David Hopley

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.

The significance of coral reefs as global carbon sinks— response to Greenhouse

Coral reefs are net sinks for C, principally as CaCO3 accretion. It is possible to predict quite accurately the rate of production, given adequate information about any particular reef environment. The best data set for an extensive region is that for the Great Barrier Reef (GBR). Careful analysis of this region and the incorporation of previously documented present day system calcification rates suggest net production (G) from G = 1 (kg CaCO3 m−2 yr−1) for fringing reefs, to G = 1.9 for planar (infiled platform) reefs, G = 3 for ribbon reefs and lagoonal reefs. The 20,055 km2 of reefs in the GBR are thus estimated to average G = 2.4, resulting in a total production of ∼ 50 million tonnes yr−1. In a 50–100 year Greenhouse scenario of rising sealevel, we predict that recolonisation of present day reef flats will be extensive and prolific. Production will increase substantially, and this could be by as much as ∼ 40% (ranging from 0% for deep shoals to 180% for fringing reefs) to give ∼ 70 million tonnes yr−1 if the rate of sealevel rise reaches or exceeds 6–8 mm yr−1 We estimate 115,000 km2 of oceanic atolls worldwide. Drawing on points equivalence from the detailed analysis of the GBR, we estimate the atolls presently produce 160 million tonnes yr−1. We predict that a similar ∼ 40% increase could be possible in the next 100 years or so resulting in a production of 220 million tonnes. Accepting an existing estimate of 617,000 km2 for reefs worldwide, drawing from our projections from the GBR and the atolls, and making some assumptions about the remaining reef types (we suggest fringing reefs to dominate) we estimate global reef production at the present time to be ∼ 900 million tonnes yr−1. Within the next 100 years or so, we suggest this rate could almost double to ∼ 1800 million tonnes. In the long term (several centuries) we predict that the continuing trend of recolonisation, particularly of fringing and planar reefs could result in the production of > 3000 million tonnes yr−1 if rates of sealevel rise approaching or exceeding 6–8 mm yr−1 are achieved. Eventually (> 500 yr), reefs could actually “drown” due to inability to match the rate of sealevel increase if that rate significantly exceeds 6–8 mm yr−1. Thus, coral reefs at present act as a sink for 111 million tonnes C yr−1, the equivalent of 2% of present output of anthropogenic CO2. In the short term Greenhouse scenario (100 yr) we predict this could increase to the equivalent of ∼ 4% of the present CO2 output. In the much longer term (several centuries), if all trends continue, this could increase to the equivalent of as much as ∼ 9% of the present CO2 output. Unfortunately, we also predict that this considerable sink for C will be most likely of negative value in alleviating Greenhouse because of the immediate effect of CaCO3 precipitation is to raise the PCO2 of the surface oceans — ie, ot encourage CO2 efflux to the atmosphere. We do not attempt to quantify this effect. Other Greenhouse changes such as seawater temperature increase, changes in cloud cover, increased rainfall and runoff, increased storm activity, and changes in dissolved CO2 concentration and surface ocean circulation may complicate the reef response. However, we suggest that sealevel rise will be the dominant influence, at least during the next 100 years or so.

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.

