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The shells of large benthic foraminifers (LBF) are key contributors to the development and maintenance of coastal landforms in the Pacific as well as to Paleogene and Miocene carbonates deposited along the Neotethys Seaway and tropical Pacific islands. The current study assessed growth and fecundity of two species, Marginopora vertebralis and Amphistegina lobifera, collected from sites in Viti Levu, Fiji, based on shell diameter, shell weight, fecundity, and survival. Specimens were cultured without supplemental nutrients or food for 15 months under controlled laboratory conditions. Physicochemical parameters, including salinity, alkalinity, pH, and temperature, varied by

The instrumental record reveals that tropical cyclone activity is sensitive to oceanic and atmospheric variability on inter-annual and decadal scales. However, our understanding of the influence of climate on tropical cyclone behaviour is restricted by the short historical record and the sparseness of prehistorical reconstructions, particularly in the western North Pacific, where coastal communities suffer loss of life and livelihood from typhoons annually. Here, to explore past regional typhoon dynamics, we reconstruct three millennia of deep tropical North Pacific cyclogenesis. Combined with existing records, our reconstruction demonstrates that low-baseline typhoon activity prior to 1350 ce was followed by an interval of frequent storms during the Little Ice Age. This pattern, concurrent with hydroclimate proxy variability, suggests a centennial-scale link between Pacific hydroclimate and tropical cyclone climatology. An ensemble of global climate models demonstrates a migration of the Pacific Walker circulation and variability in two Pacific climate modes during the Little Ice Age, which probably contributed to enhanced tropical cyclone activity in the tropical western North Pacific. In the next century, projected changes to the Pacific Walker circulation and expansion of the tropics will invert these Little Ice Age hydroclimate trends, potentially reducing typhoon activity in the deep tropical Pacific. Atmospheric circulation shifts during the Little Ice Age led to greater typhoon generation in the tropical North Pacific according to a comparison of sediment proxy records of past storm activity and outputs of general circulation models.

Sea-level rise and increased storminess are expected to destabilize low-lying reef islands formed on coral reef platforms, and increased flooding is expected to render them uninhabitable within the coming decades. Such projections are founded on the assumption that islands are geologically static landforms that will simply drown as sea-level rises. Here, we present evidence from physical model experiments of a reef island that demonstrates islands have the capability to morphodynamically respond to rising sea level through island accretion. Challenging outputs from existing models based on the assumption that islands are geomorphologically inert, results demonstrate that islands not only move laterally on reef platforms, but overwash processes provide a mechanism to build and maintain the freeboard of islands above sea level. Implications of island building are profound, as it will offset existing scenarios of dramatic increases in island flooding. Future predictive models must include the morphodynamic behavior of islands to better resolve flood impacts and future island vulnerability.

Understanding the positional uncertainty of vector objects within a GIS has been widely discussed and a range of approaches have been applied to quantify the uncertainty of such objects. Recently, a number of studies have adopted remote sensing approaches to examine the erosion of low-lying atoll islands. These studies are underpinned by a comparison of shoreline positions interpreted from remote imagery and represented as line features, and/or island areas represented as polygons. However, the effects of image resolution and scale of interpretation have rarely been considered when examining the results of such studies. In order to examine the control of image resolution and scale of interpretation on the accuracy of shoreline positions, repeated digitisation of islands was undertaken under a range of conditions similar to those applied in an operational setting. High-resolution (40 cm) satellite imagery was resampled to coarser resolutions and islands repeatedly digitised at a range of scales. Results quantify seemingly intuitive relationships, whereby shorelines digitised from coarse imagery and at small scales are less accurate than those digitised from high-resolution imagery at large scales. Results of this study provide a useful guide for selecting imagery for shoreline change studies and understanding the positional uncertainty of vector objects within a GIS.

Large uncertainty surrounds the future physical stability of low-lying coral reef islands due to a limited understanding of the geomorphic response of islands to changing environmental conditions. Physical and numerical modelling efforts have improved understanding of the modes and styles of island change in response to increasing wave and water level conditions. However, the impact of sediment supply on island morphodynamics has not been addressed and remains poorly understood. Here we present evidence from the first physical modelling experiments to explore the effect of storm-derived sediment supply on the geomorphic response of islands to changes in sea level and energetic wave conditions. Results demonstrate that a sediment supply has a substantial influence on island adjustments in response to sea-level rise, promoting the increase of the elevation of the island while dampening island migration and subaerial volume reduction. The implications of sediment supply are significant as it improves the potential of islands to offset the impacts of future flood events, increasing the future physical persistence of reef islands. Results emphasize the urgent need to incorporate the physical response of islands to both physical and ecological processes in future flood risk models.

