DISLOCATION OF TSUNAMI BLOCKS ON BONAIRE (LEEWARD ANTILLES) DATED BY COSMOGENIC NUCLIDES (36CL)

Onshore coarse-clast deposits represent a valuable geo-archive to investigate Holocene high-energy wave events (tsunamis, storms). However, in the case of blocks and boulders, dating the time of dislocation is still a major challenge. On Bonaire (Leeward Antilles), a large dataset of ¹⁴C and ESR ages for coral rubble has been established within the past decade in order to infer the timing of distinct tsunamis. This approach is limited due to the facts that (i) the death of an organism (i.e. the coral) does not necessarily represent the age of the event inducing the onshore transport; (ii) periodic strong storms and tsunami may contribute to coral rubble accumulation; (iii) the largest blocks as the strongest evidence for the occurrence of paleotsunamis lack datable material. Due to (i) and (ii), ¹⁴C and ESR ages scatter widely. In order to overcome these problems, we apply terrestrial cosmogenic nuclides (TCN, in particular ³⁶Cl) in case of the the largest blocks (up to 150 t) in order to directly date the transport event(s), i.e.the inferred tsunami(s). This dating method has hitherto been disregarded in the coastal environment, particularly in the context of block deposits. Along the eastern coast of Bonaire, blocks were quarried from the cliff edge of the MIS 5.5 coral reef platform and transported further inland by high-energy waves. The following characteristics of the megaclasts are fundamental for the success of the presented dating approach:

(1) due to the lithology (aragonite, calcite), concentration measurements of ³⁶Cl are performed;

(2) only large and thick boulders and blocks (>50 t, >2 m thickness) for which tsunami transport was inferred were sampled;

(3) since the boulders stem from the edge of the coral reef platform, they had been exposed to cosmic radiation prior to the transport event(s) and had already accumulated a certain amount of TCN. To avoid this problem of inheritance, we only sampled the thickest clasts, and those having experienced a 180° overturn during transport and exposed a “blank” side to cosmic rays only since the event. The complete overturn is attested by the presence of inactive rock pools in upside-down position and bioerosive notches.