Roger F. McLean

New model of reef-island evolution: Maldives, Indian Ocean

A new model of reef-island evolution, based on detailed morphostratigraphic analysis and radiometric dating of three islands in South Maalhosmadulu Atoll, Maldives, is presented. Islands initially formed on a foundation of lagoonal sediments between 5500 and 4500 yr B.P. when the reef surface was as much as 2.5 m below modern sea level. Islands accumulated rapidly during the following 1500 yr, effectively reaching their current dimensions by 4000 yr B.P. Since then the high circum-island peripheral ridge has been subject to seasonal and longer-term shoreline changes, while the outer reef has grown upward, reducing the energy window and confining the islands. This new model has far-reaching implications for island stability during a period of global warming and raised sea level, which will partially reactivate the energy window, although it is not expected to inhibit upward reef growth or compromise island stability.

ATOLL REEF-ISLAND FORMATION AND RESPONSE TO SEA-LEVEL CHANGE : WEST ISLAND, COCOS (KEELING) ISLANDS

Abstract Reef islands around the margin of coral atolls generally comprise unconsolidated Holocene sands and gravels, overlying a reef flat or cemented conglomerate platform. Such islands have accreted within the last 3000–4000 years, since sea level has reached a level close to present and the reef flat and conglomerate platform have formed. Island morphology consists of an oceanward ridge, a less distinct lagoonward ridge, and low-lying central depression. Several alternative models of how such reef islands might have developed are examined in relation to chronology and sediment provenance, particularly in the context of the Cocos (Keeling) Islands where this issue has been debated since Darwin visited the atoll. Which of these models appears most appropriate for an elongate reef island on the atoll margin is assessed using conventional radiocarbon dating of coral shingle and accelerator mass spectrometry (AMS) radiocarbon dating of individual sand grains from pits across West Island. The dating results suggest that both coral clasts and individual grains of various components are generally reliable and replicable indicators of the chronology of island accumulation, implying rapid transport of skeletal material, after death of the contributing organisms, across the reef flat zone, and relatively little reworking. The central part of West Island appears to have formed first, with oceanward accretion up until about 2000 years BP. Gradual oceanward accretion with lesser lagoonward extension has continued beyond 2000 years BP at the northern and southern ends of the island, and a sequence of lagoonward recurving spits has formed adjacent to the inter-island passage at the southern end of the island. Radiocarbon dating of fossil microatolls indicates a gradual fall of sea level over this period which appears to have had little effect on the pattern of island accretion.

Geological effects of tsunami on mid-ocean atoll islands: The Maldives before and after the Sumatran tsunami

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.

Sequential studies of hurricane deposit evolution at Funafuti atoll

Abstract Successive surveys of a massive coral rubble rampart deposited during a tropical cyclone in 1972 indicate lagoonward movement. The deposit is expected to connect with existing islands and to add significantly to the atolls land area.

Relationships between grain size, size-sorting, and foreshore slope on mixed sand - shingle beaches

Abstract The variation of mean grain size (Mzφ), sorting (σI), and beach face slope over mixed-sand shingle beaches at two areas along the east coast, South Island, New Zealand, it analysed in order to determine the nature of the relationships between these three variables. The study is confined to mean grain sizes between 0.25 and 16.0 mm, and the sorting and slope values associated with this range of sizes. Trend analysis of the data has been performed by fitting curves with the aid of a computer. As distinct from the linear average relationship betweeen size and slope distinguished by many workers, it is demonstrated that for given beaches curvilinear trends in size/sorting are reflected in slope/size relationship. Thus, the best sorted sediments are found to be associated with the steepest gradients on the curves, while zones of poor sorting are associated with “plateaus” in the size/slope trend. It is suggested that size is the primary control of sorting trends in the sediments examined whilst hydrau...

From Foreshore to Foredune: Foredune Development Over the Last 30 Years at Moruya Beach, New South Wales, Australia

Abstract Between 1974 and 1976, a series of east-coast cyclones in the western Tasman Sea resulted in extensive coastal erosion along southeastern Australia. In many beach compartments, the backshore and incipient foredune were completely removed, and the sea cut back into the swale and/or second dune ridge. This occurred at Moruya Beach, where a profile monitoring program had been established in 1972—a program that continues to this day. Here we report field evidence describing the initial condition of the beach, its subsequent erosion (such that the position of the initial backshore became the foreshore), and how this foreshore became reconstituted as a backshore ultimately developing into the present foredune. Critical to the formation of the frontal dune was the presence of a broad backshore berm at an elevation of 2.3 to 2.8 m above local mean sea level (MSL). Achievement of this elevation did not, by itself, guarantee foredune development. Rather, there is also a width threshold to the berm, which at Moruya is at least 30 m. While the berm reached either this elevation (2.3 to 2.8 m) or width (>30 m) on several occasions prior to the formation of the incipient foredune, it was only when both conditions were satisfied that the embryo foredune developed into an incipient foredune. This was in the late 1970s and early 1980s. Incremental vertical growth of the foredune took place over the next 15 years, from an elevation of ca. 3.0 m up to 5 m above MSL. Initially, the position of the newly accreted foredune was well seaward of its prestorm (1972) position, but in the last few years it has tended to migrate inland, though the geographic position of the mean and high water level intercepts have not migrated with it.

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.

A late Quaternary vegetation history for far northern New Zealand

Pollen diagrams are described for 2 sites near Cape Regina from far N North Island. Forest persisted throughout the last 17 000 yr and while initially containing cooler elements change indicating a maximum in diversity and complexity was attained between 10 000-6800 BP. This was the warmest and probably most mesic environment represented in the record. Nothofagus was never an important element and Dacrydium cupressinum, Podocarpus spp. and tree ferns dominated throughout. Agathis australis expanded from c11 500 BP but declined after 3000 BP, perhaps associated with increased droughtiness. Shrubland also occurred throughout the record; natural fire was an important component in their function. Fire incidence increased in the recent past suggesting it was an agent in forest destruction following human occupance

Sea level change in the Holocene on the northern Great Barrier Reef

Detailed studies, utilizing a range of both well controlled sea level criteria and dates, are required if Holocene time-sea level curves are to be established with any degree of confidence. This paper is restricted to an interpretation of Expedition results from the northern Great Barrier Reef, excluding those from the drill core. Extensive colonies of emergent fossil corals in growth position indicate that present sea level was first reached about 6000 a b. p. Elevations of cay surfaces, cemented rubble platforms, microatolls, coral shingle ridges, reef flats and mangrove swamps, referenced to present sea level show an array of heights. However, levels of particular features are accordant on many reefs: it is believed that these can be related to particular sea levels. Radiometric dating provides the time framework. Ages of samples from similar deposits on different reefs are surprisingly consistent. Oscillations in sea level since 6000 a b.p ., relative to present sea level, are identified with varying degrees of confidence. This history of relative sea level does not separate eustatic from noneustatic components.

Coral islands defy sea-level rise over the past century: Records from a central Pacific atoll

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.