xinyadong stalagmites in Southwest China (Wang et al. 2010); (c) the δ13C record of peat in Taishizhuang, North China (Jin & Liu 2002); (d) the Rb/Sr record of Cuoe Lake sediments in the Tibetan Plateau, Northwest China (Wu et al. 2006) and (e) the δ18O record of Shennongjia stalagmites, Central China (Zhu et al. 2006).

Close modal

Cooling events during 5737–5422 cal.aBP

From 5611 to 5422 cal.aBP, the pollen content of woody plants decreased sharply in the KY-01 sediment core, the pollen content of herbaceous plants increased significantly, and the Ostracod content decreased slightly, which reveals that the temperature decreased greatly and the climate became dry and cold. Many studies in many regions of the world have also recorded climate cooling and drying events in the middle of the Holocene, which is generally referred to as the 5.5 kaBP event. Magny et al. (2006) summarized a series of marine and land environmental proxy indicators in the southern and northern hemispheres, which proved that the 5.5 kaBP event had global characteristics. There were also many regions in China that responded to this incident (Figure 14). In South China, the concentration of chlorophyll in the sediments of Lake Mar reached their lowest value between 5,500 and 5000 cal.aBP. In Northeast China, Hanebuth et al. (2011) studied Jinchuan peat and found that the δ13C value began to rise around 5000 cal.aBP, revealing drying of the climate. In the Southwest and Northwest of China (Yao & Thompson 2017), the δ18O values in the Dunde ice core and Donggedong stalagmite increased at 5500–6000 cal.aBP, indicating that the temperature suddenly decreased and the climate became dry. Through comparison, it is found that the cooling event noted in the Yellow River Delta between 5611 and 5422 cal.aBP is consistent with those of other regions that occurred during the same period.

Figure 14

Typical records of the 5.5 kaBP cold event in the monsoon area of China (gray bars indicate dry-cold period). (a) The pollen content of woody plants in the KY-01 sediment core from the Yellow River Delta; (b) the δ18O record of the Dunde ice core, Northwest China (Yao & Thompson 2017); (c) the δ13C record of Jinchuan peat, Northeast China (Hoang et al. 2016) and (d) the absorption rate of chlorophyll a in sediments of Huguangyan Maar Lake.

Figure 14

Typical records of the 5.5 kaBP cold event in the monsoon area of China (gray bars indicate dry-cold period). (a) The pollen content of woody plants in the KY-01 sediment core from the Yellow River Delta; (b) the δ18O record of the Dunde ice core, Northwest China (Yao & Thompson 2017); (c) the δ13C record of Jinchuan peat, Northeast China (Hoang et al. 2016) and (d) the absorption rate of chlorophyll a in sediments of Huguangyan Maar Lake.

Close modal

Comparison of Holocene climate and environment changes in Shandong Province

In the study of Holocene environmental evolution in other areas of Shandong Province, we can find the similar research results with this paper. Based on the analysis of sporopollenin data of three boreholes in Shandong Province, Bian (2004) thinks that 7000–3000 cal.aBP, subtropical vegetation types are generally growing in Shandong Province with warm-humid climate, and there are two large-scale cooling events during this period. Ding et al. (2011) analyzed the magnetic susceptibility, chromaticity and grain size of the Shanchengcun section in Zhangqiu, Shandong Province, 8500–3100 aBP, the average grain size and coarse silt content were the lowest in the whole section, the magnetic susceptibility value further decreased in the middle and a lower part and the redness and yellowness value increased, which was the highest value in the whole section, indicating that the temperature increased and the precipitation increased in this stage. Zhang et al. (2005) analyzed the sporopollenin, foraminifera and grain size in the northern plain of Shandong Province of two boreholes (A1 and A5), 10000–4000 aBP, the aquatic herbaceous plants are dominant, the warm and humid vegetation in the vegetation assemblage increases significantly and reaches peak in the late stage and a large number of marine foraminifera are found, indicating that this stage is a transitional sedimentary environment between land and sea, with warm-humid climate. From the above analysis, it can be seen that the results of carbonate content, magnetic susceptibility, Ostracoda, foraminifera and sporopollenin analyses of hole KY-01 in the Yellow River Delta are consistent with those of Shandong and other regions in China.