Coral resilience to ocean acidification is largely determined by the degree of physiological control corals can exert on their calcifying fluid carbonate chemistry. In this study, the boron isotopic composition (δ11B) of a Porites colony growing on a reef flat on Kiritimati Island in the equatorial central Pacific is examined to quantify the sensitivity of calcifying fluid pH (pHcf) to ambient environmental conditions. Skeletal δ11B along the growth axis of one annual growth band was determined with bulk analysis and by laser ablation (LA) MC‐ICP‐MS. Furthermore, the oxygen and carbon isotopic composition, trace element ratios, and skeletal density were quantified. Sclerochronological data were interpreted in the context of simultaneous recordings of reef flat seawater pH (pHsw), temperature, salinity, and water depth, and by measurements of these parameters on the fore‐reef. A recent model of pHcf upregulation, after optimization with seasonally resolved data, was used to simulate pHcf variability on a diurnal scale. Results showed that on a seasonal scale, Porites pHcf is upregulated compared to ambient seawater: both bulk and LA‐MC‐ICP‐MS derived δ11B resulted in a mean pHcf of 8.35 pH units. Calcifying fluid pH upregulation primarily followed variations in seawater temperatures, that is likely related to the control of temperature on calcification rate. On the reef flat, the diurnal range in pHsw was substantially higher (0.29 pH units) than on the fore‐reef (0.07 pH units). However, model results suggest that the high diurnal variability in reef flat pHsw resulted only in a limited variability in Porites pHcf.

Tropical coral reef-lined coasts are exposed to storm wave-driven flooding. In the future, flood events during storms are expected to occur more frequently and to be more severe due to sea-level rise, changes in wind and weather patterns, and the deterioration of coral reefs. Hence, disaster managers and coastal planners are in urgent need of decision-support tools. In the short-term, these tools can be applied in Early Warning Systems (EWS) that can help to prepare for and respond to impending storm-driven flood events. In the long-term, future scenarios of flooding events enable coastal communities and managers to plan and implement adequate risk-reduction strategies. Modeling tools that are used in currently available coastal flood EWS and future scenarios have been developed for open-coast sandy shorelines, which have only limited applicability for coral reef-lined shorelines. The tools need to be able to predict local sea levels, offshore waves, as well as their nearshore transformation over the reefs, and translate this information to onshore flood levels. In addition, future scenarios require long-term projections of coral reef growth, reef composition, and shoreline change. To address these challenges, we have formed the UFORiC (Understanding Flooding of Reef-lined Coasts) working group that outlines its perspectives on data and model requirements to develop EWS for storms and scenarios specific to coral reef-lined coastlines. It reviews the state-of-the-art methods that can currently be incorporated in such systems and provides an outlook on future improvements as new data sources and enhanced methods become available.

Ebb-tidal deltas are important coastal sedimentary features that influence broad-scale coastal system behaviour. However, sediment transport within and around tidal deltas is complex, making it very difficult to understand the role they play within coastal sedimentary systems. Here we present details of a new method to examine ebb-tidal delta processes using long records of cloud-masked Landsat 7 and 8 satellite imagery. Our approach is similar to that adopted in nearshore process studies, in which time-averaged collections of digital photographs from terrestrial cameras are used to differentiate between broken and unbroken water based on image tone. Using the Google Earth Engine we are able to rapidly segment 100s of Landsat images into broken and unbroken water classes based on a threshold value. By then calculating the frequency of wave breaking at the Manukau ebb-tidal delta (METD) near Auckland, New Zealand, we can repeatedly map geomorphic features such as channels, linear bar formation, channel margin linear bars, swash bars, and the terminal lobe that marks the outer limit of wave breaking. Using a ~17-year long record of observations we observe >2 km of northward alongshore channel migration and the potential emergence of a new channel to the south. The approach is simple, quick and with the growing constellations of earth observation satellites, has easy application to a range of other coastal systems.

