Metagenomics of tsunami deposits

Onshore deposits of tsunamis provide information on the long-term frequency-magnitude patterns of events, which may not be covered by historical or instrumental records. Such information is crucial to assess coastal hazards and mitigation measures against the loss of life and assets. The identification of tsunami deposits in the coastal sedimentary record and the reconstruction of flooding processes requires reliable proxies, which are based on studies of recent tsunami deposits. Microfossils (e.g. foraminifera, ostracods, diatoms) are often applied to recognize tsunami deposits and differentiate them from storm deposits. In terms of foraminifera, tsunami deposits mostly contain allochthonous associations dominated by benthic intertidal to inner shelf taxa. Specimens may originate from outer shelf to bathyal depths; even planktonic forms may occur. Furthermore, changes in test numbers, taphonomy, size or adult/juvenile ratios compared to background sedimentation are common (Engel et al., 2016). However, dissolution of microfossils often prevents identification and reduces their value as a proxy (e.g. Yawsangratt et al., 2012). We address the problem of post-depositional dissolution of foraminiferal tests in tsunami deposits by applying high- throughput metagenomic sequencing techniques to identify foraminiferal associations based on DNA remains. Metagenomics (or environmental genomics) is related to sequencing DNA directly from the sediment record, where the genetic material may persist for tens of thousands of years. Among the broad range of organisms tackled in metagenomic studies so far, foraminifera (single-celled protists) were chosen as they show a water depth-related zonation in subtidal environments and were the first group to have been identified successfully in palaeo-tsunami deposits by their DNA (Szczuciński et al., 2016). The core study area are the Shetland Islands, exposed to the mega-tsunami triggered by the early Holocene Storegga submarine slide off the coast of Norway. Tsunami run-up of more than 25 m left a distinct landward-thinning sand layer with an erosive lower contact, locally large rip- up clasts, fining-upward sequences and marine diatoms in coastal lakes and peat lowlands. In addition to Storegga tsunami deposits, two younger tsunami deposits dated to c. 5 and 1.5 ka (Bondevik et al., 2005) were sampled during a field campaign in March 2018. Preliminary microscope analysis reveals rich foraminiferal associations in the shallow subtidal muddy sands of protected fjords, which represent the main source area for tsunami deposits. The onshore tsunami deposits, however, vertically confined by ubiquitous dystrophic peat, are void of any carbonate, which seems to have quickly dissolved after deposition in the low-pH environment. This setting paves the way for developing a new eDNA-based proxy to support the identification of tsunamis in the sedimentary record. Funding by a BELSPO BRAIN-be pioneer grant (BR/175/PI/GEN-EX) is gratefully acknowledged. References: Bondevik, S., Mangerud, J., Dawson, S., Dawson, A., Lohne, Ø., 2005. Evidence for three North Sea tsunamis at the Shetland Islands between 8000 and 1500 years ago. Quat. Sci. Rev. 24, 1757–1775. Engel, M., Oetjen, J., May, S.M., Brückner, H., 2016. Tsunami deposits of the Caribbean – Towards an improved coastal hazard assessment. Earth Sci. Rev. 163, 260–296. Szczuciński, W., Pawłowska, J., Lejzerowicz, F., Nishimura, Y., Kokociński, M., Majewski, W., Nakamura, Y., Pawlowski, J., 2016. Ancient sedimentary DNA reveals past tsunami deposits. Mar. Geol. 381, 29–33. Yawsangratt, S., Szczuciński, W., Chaimanee, N., Chatprasert, S., Majewski, W., Lorenc, S., 2012. Evidence of probable paleotsunami deposits on Kho Khao Island, Phang Nga Province, Thailand. Nat. Hazards 63, 151–163.

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