Tropical Sand Cays As Natural Paleo-Cyclone Archives

Chen Tianran (Orcid ID: 0000-0001-6828-0852) Feng Yue-xing (Orcid ID: 0000-0002-2944-9632)

Tropical sand cays as natural paleo-cyclone archives

Tianran Chen1,2*, George Roff3, Yuexing Feng4, and Jianxin Zhao4

1 CAS Key Laboratory of Ocean and Marginal Sea Geology (OMG), South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China. 2 Innovation Academy of South China Sea Ecology and Environmental Engineering, Chinese Academy of Sciences, Guangzhou 510301, China. 3 School of Biological Sciences, University of Queensland, St Lucia, Queensland 4072, Australia. 4 Radiogenic Isotope Facility (RIF), School of Earth and Environmental Sciences, University of Queensland, St Lucia, Queensland 4072, Australia.

Corresponding author: Tianran Chen (E-mail: [email protected])

Key Points: • U/Th ages of pristine Acropora corals are reliable indicators of time of the cay deposition • Trend and frequency of paleo-cyclones can be recorded on decadal to centennial time- scales • Acropora branches within cyclone-generated layers can record specific paleo-typhoon events

This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1029/2019GL084274

Abstract Sand cays are valuable paleo-archives that can significantly increase our understanding of Holocene tropical cyclone variability. Here we conducted detailed sedimentological and chronological analyses from a 195-cm-depth pit excavated on Guangjin Island (northern South China Sea), a cay influenced by frequent tropical cyclones. Radiometric dating of multiple deposits revealed that foraminifera, soft coral spicules, and gastropod shells yielded variable age distributions, while U/Th ages of pristine Acropora branches provided a clear record of deposition and cay formation. Based on this robust chronostratigraphy, the proportions of > 2mm grain-size fraction within the deposits corresponded with the frequency of paleo-typhoons recorded by historical records in recent centuries. U/Th ages (AD 1687±12, AD 1735±6, and AD 1813±5) of Acropora branches from the deposits matched with three known historical typhoon events. Our results highlight the potential of cyclone-deposited sand cays as new archives for recording paleo-cyclones. Plain Language Summary Determining accurate chronologies of sand cay formation is key to understanding their temporal dynamics and stability and their potential as paleo-archives. We reconstruct the growth of a sand cay strongly influenced by frequent tropical cyclones in the South China Sea. AMS 14C and U/Th dating of coral, soft coral spicules, mollusk, and benthic foraminifera revealed substantially different ages among calcareous components within layers. Relatively small benthic foraminifera, including Amphistegina sp., Rotalia calcarinoides, and Calcarina hispida, yielded abnormally old ages, suggesting a disparity of centuries to millennia between the organism death and final deposition. U/Th ages derived from Acropora branches with eroded surfaces exhibited variable but older ages, while AMS 14C ages from gastropod and large benthic foraminifera exhibited younger ages within the same layers. Taphonomically pristine Acropora branches commonly recorded in ‘cyclone-deposit’ layers were consistently the youngest component of all radiometric ages, and align with paleo-typhoon records throughout the Little Ice Age. Age distribution pattern of the dated coral fragments also provides a detailed record of the island formation history.

1 Introduction Tropical cyclones have devastating impacts on natural communities, human lives, and economy, especially in coastal areas over tropical cyclone basins [Woodruff et al., 2013]. Projections of future tropical cyclone activity associated with global change are of increasing importance. Although simulations suggest that global warming may lead to an increase in tropical cyclone destructiveness [Emanuel, 2005; Patricola and Wehner, 2018], uncertainties still exist in predicting shifts in the trend and frequency of cyclones in a warming climate, largely due to limited paleo-records and short-term instrumental observations. Extended historical and geological paleo-cyclone records are important to increase the understanding of tropical cyclone variability on decadal to centennial to millennial time scales.