Tuck, M.E.; Ford, M.R.; Masselink, G., and Kench, P.S., 2018. Physical Modelling of Reef Platform Hydrodynamics. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 491–495. Coconut Creek (Florida), ISSN 0749-0208.Low-lying coral reef islands are considered extremely vulnerable to the implications of climate change, particularly sea level rise. Over the last decade numerical modelling has been the primary method used to predict the future state of coral reef islands in response to rising sea level. However, disregarding complex island morphodynamics, numerical models treat islands as geomorphically inert landforms despite evidence showing islands fluctuate in size, shape and position on the reef flat across a range of timescales. In order to improve our understanding and incorporate reef island morphodynamics into predictive models, a physical modelling methodology is applied to a motu, a gravel island generally located on the windward side of coral reef atolls. Physical modelling experiments took place in the Coastal Basin at the COAST laboratory, Plymouth University, UK. A 1:50 scale reef platform was constructed based on the morphology of the reef platform on the windward side of Funafuti atoll, Tuvalu. Wave processes on the reef platform have a strong influence on island geomorphology and therefore accurate replication of the hydrodynamic processes are a crucial aspect of successfully physically modelling island morphodynamics. This paper details the reef platform hydrodynamics observed during 18 experiments where wave height, wave period and water level were varied. Results highlight the successful application of physical modelling to reproduce field observations of reef platform hydrodynamics in the flume, demonstrating the accuracy of the scaled physical modelling experiments.

Majuro is a low-lying island perched on a coral atoll in the central Pacific Ocean and is home to nearly 28,000 people. Considered highly vulnerable to the impacts of marine inundation, Majuro is expected to experience increasingly severe inundation as a result of continued sea-level rise. Popular media, academic papers, government reports, disaster declarations and other online resources are used to document 18 inundation events at Majuro over the past 36 years, which caused considerable impact to local physical and anthropogenic systems. The physical drivers and impacts of the documented inundation events are examined using tide gauge and weather observations and wave model hindcasts. The ocean-facing shorelines of Majuro experience frequent inundation caused by swell waves generated by distant storms from both the north and the south Pacific Ocean. In some instances, complete overwashing of the island by swell waves has been reported. Less frequent, although potentially far more damaging, are inundation events associated with typhoons and tropical storms, with the most recent in 1997. Inundation along the sheltered lagoon-facing shoreline of Majuro has occurred in the absence of waves due to the coincidence of high sea levels during La Nina conditions and seasonally high tides, as in 2011. Lagoon inundation also appears to have been caused by offshore swell penetrating into the lagoon, most effectively at high tide, and by locally generated wind waves within the lagoon. The classes of inundation identified in this study have unique drivers and the impacts have varying spatial and temporal characteristics in terms of impact and predictability. The inundation events are discussed with respect to the drivers of inundation and the future outlook under rising sea-level conditions.

Coral reef islands are small, low-lying landforms that provide the only habitable land for atoll nations and support a range of endemic flora and fauna. Thus, the vulnerability of these islands to projected sea-level rise is a global concern. The internal structure and modes of island accretion are integral in understanding how islands have formed and subsequently, how they will change in the future. This study employs extensive ground penetrating radar (GPR) imaging on a reef island on Huvadhoo Atoll, Maldives, combined with island coring and stratigraphic analyses, in order to infer styles of island accretion. The GPR images imply central core accretion, with vertical accretion occurring on the initial core and progradation occurring outwards in both directions. However, some transects display progradation occurring predominately in one direction, which may also indicate erosional and rollover or overwash processes that could have led to island migration. Furthermore, using inferences of major radar units and boundary changes in signal reflectors combined with detailed subsurface sedimentology, the subsurface stratigraphy of the island sediments was divided into four discrete facies: velu (lagoon infill), finolhu (initial island core), athiri (beach and paleobeach), and muraka (surface layer) facies. The GPR records on KAN provide visual evidence of the mode of island evolution to complement previous theoretical interpretations, and have demonstrated the capability of GPR to discern sedimentary facies, subsurface features, and changes in sediment characteristics on reef islands. Results from this study show past processes of island building and could infer island response to various stages of sea-level change, which has implications for the stability of reef islands in the face of projected sea-level rise.

