Time scales and modes of reef lagoon infilling in the Maldives and controls on the onset of reef island formation

Faro are annular reefs, with reef flats near sea level and lagoons of variable depth, characteristic of both the perimeter and lagoons of Maldivian (Indian Ocean) atolls. Their geomorphic development remains largely unknown, but where faro lagoons (termed velu in Maldivian) have infilled and support reef islands, these provide precious habitable land. Understanding the timing and modes of velu infilling is thus directly relevant to questions about reef island development and vulnerability. Here we use a chronostratigraphic data set obtained from a range of atoll-interior faro with partially to fully filled velu (including those with reef islands) from Baa (South Maalhosmadulu) Atoll, Maldives, to determine time scales and modes of velu infilling, and to identify the temporal and spatial thresholds that control reef island formation. Our data suggest a systematic relationship between faro size, velu infilling, and island development. These relationships likely vary between atolls as a function of atoll lagoon depth, but in Baa Atoll, our data set indicates the following faro-size relationships exist: (1) faros <∼0.5 km2 have velu that were completely infilled by ca. 3000 calibrated years B.P. (cal yr B.P.) with islands having established on these deposits by ca. 2.5 cal kyr B.P.; (2) faros >0.5 km2 but <∼1.25 km2 have velu in late stages of infill, may support unvegetated sand cays and, given sufficient sand supply, may evolve into larger, more permanent islands; and (3) faros >∼1.25 km2 have unfilled (deeper) velu which might only infill over long time scales and which are thus unlikely to support new island initiation. These new observations, when combined with previously published data on Maldivian reef island development, suggest that while the velu of the largest faro are unlikely to fill over the next few centuries (at least), other faro with near-infilled velu may provide important foci for future reef-island building, even under present highstand (and slightly rising) sea levels

Terrigenous sediment-dominated reef platform infilling: an unexpected precursor to reef island formation and a test of the reef platform size-island age model in the Pacific

Low-lying coral reef islands are considered highly vulnerable to climate change, necessitating an improved understanding of when and why they form, and how the timing of formation varies within and among regions. Several testable models have been proposed that explain inter-regional variability as a function of sea-level history and, more recently, a reef platform size model has been proposed from the Maldives (central Indian Ocean) to explain intra-regional (intra-atoll) variability. Here we present chronostratigraphic data from Pipon Island, northern Great Barrier Reef (GBR), enabling us to test the applicability of existing regional island evolution models, and the platform size control hypothesis in a Pacific context. We show that reef platform infilling occurred rapidly (~4–5 mm yr−1) under a “bucket-fill” type scenario. Unusually, this infilling was dominated by terrigenous sedimentation, with platform filling and subsequent reef flat formation complete by ~5000 calibrated years BP (cal BP). Reef flat exposure as sea levels slowly fell post highstand facilitated a shift towards intertidal and subaerial-dominated sedimentation. Our data suggest, however, a lag of ~1500 yr before island initiation (at ~3200 cal BP), i.e. later than that reported from smaller and more evolutionarily mature reef platforms in the region. Our data thus support: (1) the hypothesis that platform size acts to influence the timing of platform filling and subsequent island development at intra-regional scales; and (2) the hypothesis that the low wooded islands of the northern GBR conform to a model of island formation above an elevated reef flat under falling sea levels

Carbonate production of an emergent reef platform, Warraber Island, Torres Strait, Australia

Complex relationships exist between tropical reef ecology, carbonate (CaCO₃) production and carbonate sinks. This paper investigates census-based techniques for determining the distribution and carbonate production of reef organisms on an emergent platform in central Torres Strait, Australia, and compares the contemporary budget with geological findings to infer shifts in reef productivity over the late Holocene. Results indicate that contemporary carbonate production varies by several orders of magnitude between and within the different reef-flat sub-environments depending on cover type and extent. Average estimated reef flat production was 1.66 ±1.78 kg m² yr⁻¹ (mean ± SD) although only 23% of the area was covered by carbonate producers. Collectively, these organisms produce 17,399 ±18,618 t CaCO₃ yr⁻¹, with production dominated by coral (73%) and subordinate contributions by encrusting coralline algae (18%) articulated coralline algae, molluscs, foraminifera and Halimeda (<4%). Comparisons between these organisms production across the different reef flat zones, surface sediment composition and accumulation rates calculated from cores indicate that it is necessary to understand the spatial distribution, density and production of each major organism when considering the types and amounts of carbonate available for storage in the various reef carbonate sinks. These findings raise questions as to the reliability of using modal production rates in global models independent of ecosystem investigation, in particular, indicating that current models may overestimate reef productivity in emergent settings

Rapid reef island formation and stability over an emerging reef flat: Bewick Cay, northern Great Barrier Reef, Australia