Coral reefs are widely distributed in tropical oceans. Reefal deposits are excellent materials for identifying paleo-cyclones and -tsunamis. Those natural archives currently include storm ridges [e.g., Nott and Hayne, 2001; Hayne and Chappell, 2001; Scheffers et al., 2012; Liu et al., 2016], large uplifted coral blocks [e.g., Yu et al., 2004, 2012; Zhao et al., 2009; Liu et al., 2014; Lau et al., 2016], and lagoon sediments [e.g., Yu et al., 2009; Toomey et al., 2013; Klostermann et al., 2014]. However, well-developed storm ridge systems are not widespread on global coral reefs, and are primarily recorded from Australia [Nott and Hayne, 2001; Hayne and Chappell, 2001; Scheffers et al., 2012]. Uplifted coral blocks can provide a minimum estimate of past cyclone frequency, yet blocks can be eroded and broken post-transport, and only very strong cyclones or even tsunamis are capable of transporting large blocks [Yu et al., 2004; Yu et al., 2009; Kench et al., 2018]. Grain-size distributions in lagoonal sediment cores represent promising paleo-archives, yet sediment compaction associated with vibracoring could disrupt records [Yu et al., 2009], and clastics transported by individual cyclone event apparently do not entirely cover lagoon floor, leading to ‘signal loss’ in tracing paleo-cyclones with a small number of vibracores [Klostermann et al., 2014]. Sand cays are low-lying subaerial accumulations of unconsolidated sand and gravel deposited on reef flats throughout the tropical Indo-Pacific [Woodroffe et al., 1999; Yamano et al., 2000]. Most modern cays initiated in the mid to late Holocene [e.g., Woodroffe et al., 1999; Kench et al., 2005; Kench et al., 2014]. Construction and geomorphology of sand cays are closely related to tropical cyclones [Woodroffe, 2008; Montaggioni et al., 2018; Kench et al., 2018]. Therefore, cay deposits represent a potential archive for reconstructing Holocene tropical cyclone events. While the formation, evolution and stability of sand cays have been well studied [e.g., Kench et al., 2005; Woodroffe et al., 2007; McKoy et al., 2010; Kench et al., 2012; Kench et al., 2014; Yamano et al., 2014], few studies have attempted to explore the potential for sand cays as paleo-cyclone archives. Montaggioni et al. [2018] linked the stratigraphic sequence of an ocean-facing island on Takapoto, French Polynesia with at least four storm events, yet reconstruction of paleo-storm frequency was complicated by a lack of clear chronostratigraphy. Tropical cyclones have both positive (supply of bio-clastics for cay accretion) and negative (erosion) influence on sand cay formation [Kayanne et al., 2016], which results in mixing and reworking of cay deposits. Consequently, radiometric dating of cay deposits can be problematic, as time-lags may exist between the death of calcareous organisms and their final deposition within cays [Woodroffe et al., 2007; Liu et al., 2016; Montaggioni et al., 2018]. Previous studies have used a broad range of bioclastic materials to determine depositional chronologies, including gastropod shells [e.g., Woodroffe et al., 2007], foraminifera [e.g., Yamano et al., 2014], corals [e.g., Yamano et al., 2014; Liu et al., 2016; Montaggioni et al., 2018], and bulk sample approaches [e.g., Kench et al., 2005; McKoy et al., 2010; Kench et al., 2012], yet the accuracy of these approaches to quantifying robust chronostratigraphies is unclear. Here we reconstruct paleo-cyclone events over the past ~400 years based on the “cyclone- generated layers” in a tropical sand cay (Guangjin Island; 111°42’E, 16°27’N; Fig. 1). Firstly, we present a detailed chronostratigraphic study examining the reliability of radiometric dating

derived from multiple calcareous components in sediment profiles. Variability in ages among AMS 14C and high-precision U/Th dating of calcareous skeletons were rigorously investigated to identify the range of ages and the youngest component within layers. Secondly, we explore the timing of cay deposits against published historical documentary records to test the reliability of tropical sand cays as natural paleo-cyclone archives. 2 Field setting and methods 2.1 Study site Guangjin Island is a small vegetated sand cay, located in the south of Yongle Atoll, northern South China Sea (SCS), western Pacific (Fig. 1). The uninhabited island is mainly covered by Scaevola sericea shrubs. It is approximately 320 m in length and 200 m in width, with a platform area of ~0.06 km2. The northern SCS has among the highest frequency of tropical cyclones in the world, and Guangjin Island was exposed to 1-7 tropical cyclones (tropical storms and typhoons) per year from 1988 to 2018 (China Meteorological Administration, CMA; http://typhoon.nmc.cn/). Two tropical storms, ‘Bebinca’ and ‘Rumbia’ (Fig. 1), impacted Guangjin Island and Southern China during our fieldwork in June 2013.

