The Formation & Evolution of Caribbean Coral Reef Islands

Abstract

Reef islands are geomorphically unique environments that provide habitats for endemic species and human communities yet are increasingly vulnerable to environmental change. Research from the Indian and Pacific Oceans has identified key drivers of reef island formation and provided valuable insights into future island trajectories. However, no equivalent data are available for the Caribbean. This thesis presents the first reconstructions of Caribbean reef island formation, focussing on reef islands in Utila and Cayos Cochinos, Honduras, and identifies the timings, modes, and key geo-ecological drivers of their evolution over millennial to decadal timescales. Reconstructed chronostratigraphies revealed that the Honduran reef islands are relatively young compared to those in the Indian and Pacific Oceans, having formed primarily during four, discrete island-building episodes over the past 2,600 years. Island formation likely coincided with periods of heightened wave energy and slowing rates of sea-level rise which both had critical roles in island initiation and evolution. Since formation, island evolution has continued with satellite-based shoreline change analyses revealing the Honduran islands to be naturally dynamic over shorter, decadal, timescales. However, where larger islands (>2,500 m2) have typically accreted or remained stable over recent years, smaller islands (<1,500 m2) predominantly eroded, raising critical questions about size thresholds for island persistence under contemporary environmental stressors. Finally, examination of island sub-surface stratigraphy and surrounding reef sediment budgets suggests that contemporary and future reef island sediment supplies are likely to be reliant on high rates of reef crest sediment production from both parrotfish, and coral rubble generated during high-energy events. Collectively, the results provide the first Caribbean reef island data for global databases, and highlight their distinctiveness compared to Pacific and Indian Ocean counterparts. The research emphasises the critical interplay of sea-level change, high wave energy events, reef geomorphology, and reefal sediment supply in Honduran reef island evolution, and the necessity of both regional and island-specific knowledge for assessing reef island futures. Initially, results suggest an optimistic outlook for island persistence under future sea-level rise, provided sediment supplies remain high. However, erosive trends observed on smaller islands, and predicted accelerations of sea-level rise and reef degradation pose significant threats to reef island stability over the shorter timescales most relevant for reef island communities.

Date of Award	27 Feb 2025
Original language	English
Awarding Institution	Northumbria University Newcastle University
Supervisor	Holly East (Supervisor), Emma Hocking (Supervisor), Mike Jeffries (Supervisor), Pauline Gulliver (Supervisor), James R. Guest (Supervisor) & Dan Exton (Supervisor)