Sedimentological and microfossil records of modern typhoons in a coastal sandy lagoon off southern China coast

Hong-Shui Qi , Min Chen , Lin-Nn Shen , Feng Ci ,Ai-Mei Zhng , Qi Fng

a Third Institute of Oceanography, Ministry of Natural Resources, Xiamen 361005, Fujian Province, China

b Southern Marine Science and Engineering Guangdong Laboratory, Zhuhai 519082, Guangdong Province,China

c Observation and Research Station of Coastal Wetland Ecosystem in Beibu Bay, Ministry of Natural Resources, Beihai 536015, Guangxi Province, China

d Xiamen Municipal Bureau of Land,Resources and Housing Administration, Xiamen 361013,Fujian Province,China

Abstract To determine the characteristics and potential indicators of modern typhoon deposition in a sandy lagoon off the coast of Guangdong Province(southern China),we analysed the 210Pb,sedimentology,and microfossils of samples from ten cores obtained before and after the passage of Typhoon Rammasun in 2014.Typhoon deposition showed a thinning trend from internal areas of the lagoon to its mouth,with the maximum thickness inside the lagoon of ～35 cm. These typhoon deposits are dominated by overwash and differ from sediments deposited under normal weather conditions.Under normal weather conditions,lagoon sediment has a 210Pb curve that follows a model of exponential decay,has a unimodal granularity frequency curve,and lacks organic matter and microfossils(diatoms and foraminifera).However, 210Pb is low in the typhoon deposits,the grain size is coarse, and the granularity frequency curve is obviously bimodal. There are also abundant foraminifera in the typhoon deposits. We found a clear double-layered structure in the typhoon deposits,which was caused by strong hydrodynamic disturbance that mixed sediments originally from the offshore area with those of the lagoon. The lower layer has coarse-grained particles with medium sorting, low organic matter content, and low diatom content. The upper layer has fine-grained particles with poor sorting, high organic matter content, and abundant diatoms. The rate of fragmentation of diatoms in the upper layer was very high (40%-60%). The diatom assemblage contained offshore and freshwater species carried by storm runoff.Therefore,we believe that the sediments of this typical sand bar-lagoon environment retain evidence of typhoon events along the southern China coast that is displayed in the marked sedimentological and microfossil characteristics.

Keywords Sediment record, Microfossils, Typhoon Rammasun, Lagoon, Southern China coast

1.Introduction

As major marine natural disasters, tropical storms with sustained surface wind speed of >63 km/h occur globally at least 80 times annually (Webster et al.,2005; Xu et al., 2007). The Northwest Pacific region,including the South China Sea, is one of the regions frequently affected by tropical storms such as typhoons, and southern China is an area that is often seriously impacted.Throughout Chinese history,many individual typhoon events have resulted in more than 10,000 fatalities.Among these,two of the most serious events were the typhoons that caused at least 100,000 fatalities in Shanghai in 1696 and the Pearl River Delta in 1862.In a 45-year period(1949-1993),455 tropical storms affected China, with the southern Guangdong Province suffering most (Liang et al., 1995). Tropical storms can lead to many casualties and huge economic losses, and the resulting storm surge and huge waves can have substantial impact on fragile coastal ecosystems (Bianchette et al., 2009).

Owing to the reasonably short period over which tropical storms have been measured using modern instrumentation, the complex processes and mechanisms of their formation, development, and evolution are not well understood and the characteristics of super typhoons remain even less well known. Therefore,it is crucial to expand the time series of tropical storm data (Liu and Fearn, 1993). Obtaining information on ancient storm activity from geological records that span the past several thousand years will help improve interpretation of modern storm deposits and prediction of future storm activity. Identification and study of palaeostorm deposits require comprehensive use of techniques in the fields of sedimentology,palaeontology, and geochemistry and examination of historical documents(Das et al.,2013;Palinkas et al.,2014; Pilarczyk et al., 2016; Katsuki et al., 2017;Switzer et al.,2020).Exploration and improvement of research methods in these areas are important for palaeostorm studies, and grain size and microfossil analyses have often been used in the study of palaeostorm deposits.However,there is a general lack of similar research on modern storm deposit formation and preservation, particularly in areas with sandy coasts, which introduces uncertainty and limits the interpretation of palaeo- and modern-storm records.