2.2 Sand sampling A ~2m-depth pit was excavated on the windward side of Guangjin Island (Fig. 1c&d). Roughly 20-40g sediment samples were collected with a very thin aluminum foil at 1cm- resolution along a 195 cm-profile (Profile 1; supplementary Video_Profile 1). In order to minimize uncertainty caused by a single profile, contrastive samples were collected at 2 cm- resolution along another 120 cm-profile (Profile 2; Fig. 1e) in the same pit. Therefore, total 195 and 60 sand samples were collected from Profile 1 and 2, respectively. Sand samples were cleaned by fresh water, dried, and sieved into < 2 mm and > 2 mm grain- size classes. For each sample, the weight percent was calculated as the ratio of >2 mm fraction to the whole. Then the < 2 mm fractions were further sieved into 3 grain-size classes including < 0.5, 0.5-1, and 1-2 mm for determining sedimentary composition. Twenty-six samples (23 samples from Profile 1, and 3 samples from Profile 2) were selected and point-counted under a stereomicroscope to determine sedimentary composition (supplementary Fig. S1 and Fig. S2). 2.3 Radiometric dating To determine the reliability of individual grains for reconstruction of depositional chronology, different skeletal components from 26 sediment layers were selected for AMS 14C and high-precision U/Th dating (supplementary Table S1). For radiocarbon dating, 20-50 mg of individual component grains of mollusk, soft coral carbonate spicule, and four species of benthic foraminifera were handpicked under a binocular microscope. Pristine grains were preferentially selected in order to minimize the potential age bias due to reworking and contamination (Fig. S1). For example, individual foraminifer retained

their original spines and shells, and gastropod shells showed little sign of abrasion and retained their coloration. The carbonate samples were dated for AMS 14C ages (supplementary Table S2) in the Beta Laboratory (Miami, USA) and calibrated using Calib software with a regional Delta- R of 18 ± 37, a weighted mean of 3 Delta-R values from the Xisha Islands [Southon et al., 2002]. This regional Delta-R value is within the error of other Delta-R values reported for the SCS [Yu et al., 2010]. For U/Th dating, one to four fossil Acropora branches (Fig. S1) were taken from each sediment sample, and total 66 Acropora samples were taken from Profile 1 and Profile 2 (Table S1). Before dating, diagenetic alterations were investigated by powder X-ray diffraction (XRD) analysis. Surfaces of Acropora branches were polished and the fresh skeletons were crushed into 0.5-1 mm-size grains. Approximately 150 mg grain samples were cleaned and weighted at the Radiogenic Isotope Facility, School of Earth and Environmental Sciences, the University of Queensland, and U/Th isotopic ratio measurements performed on a multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) following the analytical protocol of Clark et al. [2014]. 2.4 Historical records of landfalling typhoons Since ancient times, local governments (cities, counties, and even villages) of Guangdong Province (Before 1988, Hainan belongs to Guangdong), Southern China (Fig. 1a) recorded natural disasters including landfalling strong tropical cyclones, or typhoons, in abundant historical documents such as local chronicles. The documentary typhoon records span at least 1,000 years, but became more detailed and rigorous after ~ AD 1400 (Ming Dynasty), corresponding to the ‘Little Ice Age (LIA)’ period [Chan and Shi, 2000; Liu et al., 2001]. The reconstructed time series [Chan and Shi, 2000; Liu et al., 2001] and events [Man and Liu, 2017] of typhoon landfall in Guangdong offer a rare opportunity to evaluate the reliability of cay deposit records in this study.

3 Results 3.1 Sedimentology This cay is formed by unconsolidated sand and gravel-sized bioclastic sediments. Based on sedimentological analyses (Fig. S2), the major skeletal components include coral fragments (dominated by branching Acropora spp.), coralline algae (e.g., Porolithon), Halimeda, mollusk fragments (dominated by gastropods), benthic foraminifera (dominated by Amphistegina sp., Rotalia calcarinoides, Calcarina hispida, and Amphisorus hemprichii), and others (e.g., bryozoan, crab shell, and echinoid spines). Corals represent the single most important contributor to the sediment (~80% of the total weight; Fig.S2c). Formation and stabilization of the cay are dependent on continual supply of coral sediments from surrounding reef slopes and flats and are thus highly vulnerable to natural and anthropogenic disturbances. Several cyclone-generated sediment layers are visually apparent, most notably at the depths of 21-24 cm, 28-36 cm, 40-48 cm, 53-57 cm, 61-64 cm, 66-71 cm, 76-89 cm, and 97-101 cm