With projections of accelerating sea-level rise, changing weather and wave regimes and increasing intensity of storm events associated with anthropogenic climate change, the physical persistence and consequently habitability of coral reef islands is a global concern. This study presents an analysis of shoreline changes on 104 coral reef islands from 16 atolls in the western equatorial Pacific nation of the Federated States of Micronesia across a period coincident with rising local sea level and a high frequency of storm events. Aerial photographs from the mid-1940s and 1970s were analysed alongside recent high-resolution satellite imagery to document shoreline changes and planform morphological adjustments in islands. Results revealed accretion has been the predominant mode of shoreline change, with 46% of the studied shorelines showing statistically significant accretion leading to a net increase of 64.37 ha (~3%) of planform land area across the archipelago. Three new islands were formed with a cumulative land area of 2.05 ha, conversely, three islands were completely eroded leading to a loss of net land area of 3.20 ha, providing a record of concurrent island formation and loss within a high storm frequency setting. Reef islands in the FSM demonstrated a range of styles of planform geomorphic adjustments including island rotation, expansion, contraction, spit extension, and migration of islands on the reef platform. Results provide empirical evidence of shoreline accretion despite local sea-level rise as well as of heterogeneity in magnitude and style of geomorphic responses of reef islands across FSM with considerable variability at inter and intra-atoll scales.

The long-term persistence of atoll islands is under threat due to continued sea level rise driven by anthropogenic climate change. One widely discussed potential impact of sea level rise is the widespread, chronic erosion of atoll islands. Despite concerns of erosion driven by sea level rise, no published evidence exists of pervasive erosion of atoll islands at a global scale. Existing studies of atoll island change have been based on small, temporally sparse samples of islands on a limited number of atolls. As a result, the global response of atoll islands coincident with sea level rise remains uncertain. Using rich collections of Landsat imagery, this study analyses changes in land area on 221 atolls in the Indian and Pacific Oceans. Results show that, between 2000 and 2017, the total land area on these atolls has increased by 61.74 km2 (6.1 %) from 1007.60 km2 to 1069.35 km2. Most of the change in land area resulted from island building within the Maldives and on atolls in the South China Sea. Since 2000, the Maldives have added 37.50 km2 of land area, while 16.57 km2 of new islands have appeared within the South China Seas Spratly and Paracel chains. Understanding the extent of land area change at the global scale improves insights into the variation in responses, and how the manipulation of land is shaping the potential habitability trajectory for some atoll communities. Results highlight the anthropogenic imprint on the size of atoll islands, thereby providing a better understanding of variations in the future trajectories of human settlement and adaptation within atoll settings.

Reef islands are at the forefront of concern for future accelerating sea-level rise since their low-lying and isolated nature puts them at higher risk of marine inundation compared to continental coastlines. However, the perceived threat of complete submersion as implied by projected future sea-level rise and current island elevations do not consider the morphologically resilient nature of reef island systems. In particular, the role of sediment supply in the resilience of these islands is still relatively poorly studied. This study presents detailed descriptions of the sedimentary characteristics and stratigraphy of two lagoonal platform islands in Huvadhoo Atoll, Maldives, that formed during periods of Holocene sea-level rise. Island subsurface stratigraphy was reconstructed by analysing the skeletal composition and textural properties of 306 sediment samples from 37 cores extracted across the islands. Island sediments were dominated by coral sands with varied proportions of secondary constituents (molluscs, Halimeda, foraminifera, and crustose coralline algae). Downcore variations in composition show that the proportion of coral sands decrease with depth and the proportion of molluscs and Halimeda increase with depth (with the exception of cores that terminated on lagoon infill). The increased proportion of Halimeda and molluscs in these early island deposits may have resulted from the catch-up growth strategy of the reef during the mid-Holocene highstand as both organisms have high turnover rates and directly contribute to sediment production after death. The sedimentological response of increased Halimeda and molluscs highlights the resilient and dynamic nature of reef islands and the ability of reefs to adjust ecologically to changing sea levels.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/gy3zhqocMGw

Using coarse-scale approaches, existing national assessments of vulnerability and adaptation highlight physical land instability as a major threat to atoll island nationhood. However, such evaluations are bereft of detailed, ground-truthed analyses of the physical impacts of climatic change on reef islands, treating islands as homogenous in both biophysical and social characteristics. The distinct geomorphic context of two proximate reef islands (Jeh and Jabat) in the Marshall Islands was examined through conventional land survey techniques. A template documenting the nuances in island topography was used to evaluate simple inundation scenarios, reflecting current and future sea-level changes under storm surge conditions. The variations in local scale community exposure to inundation were discernible. The study highlights the importance of treating coarse-scale assessments with caution and underscores the need for continued commitment to resolving variations in community experiences to environmental change. Notions of risk and exposure are complex and embedded in both the biophysical and social contexts of each island community. Despite a number of targeted urban vulnerability studies in the Pacific there remains a need for efforts to document localised differences in experience to better inform contemporary adaptation efforts.

