Scott G. Smithers

Caribbean-wide decline in carbonate production threatens coral reef growth.

Global-scale deteriorations in coral reef health have caused major shifts in species composition. One projected consequence is a lowering of reef carbonate production rates, potentially impairing reef growth, compromising ecosystem functionality and ultimately leading to net reef erosion. Here, using measures of gross and net carbonate production and erosion from 19 Caribbean reefs, we show that contemporary carbonate production rates are now substantially below historical (mid- to late-Holocene) values. On average, current production rates are reduced by at least 50%, and 37% of surveyed sites were net erosional. Calculated accretion rates (mm year−1) for shallow fore-reef habitats are also close to an order of magnitude lower than Holocene averages. A live coral cover threshold of ~10% appears critical to maintaining positive production states. Below this ecological threshold carbonate budgets typically become net negative and threaten reef accretion. Collectively, these data suggest that recent ecological declines are now suppressing Caribbean reef growth potential.

Holocene reef growth in Torres Strait

The platform and fringing reefs of Torres Strait are morphologically similar to reefs of the northern Great Barrier Reef to the south, except that several are elongated in the direction of the strong tidal currents between the Coral Sea and the Gulf of Carpentaria. Surface and subsurface investigations and radiocarbon dating on Yam, Warraber and Hammond Islands reveal that the initiation and mode of Holocene reef growth reflect antecedent topography and sea-level history. On the granitic Yam Island, fringing reefs have established in some places over a Pleistocene limestone at about 6 m depth around 7000 years BP. Emergent Holocene microatolls of Porites sp. indicate that the reefs have prograded seawards while sea level has fallen gradually from at least 0.8 m above present about 5800 years BP. On the Warraber Island reef platform drilling near the centre indicated a Pleistocene limestone foundation at a depth of about 6 m over which reefs established around 6700 years BP. Reef growth lagged behind that on Yam Island. Microatolls on the mature reef flat indicate that the reef reached sea level around 5300 years BP when the sea was around 0.8–1.0 m above present. On the reef flat on the western side of Hammond Island bedrock was encountered at 7–8 m depth, overlain by terrigenous mud. A progradational reef sequence of only 1–2 m thickness has built seaward over these muds, as sea level has fallen over the past 5800 years. Reef-flat progradation on these reefs is interpreted to have occurred by a series of stepwise buildouts marked by lines of microatolls parallel to the reef crest, marking individual coalescing coral heads. Detrital infill has occurred between these. This pattern of reef progradation is consistent with the radiocarbon dating results from these reefs, and with seismic investigations on the Torres Reefs.

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.

Fringing and nearshore coral reefs of the Great Barrier Reef:episodic Holocene development and future prospects

Abstract The Holocene growth of fringing and nearshore reefs on the GBR is examined. A review of data from 21 reefs indicates that most grow upon Pleistocene reef, boulder, and gravel, or sand and clay substrates, with no cored examples growing directly over rocky headlands or shores. Dated microatolls and material from shallow reef-flat cores indicate that fringing and nearshore reefs have experienced several critical growth phases since the mid-Holocene: (1) from initiation to 5500 YBP, optimum conditions for reef and reef-flat growth prevailed; (2) from 5500–4800 YBP, reef-flat progradation stalls in almost 50% of the reefs examined; (3) of reefs prograding post-4800 YBP, approximately half ceased active progradation around 3000–2500 YBP; (4) reefs prograding to present do so at rates well below mid-Holocene rates; (5) a group of nearshore reefs has established since 3000 YBP, in conditions traditionally considered poor for reef establishment and growth. Importantly, many of the reefs that appear to have grown little for several millennia are veneered by well-developed coral communities. Although local conditions no doubt exert some influence over these growth patterns, the apparent synchronicity of these growth and quiescent phases over wide geographical areas suggests the involvement of broader scale influences, such as climate and sea-level change. Recognition and understanding these phases of active and moribund reef growth provides a useful longer term context in which to evaluate reported current declines in fringing and nearshore reef condition.

