Evolution of Palaeoenvironment of the South Yellow Sea Since the Last Deglaciation

WANG Haoyin, LI Guangxue, *, ZHANG Yang, LIU Yong, LIU Dongsheng, DING Dong, XU Jishang, MA Yanyan, QIAO Lulu, WANG Liyan, and LI Qian

Evolution of Palaeoenvironment of the South Yellow Sea Since the Last Deglaciation

WANG Haoyin1), 2), LI Guangxue1), 2), *, ZHANG Yang1), 2), LIU Yong1), 2), LIU Dongsheng1), 2), DING Dong1), 2), XU Jishang1), 2), MA Yanyan1), 2), QIAO Lulu1), 2), WANG Liyan1), 2), and LI Qian1), 2)

1),,,266100,2),,266100,

The sediments in core YS01 recovered from the South Yellow Sea mud deposit zone contain abundant benthic forami- nifera, particularly shallow continental shelf species, but rare of planktonic foraminifera. The benthic foraminifera are dominated by stenohaline cold shallow-water species and euryhaline brackish-water species. In this paper, the palaeoenvironmental changes were discussed based on the grain-size compositions and benthic foraminiferal assemblages. Six different benthic foraminiferal assemblages were discriminated by species analysis from the foraminiferal fauna. According to AMS14C dating data in core YS01, we identified four main stages of marine environmental changes since the last deglaciation: a near-shore depositional stage (13.1–9.5kyrB.P.), a transitional stage from near-shore deposition to shallow-sea deposition (9.5–5.6kyrB.P.), a high sea level stage with shallow-sea deposition (5.6–2.9kyrB.P.), and a stable shallow-sea depositional stage (2.9kyrB.P. to the present).

benthic foraminifera; last deglaciation; South Yellow Sea; palaeoenvironmental change

1 Introduction

The Yellow Sea is a semi-enclosed shallow sea located between mainland China and Korean Peninsula. The total area is approximately 3.8×105km2, with an average water depth of 44m. The Yellow Sea Warm Current passes over the mud deposit zones in the central and south-eastern Yellow Sea. The other main hydrodynamic factors include Yellow Sea Coastal Current, Kuroshio Current, and Tsushima Current (Fig.1). All of these currents notably influ- ence the distribution of water masses and sediments (Wang, 2014). The mud deposit area in the central South Yellow Sea is the broadest mud patch in the Yellow Sea (Wang, 1988) which was associated with a cyclonic eddy current system at high sea level with a stable sedimentary environment and preserved a relative high-reso- lution sediment record of the late Holocene (Liu, 1999). The interaction between northward inflow of the Yellow Sea Warm Current and southward inflow of the Yellow Sea Coast Current may be the main hydrodynamic factor that engenders the eddy circulation system (Hu, 1984; Shen, 1993) which may account for the mud sediment deposit in the distal mud area. The central South Yellow Sea mud deposit zones, where water depth can exceed 70m, with average of 55m, has an approximately 6.1×104km2in area. Previous studies indicated that this mud area is one of the modern depocenters on the continental shelf sea in eastern China, with depositional rate from 1.0 to 1.3mmyr−1approximately (De Master, 1985; Li, 2014; Wang, 2014a; Fig.1).

Global sea levels have risen since the last deglaciation, and in response, the Yellow Sea sedimentary environment has changed radically and received massive deposits of terrigenous fine particulate matter, resulting in the formation of different mud sedimentary areas under the action of a complex ocean dynamics system (Li, 1997). As the most effective marine paleoenvironmental information source, massive mud sediments provide excellent materials for investigating the Yellow Sea Warm Current and the related formation about the evolution of cold water and the sedimentary environment.