It is difficult to study microfossils in modern storm deposits because it is challenging to conduct sampling in an appropriate location before the occurrence of a severe typhoon event. In the past 30 years, relevant research has been undertaken mainly in coastal areas of the western Atlantic. For example, Hawkes and Horton (2012) compared the volume, particle size distribution, organic matter content, and foraminiferal assemblage of overwash deposits associated with Hurricane Ike (landfall: 13 September 2008) at three sites on the Galveston and San Luis islands (Texas,USA). Following the passage of Hurricane Andrew(1992),which swept along the Louisiana coast(USA),it was found that the diversity of diatom species was higher in sediments associated with the storm, while the number of marine species decreased, concurrent with low nitrogen content and poor sorting (Parsons,1998). However, salt marsh sediments from the Mississippi and Alabama coasts,recovered during the 2005 storm season,showed almost no foraminifera in stormrelated sediments,whereas many foraminifera groups typical of salt marsh environments were found in the sediments unaffected by the storm. The absence of foraminifera in storm-related sediments might be attributable both to their inherent rarity in beach and dune sands as well as to post-depositional changes(Horton et al., 2009). The diatom distributions in surface sediments on the inner shelf of the East China Sea were obviously modified by Typhoon Morakot, causing fresh water species to be transported 100 km offshore and seasonal patterns completely upset (Chen et al.,2019). Sedimentological characteristics of tropical cyclone overwash sediments from Vanuatu, South Pacific were studied in 2018 (Hong et al.,2018). Studies in the Philippines had shown that Typhoon Haiyan deposits have the same sedimentary and stratigraphic characteristics as both storm and tsunami deposits(Soria et al., 2017). The coastal flood caused by Typhoon Haiyan left two different sediment assemblages in Hernani, central Philippines (Soria et al.,2018). Given the paucity of related research, new observations and additional data on modern typhoon deposition are urgently required.

Fig. 1 Study area and path of Typhoon Rammasun in July 2014 (modified from typhoon track data from http://map.weather.gov.cn/). The coastal current was modified from Bao et al. (2005).

Little research has been undertaken on modern typhoon deposition in sandy coastal areas of the typhoon-prone Asia Pacific region. Therefore, this study investigated the grain sizes and microfossil characteristics of sediments from a lagoon (Niulanchong)on the coast of southern China(Hailing Island).The research objectives were to improve both the identification of sandy coastal storm deposits and the methods used for palaeostorm interpretation, and provide a scientific basis for the application of these proxies in subsequent palaeostorm studies.

The remainder of this paper is organised as follows.Section 2 presents the characteristics of the area and the typhoon studied. The materials and methods adopted in the study are described in Section 3. Section 4 comprises a comprehensive analysis of the results. A discussion is presented in Section 5 and our conclusions are stated in Section 6.

2.Regional setting

2.1. Study area and surveys

Deposition in a lagoon environment is relatively homogeneous under normal weather conditions, and the sedimentary environment will generally only change when subjected to external disturbance,such as a strong storm, which will leave a trace in the sedimentary record. Niulanchong Lagoon on Hailing Island in southeastern China is a suitable location for studying the deposition of modern typhoon-related sediments.Hailing Island (21°33′-21°40′N, 111°47′-112°01′E) is located to the south of Yangjiang City (Guangdong Province) and south of the Tropic of Cancer (Fig. 1).Hailing Island is the fifth largest island in Guangdong Province, with a land area of 108 hm2and a total coastline length of 141.7 km. The central part of the island is mainly hilly (Tang et al., 2005). Hailing Island has a subtropical climate with annual average temperature of 23°C and annual average rainfall of 1816 mm (which falls mainly during April-October).The direction of the strong prevailing wind is from the southeast, and the coastal current flows toward the southwest. The coastal current is driven by oceanic barotropic and baroclinic effects caused by freshwater runoff along the west coast of Guangdong Province(Bao et al., 2005). The area has an irregular semi-diurnal tide with an average tidal range of 1.57 m in nearshore areas of Hailing Island. The wave direction is predominantly NNE and NE, with annual frequency of 34%and 23%,respectively,and the mean wave height is 0.5 m (Sun, 2006). Each summer, the study area generally experiences approximately two to three typhoons(Cai et al.,2012).

Fig.2 a Outline of Hailing Island and the study area,and b The locations of the cores obtained in Niulanchong Lagoon before and after the passage of Typhoon Rammasun;c The beach area and Niulanchong Lagoon in July 2014;d Niulanchong lagoon in July 2014;e The offshore area in July 2014; f Section view of Niulanchong Lagoon and the dune before it. The Google Earth photograph was taken on October 2014.

Niulanchong Lagoon, which was selected as the study area,lies behind the beach of a small bay in the middle of the southern coast of Hailing Island(Fig.2).The backshore area comprises a 20-m-wide berm and a 1-m-high sand dune covered with vegetation. We conducted two surveys in the small lagoon before and after the passage of Typhoon Rammasun in 2014. The beach area gradually recovered after the impact of the typhoon,and Niulanchong Lagoon was further filled by overwash deposits.The upper profile of the beachface is steep and there is a sand bar on the lower profile.It should be noted that the lagoon had become completely filled in by 2020 with only tidal channels remaining.