(Fig. 1e). Based on sieved grain-size classes, sediments in these layers are dominated by coarse sand (> 2 mm) and gravels mainly composed of small (< ~1 cm in length) Acropora branches with obvious abrasion (e.g., Fig. S1g) and relatively large (> ~2 cm in length) Acropora branches with taphonomically pristine surfaces (less abrasion and intact corallite structures; Fig. S1f). In contrast, non-cyclone-generated sediment layers (mostly deeper than 100 cm in the pit; supplementary Video_Profile 1) were characterized by fine sand (< 2 mm) and Acropora branches with relatively small sizes (< ~1 cm). 3.2 Chronology The chronology for the pit at Guangjin Island was estimated based on radiocarbon dating of 28 benthic foraminiferal samples (including 4 species of Amphistegina sp., Rotalia calcarinoides, Calcarina hispida, and Amphisorus hemprichii), 9 gastropod shell samples, and 2 soft coral spicule samples, and U/Th dating of 66 fossil Acropora coral samples (Table S1). The AMS 14C and MC-ICP-MS U/Th dating results (supplementary Tables S2 and S3, respectively) indicate that different calcareous components yield substantially different ages, and significant deviations in U/Th and 14C ages were also found within each component (Fig. 2a). Benthic foraminifera including Amphistegina sp., Rotalia calcarinoides, and Calcarina hispida generally yielded the oldest ages among the calcareous components used for AMS 14C dating (Fig. 2a). While the large benthic foraminifera Amphisorus hemprichii, soft coral carbonate spicules, and gastropod shells yielded relatively young ages within layers (Fig. 2a). Most U/Th dating with individual Acropora fragments yielded highly variable and old ages (Fig. 2a). Within the cyclone-generated layers, however, U/Th ages determined for a few pristine Acropora corals represent the youngest component of all radiometric dates (Fig. 2). Within the past millennia, the LIA has the most U/Th ages of corals (supplementary Fig. S3).

4 Discussion 4.1 Chronostratigraphy Small benthic foraminifera including Amphistegina sp., Rotalia calcarinoides, and Calcarina hispida exhibited well-preserved fine thorns and walls (Fig. S1a&b), seemingly suggesting little transportation or reworking, but generally yielded the oldest ages among the calcareous components used for radiocarbon dating (Fig. 2a). These results suggest that the three foraminifera species are not appropriate for constraining chronostratigraphy of the cay. The age discrepancies between Amphistegina sp., Rotalia calcarinoides, and Calcarina hispida shells strongly indicate the incorporation of older and/or reworked shells within layers. In order to sufficient quantity for AMS 14C dating, dozens of shells of the three foraminifera species with small size were hand-picked under an optical microscopy, which may not be able to accurately distinguish pristine shells from reworked. Chronologies from these microfossil shells are likely to be affected by “time-averaging” [Kowalewski, 1996] caused by mixing of a large number of shells with various origins and ages. Similar results have been reported from other reef regions,

