Colin D. Woodroffe

The impact of sea-level rise on mangrove shorelines:

There are extensive mangrove forests along the tropical shorelines of the world; they are particularly well developed in, though not exclusively restricted to, areas that are sheltered and muddy. Stratigraphic studies of the sediments underlying both the mangrove forests themselves and associated wetlands indicate that there have been considerable changes in the extent of these wetlands as a result of past sea-level fluctuations. Further changes in the area covered by mangroves are to expected if the sea rises at the rates anticipated as a result of the greenhouse effect. The exact response of a mangrove coastline will depend upon shoreline topography, sources of sediment, rates of sediment supply, and the rate of sealevel rise. The stratigraphy and chronology of Holocene deposits on mangrove shorelines allow reconstruction of the way that mangrove forests have responded

Fringing reef growth and morphology: a review

Abstract Fringing reefs are generally not simple veneers of coral growth along tropical shorelines. Extensive research over the past few decades, based on radiocarbon dating of Holocene reef deposits, has indicated that they can develop in a complex variety of ways even though the surface morphology may appear relatively simple. The principal factor that appears to determine the growth and morphology of fringing reefs is the available accommodation space. Sea-level fluctuations are important, primarily because the sea surface determines the absolute accommodation space for a given reef. This means that a reef established during a period of sea-level rise will be able to accrete vertically as space is created above it. If, however, the reef establishes at, or grows to, the sea surface, thereby occupying all the available accommodation space, it can no longer accrete vertically and begins to build laterally. The morphology and chronostratigraphy of a range of Holocene fringing reefs are described, on the basis of which six fringing reef growth models are identified. In model A, the fringing reef is established at depth and primarily accretes vertically towards the sea surface. Reef growth in model B initiates at sea level and due to the lack of vertical accommodation space grows laterally. Model C has a similar morphology to model B; however, the reef progrades over a non-reefal sediment wedge. Episodic lateral and vertical growth occurs in model D, with a stepwise progradation of the reef front. The remaining models are characterised by seaward reef framework behind which unconsolidated sediments accumulate. In model E, reef-crest growth forms a barrier leading to the development of a backreef lagoon. Model F has a similar morphology to model E, except that the reef crest is formed by hurricane rubble accumulation rather than framework accretion, and is periodically reworked.

Landscape variability and the response of Asian megadeltas to environmental change

Deltas, occurring at the mouths of river systems that deposit sediments as they enter the sea, are some of the most dynamic sedimentary environments. They contain a long, and often economically significant, sedimentary record of their response to past episodes of climate and sea-level change. Geological investigation of these deposits, and the processes controlling sedimentation, provide insights into the response of deltas to environmental change, which in turn may offer rational and cost-effective strategies for the sustainable management of natural resources and land use in these dynamic systems in the face of future environmental change.

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.

Estuarine infill and coastal progradation, southern van diemen gulf, northern Australia

Abstract There are several estuaries associated with the pronouncedly seasonal rivers which drain northwards from the Middle Proterozoic sandstone Arnhem Land plateau, and the Tertiary Koolpinyah land surface, into the macrotidal van Diemen Gulf, in the Northern Territory of Australia. The Holocene development of these, investigated in greatest detail for the South Alligator River with an upland catchment of > 10,000 km2. through drilling, palynology and radiocarbon dating, comprises both estuarine infill and coastal progradation. Three phases of estuarine infill can be recognised: (i) a transgressive phase (8000-6800 years B.P.) of marine incursion; (ii) a big swamp phase (6800-5300 years B.P.) of widespread mangrove forest development; and (iii) a sinuous/cuspate phase of floodplain development since 5300 years B.P., during which the tidal river has meandered and reworked earlier estuarine sediments. Since 6000 years B.P., the South Alligator coastal plain has prograded at a decelerating rate, with two phases of chenier ridge formation. A similar pattern of estuarine infill, and decelerating coastal plain progradation, is demonstrated for the Adelaide and Mary Rivers, both with catchments of > 6000 km2. The southern shore of van Diemem Gulf appears to have changed its overall position little during the last 2000 years. The major source for the clay, silt and fine sands which have infilled the estuary and coastal plain has been from seaward. Dispite the similarity of development, coastal sediment build up has had different effects on the morphology of each tidal river. The Adelaide has undergone a major diversion and no longer flows directly into van Diemen Gulf, but occupies a former fluvial course, and the Mary has been blocked entirely, and its former estuarine palaeochannels have been infilled with tide-transported sediment.