Many researchers have carried out extensive researches on mud sediments in the Yellow Sea in recent years (Zheng, 1979; Hao, 1980; Wang, 1988; Zheng, 1988; Sun, 2014; Wang, 2014b). Since the early 1990s, understanding the marine environment by using the changes in benthic foraminiferal assemblages and oxygen and carbon isotope compositions since the last deglaciation has been the key subject of research in the South Yellow Sea mud areas (Wang, 2009). Kim and Ku- cera (2000) analyzed the foraminifera in core samples from the central South Yellow Sea mud area and concluded that the Holocene transgression started at 15.09cal.kyrB.P. and the modern Yellow Sea circulation began to form between 8.47 and 6.63cal.kyrB.P. Furthermore, these authors analyzed the δ18O values of foraminifera in various areas of the South Yellow Sea and concluded that the paleosalinity of the Yellow Sea has risen since the Holocene (Kim and Kennett, 1998). Li(2000) analyzed samples from core YSDP102, which was collected in the southeastern Yellow Sea mud area, and prelimina- rily concluded that the Yellow Sea warm current and the accompanying southeastern Yellow Sea cold water formedapproximately 6400 years ago. The Yellow Sea Warm Cur-rent significantly strengthened over time and has remained up to the present. Based on benthic foraminifera and stable isotope compostitions, since 8.4cal.kyrB.P., the South Yellow Sea has evolved from a low-salinity estuarine en- vironment to a low-salinity neritic environment, and then to its modern shelf sea environment (Xiang, 2008). Meng(1998) discussed the paleoenvironmental events and environmental changes in the South Yellow Sea since 15cal.kyrB.P. and suggested that the climate fluctuated sharply during the last deglaciation and the transition from the late glacial to the postglacial period. Zhuang(2002) hypothesized that the South Yellow Sea mud areas began to form at approximately 9.7cal.kyrB.P. Wang(2009) found fine-grained sediments on top of a postgla- cial sedimentary sequence within postglacial transgres- sion sedimentary record in the mud area of the northern South Yellow Sea, which suggests that modern Yellow Sea circulation began to form after the high sea level period in the middle Holocene and has continued to the present.

This paper was based on the core YS01 collected from the South Yellow Sea central mud area and high resolution AMS14C dating, benthic foraminiferal analysis and the particle size analysis have been carried out to discuss the sedimentary characteristics in the central southern Yel- low Sea since the last deglaciation. We extracted the benthic foraminifera data and particle size parameters to reconstruct the marine paleoenvironment evolutionary history of South Yellow Sea since the last deglaciation. The structural characteristics of benthic foraminiferal assemblages in sediments have been demonstrated to explore the changes to the marine environment that have occurred in this region since 13kyrB.P.

2 Materials and Methods

Core YS01 was collected from the west-central South Yellow Sea mud area (35.5201˚N, 122.4876˚E) at a water depth of 58.5m (Fig.1). The total length of the core is 29.23m, but this study focused on the upper 12.92m.

Samples for particle size analysis were taken at 1cm intervals, and a total of 1237 bulk samples were measured using a laser particle size analyzer (Malvern Mastersizer- 2000) at the Institute of Marine Geology, Ministry of Land and Mineral Resource, China. The range of the instrument is 0.02–2000μm, the resolution ratio is 0.01φ and the relative error of repeated measurements is less than 3%. A 0.5g subsample was placed into 50mL beakers, into which distilled water and 30% H2O2were added. 24h later, add 10% HCl to the beakers and stir it to remove carbonate minerals. After 24h of standing and subsequent ultrasonic dispersion, the samples were placed into the analyzer to measure the particle size compositions.

Fig.1 Location of core YS01 in the Yellow Sea. Modified from Beardsley et al. (1985) and Su (1998).

A total of 321 samples at 4cm intervals were prepared for benthic foraminiferal analyses. The volume of each sample was approximately 95cm3. Samples were oven dried at 60℃ and weighed (10g). Diluted H2O2(10%) was added to help the disaggregation of the indurated samples when needed. After the samples were washed through a 63μm sieve, the coarse fraction was oven dried at 60℃ and stored for foraminiferal studies. The taxonomy of benthic foraminifera follows Wang (1985), Wang(1988) and Loeblich and Tappan (1994). The abundance was used to indicate the density of foraminifera.

The simple diversity () was used to indicate the species richness of foraminifera.

The Shannon index [()] was calculated to evaluate the variation of benthic foraminiferal complex diversity.

wherepis the proportion of theth species.

3 Results

3.1 Chronology

Mixed specimens of the benthic foraminifera from 14 horizons were selected for accelerator mass spectrometry (AMS)14C dating at the Woods Hole Oceanographic Ins- titution, USA, and Beta Analyses Company, USA (Fig.2). Raw radiocarbon results were converted to calendar ages following the method of Stuiver(1998) by using the CALIB 5.0.1 program.