for example, Woodroffe et al [2007] found that foraminifer Calcarina spengleri yielded unexpected old ages (4510-5900 cal BP) from the reef flat around Warraber island, Australia, and attributed this to re-deposition and vertical mixing. Compared with small benthic foraminifera, however, larger calcareous components including Amphisorus hemprichii, gastropods, and soft coral spicules yielded relatively young ages, probably because few replicated shells as possible to reduce the degree of age averaging. Similar to radiocarbon dating, U/Th ages from the 66 coral rubbles reveal a substantial range of variation (Fig. 2a). These old ages (> ~500 years) are mainly derived from Acropora branches with eroded surfaces. Compared with sediment profiles based on lagoon and back reef matrix cores from leeward environments which showed minimum mixing/reworking of older coral fragments [e.g., Yu et al., 2006; Roff et al., 2015], the cay shows a more complex and mixed age structure, probably because of severe disturbance induced by frequent tropical cyclones. However, a few pristine Acropora branches commonly collected in cyclone-generated layers were consistently the youngest component of all radiometric ages, indicating that the temporal gap between their death and deposition was minimal, or even these coral shingles came from live coral sources nearby. Because of skeletal morphology and strength, branching corals (e.g., Acropora) are more vulnerable to damage than massive corals (e.g., Porites) [Scoffin, 1993], and thus are more easily to be broken, rapidly transported across the reef flat zone, and deposit immediately during tropical cyclone events. Only these individual Acropora corals are reliable indicators of the chronology of this rapidly-accumulated sand cay under the impacts of frequent tropical cyclones. The chronostratigraphy of sediment layers between ~40-195 cm was therefore established based on the youngest U/Th ages (Fig. 2b). 4.2 Cay accretions associated with tropical cyclones Within the top 40 cm of the pit, all sediment samples collected are composed of sand and small (< ~5 mm) coral fragments with obvious abrasion (numerously reworking and re- deposition) rather than pristine coral rubbles, resulting in disordered, old ages (Fig. 2a). We speculate that colonization of vegetation occurred after ~1865 AD at the edge of the cay, resulting in creating a barrier to waves and filtering most sands and gravels, and only small, light sands were deposited during cyclones. In our fieldwork, for example, no large, pristine coral fragments were found near the pit, instead, fossil Acropora coral rubbles were randomly collected on the beach after the two severe storms (Fig. 1a). U/Th ages of four storm-deposited Acropora (X1-4; Table S3) show a similar scenario to the old ages from the pit (Fig. 2a). If the surface layer (Fig. 1d) was regarded as ‘modern’ deposits, the average rate of the cay accumulation could be 3.6 mm/year. It is worth noting that the deposition rate as defined by the youngest U/Th ages slowed down exponentially (Fig. 2b) and a rapid accumulation occurred within a 123-year window (AD 1477-1636; 103-195 cm in depth) at an average rate reaching as high as 5.8 mm/year, approximately twice the rate of accretion during the period of highly frequent cyclones (2.7 mm/year; AD 1636-1865; 40-103 cm), indicating that tropical cyclones during the LIA did not accelerate the cay accumulation, although deposited several cyclone- generated sediment layers (Fig. 1e). Tropical cyclones supplied relatively large coral gravels for

the cay accretion, but simultaneously washed away deposits especially sand-sized sediments which are the principal materials for accretion to the leeward or even lagoon. Meanwhile, strengthened waves caused by cyclones provided a larger source supply for the cay accumulation, including newly broken coral rubbles and very old, subfossil branches (Fig. 2a; X1-4, Table S3) which are likely the products of depositional cycles, resulting in considerable mixing and reworking occurred. 4.3 Paleo-cyclone records The accumulation phase of cay sediments corresponds with tropical cyclone variability (Fig. 3). Initially, the accumulation phase below ~100 cm depth appears to form the sand cay dominated by sand-size sediment (supplementary Video_Profile 1) associated with a period before ~AD 1600 where storms and typhoons were comparatively less frequent (Fig. 3b). Following cooling period (~AD 1600-1900) which has the highest typhoon frequency in the northern SCS over the recent 1000 years [Chan and Shi, 2000; Liu et al., 2001], a higher frequency of coarse deposits (Fig. 3a) were recorded between ~40 and 100 cm depth forming clear cyclone-generated layers (Fig. 1e; supplementary Video_Profile 1). On a decadal timescale, at least seven individual peaks (L3-L9) correspond well with high frequencies of typhoons in Guangdong according to historical records (Fig. 3). Most notably, two ~30-year periods (~AD 1660-1690 and ~AD 1850-1880) with the most frequent typhoon strikes match with the three distinctly large peaks centered around AD 1687±12 and AD 1865±3 in the weight percent curves. These results suggest that the typhoon frequency can be accurately recorded by the cay on decadal to centennial time-scales. Key U/Th ages yielded from pristine Acropora corals within cyclone-generated layers, including AD 1687±12, AD 1735±6, and AD 1813±5, are contemporaneous with three paleo- typhoon events (AD 1685, AD 1735, and AD 1818) recorded by historical documents from Hainan (Man and Liu, 2017). In China, the term ‘tropical cyclone’ is classified into 6 categories including ‘tropical depression’, ‘tropical storm’ (>17.1 m/s), ‘severe tropical storm’ (>24.4 m/s), ‘typhoon’ (>32.6 m/s), ‘severe typhoon’ (> 41.4 m/s), and ‘super typhoon’ (> 50.9 m/s), based on the maximum wind speeds (CMA, http://www.cma.gov.cn). Such classification schemes did not exist in ancient China, and the term ‘typhoon’ as used in historical paleo-typhoon reconstructions [e.g., Liu et al., 2001; Man and Liu, 2017] tends to represent damage resulting from these events (e.g., victims, destroyed houses and boats, and flooded croplands) rather than actual typhoon strength. Based on our observations, some severe storms (e.g., ‘Rumbia’, Fig. 1a) were as destructive as historical records of typhoon. Therefore, paleo-cyclone events recorded by historical documents and from the cay paleo-archives may be classified as either typhoons or severe storms, according to modern classification scheme. While high-precision U/Th ages of Acropora branches within cyclone-generated layers of the cay may be an accurate marker of an individual high-impact paleo-cyclone, the peaks in coarse fraction deposits may also result from (and be interpreted as) the cumulative effect of multiple disturbances during periods of high typhoon frequency (Fig. 3) rather than attributed to individual typhoons.