2.2. Typhoon Rammasun

The tropical disturbance first monitored on 9 July 2014 in the Northwest Pacific developed into the ninth named storm of the season,called Rammasun,on July 12. This system strengthened continuously and was classified as a typhoon on July 14. It made landfall across the Philippines as a strong typhoon on July 15 and then intensified into a super typhoon as it crossed the South China Sea.At approximately 15:30 UTC+8 on July 18, Typhoon Rammasun hit Wenchang City(Hainan Province) as a super typhoon (wind speed:216 km/h, surface pressure: 910 hPa), the strongest typhoon to make landfall across southern China since 1973.At 19:30 LTon the same day,without weakening in intensity,Rammasun hit Xuwen County(Guangdong Province), becoming the only typhoon since 1949 to make landfall twice as a super typhoon (http://typhoon.weather.com.cn/). On the afternoon of July 18, the hourly precipitation recorded in Hainan Province and in many places of southern Guangdong Province exceeded 40 mm; rainfall in Wenchang reached 69.1 mm and wind speed reached 160 km/h.The wind speed in Zhanjiang City (Guangdong Province) and Wenchang City (Hainan Province) was generally >150 km/h. The maximum wind speed recorded during this event was in Xuwen (215 km/h).On the morning of July 19, Typhoon Rammasun again made landfall, over the coastal area of Fangcheng Port(Guangxi Province).At this time,the surface wind speed was >200 km/h and the hourly rainfall was>60 mm (Wang et al., 2019). During its passage, this typhoon affected coastal areas of the Philippines and Hainan, Guangdong, and Guangxi provinces in China,over a period of at least 9 days.The track of Typhoon Rammasun can be considered representative of western Pacific tropical cyclones in that it passed through the Bashi Channel (Philippines) and then travelled westward to southern China(Guangdong and Hainan provinces) and Vietnam (Wang et al., 2018).

3.Materials and methods

3.1. Sample information

Five core samples (HLD1-A to HLD5-A) were collected from Niulanchong Lagoon(Fig.2b)on 13 July 2014 before the start of the typhoon season (Table 1).Typhoon Rammasun made landfall across coastal areas of southern China on July 18 and affected the study area (Wang et al., 2019; Yang et al., 2019). Subsequently, on 23 August 2014, after the passage of Typhoon Rammasun,five more core samples(HLD1-B to HLD5-B) were collected from the same sites (Fig. 2b)based on positioning using a DGPS (Trimble SPS 332;accuracy: ±0.5 m). The core samples were collected manually using a gravity sampler. The locations of the sampling sites and details of the sediment cores are shown in Table 1 and Fig.2. Preliminary description of the core samples (e.g., colour and petrography) was performed under laboratory conditions. Each core sample was separated longitudinally and continuous sampling was performed at 2-cm intervals.

Table 1 Details of sediment cores recovered from the study area(Niulanchong Lagoon).

Sample grain size,210Pb,diatoms,and total organic carbon (TOC) were analysed at the Third Institute of Oceanography,Ministry of Natural Resources of China.The foraminiferal analysis was completed at the Ocean University of China.The X-ray scanning for core photos was performed at the Second Institute of Oceanography, Ministry of Natural Resources of China.

3.2. Grain size analysis

All sorted core samples were selected for grain size analysis based on 4-cm sampling intervals.Overall,197 samples were selected for analysis.

We determined the mean grain size (Mz; unit: Φ),sorting coefficient (So), and skewness (Ski) of the sediment samples using laser diffraction particle size analysis and the sieving method.The coarse fraction of a sample(>1 mm)was first sieved,dried,and weighed to calculate the percentage of particles >1 mm, and this fraction was then sieved again at intervals of 1/2Φ. The fine particle fraction was analysed using a Mastersizer 2000 laser particle size analyser (test range: 0.02-2000 μm). We used the Fork and Ward(1957)'s formula to calculate Mz, So, and Ski (with standard deviation; Fork and Ward, 1957; Blott and Pye, 2001):

3.3.210Pb

From the HLD1-A and HLD1-B cores, 13 and 23 samples(including parallel samples)were selected for210Pb analysis,respectively.

The210Pb activity was analysed from dry samples(4 g) using a 7200-8 alpha spectrometer (Canberra,France). The analysis procedure followed the method of Swarzenski (2014) and used the constant rate of supply model. All sediment radionuclide concentrations are reported in dpm/g dry weight.

3.4. Diatom sample processing

All sorted core samples were selected for diatom analysis based on 2-cm sampling intervals.Overall,389 samples were selected for diatom analysis.

Fig. 3 Comparison of 210Pb values at sampling site HLD1 before (HLD1-A) and after (HLD1-B) the passage of the Typhoon Rammasun.

We prepared the samples following a modified version of the method of H¨akansson (1984). Samples(～5 g)were dried in an oven(60°C)and weighed.They were then treated with 10% HCl and 30% H2O2to remove carbonate and organic material, respectively,and washed in distilled water to remove the chemicals from the solution.The samples were soaked in distilled water for 24 h and then scattered using an ultrasonic dispersion instrument(120 Hz)for 2 min.We chose not to consider diatoms in the microplankton size fraction in this study. Therefore, the samples were filtered through a 15-μm sieve to remove micro-diatoms and other material finer than 15 μm. The suspension containing the diatoms was concentrated to a volume of 2 mL. After homogenization, a subsample of the suspension was transferred to a cover slip and air-dried.Three permanent slides were made from each sample using Canadian balsam (H¨akansson, 1984) for identification under an optical microscope (Olympus BX51,objective lens 40 ×, and ocular lens 10 × ).

In counting the broken valves of the diatoms, at least half of the valves of Centricae must be incomplete, whereas the Pennatae must be incomplete on the side of the shell seam(Lopes et al.,2006).The rate of diatom fragmentation was defined as the ratio of the number of broken valves to the total number of valves (i.e., the sum of the broken and complete valves).