We can get the Marine Reservoir Effect value for core YS01, Δ=−100±36, according to the average Δvalue for western and eastern South Yellow Sea which provided by CALIB 5.0.1 (Table 1). Based on these 14 calendar ages, we obtained a calendar age of 12760yrB.P. for the oldest (1292cm) portion of the core using linear interpolation (Fig.3).

Fig.2 Profile photographs of the core YS01. The bars show the ages, which were calculated based on AMS14C dating performed at the Woods Hole Oceanographic Institution, USA (red bars), and the Beta Analytic Radiocarbon Dating Laboratory, USA (blue bars).

Table 1 AMS14C ages of foraminifera samples in core YS01

Notes:aWoods Hole Oceanographic Institution, USA;bBeta Analytic Radiocarbon Dating Laboratory, USA.

3.2 Grain Size Compositions

Based on the lithology and grain size characteristics, core YS01 can be divided into two main depositional units (DU1 and DU2) (Fig.4).

1) DU1 (1292–1082cm) can be divided into a lower part and an upper part.

Lower part: DU1a (1292–1210cm) is gray to grayish-brown silty clay. This section also includes lenticular rust-colored clay, bivalve fragments and plant roots in the top part. The median grain size ranges from 10.4μm to 16.4μm, and the mean grain size is 13.1μm. The silt content is high, and the average content of silt is 70.4%, the average content of clay is 23.3%, and the average content of sand is 6.3%. The average sorting coefficient is 1.9, the average skewness is 0.20, and the average kurtosis value is 0.88. The moisture content is 15.6%–22.0% in this section (Fig.4).

Fig.3 Down-core variation of ages and sedimentation rates in core YS01. The AMS14C ages are shown with the red line, and the depositional rates are shown with the blue line.

Upper part: DU1b (1210–1082cm) is grayish-brown to dark gray clay with some fine sand (Fig.4). The ave- rage mean grain size is 10.2μm. At the top of this part (1123–1130cm), bioturbation is present, and the burrows are filled with bivalve fragments. At the bottom of this section, dense pores approximately 1–2mm in diameter can be seen. This section is in erosional contact with DU2.

2）DU2 (1082–0cm) can also be divided into 2 parts.

Lower part: DU2a (1082–800cm) is dark gray, gray brown or gray clay.

The median grain size is between 5.1 and 7.7µm, with an average of 6.1µm. This section has an average content of 34.9% clay, 64.4% silt and 0.7% sand. This indicates that the sorting coefficient is high and that the sediment particles are fine.The average sorting coefficient is 1.7, the average skewness is 0.07, and the average kurtosis is 1.01 (Fig.4). Sporadic bivalve fragments are present. Dense spots of black-gray organic matters are present at 1075–1080cm. At 820–830cm and 995–1025cm, biological bur- rows can be observed.

Upper part: DU2b (800–0cm) is characterized by gray to grayish-brown clayey silt. The median grain size ranges from 3.9μm to 5.7μm, and the mean grain size is 5.0μm. The average clay content is 40.1%, the average silt content is 59.8%, and the average sand content is 0.1%(Fig.4). The sample has horizontal bedding, and bivalves with a diameter of approximately 5mm are present at 232cm and 561cm. These data suggest that the hydrodynamic con- ditions were weak during the deposition of sediments.

3.3 Benthic Foraminiferal Community

Core YS01 contains abundant benthic foraminifera. We analyzed 63080 tests from 321 samples with benthic foraminifera and identified a total of 59 benthic foramina- fera species (including undefined species) belonging to 35 genera. Additionally, 220 planktonic foraminifera tests were found in two horizons: the segment at 888–890cm with 215 tests (and), and the segment at 908–910cm with 5 tests (). All foraminiferal tests belong to common species that are widely distributed in the Quaternary strata in the Yellow Sea and the East China Sea and represent coastal to shallow shelf sea sedimentary environments (Murray, 1971; Hao, 1980; Wang, 1980; Wang, 1985; Wang, 1988; Zheng, 1988).

Fig.4 Variation of the grain-size compositions and main parameters for the core YS01.

The abundance of benthic foraminifera in core YS01 was approximately 20 to 30g−1, with the maximum value of 158g−1. From 0–620cm, the foraminiferal abundance changed gradually, with an average abundance of 18g−1. In contrast, from 620–1292cm, the abundance changed dramatically, with a maximum value of 158g−1(at 1086cm) and an average abundance of 22g−1(Fig.5).

