Storm deposition layer on the Fujian coast generated by Typhoon Saola (2012)

Typhoons have a significant effect on the marine depositional environment and depositional process. In this paper, we used the high-resolution Chirp sonar sub-bottom profiler and radioisotope detection techniques to examine the storm-deposited layer formed in the seawater near the path of Typhoon Saola along the coast of Fujian, China. The thickness of the typhoon-deposited layer acquired using these two methods was 10–25 cm. The thickness, sediment grain size, and δ13C values of the deposited sedimentary layer indicated that it was mainly matter from the re-suspension and redistribution of seafloor sediments. The particle sizes of the sediments in the storm-deposited layer became coarser, indicating that the fine-grade compositions spread over a wider range out of the coastal zone.


Typhoon Saola
Typhoon Saola was first formed east of the Philippines on July 28, 2012 and moved northwest after its formation. It was strengthened to a typhoon on the afternoon of July 30 and intensified to a strong typhoon at 14:00 on August 1. Typhoon Saola landed on Hualien, Taiwan at 19:00 on August 1. The typhoon gradually weakened and landed second time on Fuding, Fujian at 22:00 on August 2. The maximum wind at the center of the typhoon at the second landing was 33 m/s. The typhoon continued to move into inland Fujian after landing and weakened into a tropical storm. The numbering stopped on the evening of August 3 ( Fig. 1) 15,16 . Typhoon Saola had a high intensity but with a slow moving speed. In combination with a southwest monsoon, the local rain was heavy, affecting a wider spatial range over a long time period. The typhoon significantly affected the coastal marine sedimentary environment by altering material supply, transportation and deposition.

Results
The study area was located in the mud depo-area off the Zhejiang-Fujian coast (Fig. 1), where the seafloor sediments were dominated by fine-grained clayey silt and silty clay. The thickness of muddy sediment, which was mainly discharged by Yangtze River and transported southward by the coastal current (namely the Minzhe Coastal Current), was more than 30 m in the deposition center and the sedimentary structure of stratum of the upper sediment layer was relatively uniform without any sub-reflection 17,18 .
The characteristics of the shallow stratum profile on the 4 survey profilers are shown in Fig. 2. Typhoon Saola passed through profilers L1 and L3, and its path was generally parallel to profiler L2 ( Fig. 1) (measured on August 14, 2012). And the profiler SL60 (measured on August 19, 2012), located in the inner-shelf off the Minjiang Estuary, was treated as a reference line (far away from the typhoon path and beyond the typhoon's direct impact). Profiler L1 (18.59 km), located in the coastal shallow water, had a water depth between 12 and 20 m from the west side to east side. The physical characteristics of sediment and the structure of stratum were relatively uniform in the whole upper-most 10-20 m layer of the chirp reflection images ( Fig. 2a; a-2). However, in the profilers L2 (22.29 km) and L3 (10.91 km), extending from the coastal shallow water to deep-water area from the north side to the south side, there exist double reflections on the near-surface with the thickness of upper sediment layer was about 20 cm in the chirp reflection images ( Fig. 2b; b-2 and 2c; c-2). The double reflections both in profilers L2 and L3 were distributed across the entire profilers and slightly weakened near the coast ( Fig. 2b; b-2 and Fig. 2c; c-2). In the reference profiler SL60 (8.38 km), which extended from the coastal shallow water to deep-water area, the physical characteristics of sediment and the structure of stratum were relatively uniform in the whole upper-most 10-20 m layer in agreement to the profiler L1 of the chirp reflection images ( Fig. 2d; d-2).
