Holocene sea levels of Bonaire (Leeward Antilles) and tectonic implications

Future relative sea-level (RSL) rise is one of the most serious coastal hazards worldwide, in particular in the Caribbean region. RSL is a function of global (glacioeustasy, steric effect), regional (e.g. glacioisostatic adjustment [GIA], gravitational effects inducing deformation of the Earth, upper/lower mantle viscosity, etc.) and local (lithosome compaction, vertical neotectonic movement) factors which are connected via a complex pattern of interference. Information on past RSL supports inferences on upper limits of ice shield ablation, estimates of anthropogenic contribution to historical and future RSL rise and calibration of rheological models of the earth. We present the first RSL curve for the island of Bonaire (S Caribbean). Forty-two ¹⁴C ages from 20 sediment cores taken from nine different sedimentary archives along the coast were used. The sedimentary environment of each index point was linked to a palaeo-water depth based on literature and field observations. The index points trace a local RSL history of decelerating rise since 7000 – 6000 years ago and subsequent asymptotical approximation, similar to existing RSL curves from Curaçao and Venezuela. The results were compared to a simulation of Holocene sea-level history (reference model) which considers global effects and regional GIA (including consequences for geopotential and vertical position of earth surface). Even though the central and NW parts of Bonaire experienced slow tectonic uplift of up to 50 cm since the mid-Holocene and tentative correction for compaction was applied, the new RSL curve for Bonaire runs slightly below the reference model. A further outcome of this study is the detection of a so far unknown micro-graben structure with a vertical slip rate of c. 1.5 mm yr^–1 at Boka Bartol, which was identified based on depth discrepanciesof well-dated isochronous high-energy wave deposits.