Regional sea-level comparison

Since the middle of the 20th century, a large number of scholars have carried out long-term and systematic research on the Holocene sea-level changes in eastern China and obtained a great deal of basic results. Based on the analysis of the sedimentary structure and foraminiferal characteristics of eight boreholes in the South Bank of Laizhou Bay, Tian et al. (2016) and others established the Holocene relative sea-level change history of Laizhou Bay; Huang et al. (2016) and others have discussed the sedimentary paleoenvironment and paleoclimate history of the Pearl River Delta; Yang et al. (2015) and others analyzed the lithology, particle size and burning loss characteristics of Na6 and na9 borehole sections in Qingaowan, Southeast Guangdong; Xian & Jiang (2005) and others studied the evolution characteristics of Holocene sedimentary environment, climate and sea level in the Yellow River Delta by using the grain size, mineral and element geochemical testing information of borehole sediments. From the above analysis, we can see that the results of ostracods, foraminifera and sporopollenin analysis analyses of KY-01 well in the Yellow River Delta reflect the changes in the sea level, which are consistent with the research results of other areas in China.

From information on the evolution of climate and sea level recorded by environmental proxy indexes in sediment since the Holocene (7656 cal.aBP), it can be seen that the evolution of the regional climate in the Yellow River Delta is affected by global environmental change. However, due to the unique geographical location, regional geographical features and the characteristics of the Yellow River Basin, environmental change in this region has more complexity, and the environmental evolution has regional characteristics. Changes in the climate of the Yellow River Delta from 7656 cal.aBP to 1855AD can be divided into the following three stages.

During 7656–4145 cal.aBP, the climate was warm-humid, and seawater moved onto the land. The total number of marine ostracods was the largest. During 4145–2544 cal.aBP, the pollen of woody and algal plants decreased significantly, while the pollen is suited to saline and alkaline land conditions increased rapidly; the total number of marine ostracods decreased significantly, and the climate changed from warm-humid to dry-cold. During 2544 cal.aBP–1855AD, transgression and regression alternately occurred, the sea level was relatively low, beach land or saline-alkali land formed in large areas and the climate was drier and colder than in the previous stage. In addition, the climatic characteristics of the Yellow River Delta recorded the obvious cold dry events during the middle of the Holocene between 5737–5422 and 4451–4081 cal.aBP, which is consistent with the geological and climatic records of the whole country and even the whole world.

This work was supported by the National Natural Science Foundation Project (Nos 41602356, 41977262 and 41471160) and Open Research Fund Program of Shandong Key Laboratory of Depositional Mineralization & Sedimentary Minerals (No. DMSM2018024), Study Abroad Program by the Government of Shandong Province, China (No. 201902006), Open Research Fund Program of Shandong Provincial Key Laboratory of Water and Soil Conservation and Environmental Protection (No. STKF201917), Open Research Fund Program of Shandong Key Laboratory of Eco-Environmental Science for Yellow River Delta (Binzhou University) (No. 2019KFJJ03), Major scientific and technological innovation projects in Shandong Province (No. 2018CXGC0307), Geological Exploration Fund of Shandong Province (Nos 2016(7), 2013(55) and 2018(8)), Determination Technology of Sedimentary Environment Since Holocene in the Yellow River Delta, the Determination Technology of Accurate Age of Sedimentary Strata in the Yellow River Delta and the Key Technology Research of the Division of Sedimentary Facies in the Yellow River Estuary Area. The authors thank the reviewers for their very valuable comments, which greatly improved the quality of the paper.

All relevant data are included in the paper or its Supplementary Information.

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