Due to the uniform physical characteristics of seafloor sediments in the study area, the reflection of stratum should be uniform without any sub-reflections just as shown in the reference profiler SL60 ( Fig. 2d; d-2) and other previous studies 17,18 . On the contrary, in the Typhoon Saola active area (Profilers L2 and L3), a distinct double reflections appeared in the near-surface sediments in the relatively deep-water area ( Fig. 2b; b-2 and Fig. 2c; c-2). The double reflections indicate significant changes in the physical properties of the sediments, which might be the newly formed or transformed sediment layer caused by the typhoon. The thickness of the newly sediment layer could be discerned in the shallow stratum profiles and was relatively thin (approximately 20 cm in thickness). However, there were no double reflections in the profiler L1, which is also located in the typhoon-impacted area. Possible mechanism will be analyzed in the discussion section by elaborating and comparing the distributions of sediment physical characteristics in different cores ( Fig. 2a; a-2).
The distribution features of 210 Pb ex activity, particle size and δ 13 C in the 4 box-type samples collected in the study area, along with a reference core (MJK9, collected in the profile SL60 in April, 2010) are shown in Fig. 3. The sediments were relatively homogeneous and mainly composed of fine-grained clayey silt in all the 5 cores, consistent with previous findings 17,18 . The distributions of δ 13 C is shown as a steady pattern with little fluctuations from the top to bottom of sediments with value generally ranging from − 23 to − 22.4‰ in the 4 cores (Fig. 3). In the core ND-4, the value of δ 13 C was decreased to around -26‰ with the increase of sand content in the 2-14 cm depth of sediments (Fig. 3d). The distributions of 210 Pb ex (less than 2 dpm/g) and average grain size (range from 7 to 7.4 Φ ) in the core ND-1 were relatively uniform from the top to bottom (Fig. 3a). In the cores of ND-2, ND-3 and ND-4, the sediments can be divided into two parts according to the distributions of 210 Pb ex and grain size. The thickness of upper parts was 18, 26 and 14 cm, respectively, in the core of ND-2, ND-3 and ND-4 ( Fig. 3a-c). The boundary of the two layers corresponded with color variations (brown in upper layer and dark brown in bottom layer) in the sediments (Fig. 3). The content of 210 Pb ex was relatively high (about 4, 4-5 and 3-4 dpm/g, respectively, in the core of ND-2, ND-3 and Nd-4) without significant vertical decay in the top sediments, while it was gradually reduced in the bottom layer. In the upper sediment layer, the grain size was significantly larger than that in the lower sediment layer in the 3 cores. The variations of the content of 210 Pb ex , average grain size and sand content in the sediments were similar in the 3 cores. The distribution of 210 Pb ex in the reference core MJK9 showed a different pattern, with high value and significant vertical decay (from 5 decreased to 2 dpm/g) in the top 28 cm, while it was low and stable (less than 2 dpm/g) in the bottom layer. The average grain size in the upper layer was slightly larger than that in the bottom layer (Fig. 3e).
Based on the distributions of 210 Pb ex content, color and average grain size, the sediments of the 4 cores in typhoon-active area can be divided into 2 parts. The top part was generally 10-25 cm thick, with the 210 Pb ex content relatively high and uniform vertically, which showed typical characteristics of mixed sediment layer. Judging from fresh yellow brown color and the high 210 Pb ex content, this layer might be the product of strong perturbation or simultaneous short-term deposition caused by typhoon process. The sediment grain sizes in the upper layer of ND-2, ND-3 and ND-4 were larger than those in the bottom part, which indicated a coarsening of sediment ( Fig. 3b-d). In the bottom part, the 210 Pb ex content gradually decreased with increasing depth, and the color was mainly dark brown, which indicated that the sediments in this part was generally stable and continuously deposited. There were significantly rough fractures between the two parts (in Fig. 3). In addition, the thickness of the upper sediment layer in the 4 box-type samples corresponded well with the shallow stratum profile, which might be the storm-deposited or storm-transformed sedimentary layer formed by Typhoon Saola. However, the distribution of 210 Pb ex content and average grain size of sediments in the reference core MJK9 showed a typical stable and continuous deposition signature without sediments mixing and fresh brown color.