A model that resolves reef island formation in relation to both reef platform substrate development and mid-Holocene sea-level change is presented for Bewick Island, northern Great Barrier Reef, based on morphostratigraphic analysis and radiometrically dated island sediments and reef corals. On Bewick Island, microatolls record reef-fl at development at higher sea level (+ 1.5 m) by 6500 yr B. P. Island building began on a partially emergent reef flat 5000-4000 yr B. P., when sea level was 0.5 m above present. As sea level fell to its present level, the reef platform process window closed and the island core stabilized. Results present the first unequivocal evidence of island building directly over a reef flat comprising microatolls, and the first detailed model of island formation from the Great Barrier Reef. The model demonstrates that the interplay of sea level and reef surface elevation can vary between sites but their convergence is critical for island initiation. Future trajectories of island change will vary, dependent on the unique relative sea level and substrate depth conditions that govern island formation

Monsoonally influenced circulation around coral reef islands and seasonal dynamics of reef island shorelines

Synchronous measurements of reef flat waves and nearshore currents were made around the perimeter of three coral reef islands in the central Maldives. Nearshore current velocities were shown to be tidally modulated in most instances with maximum velocities associated with lower tidal stages. Peak currents were consistently observed on the windward and lateral flanks of island shorelines (0.3–0.4 ms− 1). The relatively low elevation of reef surfaces and medium to low energy incident wave energy (Hs ~ 0.2–0.3 m) accounts for low tidal stage peaks in velocity as this corresponds to the period when wave setup-induced current processes are maximised. Significantly, all nearshore currents were unidirectional and alongshore. The generation of unidirectional alongshore flow patterns results from island configuration which deflects flow and variations in reef geometry that control: spatial variations in wave-setup around the reef platform edge and at the island shoreline, which can drive alongshore currents; the presence of a nearshore moat that channelizes flow in the nearshore: and variations in reef geometry where topographic low points act as preferential drainage points. Results show that changes in monsoon conditions, that modulate wind and wave patterns from the west to northeast, can force substantial changes in reef platform and nearshore current flows, but that the degree of change is dependent on reef platform shape and position in the atoll. The circular study island was found to exhibit a near-complete reversal in unidirectional flow from northeast flow in the westerly monsoon to southwest flow in the northeast monsoon. However, the elongate and triangular shaped reef platforms exhibited more localised changes in flow patterns. Significantly, those shorelines that exhibit the greatest changes in shoreline currents correspond to the sections of coast that exhibit the largest morphological adjustments between seasons. Results provide significant insights for the morphodynamics of reef platform islands. In particular, the nearshore process regime is dominated by alongshore processes. Furthermore, reef platform shape and the magnitude of change in direction of wave approach to reef platforms provide critical indicators of the morphodynamic behaviour of reef platform islands

Holocene reef growth in the Maldives: evidence of a mid-Holocene sea-level highstand in the central Indian Ocean

Radiometrically calibrated ages from three reef cores are used to develop a Holocene reef growth chronostratigraphy and sea-level history in the Maldives, central Indian Ocean. Last interglacial reef (U-series age 122 ± 7 ka) was encountered at 14.1 m below mean sea level. An age of ca. 8100 calibrated (cal) yr B.P. immediately overlying this Pleistocene surface records the initiation of Holocene reef growth. Massive in situ corals occur throughout the cores and the consistency of the three age-depth plots indicate that the reef grew steadily between 8100 and 6500 cal yr B.P., and at a decreasing rate for the next 2 k.y. The position of modern sea level was first achieved ca. 4500 cal yr B.P. and sea level reached at least 0.50 ± 1 m higher from 4000 to 2100 cal yr B.P. before falling to present level. Emergent fossil microatolls provide evidence of this higher sea level. Results are significant to two long-standing issues relating to Maldivian sealevel history. First, the ambiguity of a late Holocene highstand has been resolved with clear evidence of its existence reported here. Second, the uncertainty of the regional pattern of sea-level change in the central Indian Ocean has been clarified, the Maldivian results broadly agreeing with island records in the eastern, rather than western Indian Ocean. Our results provide the first field evidence confirming geophysical model projections of a highstand 4–2 k.y. ago in the central Indian Ocean, though the observed level (+0.50 ± 0.1 m) is lower than that projected

Impacts of Cyclone Yasi on nearshore, terrigenous sediment-dominated reefs of the central Great Barrier Reef, Australia