3.5. Foraminifera sample processing

Overall, 14 samples from the HLD1-B core were analysed for foraminiferal content. For foraminiferal analysis,the samples(63 μm-0.5 mm)were subjected to a heavy liquid (zinc iodide (ZnI2); specific gravity:2.2-2.3).When a sample exceeded 5 g,approximately 5 g was subsampled and used for analysis; however,when a sample was less than 5 g,the entire sample was used. Each sample was transferred to a 50-mL centrifuge tube, to which 10 mL of the heavy liquid was added. The mixture was then centrifuged for 2 min(3000 rpm)and subsequently left for 15 min to settle.The supernatant was transferred to a numbered beaker. This process was repeated and the resultant pellet was retained. The contents of the beaker were washed with 10‰ acetic acid through a silk screen(aperture: 0.01 mm). The residue was transferred to numbered filter paper to remove excess water and then dried at 60°C. After drying, the material was transferred to a numbered paper bag for reference.Foraminiferal identification was conducted using the banding method with a binocular stereomicroscope.Generally, specimens were identified to species level and no less than 200 shells were counted in each sample. The identified foraminifera were placed into numbered small paper bags for inspection.

4.Results

4.1. Comparison of 210Pb results before and after passage of Typhoon Rammasun

Fig. 4 Sedimentary grain size frequency curves of the ten core samples obtained before and after the passage of the Typhoon Rammasun.

Fig. 5 Sedimentological analysis results of the ten core samples obtained before and after the passage of the Typhoon Rammasun.

To identify the thickness of the sedimentary disturbance caused by Typhoon Rammasun,210Pb was measured in the samples obtained from cores before(HLD1-A) and after (HLD1-B) the passage of Typhoon Rammasun.The location of HLD1 in the innermost part of the lagoon means that it is less affected by the open sea under normal weather conditions and that the sedimentary environment is reasonably stable. Analysis of the210Pb results from HLD1-A and HLD1-B(Fig. 3) indicates that210Pb activity decreased markedly with depth; however, these two cores show notably different characteristics. In the 0-4-cm layer of HLD1-A,210Pb activity showed obvious mixing,likely reflecting a bioturbation layer. In HLD1-B,210Pb activity was strongly mixed in the 0-30-cm layer. The activity of210Pb showed a regular exponential decline below 30 cm. This notable change occurred over a short period (40 d), which suggests that it was caused by the typhoon event. However, this phenomenon of210Pb mixing in the sedimentary layer obviously cannot be used as the only form of identification of typhoonrelated deposition owing to its ambiguity.

4.2. Grain size characteristics

4.2.1. Before the passage of Typhoon Rammasun

The sedimentary grain size frequency curve of each of the five core samples obtained before the passage of the typhoon consisted of a single-peaked curve,except for HLD5-A,which had a double-peaked curve at the 1-cm layer obviously (Fig. 4). The sediments of these cores were composed mainly of sand, that is, sand content was almost >98%. The grain size bins were concentrated in the range of 100-1000 μm (Fig. 4).Overall, the sediment composition was reasonably simple(Fig.5).

The grain size parameters of HLD1-A changed greatly in the vertical and could obviously be divided into two parts. The Mz value in the upper layer(0-10 cm)of HLD1-A indicated fine grains,the So value indicated poor sorting,and the Ski value was positively biased, whereas the opposite was true below 10 cm.The grain size parameters of HLD2-A changed little in the vertical, that is, the Mz value was in the range of 1.05-2.21Φ. The So values were between 0.49 and 1.13,indicating good separation performance,and the Ski values in the range of −0.34 to 0.31 were nearly symmetrical. There were three fine layers at 0-5 cm,30-37 cm, and 42-48 cm in HLD3-A. The sorting was poor and the Ski value was positively biased in each of the three fine layers. The grain size parameters of HLD4-A remained unchanged above 50 cm; however,the sediment at the bottom (50-58 cm) was fine and the sorting was poor.The average particle size was fine and the sorting was poor in the 0-6-cm layer of HLD5-A,whereas these parameters were uniform in the lower part of the core. The Ski values were reasonably uniform and nearly symmetrical.

Each of the HLD1-A samples showed TOC values of<0.4% and TOC was almost zero below the surface layer. The other four core samples contained no organic matter.

4.2.2. After the passage of Typhoon Rammasun

Except for HLD4-B, the sedimentary grain size frequency curves of the sediment cores after the passage of the typhoon were notably different to those before the typhoon(Fig.4). HLD1-B had a double-peaked frequency curve at 5 cm, and a primary (sharp) and secondary (gentle) double-peaked shape at 29 cm. This result was consistent with the210Pb results, which confirmed that the depth of the typhoon deposit at this core was approximately 30 cm. The sedimentary grain size frequency curves of HLD2-B and HLD3-B both showed double-peaked curves, and did not become single-peaked curves until 35 cm. The grain size frequency curve of HLD5-B showed bimodal morphology in the layers above 15 cm and below 41 cm.

