8Eric Simon, University of North Texas. 9 10 Washover sedimentation resulting from modern typhoon strikes on the Gulf of Thailand coast 11 forms anomalous sand layers in low-energy coastal environments including marshes, ponds and 12 swales. The primary diagnostic for recognizing prehistoric typhoon-deposited sand layers in the 13 geologic record are sharp upper and lower contacts between coarser-grained transported sand 14 layers and finer-grained in-situ sediments. In this first paleotempestology study in Thailand, 15 cores from two low-energy settings on the Gulf of Thailand coasta coastal marsh near Cha-am 16 and beach ridge plain swales near Kui Burireveal geologic evidence of up to 19 typhoon 17 strikes within the last 8000 years. The sand layers have sharp upper and lower contacts with 18 enclosing finer sediments. Some sand layers also contain other evidence of a sudden powerful 19 landward-directed surge of ocean water, including gravel-sized clasts, offshore foraminifera, 20 abundant shell fragments, plant debris and mud rip-up clasts. Sand layers record eleven typhoon 21 strikes at Cha-am, ranging in age from AD 1952 to 7575 cal yr BP, and eight typhoon strikes at 22 Kui Buri, ranging in age from 4075 to 7740 cal yr BP. Bayesian age-depth models, derived from 23 © 2015. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ 2 eight AMS radiocarbon dates, suggest that the frequency of typhoon strikes was 2-5 times 24 greater from 3900-7800 cal yr BP compared to 0-3900 cal yr BP. Possible explanations for this 25 variability in the typhoon record are that typhoons were more frequent and/or more intense in 26 Southeast Asia in the mid-Holocene because of climatic changes associated with the Mid-27 Holocene Warm Period or that the record reflects site sensitivity changes resulting from a mid-28 Holocene sea-level highstand. The preliminary finding of a possible link between warmer 29 conditions and a greater frequency of intense typhoon strikes could have important societal 30 implications, given possible consequences of ongoing global warming.31 32

Grain size, as one of sedimentological proxies, coupled with a detailed description of the sedimentary structures and luminescence dating were used to unveil the sediment sources and transport process of the Holocene ancient coastal storm events recorded in the beach ridge plain, wet swale and muddy environments at Prachuap Khiri Khan, in the Southern Peninsula of Thailand. In this study, a total of 141 sand samples were collected from the shore-normal ridge-swale topography and analyzed for layers of candidate storm deposits, revealing at least 21 candidate coastal storm events. The grain size distribution of beach sediments was, in general, unimodal, while the candidate storm sediments revealed a mixed combination of multimodal, bimodal and unimodal distributions. Plots of mean grain size against skewness and kurtosis and of skewness against kurtosis could differentiate storm deposits from shore-normal beach sediments. Sedimentary structures preserved in the ancient coastal storm deposits included parallel and inclined landward laminations, mud rip-up clasts, layers of shell fragments, a pebble grain, normal and reverse grading and sharp lower and upper contacts. Candidate storm layers overlain on a dry beach ridge intervened with mud in a swale showed a finer and thinner landward deposit. Marine shell fragments, smaller foraminifers, ostracod and scaphopod (tusk shell), were well preserved. Based on optically stimulated luminescence dating and a correlated accelerator mass spectrometry age, multiple layers of sand derived from different frequencies of coastal storms were deposited over the middle to late Holocene.

Sedimentary evidence of storms and fluvial floods (FFs) is crucial for a better understanding of such events in coastal zones. In this study, we analyzed the sedimentary characteristics of the coastal storm and FF deposits at the Hoa Duan barrier, Thua Thien Hue, central Vietnam. Analyses of the sedimentary structures and properties (grain size distribution, composition, roundness, and sphericity) and loss on ignition revealed that the storm sediments were comprised of coarser grains with a low organic and carbonated content, and with sedimentary structures, including parallel and inclined landward lamination, multiple sets of normal and reverse grading, mud rip-up clasts, and sharp and erosional contacts (both top and bottom) with finer-grain layers. Conversely, the FF sediments had only fine to very fine grains, with dominant high organic and carbonate contents, and only exhibited sedimentary structures of sharp erosional top and bottom contacts with coarser-grained layers. The clearest differentiation to distinguish coastal storm layers from inland FF layers was obtained by plotting the mean grain size against the sorting. The results of optically stimulated luminescence dating suggested that two storm layers and one FF layer were deposited during the last 130 ± 10 years. Moreover, two layers were deposited by storms and one by a FF prior to that (>130 ± 10 years). The identification of the sedimentary diagnostic key of these two hazards can help to improve the understanding of the geomorphological evolution of the studied site and the other parts of this coastal region in order to remind the coastal community to prepare for future coastal hazards well.

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