Fig.5 Abundance, simple diversity (S) and Shannon index (H(s)) of benthic foraminiferal community in core YS01.

The simple diversity value (), similar to abundance, changed dramatically in this core. The highest simple di- versity value in the core is 24 species, while the lowest value was 0. The change of the Shannon index was similar to that of the simple diversity value. The majority of the Shannon index values were between 2 and 2.5 (Fig.5).

Classified by the texture of foraminiferal tests, the foraminiferal assemblages were dominated by hyaline tests, with an average content of 95.2%. Agglutinated tests were the second abundant at 4.3%, and porcelaneous tests were the least abundant, with an average content of only 0.5% in core YS01 (Fig.6).

3.4 Benthic Foraminiferal Assemblages

The characteristics of the benthic foraminiferal fauna in core YS01 are remarkable because the dominant species in the samples are near-shore and shallow-water species. There are six kinds of foraminiferal assemblages in the core (Fig.7). Additionally, the distribution of foraminifera in the eastern Yellow Sea surface sediments are used as references (Wang, 1980; Wang, 1985; Wang, 1988; Lei and Li, 2016), based on the assumption of corresponding regional sedimentary environment. We divided the core into six groups based on the benthic fora- miniferal assemblages (Fig.7).

Fig.6 Relative contents of hyaline tests, agglutinated tests and porcelaneous tests in core YS01.

Group I (1292–1082cm): thevar.-assemblage. This assemblage is dominated byvar. (average: 39.5%) and(average: 32.7%). Other common species include(8.0%),(6.5%),(5.8%), and(4.0%).

Group II (1082–800cm): the-assemblage. This assemblage is dominated by(average: 22.4%) and(average: 22.0%). Common species include(14.7%),(10.4%), and(6.6%).

Group III (800–621cm): the-assemblage. This assemblage is do- minated by(average: 15.8%) and(average, 13.0%). Other common species include(9.4%),(7.4%) and(6.9%). Note that the average content ofreaches the highest value in this section.

Group IV (621–408cm): the-assemblage. This assemblage is do- minated by(average: 23.5%) and(average: 17.0%). Other common species in- cludeand.

Fig.7 The basic indexes and main species contents of the benthic foraminifera in core YS01.

Group V (408–85cm): theassemblage. The high level ofabundance (28.2%) in this assemblage is the defining characteristic. The other obvious feature is the high content of,which is significantly higher in this assemblage than in other assemblages, and the average content is 23.2%.

Group VI (85–0cm): the-assemblage. This assemblage is dominated by(average: 25.3%) and(average: 12.3%). In addition,(15.0%) and(8.7%) are also common.

4 Discussion

In the 1990s, researches on Greenland ice core δ18O data led to the theory of alternating temperatures. Evidences for the alternation of glacial and interglacial periods are common in the global scope (Grootes, 1993). Studies in recent decades have found that stalagmites in southern China can be used to construct a high-precision absolute time scale. Therefore, these records have replaced that in ice core as new global paleoclimate indicators (Henderson, 2006; Wang, 2009). This paper compared the records of δ18O values in the Dongge Cave stalagmite, δ18O values in the Greenland GISP2 ice core, and foraminiferal data in core YS01 (Fig.8).

Environmental factors, including temperature, salinity, water depth and sediment features, regulate the distribution of benthic foraminifera (Murray, 1971; Lei and Li, 2016; Petersen, 2016; Wang, 2016). According to the foraminiferal species compositions and the AMS14C dating data, the paleoenvironment evolution in the studied area can be divided into the following stages:

1) Near-shore depositional stage (13.1–9.5kyrB.P., 1292–1082cm):The depositional unit corresponding to this period is DU1. The foraminiferal assemblage associated with this stage is assemblage I. The sediments are composed of gray silty clay and silty sand. The forami- nifera at this stage are dominated byvar.,, a number ofand the cold- water species(Fig.8).var. is the most widely distributed near-shore euryhaline species in the world, and it is widely distributed in offshore deposits around China. This species is a typical shallow-water spe- cies and is mainly found in water depths of less than 20m in the South Yellow Sea (Wang, 1980, 1988; Wang, 1985).is a typical near-shore and shallow- water species that is mainly distributed in waters with depths of less than 30m in the South Yellow Sea and the Bohai Sea (Wang, 1988). Theis mainly distributed in the Yellow Sea coastal current area of the South Yellow Sea (Wang, 1988; Lei and Li, 2016).is widely distributed in the modern shelf sea area and occurred in the Quaternary layers. It is common at depths below 50m along the inner continental shelf in intertidal zones and estuary areas (Wang, 1980).

