May, S.M.

Dating the transport/deposition time of supratidal coarse-clast deposits is difficult, limiting their value for inferring frequency-magnitude patterns of high-energy wave events. On Bonaire (Leeward Antilles, Caribbean), these deposits form prominent landforms, and transport by one or several Holocene tsunamis is assumed at least for the largest clasts. Although a large dataset of 14C and electron spin resonance (ESR) ages is available for major coral rubble ridges and ramparts, it is still debated whether these data reflect the timing of major events, and how these datasets are biased by the reworking of coral fragments. As an attempt to overcome the current challenges for dating the dislocation of singular boulders, three distinct dating methods are implemented and compared: (i) 14C dating of boring bivalves attached to the boulders; (ii) 230Th/U dating of post-depositional, secondary calcite flowstone and subaerial microbialites at the underside of the boulders; and (iii) surface exposure dating of overturned boulders via 36Cl concentration measurements in corals. Approaches (ii) and (iii) have never been applied to coastal boulder deposits so far. The three 14C age estimates are older than 40 ka, i.e. most probably beyond the applicability of the method, which is attributed to post-depositional diagenetic processes, shedding doubt on the usefulness of this method in the local context. The remarkably convergent 230Th/U ages, all pointing to the Late Holocene period (1.0–1.6 ka), are minimum ages for the transport event(s). The microbialite sample yields an age of 1.23 ± 0.23 ka and both flowstone samples are in stratigraphic order: the older (onset of carbonate precipitation) and younger flowstone layers yield ages of 1.59 ± 0.03 and 1.23 ± 0.03 ka, respectively. Four coral samples collected from the topside of overturned boulders yielded similar 36Cl concentration measurements. However, the computed ages are affected by large uncertainties, mostly due to the high natural chlorine concentration. After correction for the inherited component and chemical denudation since platform emergence (inducing additional uncertainty), the calculated 36Cl ages cluster between 2.5 ± 1.3 and 3.0 ± 1.4 ka for three of four boulders whilst the fourth one yields an age of 6.1 ± 1.8 ka, probably related to a higher inheritance. These 230Th/U and 36Cl age estimates are coherent with a suggested tsunami age of < 3.3 ka obtained from the investigation of allochthonous shell horizons in sediment cores of northwestern Bonaire. Whilst 230Th/U dating of post-depositional calcite flowstone appears to be the most robust and/or accurate approach, these results illustrate the potential and current limitations of the applied methods for dating the dislocation of supralittoral boulders in carbonate-reef settings.

Abstract:

We present sediment cores from seven coastal geoarchives on Bonaire, southern Caribbean, containing layers of high- energy sedimentation. Tsunami deposition is inferred for some layers based on the presence of allochthonous reefal shells including articulated specimens and a high percentage of angular fragments, planktonic foraminiferal taxa and those from the deeper shelf (below storm wave base), basal unconformities and hiatuses of >1000 a, rip-up clasts, thin depositional sequences comprising basal traction carpets overlain by normally graded sand, a proximal sediment source (littoral) in the lower part of the deposit and a broad mixture (littoral, shelfal, terrestrial) in the upper part, and the lack of deposition during recent hurricane flooding. Several tsunami layers were precisely dated to 3300-3100 cal BP, whereas the record of further candidate tsunamis is more disjunct. Additional tsunami evidence is provided by the largest coastal boulders (up to 150 t; a-axis up to 10 m). 

Abstract:

A sediment record of three alluvial sites along the east- and northeast-oriented shore of Bonaire (Netherlands Antilles) provides evidence for the recurrence of several extraordinary wave impacts during the Holocene. The interpretation of onshore high-energy wave deposits is controversially discussed in recent sedimentary research. However, it represents a powerful tool to evaluate the hazard of tsunami and severe storms where historical documentation is short and/or fragmentary. A facies model was established based on sedimentary and geochemical characteristics as well as the assemblage and state of preservation of shells and shell fragments. Radiocarbon data and the comparison of the facies model with both recent local hurricane deposits and global “tsunami signature types” point to the occurrence of three major wave events around 3300, 2000–1700 and shortly before 500 BP. Since (i) the stratigraphically correlated sand layers fulfill several sedimentary characteristics commonly associated with tsunamis and (ii) modern strong hurricanes left only little or even no sediment in the study areas, they were interpreted as tsunamigenic. However, surges largely exceeding the energy of those accompanying modern hurricanes in the southern Caribbean cannot entirely be ruled out. The results are partially consistent with existing chronologies for Holocene extreme wave events deduced from supralittoral coarse-clast deposits on Aruba, Bonaire and Curaçao as well as overwash sediments from Cayo Sal, Venezuela.