Aeolian Sedimentation on Equatorial Reef Islands

• Main Author: Borrie, David
• Other Authors: Hilton, Michael
• Format: Thesis
• Language: English
• Published: University of Otago 2020
• Subjects: Maldives; aeolian sedimentation; sand cay formation; reef islands; sand; Coulter; Reef Island; Ocean acidification
• Online Access: http://hdl.handle.net/10523/9860

Aeolian landforms have been recorded on reef islands between 7° - 25° S, but never in equatorial settings. While evidence of aeolian activity has been observed, the significance of aeolian sedimentation has not been considered in the current model of reef island development. Reef islands are derived from biogenic sediments produced on the reef platform. Sea level rise and an increase in ocean acidification and extreme weather events are expected as a response to anthropogenically forced climate change. The response of reef islands to these processes is currently unknown. It is suggested that aeolian sedimentation provides a key mechanism of reef island accretion above the limit of marine processes. This research will investigate the significance of aeolian sedimentation of low-lying islands in an equatorial setting. This study presents high-frequency measurements of aeolian sedimentation and incident and near surface wind flow on a lagoon sand cay (Maaodegalaa) in the Maldives. Wind flow was recorded using sonic anemometers operating at 1 Hz over 8 days during the Iruvai monsoon in January and February 2018. WenglorTM particles counters, also operated at 1 Hz, and self-orientating swinging sediment traps, were used to measure sedimentation and particle flux at the bed during two high speed wind events, on the 31st of January and the 1st of February 2018. Island topography and vegetation cover were surveyed using RPAS (Remote Piloted Aircraft System) photogrammetry and a laser level. Sand samples were collected from the surface and through a series of pits on two landforms in Huvadhoo Atoll. Particle distribution analysis was completed at the University of Melbourne using a Beckman Coulter LS 13 320 laser diffractor. Four distinct transitional stages of island form were identified in Huvadhoo lagoon; (i) patch reefs with submarine sediment deposits; (ii) ephemeral sediment deposits which are covered at spring high tide; (iii) sand cays which rise above spring high tides and are colonised by early successional vegetation species; and (iv) forested islands with stable cores. Each island form is progressively more stable which is associated with increasing vegetation cover. Beach rock is also associated with island stability, but only one occurrence was reported in the lagoon. The subaerial surface of Maaodegalaa is comprised of medium-size coral sands. As no tsunami or over-wash events were reported since the formation of the island in its current position since 2011, these sands are likely deposited through aeolian processes. Coarse and fine layers within the stratigraphy became gradually finer closer to the surface, but an unequivocal aeolian facies could not be determined from grain size characteristics alone. Significant sediment flux was recorded on Maaodegalaa during the Iruvai monsoon (35,848.13 g/hour/m2). This was recorded in the lee of a scarp during the second recorded high-speed wind event. Higher rates of sedimentation occurred in the lee of a scarp, which indicates that flow acceleration over scarps is likely to provide higher rates of sand flux compared with flow over simple topographies. Aeolian accretion may be a significant phase in reef island development. The length of this period will differ between island morphology, seasonal forms, vegetation cover and exposure to incident wind. Ultimately it will be limited by vegetation cover, which decreases the potential for aeolian sedimentation through and increase in surface roughness and shelter. This research proposes a model where island accretion results from a combination of aeolian sedimentation and plant colonisation. The initial ephemeral surface provides an environment conducive to seed germination. The increase in surface roughness from vegetation colonisation also promotes accretion through growth of aeolian landforms. This model is independent of sea-level over decadal time scales. The period over which aeolian accretion and plant colonisation occur simultaneously is unknown, but it is likely to occur over years to decades. This research is the first to record aeolian sedimentation in an equatorial setting. It is also the first to consider aeolian accretion in a model of reef island development. However, a clear aeolian facies could not be identified using only Folk and Ward grain size statistics, and it is recommended that sediment shape and composition is analysed in further research.