In comparison with the situation before the passage of the typhoon, all the core samples (HLD1-B,HLD2-B, and HLD3-B) inside the lagoon after the passage of the typhoon had a layer of fine-grained sediment on the surface. The thickness of these finegrained deposits was 12 cm, 26 cm, and 15 cm at HLD1-B, HLD2-B, and HLD3-B, respectively. The TOC content of HLD1-B and HLD3-B was relatively high in these fine-grained layers.The grain size parameters of the upper part of HLD4-B were little changed,but the TOC value was higher at 0-6 cm and 14-34 cm. The grain size in the upper 0-12 cm of HLD5-B,located on the outermost side of the lagoon, was obviously finer,although the core did not contain organic matter.

4.3. Diatom assemblages

Overall, 84 species and varieties of diatoms belonging to 31 genera were identified in the core samples. Among them, Fragilaria capusina, Gyrosigma nodiferum,and Cyclotella comta were the main freshwater species. The main brackish water species were Actinocyclus ehrenbergii, Aulacodiscus sollittianus,Coscinodiscus blandus, Coscinodiscus centralis, Coscinodiscus curvatulus, Coscinodiscus divisus, Coscinodiscus gigas,Coscinodiscus oculus,Coscinodiscus rothii,Cyclotella stylorum, and Paralia sulcata. The main saltwater species were Actinocyclus fasciculatus,Actinocyclus ingens, Actinoptychus undulates, Amphora crassa, Amphora proteus, Biddulphia tridens, Coscinodiscus radiates, Navicula fujianensis, Navicula humerosa var. Constricta, Navicula granulate, Navicula marina, Surirella fluminensis, Trachyneis aspera, and Tryblioptychus cocconeiformis. The characteristic species compositions and diatom variations at sampling sites HLD1, HLD2, HLD3, HLD4, and HLD5 before and after the passage of Typhoon Rammasun are shown in Figs.6-10.

4.3.1. HLD1

The diatom concentrations in HLD1-A (before the passage of the typhoon) were in the range of 0-278 valves/g,which are considered as low values.The rate of diatom fragmentation was 0-12.48% (Fig. 6). The core samples contained mainly saltwater and freshwater species, indicating that the station might have been affected by the freshwater of the surrounding water channel and pools(Fig.2).

After the typhoon, the diatom concentrations in HLD1-B were in the range of 0-1507 valves/g. The content of diatoms in the upper 0-30 cm of the core had greatly increased in comparison with the situation before the typhoon, and could clearly be divided into two sections. The core samples also contained mainly saltwater species with a few freshwater species. The rate of diatom fragmentation was in the range of 0-45.31%. The highest rate of diatom fragmentation was concentrated in the upper part of the core(0-33 cm);the average value of 11.68%was higher than in HLD1-A(Fig.6).

4.3.2. HLD2

The diatom concentrations in HLD2-A (before the passage of the typhoon) were in the range of 0-80 valves/g, and the rate of fragmentation was in the range of 0-0.25% (Fig. 7). This indicated that the station was almost free of diatoms before the typhoon.

After the typhoon, the diatom concentrations in HLD2-B were in the range of 0-1012 valves/g, that is,markedly higher than before the typhoon.In the 0-35-cm layer, there were large numbers of saltwater and freshwater species, together with a few warm water species and offshore species. The offshore species and warm water species comprised mainly Thalassiosira excentrica and Azpeitia nodulifera respectively. In comparison with the diatom assemblages before the typhoon, these freshwater, warm water, and offshore species were obviously not local,but transported from other places.The rate of diatom fragmentation was in the range of 0-75%(average:18.35%),and the broken diatoms were distributed mainly in the 0-35-cm layer(Fig.7).

4.3.3. HLD3

The diatom concentrations in HLD3-A (before the passage of the typhoon) were in the range of 0-169 valves/g, and the rate of diatom fragmentation was 0-50%; both ranges are considered low. The diatoms in the core samples comprised mainly saltwater species with an average content of 20.75% in the 0-35-cm layer (Fig. 8), indicative of a normal lagoon environment.

After the typhoon, the diatom concentrations in HLD3-B were in the range of 0-212 valves/g, that is,slightly higher than before the typhoon. The rate of diatom fragmentation was 0-50%, that is, markedly higher than before the typhoon, and the broken diatoms were distributed mainly in the 0-12-cm layer(Fig. 8).

4.3.4. HLD4

In HLD4-A(before the passage of the typhoon),the diatoms were distributed mainly in the 0-5-cm and 50-58-cm layers, and the highest value was 181 valves/g. The core samples that contained the greatest number of saltwater species,warm water species,and broken diatom shells were from the 50- to 58-cm layer (Fig. 9).

After the typhoon, HLD4-B contained a rich layer of diatoms at 53-77 cm. The composition characteristics of the diatoms were very similar to those of the 50-58-cm layer of HLD4-A (Fig. 9); therefore, they can be considered to belong to the same layer. This layer was also strongly reflected in the grain size analysis results before and after the typhoon (Figs. 4 and 5). There were a small number of diatoms in the upper 0-5 cm of HLD4-B, but the rate of fragmentation (up to 20%-40%) was higher than before the typhoon (Fig. 9).