This stage was associated with coastal and estuarine facies and transitional facies sedimentary environment. The environment during this period, the early Holocene, was characterized by melting and retreating glaciers, which represented the transition from a dry and cold climate to a warm climate (Fig.8). The benthic foraminiferal assemblage primarily consists of the near-shore and shallow-water speciesvar. andalong with the estuarine intertidal species, indicating that the study site in this period was featured by a near-shore sedimentary environment.Lenticular rust- colored clay and plant roots appeared in the lower part of the core, and the abundance of foraminifera increased rapidly in the upper part, indicating that the water depth likely increased during this period.

Fig.8 Sedimentary records in core YS01 compared with other climate proxies from the Northern Hemisphere. (a), GISP2 δ18O isotope record (red; Groots et al., 1993; Stui- ver et al., 1995); (b), 5-year-resolution stalagmite δ18O iso- tope record from China Dongge Cave (brown; Wang et al., 2005); (c), reconstructed sea-level rise curves for Western Pacific regional seas since the last deglaciation (dark blue; modified from Li et al., 2014; light green squares obtained by Chough et al., 2004); (d), the water depth curve calculated by transfer function in the South Yellow Sea (light blue; Wang, 2017); (e)–(h), relative abundance of benthic foraminifera in core YS01: A. beccarii var. (orange),B.subula (green), A. ketienziensis (ocean blue) and Astro. tasmanensis (purple). The black line is the five-point running average curve of each proxy; the grey belts and yellow belts denote the four depositional stages in core YS01 since the last deglaciation.

2) Near-shore to shallow-sea transition stage (9.5–5.6 kyrB.P., 1082–800cm): The corresponding deposition unit is DU2a. The sediment in this section is gray silty clay. The content of sand is very low, less thanone percent. This periodis characterized by foraminiferal assemblage II. The dominant species in the core are the middle shelf sea species,and(Fig.7).is mainly distributed on the north- eastern shelf at water depths of 50–100m in the East China Sea and along the shelf areas with depths greater than 50m in the Yellow Sea (Wang, 1980).is mainly distributed in the sediments at depths of more than 50m in the South Yellow Sea (Wang, 1980).is a cosmopolitan species and is widely distributed in the modern Mediterranean Sea, Atlantic Ocean, Pacific Ocean, eastern South China Sea and the Yellow Sea.is mainly found at depths of 50m in cold-water environments and large numbers occurred at a water depth of 60m in the southern area of the Yellow Sea (Wang, 1980, 1988).

This period was characterized by rising temperatures with subsequent rapid sea level rise (Wang, 2014). In this stage, the tests of the foraminifera from core YS01 are mainly hyaline. The benthic foraminiferal assemblage in this stage is significantly different from that of the previous stage. The assemblages of the coastal and intertidal zones were replaced with middle and outer continental shelf species. The grain size of sediments changed little during this period. The sediments are mainly composed of silty sand and clay, reflecting a stable sedimentary environment. Based on the foraminiferal assemblage, in this stage, the water depth was significantly higher than that in the previous stage. The sedimentary environment shifted from a coastal or near-shore environment to a shallow shelf sea environment (Fig.8).

We note that planktonic foraminifera were found at the top of this unit. Accordingly, we speculate that the Yellow Sea Warm Current formed at approximately 6.8kyrB.P. This age is consistent with previous research results (Xiang, 2008), which suggest that the Yellow Sea Warm Current formed at approximately 6–7kyrB.P. (Liu, 1999; Baker, 2000; Li, 2007). Compared with that in the previous period, the occurring abundance ofin this stage (Fig.7),which indicates a middle and outer shelf sea environment, is markedly higher, sug- gesting that the water depth in the area had increased. Previous research results show that the global sea level was rising (Li, 2014). On the Pacific coasts of Australia (Baker, 2000; Lambeck, 2002), the South China Sea (Yu, 2009), Japan (Hongo, 2010) and the Philippines (Berdin, 2004), the sea level at 8kyrB.P. was higher than the current sea level. Similar re- sults have also been found in Brazil (Angulo, 1999; Ybert, 2003) and Antarctica (Hall, 2004). The foraminiferal assemblage in core YS01 are consistent with the globally high sea level observed in other regions during this period.