Liang, Y.; Kench, P.S., Ford, M.R., and East, H.K., 2016. Reef sediment supply and island building in the South Maldives. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 587–591. Coconut Creek (Florida), ISSN 0749-0208. Reef islands are small, low-lying islands composed mainly of bioclastic sands and gravels from the late Holocene. These islands, which support around 700,000 people, are vulnerable to potential threats including land submergence and shoreline erosion. Thus, the sediment supply of the surrounding coral reef is an important consideration for coastal maintenance but there is little research on the link between reef productivity and island building. This paper presents an examination of reef and island beach sediments (material and composition), as well as the eco-geomorphic zones of the contemporary reef. Two islands in Huvad...

The excavation of reef-flat sand and aggregate for engineering projects is a common practice on atoll islands, with attendant coastal management and hazard mitigation issues. To assess the impact of reef-flat excavation pits on wave transformation, a numerical study is carried out based on one-dimensional weakly dispersive, highly nonlinear Boussinesq equations. Model simulations are compared to field observations made at Majuro Atoll, Republic of the Marshall Islands (Ford et al., 2013a), at a 75-m-wide fringing reef flat with a 17-m-wide, 4-m-deep excavation pit and at an adjacent unmodified reef flat. With a calibrated empirical breaking model and uniform friction coefficient, the numerical model is shown to satisfactorily reproduce the observed sea and swell (SS) and infragravity (IG) frequency band wave heights for a moderate amplitude wave event. By removing the lateral morphological differences of the field experiment, the model shows that an excavation pit reduces/increases shoreline IG/SS wave heights compared to numerical tests without the pit, in qualitative agreement with Ford et al. (2013a). An EOF analysis demonstrates that the reduction in IG wave heights is associated with modifications of the ¼ and ¾ wavelength IG standing modes across the reef caused by the pit. Model tests are used to evaluate the effects of cross-reef pit location and width on wave transformation. Shoreline wave heights are least affected by a pit located near the reef crest, and both SS and IG wave heights increase as the pit location moves shoreward. Shoreline wave heights are an increasing function of pit width as the increase in SS wave height exceeds the decrease in IG wave height produced by the pit. The effects of incident wave height and spectral bandwidth are also examined for the reef with and without pit. It is found that both shoreline SS and IG wave heights increase with the increasing incident wave height for both profiles. The numerical model results also demonstrate that the shoreline response is sensitive to the bandwidth of the incident wave spectrum, and larger IG shoreline wave heights are excited for narrower spectral bandwidths until wave reflection limits the forcing.

Storms are important drivers of geomorphic change within atoll settings, causing rapid island erosion while also initiating island building through the generation of vast quantities of reef-derived sediment. Due to the combined effects of storms and sea-level rise, reef islands are thought to be on an erosional trajectory. Jaluit Atoll in the Marshall Islands was struck by Typhoon Ophelia in A.D. 1958, causing significant geomorphic change to reef islands. Using aerial photographs as well as recent satellite imagery we track the impacts of the typhoon and the multidecadal recovery of islands. Ophelia caused a significant reduction in total land area of Jaluit Atoll, from 9.95 km 2 to 9.45 km 2 (–5.1%), with islands on the northeast rim collectively reducing in size from 4.72 km 2 to 4.14 km 2 (–12.2%). Between 1976 and 2006, 73 of 87 islands increased in size, with the total landmass exceeding the pre-typhoon area (10.25 km 2 ). However, we observe considerable spatiotemporal variability of impacts and relaxation following the typhoon. Results indicate that despite significant typhoon-driven erosion and a relaxation period coincident with local sea-level rise, islands have persisted and grown.

Low-lying reef islands are considered highly vulnerable to the impacts of climate change. Accelerating rates of sea level rise as a result of anthropogenic climate change are expected to destabilise islands and threaten to render entire nations uninhabitable. Using historic aerial photographs and recent high-resolution satellite imagery, shoreline changes on six atolls and two mid-ocean reef islands in the Republic of the Marshall Islands were analysed. Results reveal that since the middle of the 20th century more shoreline has accreted than eroded, with 17.23% showing erosion, compared to 39.74% accretion and 43.03% showing no change. The net result of these changes was the growth of the islands examined from 9.09 km2 to 9.46 km2 between World War Two (WWII) and 2010. Analyses of shoreline changes since the 1970s show that shorelines are accreting albeit at a slower rate, with rates of change between the 1970s and 2010 of 0.29 m/dec compared with 0.77 m/dec between WWII and 1970s. The observed shoreline changes occur in the context of locally rising sea level. As sea level continues to rise there is a critical need for regular monitoring of reef islands in order to better understand the spatio-temporal variability of reef island change and guide future adaptation efforts within atoll nations.

