Bonaire’s boulder fields revisited: evidence for Holocene tsunami impact on the Leeward Antilles Max Engel * , Simon Matthias May ** Institute of Geography, Universität zu Köln, Albertus-Magnus-Platz, 50923 Köln (Cologne), Germany article info Article history: Received 30 August 2011 Received in revised form 23 November 2011 Accepted 5 December 2011 Available online 18 February 2012 Keywords: Boulder transport Tsunami Volume calculation Bulk density Coral limestone Caribbean Coastal hazard abstract Supralittoral boulders and blocks are prominent sedimentary features along rocky shorelines worldwide. In most cases, their deposition is attributed to high-energy wave events (tsunamis, severe storms). Even though tsunami waves are expected to have higher transport capacities compared to storm waves, megaclasts of up to 100 t were observed to have been moved laterally by the latter waveform. The deduction of certain extreme wave events (tsunamis, severe storms) from the boulder record thus remains a major challenge in palaeo-event research; the debate on their differentiation is ongoing. At the eastern coast of Bonaire (Leeward Antilles) in the southern Caribbean, numerous limestone blocks and boulders (up to c. 130 t) are distributed on top of a 3e6 m a.s.l. (above mean sea level) palaeo-reef platform. Disagreement exists among a number of authors concerning the transport processes involved in the formation of the boulder fields. In this paper, state-of-the-art modelling approaches of coastal boulder entrainment and transport were applied in order to provide new and more reliable data to support/challenge the working hypothesis of tsunami deposition. To improve the reliability of the boulder transport model, more realistic input parameters were provided by new DGPS measurements of the boulder dimensions and the calculation of bulk densities by taking into account the heterogeneity of the reef-rock boulders. Existing hydrodynamic equations were modified to allow for the irregular shape and real dimensions of the boulders. The results indicate that (i) boulder weight and dimension, and thus (ii) calculated wave energy and wave heights were overestimated in most of the previous studies, where calculations of boulder volume were based on multiplication of the main axes. The results of this study and wave heights observed during recent severe tropical cyclones seem to rule out storm-generated waves for the dislocation of the largest blocks. However, the majority of coarse-clast deposits may have been generated by periodic hurricane swells. The results underline the significance of more realistic field data in modelling boulder transports by waves. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Supralittoral boulders are prominent sedimentary features along rocky shorelines worldwide (Moore and Moore, 1984; Mastronuzzi and Sansò, 2000; Rubin et al., 2000; Kelletat and Schellmann, 2002; Scheffers and Kelletat, 2003; Noormets et al., 2004; Matsukura et al., 2007; Scicchitano et al., 2007; Hansom et al., 2008; Frohlich et al., 2009; Goto et al., 2009, 2010; Bourgeois and MacInnes, 2010; May et al., 2010; Regnauld et al., 2010; Switzer and Burston, 2010; Etienne et al., 2011). In the Caribbean, they have been studied in terms of their transport processes on Grand Cayman (Jones and Hunter, 1992), Jamaica (Robinson et al., 2006, 2008; Khan et al., 2010), the Bahamas (Hearty, 1997; Kelletat et al., 2004), Puerto Rico (Moya and Mercado, 2006), Anegada (Buckley et al., 2011), Barbados, Anguilla, St. Martin, Guadeloupe, Grenada (Scheffers et al., 2005; Scheffers and Kelletat, 2006), Venezuela (Schubert, 1994), as well as Aruba, Curaçao and Bonaire (Scheffers, 2002, 2005; Morton et al., 2006, 2008; Spiske et al., 2008; Pignatelli et al., 2010; Watt et al., 2010). Supralittoral boulder accumulations have been attributed to “exceptional disturbances of sea level” (Bird, 2008: p. 32) due to storm surges, tropical cyclones (known locally as hurricanes) or tsunamis. Thus, they have the potential to provide a historical context of past inundation and represent a basis for the definition of possible future inundation zones e provided that their transport mechanisms are inferred correctly. Although the extraordinarily long period of a tsunami (in the order of 5e60 min) implies a higher transport capacity compared to shorter waves induced by storms or tropical cyclones (in the order * Corresponding author. Tel.: þ49 221 4701562; fax: þ49 221 470 5124. ** Corresponding author. Tel.: þ49 221 4704841; fax: þ49 221 470 5124. E-mail addresses: max.engel@uni-koeln.de (M. Engel), mays@uni-koeln.de (S.M. May). Contents lists available at SciVerse ScienceDirect Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev 0277-3791/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2011.12.011 Quaternary Science Reviews 54 (2012) 126e141