The chronology of prehistoric high-energy wave events (tropical cyclones, tsunamis) in the southern Caribbean and their impact on coastal geo-ecosystems – a case study from Bonaire (Leeward Antilles)

Field: Earth and Planetary Sciences (Physical Sciences)

By using sediment archives along the coast of Bonaire (Leeward Antilles; political status: special municipality of the Netherlands), such as bokas (enclosed lagoons), narrow floodplains and mangrove swamps, and by investigating supralittoral blocks and boulders, the goal of this thesis is (i) to identify prehistoric high-energy wave events, (ii) to differentiate between extraordinary storms and tsunamis, and (iii) to date these events. Furthermore, the spatio-temporal impact of high-energy wave events on coastal geo-ecosystems is evaluated. A total of 45 vibracores (max. depth 11 m) and push cores (max. depth 1.62 m) was taken from all around the island (Lagun, Boka Washikemba, Lac Baai, Saliña Tam, Klein Bonaire, Boka Funchi, Wayaka, Boka Bartol, Playa Grandi). Cores from the most promising locations were investigated in terms of sedimentology, geochemistry, mineralogy and faunal remains. \nThe coasts of the Caribbean Sea are prone to tsunami hazard, proven by numerous observed events of the last 500 years. Bonaire was chosen as the site of interest for this study due to the antagonism of a total lack of historical accounts on tsunami occurrence and its massive record of coarse-clast coastal landforms and single blocks and boulders, which are attributed to deposition by extreme wave events (tsunamis, hurricanes). \nThe largest coastal blocks were studied as evidence for the occurrence of more powerful waves than those of strong tropical cyclones (locally known as hurricanes). By comparing calculated minimum storm wave heights required to quarry and move the largest blocks of Spelonk and Boka Olivia on northeastern Bonaire, with maximum wave heights observed during recent high-category hurricanes and the buoy record, little doubt remains that one or more major tsunamis occurred in Mid- to Late Holocene times. This confirmation of the previously established (and in the meantime challenged) tsunami hypothesis provides a very important framework condition for the interpretation of the subsurface overwash deposits in the coastal stratigraphical archives. \nA tentative chronology of tsunami occurrence was established, based on a comparison of subsurface overwash deposits identified in the sediment cores from the stratigraphical geoarchives with characteristic signature types of modern tsunamis and calibrated 14C data. Uncertainties in sediment interpretation, associated with the fact that most of these signature types may also occur in storm deposits, were reduced according to (i) the results from the numerical boulder study, (ii) the high tsunami potential of the southern Caribbean (abundant potential tsunami trigger mechanisms), (iii) the identification of sediment source areas which are out of reach for storm waves, and (iv) the lack of littoral and marine sand/mud input during recent high-category hurricanes. \nSeveral sites reflect long-lasting environmental change subsequent to palaeotsunami impact. At the open embayment of Lagun, this is confirmed by palynological investigations. Pollen spectra prior to the wave impact are dominated by mangrove types (Rhizophora), whereas significantly lower percentage of Rhizophora pollen was detected in the post-tsunami samples. Mangroves seem to have only gradually recovered from the impact, probably on centennial scales. In contrast to Lagun, the bokas and saliñas of Bonaire are cut off from the sea by a quasi-impermeable barrier of beachrock and coral rubble. The candidate tsunami deposit of Boka Bartol is vertically confined by mangrove peat (below) and evaporite-rich mud (above). This pattern is interpreted as the transformation of an open mangrove-fringed embayment (pre-tsunami) into a poly- to hypersaline lagoon (post-tsunami) due to the establishment or closure of the barrier of coral material during the event. The high amount of gypsum and halite crystals and thin layers of inorganic carbonate are the result of limited water input and high evapotranspiration rates during the dry season. The absence of marine faunal remains also indicates very limited exchange with the ocean. \nThe chronology of potential tsunamis based on data from the sediment cores starts with extreme wave event I (EWE I), around 3600 BP, represented by a carbonate sand deposit bracketedby man-grove peat, which was found on Klein Bonaire. A well-preserved candidate tsunami deposit from Boka Bartol with a maximum age of 3300 BP (EWE II) has counterparts on the leeward coast (Klein Bonaire, Saliña Tam, possibly between Saliña Tam and Punt’i Wekua) and the windward coast (Playa Grandi, possibly Boka Washikemba). EWE II can be identified as the best-documented candidate palaeotsunami on Bonaire. EWE XII, with a maximum age of 2000 BP, left massive shell-dominated deposits at Lagun (windward side) and carbonate sands at Saliña Tam, both of which were interpreted as tsunamigenic. Deposits of another younger, unspecified EWE XVI (pre-500 BP) were found between Saliña Tam and Punt’i Wekua, and Boka Washikemba. Based on these results, a preliminary estimation of the recurrence interval of high-energy wave events on Bonaire, which reveal magnitudes significantly exceeding those of recent high-category hurricanes and which are therefore likely to be major tsunamis, is in the order of roughly 1000 years or less. \nThis study corroborates conclusions of predecessor studies that the hazard of tsunami does exist on Bonaire, although historical accounts on tsunami ocurrence are lacking. It is strongly suggested to the authorities of Bonaire, to conduct a local analysis of tsunami risk, similar to the one recently established for the neighbour island of Curaçao. A study of modelling tsunami inundation based on the spectrum of tsunami triggering mechanisms and past inundation inferred from the overwash deposits identified in this research may be useful to determine high-risk areas, which are prone to potential future tsunami damage.