5 Conclusions and implications for future studies

We constructed a detailed chronostratigraphy of sand cay formation through AMS 14C and U/Th chronologies of individual calcareous components. Dating results showed that targeted U/Th dating of pristine Acropora branches are the best indicators of time of deposition than any other older components, and thus provided an accurate chronology of the cay accumulation. Relatively small benthic foraminifera yielded substantially old ages, probably due to mixing of reworked or redeposited microfossil shells. Based on the sedimentological and chronological analyses, we found that the variability in the weight percent of > 2mm grain-size fraction in cay sediments corresponds with paleo-typhoon records from historical documents on decadal to centurial time-scales over the past 400 years. High-precision U/Th ages of Acropora branches within cyclone-deposit layers aligned with documented paleo-typhoon events. Our results highlight the potential of cyclone-deposited sand cays as new archives for reconstructing and extending Holocene cyclone records in tropical cyclone basins. The rigorous chronostratigraphy also implicates a type of study for reconstructing reefal cay/island formation history. If more such vertical profiles could be recovered from a transection across the island-lagoon system (from windward to leeward sides), combined with other types of archives [e.g., Liu et al., 2016; Roff et al., 2015], a 3D mapping of island formation process and history can be reconstructed, and the roles of cyclones and other climatic factors can be evaluated.

Acknowledgments, Samples, and Data We thank Faye Liu (RIF) and Ai Nguyen (RIF) for U/Th dating lab assistance. This work was supported by the National Basic Research Program of China (2013CB956104), the Strategic Priority Research Program of Chinese Academy of Sciences (No. XDA13020100), the National Natural Science Foundation of China (41676049, 41476038, and 41472158), and the CAS Youth Innovation Promotion Association (2015284). The datasets presented in this study are available to download from the supplemental Table S2 (AMS radiocarbon ages for individual grains) and Table S3 (U-series dating of the Acropora coral branches). The data used to create Figure 2, 3 and S2 are available on Zenodo (https://doi.org/10.5281/zenodo.3361862).

References Chan, J.C.L, and J.E., Shi (2000), Frequency of typhoon landfall over Guangdong Province of China during the period 1470-1931, Int. J. Climatol., 20, 183-190. Clark, T.R., G., Roff, J.X., Zhao, Y.X., Feng, T.J., Done, and J.M., Pandolfi (2014), Testing the precision and accuracy of the U-Th chronometer for dating coral mortality events in the last 100 years, Quat. Geochronol., 23, 35-45. Emanuel, K (2005), Increasing destructiveness of tropical cyclones over the past 30 years, Nature, 435, 686- 688. Hayne, M., and J., Chappell (2001), Cyclone frequency during the last 5000 years at Curacoa Island, north Queensland, Australia, Palaeogeogr. Palaeoclimatol. Palaeoecol., 168, 207-219.