Response of coral reefs to climate change: Expansion and demise of the southernmost Pacific coral reef

Received 21 May 2010; revised 20 June 2010; accepted 6 July 2010; published 3 August 2010. [1] Coral reefs track sea level and are particularly sensitive to changes in climate. Reefs are threatened by global warming, with many experiencing increased coral bleaching. Warmer sea surface temperatures might enable reef expansion into mid latitudes. Here we report multibeam sonar and coring that reveal an extensive relict coral reef around Lord Howe Island, which is fringed by the southernmost reef in the Pacific Ocean. The relict reef, in water depths of 25–50 m, flourished in early Holocene and covered an area more than 20 times larger than the modern reef. Radiocarbon and uranium‐series dating indicates that corals grew between 9000 and 7000 years ago. The reef was subsequently drowned, and backstepped to its modern limited extent. This relict reef, with localised re‐establishment of corals in the past three millennia, could become a substrate for reef expansion in response to warmer temperatures, anticipated later this century and beyond, if corals are able to recolonise its surface. Citation: Woodroffe, C. D., B. P. Brooke, M. Linklater, D. M. Kennedy, B. G. Jones, C. Buchanan, R. Mleczko, Q. Hua, and J. Zhao (2010), Response of coral reefs to climate change: Expansion and demise of the southernmost Pacific coral reef, Geophys. Res. Lett., 37, L15602, doi:10.1029/2010GL044067.

Reef-island accretion and soil development on Makin, Kiribati, central Pacific

The late Holocene accretionary history of reef islands on Makin, the northernmost table reef of the Gilbert atoll chain, western Kiribati, has been reconstructed based on conventional radiocarbon dating of coral shingle and bulk sand samples, and AMS radiocarbon dating of individual grains. Makin is geomorphologically and sedimentologically simple, comprising one main island with only a minor residual lagoon, and composed of sand dominated by the reef-flat foraminifera Calcarina spengleri, Amphistegina lobifera and Baculogypsina sphaerulata. Deposition commenced in mid platform around 2500 years ago; in situ fossil coral (Heliopora) dated at 2400±80 years BP indicates that sea level was 0.4–0.5 m above present at that time. Progradation occurred progressively to westward at a relatively constant rate of 200–300 m ka−1. However, progradation was interrupted on the eastern side by lagoon encapsulation around 1400 years ago. The soil and vegetation characteristics are uniform over most of the reef islands as a result of clearance and the planting and maintenance of coconuts, and do not reflect reef-island depositional history. Remnant stands of Pisonia forest, with an associated phosphatic soil, are not related to any particular geomorphological feature, or period of deposition. The reef islands are continuing to accrete as a result of production of foraminifera on the reef flat.

Deltaic and estuarine environments and their Late Quaternary dynamics on the Sunda and Sahul shelves.

Abstract Deltaic and estuarine environments have been, and continue to be, some of the most rapidly changing environments. Those associated with the Sunda shelf generally receive large volumes of sediment and were characterised by a diverse and productive vegetation before much of it was cleared and converted for agriculture, silviculture or urban development. By contrast estuaries in northern Australia receive far less sediment supply, and record a much less modified pattern of landform change during the Holocene. Three periods of change are discussed: first, the long-term geological development and response of deltaic–estuarine plains to eustatic cycles of sea-level change, particularly postglacial sea-level rise to present; second, Holocene development of deltaic–estuarine environments, dominated by patterns of coastal progradation and distributary migration, under relatively stable sea level; and third, the impact of human modifications. These observations provide a framework within which response of the deltaic–estuarine environments to future, anticipated environmental change can be assessed.

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.