Tropical Cyclone (TC) Yasi (Category 5) was a large (~ 700 km across) cyclone that crossed Australia's Queensland coast on the 3rd of February 2011. TC Yasi was one of the region's most powerful recorded cyclones, with winds gusting to 290 km/h and wave heights exceeding 7 m. Here we describe the impacts of TC Yasi on a number of nearshore, turbid-zone coral reefs, that include several in the immediate vicinity of the cyclone's landfall path (King Reef, Lugger Shoal and Dunk Island), as well as a more distally located reef (Paluma Shoals) ~ 150 km to the south in Halifax Bay. These reefs were the focus of recent (between 2006 and 2009) pre-Yasi studies into their geomorphology, sedimentology and community structure, and here we discuss data from a recent (August 2011) post-Yasi re-assessment. This provided a unique opportunity to identify and describe the impacts of an intense tropical cyclone on nearshore reefs, which are often assumed to be vulnerable to physical disturbance and reworking due to their poorly lithified framework. Observed impacts of TC Yasi were site specific and spatially highly heterogeneous, but appear to have been strongly influenced by the contemporary evolutionary stage and ecological make-up of the individual reefs, with site setting (i.e. exposure to prevailing wave action) apparently more important than proximity to the landfall path. The most significant ecological impacts occurred at King Reef (probably a result of freshwater bleaching) and at Paluma Shoals, where widespread physical destruction of branched Acropora occurred. New coral recruits are, however, common at all sites and colony re-growth clearly evident at King Reef. Only localised geomorphic change was evident, mainly in the form of coral fracturing, rubble deposition, and sediment movement, but again these impacts were highly site specific. The dominant impact at Paluma Shoals was localised storm ridge/shingle sheet deposition, at Lugger Shoal major offshore fine sediment flushing, and at Dunk Island major onshore coarse sand deposition. There was little geomorphic change evident at King Reef. Thus whilst small-scale and taxa specific impacts from Cyclone Yasi are clearly evident, geomorphological changes appear minor and ecological impacts highly variable between sites, and there is no observed evidence for major reef structural change. The study suggests that the vulnerability of reefs to major physical disturbance events can be extremely site specific and determined by interacting factors of location relative to storm path and pre-event geomorphology and ecology

Tsunami as agents of geomorphic change in mid-ocean reef islands

Low-lying atoll islands appear highly vulnerable to the effects of climate change and extreme natural events. Potentially disastrous effects of future sea-level rise have been inferred in many studies, and the actual impacts of tropical storms on island destruction and formation have been well documented. In contrast, the role of tsunami in the geomorphic development of atoll islands has not been investigated. The Sumatran earthquake of 26 December 2004 generated a tsunami that reached the Maldives 2500 km away, with waves up to 2.5 m high. Observations on the geomorphic changes resulting from the tsunami are detailed here, based on pre-and post-tsunami profile measurements of island, beach and reef topography, and GPS surveys of the planform shape of islands and beaches of 11 uninhabited islands in South Maalhosmadulu atoll, Maldives. Erosional and depositional impacts were observed on all islands and these have been quantified. In general the changes were of a minor nature with a maximum reduction in island area of 9% and average of 3.75%. Rather, the tsunami accentuated predictable seasonal oscillations in shoreline change, including localised erosion reflected in fresh scarps and seepage gullies. Depositional features in the form of sand sheets and sand lobes emplaced on the vegetated island surfaces provide clear evidence that the tsunami waves washed over parts of all the islands. Both erosional scarps and overwash deposits were concentrated at the tsunami-exposed eastern sides of the islands. Impacts on leeward shores were primarily accretionary, in the form of spit and cuspate foreland extension. Whereas the nature and magnitude of intra-and inter-island impacts was variable, an east to west decline in aggregate effects was noted. Detailed consideration of the morphodynamic interaction between the tsunami waves and island morphology, show that this cross-atoll gradient resulted not just from the reduction in tsunami energy as it passed through the atoll, but also from variations in elevation of the encircling island ridge, and the quantity and distribution of sediment in the antecedent beach. A conceptual model identifying the sequence of changes to individual islands supports the observational data and the pattern of geomorphic changes resulting from the tsunami. This model leads to consideration of the longer-term impacts of the tsunami on the future stability of islands. Four scenarios are presented, each of which has a different island-beach sediment budget, and different relaxation time to achieve dynamic equilibrium

Estimating rates of biologically driven coral reef framework production and erosion: a new census-based carbonate budget methodology and applications to the reefs of Bonaire

Census-based approaches can provide important measures of the ecological processes controlling reef carbonate production states. Here, we describe a rapid, non-destructive approach to carbonate budget assessments, termed ReefBudget that is census-based and which focuses on quantifying the relative contributions made by different biological carbonate producer/eroder groups to net reef framework carbonate production. The methodology is presently designed only for Caribbean sites, but has potential to be adapted for use in other regions. Rates are calculated using data on organism cover and abundance, combined with annual extension or production rate measures. Set against this are estimates of the rates at which bioeroding species of fish, urchins and internal substrate borers erode reef framework. Resultant data provide a measure of net rates of biologically driven carbonate production (kg CaCO3 m(-2) year(-1)). These data have potential to be integrated into ecological assessments of reef state, to aid monitoring of temporal (same-site) changes in rates of biological carbonate production and to provide insights into the key ecological drivers of reef growth or erosion as a function of environmental change. Individual aspects of the budget methodology can also be used alongside other census approaches if deemed appropriate for specific study aims. Furthermore, the methodology spreadsheets are user-changeable, allowing local or new process/rate data to be integrated into calculations. Application of the methodology is considered at sites around Bonaire. Highest net rates of carbonate production, +9.52 to +2.30 kg CaCO3 m(-2) year(-1), were calculated at leeward sites, whilst lower rates, +0.98 to -0.98 kg CaCO3 m(-2) year(-1), were calculated at windward sites. Data are within the ranges calculated in previous budget studies and provide confidence in the production estimates the methodology generates