Kayanne, H., K., Aoki, T., Suzuki, C., Hongo, H., Yamano, Y., Ide, Y., Iwatsuka, K., Takahashi, H., Katayama, T., Sekimoto, and M., Isobe (2016), Eco-geomorphic processes that maintain a small coral reef island: Ballast Island in the Ryukyu Islands, Japan, Geomorphology, 271, 84-93. Kench, P.S., R.F, McLean, and S.L., Nichol (2005), New model of reef-island evolution: Maldives, Indian Ocean, Geology, 33, 145-148. Kench, P.S., R.F., McLean, S.D., Owen, M., Tuck, and M.R., Ford (2018), Storm-deposited coral blocks: A mechanism of island genesis, Tutaga island, Funafuti atoll, Tuvalu, Geology, 46, 915-918. Kench, P.S., S.D., Owen, and M.R., Ford (2014), Evidence for coral island formation during rising sea level in the central Pacific Ocean, Geophys. Res. Lett., 41, 820-827. Kench, P.S., S.G., Smithers and R.F., McLean (2012), Rapid reef island formation and stability over an emerging reef flat: Bewick Cay, northern Great Barrier Reef, Australia, Geology, 40, 347-350. Klostermann, L., E., Gischler, D., Storz, and J.H., Hudson (2014), Sedimentary record of late Holocene event beds in a mid-ocean atoll lagoon, Maldives, Indian Ocean: Potential for deposition by tsunamis, Mar. Geol., 348, 37-43. Kowalewski, M. (1996), Time-averaging, overcompleteness, and the geological record, J. Geol., 104, 317-326. Lau, A.Y.A., J.P., Terry, A.D., Ziegler, A.D., Switzer, Y., Lee, and S. Etienne (2016), Understanding the history of extreme wave events in the Tuamotu Archipelago of French Polynesia from large carbonate boulders on Makemo Atoll, with implications for furture threats in the central South Pacific, Mar. Geol., 380, 174-190. Liu, E.T., J.X., Zhao, T.R., Clark, Y.X., Feng, N.D., Leonard, H.L., Markham, and J.M., Pandolfi (2014), High-precision U-Th dating of storm-transported coral blocks on Frankalnd Islands, northern Great Barrier Reef, Australia, Palaeogeogr. Palaeoclimatol. Palaeoecol., 414, 68-78. Liu, E.T., J.X., Zhao, Y.X., Feng, N.D., Leonard, T.R., Clark, and G., Roff (2016), U-Th age distribution of coral fragments from multiple rubble ridges within the Frankland Islands, Great Barrier Reef: Implications for past storminess history, Quaternary Sci. Rev., 143, 51-68. Liu, K., C. Shen, and K., Louie (2001), A 1000-year history of typhoon landfalls in Guangdong, Southern China, reconstructed from Chinese historical documentary records, Ann. Assoc. Am. Geogr., 9, 453-464. Man, Z., and D., Liu (2017), Impacts of typhoons landing on Hainan Island in the Qing dynasty on south China, Journal of Yunnan University (Social Sciences Edition), 16(4), 59-63 (in Chinese with English abstract). McKoy, H., D.M., Kennedy, and P.S., Kench (2010), Sand cay evolution on reef platforms, Mamanuca Islands, Fiji, Mar. Geol., 269, 61-73. Montaggioni, L.F., B., Salvat, A., Aubanel, A., Eisenhauer, and B. Martin-Garin (2018), The mode and timing of windward reef-island accretion in relation with Holocene sea-level change: A case study from Takapoto Atoll, French Polynesia, Geomorphology, 318, 320-335. Nott, J., and M., Hayne (2006), High frequency of ‘super-cyclones’ along the Great Barrier Reef over the past 5000 years, Nature, 413, 508-511. Patricola, C.M. and M.F., Wehner (2018), Anthropogenic influences on major tropical events, Nature, 563, 339-346. Roff, G., J.X., Zhao, J.M., Pandolfi (2015), Rapid accretion of inshore reef slopes from the central Great Barrier Reef during the late Holocene, Geology, 43, 343-346. Scheffers, A.M., S.R., Scheffers, D.H., Kelletat, P., Squire, L., Collins, Y., Feng, J., Zhao, R., Joannes-Boyau, S.M., May, G., Schellmann, and H., Freeman (2012), Coarse clast ridge sequences as suitable archives for past storm events? Case study on the Houtman Abrolhos, Western Australia, J. Quaternary Sci., 27(7), 713- 724.