4.3.5. HLD5

In HLD5-A (before the passage of the typhoon),diatoms were present only in the samples from the surface layer (0-5 cm), and the maximum diatom concentration was 42 valves/g. The diatoms in this layer were predominantly saltwater species and there were no broken diatoms (Fig. 10).

After the typhoon, the diatom concentrations in HLD5-B were greater in the upper 0-15-cm layer,with values typically in the range of 0-167 valves/g.Similar to those before the typhoon,the diatoms were mainly saltwater species,although some warm water species, such as C. adriaticus (10-12 valves/g) that were obviously brought from outside the lagoon,were found in the 10-18-cm layer. The rate of fragmentation was markedly greater after the typhoon, with a maximum value of 66.67%,mainly in the 0-6-cm layer(Fig. 10).

Fig. 6 Characteristics of diatom abundance at sampling site HLD1 before (HLD1-A) and after (HLD1-B) the passage of Typhoon Rammasun.

4.4. Foraminifera

According to the210Pb determination, as well as grain size and diatom data,the storm deposits of HLD1-B were well preserved after the typhoon. Therefore,HLD1-B was selected for foraminiferal analysis.Overall, 14 samples (0-56 cm) were analysed and the foraminifera species were identified. Although 26 species (including varieties) of foraminifera were found, there were six dominant and characteristic species:Cavarotalia annectens,Elphidium hispidulum,Helenina anderseni, Poroeponides cribroepandus,Pseudoeponides nakazatoensis, and Quinqueloculina lamarckiana. The distribution characteristics of the main foraminifera genera and species in HLD1-B after the typhoon are shown in Fig.11.The abundance of the foraminifera was 0-135 values/50 g.

Fig. 7 Characteristics of diatom abundance at sampling site HLD2 before (HLD2-A) and after (HLD2-B) the passage of Typhoon Rammasun.

Fig. 8 Characteristics of diatoms at sampling site HLD3 before (HLD3-A) and after (HLD3-B) the passage of Typhoon Rammasun.

Fig. 9 Characteristics of diatoms at sampling site HLD4 before (HLD4-A) and after (HLD4-B) the passage of Typhoon Rammasun.

Fig. 10 Characteristics of diatoms at sampling site HLD5 before (HLD5-A) and after (HLD5-B) the passage of Typhoon Rammasun.

On the basis of the vertical distribution characteristics of the foraminifera,the core could be divided into two sections (Fig. 11). In the lower section(30-60 cm), the number of foraminifera was very small. The surfaces of these few shells were mostly worn, contaminated by iron, or filled with intra-shell sediments. In the upper section (0-30 cm), foraminifera were abundant, with a mix of fresh shells and those showing redeposition characteristics (Fig. 11).The principal dominant species in the upper part were Cavarotalia annectens, Elphidium advenum, E. hispidulum, Pararotalia nipponica, and Quinqueloculina lamarckiana. Among them, E. hispidulum and Quinqueloculina lamarckiana are common in coastal lagoon environments, while other species are common in shallow sea environments. This layer rich in foraminifera coincides with the typhoon deposit layer obtained from the aforementioned analyses of210Pb,grain size, and diatoms.

5.Discussion

5.1. Sedimentological characteristics of modern typhoon storm deposits

According to variations in210Pb activity in the samples before and after the passage of Typhoon Rammasun, sediment transport and redeposition caused by a typhoon leads to low values of210Pb content. In Chesapeake Bay (USA),210Pb has also been used specifically to study event-related deposition,whereby the low-value area of a210Pbexcurve reflects the event-affected layer (Nie et al., 2001). Wang(2009) showed that storm surge occurrence had strong correlation with low values of210Pbexactivity in a core obtained in tidal flat sediments of the Bohai Sea.Thus, our results are consistent with these studies.

Fig. 11 Foraminifera abundance in storm deposits of HLD1-B after the passage of the Typhoon Rammasun.

Combined with analysis of the grain size characteristics of HLD1-HLD5 before and after the typhoon,it appears from the sediment granularity frequency curve(bimodal)that the sediment was affected by two types of force,but dominated by one of them after the typhoon.The grain size composition represents mainly coarse particles,with a low content of silt(Fig.5).As there is no large river injection near Niulanchong Lagoon,the original sedimentary characteristics were overprinted during and just after the typhoon, which means that sediment transport was affected primarily by the storm. Each core can clearly be divided into upper and lower parts because their sedimentary characteristics are notably different. The obvious boundaries are at 30 cm in HLD1-B, 35 cm in both HLD2-B and HLD3-B, 5 cm in HLD4-B, and 15 cm in HLD5-B(Figs.5-10).As this result is highly consistent with the results of the210Pb analysis of the samples from HLD1,we suggest that these upper sediments are typhoon deposits. This would indicate that the maximum thickness of overwash deposition following the passage of Typhoon Rammasun was 35 cm. This is close to the thickness (2-28 cm) of the stormdeposited sand layer estimated by Hawkes and Horton (2012) (Table 2). Additionally, it shows that typhoon deposits gradually become thinner from the inside of the lagoon to the entrance.