The geological stability and existence of low-lying atoll nations is threatened by sea-level rise and climate change. Funafuti Atoll, in the tropical Pacific Ocean, has experienced some of the highest rates of sea-level rise (∼5.1 ± 0.7 mm/yr), totaling ∼0.30 ± 0.04 m over the past 60 yr. We analyzed six time slices of shoreline position over the past 118 yr at 29 islands of Funafuti Atoll to determine their physical response to recent sea-level rise. Despite the magnitude of this rise, no islands have been lost, the majority have enlarged, and there has been a 7.3% increase in net island area over the past century (A.D. 1897–2013). There is no evidence of heightened erosion over the past half-century as sea-level rise accelerated. Reef islands in Funafuti continually adjust their size, shape, and position in response to variations in boundary conditions, including storms, sediment supply, as well as sea level. Results suggest a more optimistic prognosis for the habitability of atoll nations and demonstrate the importance of resolving recent rates and styles of island change to inform adaptation strategies.

Wave-driven extreme water levels are examined for coastlines protected by fringing reefs using field observations obtained in the Republic of the Marshall Islands. The 2% exceedence water level near the shoreline due to waves is estimated empirically for the study sites from breaking wave height at the outer reef and by combining separate contributions from setup, sea and swell, and infragravity waves, which are estimated based on breaking wave height and water level over the reef flat. Although each component exhibits a tidal dependence, they sum to yield a 2% exceedence level that does not. A hindcast based on the breaking wave height parameterization is used to assess factors leading to flooding at Roi-Namur caused by an energetic swell event during December 2008. Extreme water levels similar to December 2008 are projected to increase significantly with rising sea level as more wave and tide events combine to exceed inundation threshold levels.

In 1905, a devastating typhoon hit Nadikdik Atoll (5°54′ N and 172°09′ E) in the southern Marshall Islands. Evidence suggests that large sections of reef islands on Nadikdik were overwashed and destroyed. Comparison of aerial photographs taken in 1945 and modern satellite imagery provides a unique record of the geomorphic adjustment of islands after the typhoon. Between 1945 and 2010 the vegetated area of islands on Nadikdik grew from 0.74 to 0.90 km2. Observed changes to Nadikdik reef islands manifested through a range of styles and were largely accretionary. Of note, the formation of a new island was tracked from an embryonic deposit to a fully vegetated and stable island over a 61 year period. Similarly, a number of previously discrete islands have agglomerated and formed a single larger island. These changes were rapid and indicate that reef island formation can occur quickly. Evidence suggests that despite the typhoon occurring over a century ago the geomorphic adjustment of islands is still on-going.

Passive integrated transponder (PIT) tags are a radio-frequency identification device widely used as a machine-readable identification tool in fisheries research. PIT tags have also been employed, to a lesser extent, to track the movement of gravel-sized clasts within fluvial and coastal systems. In this study, PIT tags were inserted into detrital coral fragments and used to establish source–sink transport pathways on a fringing reef on Majuro Atoll in the Marshall Islands. Results suggest the transport of gravel-sized material on the inter-tidal reef flat is exclusively across-reef towards the lagoon. Considerable variation in the distance travelled by fragments was observed. Fragments were largely intact and visually recognisable after almost 5 months on the reef flat. However, the branches of some recovered fragments had broken off and corallite abrasion was observed in recovered fragments. This study indicates that PIT tags are an inexpensive and powerful new addition to the suite of sediment transport and taphonomic tools for researchers working within coral reef systems.

The timing and evolution of Jabat Island, Marshall Islands, was investigated using morphostratigraphic analysis and radiometric dating. Results show the first evidence of island building in the Pacific during latter stages of Holocene sea level rise. A three-phase model of development of Jabat is presented. Initially, rapid accumulation of coarse sediments on Jabat occurred 4800–4000 years B.P. across a reef flat higher than present level, as sea level continued to rise. During the highstand, island margins and particularly the western margin accreted vertically to 2.5–3.0 m above contemporary ridge elevations. This accumulation phase was dominated by sand-size sediments. Phase three involved deposition of gravel ridges on the northern reef, as sea level fell to present position. Jabat has remained geomorphically stable for the past 2000 years. Findings suggest reef platforms may accommodate the oldest reef islands in atoll systems, which may have profound implications for questions of prehistoric migration through Pacific archipelagos.