Discussion and Conclusions
The Chirp sonar sub-bottom profiler can distinguish and detect the reflections of shallow stratum due to the different physical properties of sediments. The 210 Pb ex content in sediments reflects the depositional features. The results from the two methods corresponded well to each other in this study. From the shallow stratum profile in the Typhoon Saola active area and the distribution of 210 Pb ex content in the sediments, a 20 cm-thick sediments formed (or disturbed) by Typhoon Saola was distinguished in the mud depo-area off the Zhejiang-Fujian coast. The distribution patterns of 210 Pb ex , showing that a gradually radioactive decaying sediment layer covered by a stable and high-content 210 Pb ex top sediments in the typhoon-affected area, were significantly different from that in a reference station (MJK9), in which the 210 Pb ex content gradually decayed from top to bottom sediments (Fig. 3). It was puzzling that there were no double reflections in the profiler L1 as mentioned above ( Fig. 2a; a-2). The 210 Pb ex content in the whole core of ND-1 was very low (< 2 dpm/g) compared with other cores (mainly higher than 4 dpm/g). This distribution pattern indicated that the sediment in the top of core ND-1 was mainly old deposit, and there was no sediment deposition during the typhoon processes. At the other stations, the variations in 210 Pb ex content indicated that the storm-deposited (disturbed) layer was relatively thick. The difference in the physical properties between the upper and bottom sediment layers was significant, and the storm-deposited (disturbance) layer was identified on the shallow stratum profile.
The strong dynamic processes of the typhoon disturbed the seafloor sediments and increased the suspended particle content in the seawater. In addition, the heavy rainfall caused by the typhoon increased the sediment flux into the sea and the particle content in the seawater. The suspended particles were carried by typhoon-induced current and spread in a large area. After the typhoon, the suspended particles deposited quickly and formed the storm-deposited layer. The δ 13 C value of the sediments in the 4 cores sediment did not vary significantly (Fig. 3), which might be an indicator that the material source in the entire sample was homologous. According to the previous studies, the seafloor sediments in the study area were mainly from Yangtze River, although there are several small rivers run into the sea around the study area, including the Min River, Ao River, Huotong Xi and Jiao Xi. Among these small rivers, the discharge of water and sediment of the Min River are the largest (average 7.5 Mt per year, while all other rivers yield approximately 3 Mt per year). The sediments from the Min River are mainly deposited near the estuary and transported south. The sediments discharged by the Huotong Xi and Jiao Xi are mainly deposited in Sansha Bay. As the sediments in the study area were mainly from the Yangtze River, carried by southward coastal currents in Fujian and Zhejiang, the mineralogical and geochemical properties of sediments were relatively stable and uniform 18 . Therefore, the sediments in the storm-deposited layer formed by Typhoon Saola were mostly likely the re-suspension and re-distribution of seafloor sediments in typhoon-affected seawater that were deposited previously by Yangtze River. Except for ND-1, the sediment particle size in the top layer at the stations became coarser than the lower layer (Fig. 3), possibly because transporting coarse re-suspended sediments from coastal area to wider seawater was harder than transporting fine particles. Using the detection data of the shallow stratum profile over a large area, we roughly calculated the redistributed amount of sediment caused by Typhoon Saola and the thickness of the storm-deposited layer can be accurately determined through the 210 Pb ex content. This process provides a new approach for evaluating the impact of typhoon on the modern sedimentary process.
The strong dynamic process during the typhoon period disturbed the seafloor sediment and caused the sediment re-suspension. The re-suspended particles were carried by typhoon-driven currents and spread over a wider area, which increased significantly the transport flux of the sediment and promoted the materials re-distribution in the coastal and continental area, especially in the longitudinally direction to the outer continental shelf, which might be limited by the strong northward-flowing Taiwan Warm Current under the calm marine situation in summer 6 . In the shallow coastal area, due to the strong sediment re-suspension and the re-transport, the newly sediment is hardly to be reserved and form new sedimentary stratum. However, in the relatively deep water area, the re-suspended and re-transported sediments, especially the relatively coarse ones, can deposit quickly and form a storm-induced sediment stratum.
In summary, this study used the Chirp sonar sub-bottom profiler and radioisotope method to determine the storm-deposited layer formed (disturbed) by Typhoon Saola in a mud depo-area off the Zhejiang-Fujian coast. By combining the thickness of the sediment layer (detected by both sonar profiler and 210 Pb ex content) and the distributions of sediment grain size and the δ 13 C values, we preliminarily examined the material source and deposition process of the storm sediment layer. The results indicated that a 10-25 cm storm-deposited (disturbed) layer was formed near the path of Typhoon Saola in the Fujian coastal area, and the sediment mainly came from the re-suspension of seafloor materials. The grain size in the storm-deposited layer became coarser, which indicated that the fine-grained compositions spread across a wider range after the typhoon, thus increasing the material transport flux in the seawater. Typhoons play a significant role in the modern marine sedimentary process, and the combining use of the shallow stratum profile detection technique and radioactive geochemical method provides a new approach for estimating typhoon effects on modern sedimentary processes.

Methods
A high-resolution EdgeTech 0512i Chirp Sonar Sub-bottom Profiler (frequency range: 1-10 kHz/5 ms) was used to obtain 3 seismic lines (51.79 km) on August 14, 2012 around the path of Typhoon Saola in a mud depo-area off the Zhejiang-Fujian coast, and a reference seismic line SL60 (8.38 km) was obtained on August 19, 2012 in the inner-shelf off the Minjiang Estuary using the same profiler (Fig. 1). An acoustic velocity of 1500 ms −1 was used to calculate water depth and sediment thickness 16 . Five shallow cores (43-64 cm) were collected using a box corer along 4 seismic lines, among them the core MJK9 in the line SL60 was collected on April 25, 2010 and its detail information can be found in a previous study 19 (Fig. 1). The cores were cut into 1-cm intervals in the laboratory, and the grain size and δ 13 C of every subsample was measured using a laser particle size analyzer (Mastersizer, 2000), with a measuring error within 3%, and an elementary analysis-isotope ratio mass spectrometers (EA-IRMS) (Flash EA 1112 HT-Delta V Advantages), with a measuring error within ± 0.2%. The 210 Pb radioisotope activities of the sediment were analyzed in 3-to 4-cm intervals by gamma spectrometry.