Scoffin, T.P. (1993), The geological effects of hurricanes on coral reefs and the interpretation of storm deposits, Coral Reefs, 12, 203-221. Southon, J., M., Kashgarian, M., Fontugne, B., Metivier, and W.W.S., Yim (2002), Marine reservoir corrections for the Indian Ocean and Southeast Asia, Radiocarbon, 44, 167-180. Toomey, M.R., J.P., Donnelly, J.D., Woodruff (2013), Reconstructing mid-late Holocene cyclone variability in the central Pacific using sedimentary records from Tahaa, French Polynesia, Quaternary Sci. Rev., 77, 181- 189. Woodroffe, C.D. (2008), Reef-island topography and the vulnerability of atolls to sea-level rise, Global Planet. Change, 62, 77-96. Woodroffe, C.D., R.F., McLean, S.G., Smithers, and E.M., Lawson (1999), Atoll reef-island formation and response to sea-level change: West Island, Cocos (Keeling) Islands, Mar. Geol., 160, 85-104. Woodroffe, C.D., B., Samosorn, Q., Hua, and D.E., Hart (2007), Incremental accretion of a sandy reef island over the past 3000 years indicated by component-specific radiocarbon dating, Geophys. Res. Lett., 34, L03602, doi:10.1029/2006GL028875 Woodruff, J.D., J.L., Irish, and S.J., Camargo (2013), Coastal flooding by tropical cyclones and sea-level rise, Nature, 504, 44-52. Yamano, H., G., Cabioch, C., Chevillon, and J. Join (2014), Late Holocene sea-level change and reef-island evolution in New Caledonia, Geomorphology, 222, 39-45. Yamano, H., T., Miyajima, and I., Koike (2000), Importance of foraminifera for the formation and maintenance of a coral sand cay: Green Island, Australia. Coral Reefs, 19, 51-58. Yu, K.F., Q., Hua, J.X., Zhao, E., Hodge, D., Fink, and M., Barbetti (2010), Holocene marine 14C reservoir age variability: Evidence from 230Th-dated corals in the South China Sea, Paleoceanography, 25, PA3205, doi:3210.1029/2009PA001831. Yu, K.F., J.X., Zhao, K.D., Collerson, Q., Shi, T.G., Chen, P.X., Wang, and T.S. Liu (2004), Strom cycles in the last millennium recorded in Yongshu Reef, southern South China Sea, Palaeogeogr. Palaeoclimatol. Palaeoecol., 2004, 210, 89-100. Yu, K.F., J.X., Zhao, G., Roff, M., Lybolt, Y.X., Feng, T.R., Clark, and S., Li (2012), High-precision U-series ages of transported coral blocks on Heron Reef (southern Great Barrier Reef) and storm activity during the past century, Palaeogeogr. Palaeoclimatol. Palaeoecol., 337-338, 23-36. Yu, K.F., J.X., Zhao, Q., Shi, and Q.S., Meng (2009), Reconstruction of storm/tsunami records over the last 4000 years using transported coral blocks and lagoon sediments in the southern South China Sea, Quaternary Int., 195, 128-137. Yu, K.F., J.X., Zhao, P.X., Wang, Q., Shi, Q.S., Meng, K.D., Collerson, T.S., Liu (2006), High-precision TIMS U-series and AMS 14C dating of a coral reef lagoon sediment core from southern South China Sea, Quaternary Sci. Rev., 25, 2420-2430. Zhao, J.X., D.T., Neil, Y.X., Feng, K.F., Yu, and J.M., Pandolfi (2009), High-precision U-series dating of very young cyclone-transported coral reef blocks from Heron and Wistari reefs, southern Great Barrier Reef, Australia, Quaternary Int., 195, 122-127.

Figure 1 Location of the sampling site. Base maps were downloaded with Google Earth (Google Inc.). (a) The South China Sea and (b) Yongle Atoll. The pink and yellow lines represent the traces of the two tropical storms ‘Bebinca’ and ‘Rumbia’; (c) Map of Guangjin Island and the location of excavation; (d) Photo of the pit excavated for sediment sampling; (e) The sediment profile (Profile 2) in the pit. Cyclone-generated layers (L-1 to L-9) characterized with coarser sediments mainly exist in the top 100 cm of the pit. Another longer profile (Profile 1) is completely exhibited in the supplementary Video_Profile 1.

Figure 2 (a) Calibrated radiocarbon and U/Th ages (2 σ) of different calcareous components; (b) Determined chronostratigraphy of this pit, based on the youngest coral U/Th ages in each layer. The dashed line represents a second order polynomial fit of youngest coral U/Th ages.

Figure 3 (a) Weight percent curves of >2 mm sieve fraction from Profile 1 and Profile 2. Cyclone-generated layers (L1~L9) were also shown with the peaks. U/Th ages of pristine Acropora branches within cyclone- generated layers were indicated with red arrows; (b) Timeseries of historical typhoon landfalls in Guangdong during the ‘Little Ice Age’ period (modified from [Liu et al., 2001] and [Chan and Shi, 2000]).