The effects of water level variations on breaking wave setup over fringing reefs are assessed using field measurements obtained at three study sites in the Republic of the Marshall Islands and the Mariana Islands in the western tropical Pacific Ocean. At each site, reef flat setup varies over the tidal range with weaker setup at high tide and stronger setup at low tide for a given incident wave height. The observed water level dependence is interpreted in the context of radiation stress gradients specified by an idealized point break model generalized for nonnormally incident waves. The tidally varying setup is due in part to depth-limited wave heights on the reef flat, as anticipated from previous reef studies, but also to tidally dependent breaking on the reef face. The tidal dependence of the breaking is interpreted in the context of the point break model in terms of a tidally varying wave height to water depth ratio at breaking. Implications for predictions of wave-driven setup at reef-fringed island shorelines are discussed.

Low-lying coral reef islands are considered extremely vulnerable to the impacts of climate change, particularly sea-level rise and increasing storm magnitude. Inundation and erosion are expected to destabilise reef islands and render them uninhabitable within decades. However, such assertions are founded upon the assumption that reef islands are geomorphologically inert landforms. Whereas, recent planform and cross-sectional analysis of reef island morphodynamics demonstrates that many islands are highly dynamic landforms changing in size, shape and position on the reef platform. This paper describes the first physical modelling experiments to model the whole-of-island morphodynamic response of reef islands to changes in sea level and wave regime. A wave basin is utilised to construct a 1:50 scale three-dimensional model of Fatato Island (Tuvalu) and document the whole-of-island response across a broad range of wave and water-level conditions. Whole-of-island results corroborate flume studies and provide new insights into the modes and styles of island change. Spit rotation, as well as lagoonward island recession and vertical oceanward crest accretion, are identified as the core island responses to increasing water level and wave conditions. However, results show that the rate and magnitude of physical adjustment is strongly dependent on the rate and magnitude of sea-level rise and wave conditions. Results challenge existing models of future island susceptibility to wave driven flooding, demonstrating that washover processes can provide a mechanism to build and potentially maintain island freeboard above sea level. These insights highlight an urgent need to incorporate island morphodynamics into flood risk models in order to produce accurate assessments of future wave-driven flood risks and better resolve island vulnerability.

The magnitude 9.0 Tohoku earthquake on 11 March 2011 generated a tsunami which caused significant impacts throughout the Pacific Ocean. A description of the tsunami within the lagoons and on the surrounding fringing reefs of two mid-ocean atoll islands is presented using bottom pressure observations from the Majuro and Kwajalein atolls in the Marshall Islands, supplemented by tide gauge data in the lagoons and by numerical model simulations in the deep ocean. Although the initial wave arrival was not captured by the pressure sensors, subsequent oscillations on the reef face resemble the deep ocean tsunami signal simulated by two numerical models, suggesting that the tsunami amplitudes over the atoll outer reefs are similar to that in deep water. In contrast, tsunami oscillations in the lagoon are more energetic and long lasting than observed on the reefs or modelled in the deep ocean. The tsunami energy in the Majuro lagoon exhibits persistent peaks in the 30 and 60 min period bands that suggest the excitation of closed and open basin normal modes, while energy in the Kwajalein lagoon spans a broader range of frequencies with weaker, multiple peaks than observed at Majuro, which may be associated with the tsunami behavior within the more irregular geometry of the Kwajalein lagoon. The propagation of the tsunami across the reef flats is shown to be tidally dependent, with amplitudes increasing/decreasing shoreward at high/low tide. The impact of the tsunami on the Marshall Islands was reduced due to the coincidence of peak wave amplitudes with low tide; however, the observed wave amplitudes, particularly in the atoll lagoon, would have led to inundation at different tidal phases.