The typhoon sediments can be divided approximately into two layers from bottom to top in four of the cores (i.e., HLD1-B, HLD2-B, HLD3-B, and HLD5-B). Stratification boundaries are located respectively at 12 cm, 26 cm, 15 cm, and 12 cm in HLD1-B,HLD2-B,HLD3-B,and HLD5-B.In comparison with the lower sediments, the grain sizes of the upper sediments are finer, but the sorting is poorer and the grain size frequency curve is obviously bimodal(Figs.4 and 5). However, only HLD2-B and HLD3-B showed obvious change in the stratification on the X-ray images (Fig. 5). This is because the grain size varies markedly between the two layers of the two cores.It is suggested that these sediments were formed by direct mixing of the coarse and fine particles in the lagoon environment without modification.Core HLD4-B was obtained from the mouth of the lagoon and showed no obvious stratification. In HLD1-B, HLD3-B,and HLD4-B, the upper parts of the cores represent storm deposits or have high levels of organic matter.These phenomena reflect the characteristics of typhoon deposition and hydrodynamics, where some of the water flows directly into the lagoon through the tidal channel at the lagoon mouth and some flows into the lagoon by crossing the sand bar. Furthermore,typhoon rainfall and flooding will wash the coastal areas and bring organic matter and other fine-grained materials into the lagoon. In the early stages of a strongly dynamic environment, coarse-grained sediments are deposited first and maintain good to medium sorting.In the latter stages,owing to weakening of the dynamic environment, organic matter and other fine-grained materials are deposited, which tend to be poorly sorted and have an obvious bimodal granularity frequency curve.

Table 2 Comparison of the sedimentary and microfossil characteristics of the deposits of Typhoon Rammasun and other modern storms.

Table 2-(continued)

Fig.12 Sedimentological and microfossil characteristics of modern typhoon deposits in sandy sediments along the southern coast of China.

5.2. Microfossil records of modern typhoon storm deposits

At all five sampling sites, we found a high proportion of broken diatoms (mainly 40%-60%) in the typhoon storm deposits,especially in the upper layer,and a rather high total diatom abundance compared with sediments deposited under normal weather conditions.Pre-typhoon deposits in the sandy lagoon,which were originally relatively barren, lacked diatoms. As a result of the passage of the typhoon, diatoms in offshore waters outside the sand bar were brought into the lagoon, which increased the total diatom concentrations in the typhoon deposits.Generally, there were more diatoms in the upper layer than in the lower layer because the size and specific gravity of diatoms make them easier to deposit with fine-grained sediment, which causes them to accumulate in the upper layer. The higher proportion of broken diatoms represents strong typhoon hydrodynamics,which broke the valves and deposited them in the upper layer. In the study area, the foraminifera content is very low in the pre-typhoon sediments, but there are more foraminifera in the typhoon deposits (HLD1-B; Fig. 11). Owing to the large size of foraminiferal shells,they are distributed throughout the typhoon deposits of HLD1-B. This indicates that the sandy lagoon in general lacks foraminifera under normal conditions and that a large number of foraminifera were transported into the lagoon from offshore waters during the typhoon.This is consistent with the study of Hawkes and Horton(2012) (Table 2). They also found species endemic to the coastal environments of the Gulf of Mexico in storm overwash sand deposits.

In the study area,only the sediments at HLD1 and HLD2, close to the interior of the lagoon, contained freshwater diatom species. There were some freshwater diatoms in the upper layer that were derived from typhoon rainfall and flood scouring, which is consistent with the situation reflected by the sedimentary characteristics. Additionally, small numbers of warm water species and offshore species of diatoms were found in the typhoon deposits in HLD1-B and HLD5-B, but not in the pre-typhoon sediments,which indicates that some offshore diatoms were carried into the lagoon by the typhoon. The foraminifera also reflected this same pattern, indicating that some freshwater shells and warm water species found in the typhoon deposits were transported into the lagoon from open offshore waters.

5.3. Identification of storm deposits along a sandy coast

On the basis of our study of Niulanchong Lagoon,the characteristics of modern typhoon deposition in a sand bar-lagoon environment along the coast of southern China were revealed.Under normal weather conditions, sediments in the lagoon are transported from the backshore sand. These sediments have a normal210Pb curve(exponential decay model;Fig.3)and a single-peaked granularity frequency curve. As the sandy coast is reasonably barren and low in biological productivity,it lacks organic matter,diatoms,and foraminifera.