Late Holocene initiation and growth of a nearshore turbid-zone coral reef: Paluma Shoals, central Great Barrier Reef, Australia

It is now well known that corals and coral reefs can grow and survive in turbid water (e.g. Marshall and Orr 1931; Anthony 2000). However, with the exception of a few excellent studies (Hopley et al. 1983; Johnson and Risk 1987; Hopley 1994; Tudhope and Scoffin 1994; Kleypas 1996), descriptions of the internal structure and growth history of turbid-zone reefs are rare compared to those of clearer water systems. Here we outline the internal structure and Holocene growth history of the largest and most southern reef (hereafter referred to as South Shoal) at Paluma Shoals, a group of nearshore turbid-zone reefs in Halifax Bay, approximately 50 km northwest of Townsville (Fig. 1a).

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.

Microatolls as sea-level indicators on a mid-ocean atoll

Microatolls are discoid corals with flat upper surfaces that develop when upward coral growth is constrained by exposure at low tide. They have been widely viewed as important and relatively precise indicators of modern and paleo sea-level positions because of their sensitivity to the water/air interface, though this has rarely been directly established by survey to a precise datum. This study involved an accurate survey of 282 microatolls growing in a range of intertidal environments (open reef flats, interisland passages, lagoons) across the Cocos (Keeling) Islands, a mid-ocean atoll in the Indian Ocean. The upper surface elevation of modern microatolls was found to vary through more than 40 cm across the environments in which these corals occur on Cocos, representing more than 30% of the spring tide range. Much of the variation reflects subtle ponding at low water levels across extensive low-gradient reef surfaces associated with interisland passages. Microatolls in open reef flat habitats are typically constrained within the narrowest and most consistent elevation range around the atoll. The environment in which individual fossil microatolls occurred when they were alive is significant in reconstructing former sea levels.

From barriers to limits to climate change adaptation: path dependency and the speed of change

Research on the barriers and limits to climate change adaptation identifies many factors, but describes few processes whereby adaptation is constrained or may indeed fail to avoid catastrophic losses. It often assumes that barriers are by and large distinct from limits to adaptation. We respond to recent calls for comparative studies that are able to further knowledge about the underlying drivers of barriers and limits to adaptation. We compare six cases from across Australia, including those in alpine areas, rivers, reefs, wetlands, small inland communities, and islands, with the aim of identifying common underlying drivers of barriers and limits to adaptation. We find that the path-dependent nature of the institutions that govern natural resources and public goods is a deep driver of barriers and limits to adaptation. Path-dependent institutions are resistant to change. When this resistance causes the changes necessary for adaptation to be slower than changes in climate, then it becomes a limit to adaptation.

Mid-Pacific microatolls record sea-level stability over the past 5000 yr

There has been geographical variation in sea level since rapid postglacial melting of polar ice ceased similar to 6 k.y. ago, reflecting isostatic adjustments of Earth and ocean surfaces to past (and ongoing) redistribution of ice and water loads. A new data set of over 100 fossil microatolls from Christmas (Kiritimati) Island provides a Holocene sea-level record of unparalleled continuity. Living reef-flat corals grow up to a low-tide level. Adjacent fossil microatolls, long-lived Porites corals up to several meters in diameter, occur at similar elevations (±0.1 m), and extensive fossil microatolls in the island interior are at consistent elevations within each population. Collectively, they comprise an almost continuous sequence spanning the past 5 k.y., indicating that, locally, sea level has been within 0.25 m of its present position, and precluding global sea-level oscillations of one or more meters inferred from less stable locations, or using other sea-level indicators. This mid-Pacific atoll is tectonically stable and far from former ice sheets. The precisely surveyed and radiometrically dated microatolls indicate that sea level has not experienced significant oscillations, in accordance with geophysical modeling, which implies that the eustatic contribution from past ice melt and the isostatic adjustment of the ocean floor to loading largely cancel each other at this site.