3) Shallow-sea depositional stage (5.6–2.9kyrB.P., 800–408cm): The sediment in this section is silty clay. The fo- raminiferal assemblages include groups III and IV. These dominate assemblages were characterized by benthic foraminifera with hyaline tests such as.,.and.(Fig.7). They are typical species living in the present shallow shelf sea of China (Wang, 1980, 1988), indicating a normal marine condition. Furthermore, the test compositions of benthic foraminifera also changed with a considerable increase of agglutinated shells, especially the species.with the highest average content in this section (Fig.7).

In general, agglutinated foraminifera build tests from small sediment particles cemented together, while calca- reous species require a specific amount of dissolved CaCO3to build calcite tests. Therefore, the agglutinated species reach their peak abundance in areas with low CaCO3,., low salinity, turbulent or deep-sea marine environment (Greiner, 1970; Murray, 2006; Wu, 2015). However, in this stage, the assemblage of core YS01 is clearly do- minated by shallow water species and the seafloor is co- vered by a mixture of silt and clay, indicating that the ma- rine environment was close to the normal shallow sea,., lower energy sedimentary environment (Fig.2). Consequen- tly, we suggest that the typical agglutinated foraminifera species (Fig.7) in the core YS01 during this period were primarily controlled by the sediment type (Li, 2010), possibly correlated with fine-grained sediments due to the raised sea level.

Based on previous research results of central mud deposits located in the South Yellow Sea, global sea level reached the highstand state at about 6–7kyrB.P., though the low amplitude fluctuations of sea level occurred (Zong, 2004; Li, 2014; Wang, 2014a). The sea level in this time maintained a high and relatively stable state (Liu, 2004; Fig.8). As thus, during this period the study area is featured by the combination of relative modern shallow sea foraminiferal species and fine sediments, in- dicating a typical and stable shallow shelf sea sedimentary environment.

4) Modern depositional stage (2.9kyrB.P. to present, 408–0cm): This part corresponds to the upper sedimentary unit DU1a. The section contains mainly gray silty clay and a grayish-brown silty clay layer. The foraminiferal as- semblages are groups V and VI. The shallow-sea species,and, are most abundant. The content ofis high, andandare also common in this segment. This combination reflects a shallow shelf sea environment with cold-water conditions (Wang, 1980). Furthermore, these assemblages are consistent with the current benthic foraminiferal assemblage, indicating that the marine environment in the area approached that of the present during this stage (Fig.8).

5 Conclusions

The sediments from core YS01 contain abundant benthic foraminifera but rare planktonic foraminifera. The ben- thic foraminifera are dominated by stenohaline cold shallow-water species and euryhaline brackish-water species. Based on the lithology and the particle size compositions, the core YS01 was divided into two main depositional units, DU1 and DU2. And the foraminiferal communities can be grouped into six assemblages.

Thus it can be seen that the changes of marine environment in this region are basically consistent with the global environment changes since the last deglaciation by comparing the particle size compositions and foramina- feral distribution in core YS01 with other proxies, which shows a good correspondence. In general, the marine environment in this area evolved from near-shore to shallow water environment since the last deglaciation. We identified four stages of marine environmental evolution since the last deglaciation: the near-shore depositional stage (13.1–9.5kyrB.P.); the transitional stage from near-shore deposition to shallow-sea deposition (9.5–5.6kyrB.P.); the high sea level stage with shallow-sea deposition (5.6–2.9kyrB.P.), and from 2.9kyrB.P. to the present, which is a stable shallow-sea depositional stage.

Acknowledgements

This work was supported by the China Geological Sur- vey Project (Nos. 121201005000150004 and GZH20110 0202), and by the Taishan Scholar Project.

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. E-mail: estuary@ouc.edu.cn

April 25, 2019;

August 18, 2019;

January 13, 2020

(Edited by Chen Wenwen)