Under the action of a strong typhoon, waves will disturb sediments and rush into the lagoon and inside the sand bar,forming typhoon deposits.However,our lagoon is connected to the open sea by tidal channels.If typhoon sediments entered the lagoon mainly through the tidal channels, we would have expected to find more alien species at HLD5 than at HLD1. In fact, we found the reverse to be true. Therefore,typhoon deposits in our study area were dominated by overwash. Compared with deposition under normal weather conditions,these typhoon deposits have very obvious sedimentary and fossil characteristics.Generally, they have low210Pb activity, coarse particle size, and obvious bimodal granularity frequency curve (Fig. 12). In HLD1-B, the storm deposits with strong variations of210Pb activity also exhibited the characteristic bimodal granularity frequency curve.These typhoon deposits also contained abundant foraminifera,most of which originated from offshore areas.This is consistent with the study of Hawkes and Horton (2012) in Galveston, and indicates that foraminifera are very important for the confirmation of storm deposits. Typhoon deposits had an obvious double-layer structure in this study attributable to the hydrodynamic processes of a typhoon (Fig. 12).The lower layer had coarser particle size, medium sorting, and low organic matter content. The upper layer had fine particle size, poor sorting, relatively high organic matter content, and abundant diatoms.These diatoms had a very high rate of fragmentation(40%-60%)caused by friction and washing of diatoms and gravel under the action of strong dynamic flow.These diatoms also included exotic offshore species and freshwater species carried in by storm runoff.These freshwater species might have come from the northeastern waterway of Niulanchong Lagoon and surrounding pools.

Analysis indicates that this deposit is a wellpreserved typhoon-event layer in the lagoon of this typical sand bar-lagoon system on the coast of southern China. Based on the sedimentological and micropalaeontological characteristics of the records from HLD4, we can find clear evidence of the most recent typhoon event (～50-58 cm in HLD4-A and～50-80 cm in HLD4-B;Fig.9);thus,their combination can be used as a reliable indicator of palaeostorm deposits.

6.Conclusions

This study characterized modern typhoon deposits from cores recovered from a typical sandy lagoon(Niulanchong Lagoon)on the southern coast of China before and after the passage of Typhoon Rammasun using210Pb,sedimentological, and micropalaeontological (microfossil) analyses. We found that the maximum depth of sediments influenced by Typhoon Rammasun in the lagoon was approximately 35 cm. The deposits formed by the typhoon, which gradually thinned from the maximum thickness inside the lagoon to approximately 5 cm atthe mouth,weremainlydominated byoverwash.

Under normal weather conditions, the sandy lagoon sediments have a210Pb curve that follows an exponential decay model and a single-peaked grain size frequency curve,and lack organic matter,diatoms and foraminifera in this environment. The sediment transport and redeposition caused by a typhoon lead to low210Pb content. Compared with normal weather conditions,the mean grain size of the typhoon deposits is coarser and the frequency curve appears bimodal.Foraminifera in the typhoon sediments,most of which came from offshore, are abundant.

The internal structure of the typhoon deposits has obvious double-layered characteristics. The lower layer consists of coarse particles,with medium sorting,and low organic matter content, whereas the upper layer has fine particles, with poor sorting, and high organic matter content,and is diatom-rich.The rate of fragmentation of the diatoms is very high (40%-60%).The diatom assemblage contains both offshore species and freshwater species carried by storm overwash and storm runoff, respectively.

This study showed that the effects of modern typhoon events can be recorded in the sediments of a typical backshore lagoon along the coast. Sedimentological and microfossil characteristics can be used as important indicators of typhoon deposition in sandy lagoons,and thus can identify palaeostorm events in the geological record. However, these indicators must be interpreted comprehensively because application of any single indicator has a multiplicity of interpretations.

Abbreviations

LTLocal Time.

UTCUniversal Time Coordinated.

Funding

This study was supported by the Key Program of National Natural Science Foundation of China (Grant No. 41930538), the National Science Foundation of China (Grant No. 41306083), the National Key Research and Development Program of China (Grant No. 2019YFE0124700), and the Scientific Research Foundation of Third Institute of Oceanography, Ministry of Natural Resources of China (Grant Nos.2019018, 2019026, and 2020017).

Availability of data and materials

The datasets used or analysed during the current study are available from the corresponding author on reasonable request.

Authors' contributions

HSQ proposed the conceptualization, wrote the original draft and took the supervision; MC provided resources and methodology, did formal analysis and data curation, and wrote and reviewed the original draft; LNS participated in the investigation, data curation, and formal analysis; FC contributed some suggestions and modifications to the manuscript; AMZ participated in the investigation and validation; and,QF participated in the investigation and data curation.All authors read and approved the final manuscript.

Competing interests

We understand that the corresponding author is the sole contact for the editorial process.He is responsible for communicating with the other authors about progress,submissions of revisions and final approval of proofs. We confirm that we have provided a current,correct email address which is accessible by the corresponding author.

We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed.We further confirm that the order of authors listed in the manuscript has been approved by all of us.

We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual property.In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property.

All listed authors declare that they have no competing interests.

Acknowledgements

We thank all those who helped to collect samples and data during the surveys.We would also like to thank Dr.Hai-Yan Long of the Ocean University of China for her foraminiferal analysis and identification, Ms. Jia-Xuan Li for her contribution in data processing and mapping,and Dr. Wu Men for his help in210Pb analysis. We acknowledge the financial support received from the Key Program of National Natural Science Foundation of China (Grant No. 41930538), the National Science Foundation of China(Grant No.41306083),the National Key Research and Development Program of China(Grant No. 2019YFE0124700), and the projects sponsored by the Scientific Research Foundation of Third Institute of Oceanography, Ministry of Natural Resources of China(Grant Nos.2019018,2019026,and 2020017).