Wotje Atoll is located at 9°25′N and 170°04′E within the Republic of the Marshall Islands in the central Pacific Ocean. As on other atolls, the islands perched along the rim of Wotje are low-lying and considered highly vulnerable to the impacts of climate change. A widely anticipated impact of continued sea level rise is the chronic erosion of island shorelines. Using a combination of aerial photographs and satellite imagery shoreline changes are assessed over a 67-year period characterized by rising sea level. Results indicate that between 1945 and 2010 shoreline accretion is more prevalent than erosion, with an average Net Shoreline Movement (NSM) of + 1.74 m, indicating accretion. Shorelines were accretionary along the lagoon, ocean and channel facing shorelines, as well as on elongate spits and small islands. A high-frequency assessment of shoreline change on a subset of islands in the east of Wotje reveals that islands were stable, with a balance between shoreline accretion and erosion. Shorelines interpreted from high resolution satellite imagery captured between 2004 and 2012 indicate that shorelines within this sample of islands are largely in an erosive state. The post-2004 shift toward erosion may be sea level rise induced, or part of an unresolved shoreline oscillation. This study demonstrates the critical need for improved shoreline change monitoring within atoll settings in order to assess sea level rise impacts along island shorelines.

Ford, M.R.; Becker, J.M., and Merrifield, M.A., 2013. Reef Flat wave processes and excavation pits: observations and implications for Majuro Atoll, Marshall Islands. An experimental deployment of pressure sensors was undertaken to assess the impact of reef flat excavation pits on wave processes at Majuro Atoll, Marshall Islands. Experiments were undertaken on two sections of an 80-m-wide fringing reef flat, one modified by the excavation of a 17-m wide, 4- to 5-m deep pit and the other an unmodified reef flat of comparable width, topography, and incident wave energy. A wave-driven inundation event during the experiment led to minor amounts of debris overwashing the road surface. The event was associated with southerly swell that was normally incident to the study site at high tide, resulting in enhanced shoreline energy in both the sea and swell and the infragravity frequency bands. The shoreline with the excavation pit received slightly smaller wave heights (∼8%) for all wave conditions, includi...

FORD, M., 2012. Shoreline changes on an urban atoll in the central Pacific Ocean: Majuro atoll, Marshall Islands. Majuro is the capital and most populated atoll in the Republic of the Marshall Islands and is located approximately 3700 km W–SW of Oahu, Hawaii. Like other atolls, Majuro is considered highly vulnerable to sea level rise. One of the widely perceived impacts of sea level rise on atoll islands is widespread chronic erosion. Using a combination of aerial photos and satellite imagery, this study presents an analysis of shoreline change over a 34- to 37-year study period, characterized by rapidly increasing population, coastal development, and rising sea level (3.0 mm y−1). Results show most (93%) urban and rural villages have increased in size over the study period. Shoreline change analysis indicates the urban area has expanded both toward the lagoon and onto the ocean-facing reef flat. Shoreline change within the urban area of Majuro has been largely driven by widespread reclamation fo...

The generation and composition of carbonate sediment within tropical carbonate settings is controlled by the skeletal production of CaCO3 in conjunction with physical, biological and chemical processes which act to break down and disperse skeletal remains. Using the results of detailed tumbling barrel experiments, this paper discusses the role the physical durability of common constituents of reef sediment plays in the composition of coral reef deposits. The durability of the skeletal remains of six reef sediment constituents was determined experimentally using tumbling barrels. Results indicate that constituent durability varies considerably amongst common reef sediment constituents. Calcareous algae Halimeda was the least durable constituent tested, followed by larger benthic foraminifera Baculogypsina sphaerulata and Marginopora vertebralis and the pelecypod Fragum fragum. Two species of branching coral (Acropora sarmentosa and Acropora nasuta) were found to be the most resistant to physical destruction. These findings provide increased power to interpret reef and island deposits and the potential role skeletal durability plays in the retention of constituent skeletons within coral reef associated sedimentary systems.

Low-lying coral islands are fragile landforms susceptible to long-term sea-level rise and extreme events, such as hurricanes and tsunamis. The Sumatran earthquake of 26 December 2004 generated waves that reached the Maldives 2500 km away. Observations of the effects of the tsunami are presented here, based on pre- and post-tsunami topographic and planform surveys of 13 uninhabited Maldivian islands. The surveys showed there was no substantial island erosion and no significant reduction in island area. Rather, the tsunami accentuated predictable seasonal oscillations in shoreline change, including localized retreat of exposed island scarps by up to 6 m, deposition of cuspate spits to leeward, and vertical island building through overwash deposition of sand sheets up to 0.3 m thick, covering up to 17% of island area. These results have implications for island stability indicating that low-lying reef islands are physically robust and the geological signature of tsunamis on atoll island development is minor.