Encyclopedia of Modern Coral Reefs: structure, form and process

The impact of sea level rise on coral reef flats was one of the first considerations raised in relation to climate change and coral reefs. Most publications in the 1980s considered the impact to be a beneficial one. This was especially so in the Indo-Pacific area, where isostatic adjustments had produced a sea level at or above its present position for over 6,000 years. Many reefs are now adjusted to this level with lagoons infilled, sediments dominating the reef flat and living corals limited to shallow pools (Figure 1). Such reef flats are too shallow for at least half the tidal cycle for the transmission of waves with sufficient energy to entrain and transport all but the finest sediments. Many general references on Greenhouse effects emphasized the rejuvenation of reef tops (e.g., Henderson-Sellers and Blong, 1989) whilst others went as far as suggesting reefs could be drowned and many ecosystems eliminated (e.g., Falk and Brownlow, 1989). Some scientific assessments suggested that renewed coral growth would make reef flats aesthetically more pleasing (e.g., Hopley and Kinsey, 1988).[Extract] Estimations of coral reef accretion rates can be undertaken using a number of techniques. Most tedious is the use of growth rates from individual organisms (Chave et al., 1972). More recent methods include the measurement of total reef metabolism and calcification (Kinsey, 1985) or estimates of rates during the Holocene from dated drill cores (Davies, 1983; Davies and Hopley, 1983).[Extract] Because of the complexity of coral reefs and difficulties in ground survey, the reef environment was one of the earliest to take advantage of remote sensing techniques (Hopley, 1978). Both aircraft and balloons (e.g., Rutzler, 1978) formed the initial platforms, usually for vertically mounted cameras using black and white film. On the Great Barrier Reef (GBR), the earliest vertical aerial photography was in 1925, when the Royal Australian Air Force photographed the Low Isles at a scale of 1:2,400 in 1928 for the Yonge Expedition (see Great Barrier Reef Committee). Simultaneously, Umbgrove (1928, 1929) was photographing reefs in Indonesia to aid the extensive work he was carrying out there.Sea level is the local height of the oceans surface, usually measured to a datum referenced to a tidal position established from a record in which high-frequency motions such as wind waves and periodic changes (e.g., due to the tides) have been averaged out. Local sea level fluctuates regularly with tides and irregularly in response to factors including wind and currents, water temperatures and salinities, and atmospheric pressure. Relative sea level is the elevation of the sea surface relative to the land at a given location. Global or eustatic sea-level fluctuations occur as the volume of water in the earths oceans changes when ice caps and glaciers grow or melt, or as large-scale changes in the configuration of ocean basins and continental margins occur through plate tectonics. There are also regional and local isostatic processes that produce spatially different patterns of relative sea-level change, including thermal expansion of surface waters, changes in meltwater load, crustal adjustment of areas directly or indirectly affected by ice on- and offloading (see Glacio-Hydro Isostasy), coastal uplift or subsidence due to tectonic processes (see Earthquakes and Emergence or Submergence of Coral Reefs), and subsidence due to aquifer depletion or sediment compaction. Sea-level indicators are used to determine relative sealevel changes at a location. Locating a sea-level indicator and determining its age and elevation relative to its modern counterpart can establish relative sea-level change. Sea-level indicators on coral reefs include a range of biological, geomorphological, sedimentological, and chemical features that provide information on the position of the sea surface at the time that they lived or were formed. They usually comprise features with a known relationship to a tidal position or datum.The earliest work on geohydrology applicable to coral cays was undertaken separately by Ghyben and Herzberg in the late 1800s and early 1900s, who determined the shape and thickness of a freshwater lens that forms under coral cays once they reach a minimum size. The relationship, which is based on the different densities of freshwater and saltwater, is expressed in the Ghyben–Herzberg equation: z = (ρf/(ρs - ρf))h, where h is the distance above sea level to the water table (phreatic surface), z is the distance below sea level to the freshwater–saltwater interface, and rs and rf are the densities of saltwater and freshwater, respectively. Using densities of 1.00 g cm-3 for freshwater and 1.025 g cm-3 for saltwater gives the often quoted relationship z = 40 h. The Ghyben–Herzberg model makes an assumption of a single layer homogeneous medium, and a system in hydrostatic equilibrium, with no mixing of fresh and salt water, giving a sharp transition between the freshwater and saltwater. This model is normally implemented with the Dupuit assumption of horizontal flow (Oberdorfer et al., 1990) and is frequently applied in resource assessments of potable water for human use on inhabited coral cay islands. In reality, this model makes assumptions that are clearly not valid inmost coral reef environments. Tidal fluctuations of the water level are assumed to be negligible, watermovement within the lens is assumed to result entirely from recharge-induced changes to the hydraulic head, outflow from the freshwater lens required to maintain mass-balance is assumed to take place at the island margin, and mixing within the framework caused by various water movements and pressure gradients (such as tidal mixing) is not considered. Perhaps most significantly, the assumption of a homogeneous medium rarely holds. In particular, differences in the reef framework above and below the Pleistocene solution unconformity (sometimes called the Thurber discontinuity) typically found 6–25 below the current reef flat level in tectonically stable areas, means that the model is fundamentally flawed. In general, a very broad transition zone between fresh and saltwater can be expected.Fringing reefs have been described as being morphologically simple (Kennedy and Woodroffe, 2002), but variation in important parameters such as the reef morphology, tidal range, and wave energy means that a widely applicable model of fringing reef water circulation does not exist. Fundamental distinctions can be made between fringing reefs with or without an enclosed lagoon, those on the windward shores (with circulation normally driven by waves) or leeward shores (with tidal and other currents dominating), and those enclosed by headlands (where topographically controlled circulation is important) or those that extend along a more or less straight shoreline. Other reef types that could be described as fringing are variously described as either fringing or barrier (e.g., Ningaloo Reef, Australia; Hearn, 1999), bank-fringing (e.g., Great Pond Bay, St Croix, U.S. Virgin Islands; Lugo-Fernandez et al., 1998b), bank-barrier (e.g., Tague Reef, St Croix; Lugo-Fernandez et al., 2004), or coral lagoons (e.g., Kaneohe Bay, Oahu, Hawaii; Hearn, 1999). Other authors have described reefs adjacent to coral cays as fringing (Daly and Brander, 2006).Coral reefs are the largest landforms built by plants and animals. Their study therefore incorporates a wide range of disciplines. This encyclopedia approaches coral reefs from an earth science perspective, concentrating especially on modern reefs. Currently coral reefs are under high stress, most prominently from climate change with changes to water temperature, sea level and ocean acidification particularly damaging. Modern reefs have evolved through the massive environmental changes of the Quaternary with long periods of exposure during glacially lowered sea level periods and short periods of interglacial growth. The entries in this encyclopedia condense the large amount of work carried out since Charles Darwin first attempted to understand reef evolution. Leading authorities from many countries have contributed to the entries covering areas of geology, geography and ecology, providing comprehensive access to the most up-to-date research on the structure, form and processes operating on Quaternary coral reefs.

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.

Positive relief over buried post-glacial channels, Great Barrier Reef Province, Australia

Abstract Continuous seismic profiling on the continental shelf, central Great Barrier Reef Province, Australia, has delineated a major sub-bottom reflector interpreted as the pre-Holocene surface. Channels eroded in this surface are infilled and overfilled by post-glacial sediments. Filling is ascribed to estuarine backfilling during sea-level rise followed by deposition of delta front deposits. In many cases these fills create positive relief over buried channels. Hence modern shelf morphology may not be an accurate guide to the positions of Pleistocene channels across the shelf.

Coral reef growth on the shelf margin of the Great Barrier Reef with special reference to the Pompey Complex

Abstract Several modes of coral reef growth are found along the edge of Australias Great Barrier Reef (GBR), determined by the morphology and slope of the shelf edge, especially between −50 m and −100 m, and the velocity of tidal currents near the surface. The simplest forms are the flood tide deltaic reefs and ribbon reefs of the far north. The shelf margin of the central GBR is characterized by lines of submerged reefs that continue on the ocean (northeastern) side of the Pompey Reefs, which are the largest and most complex in the entire GBR. Combining interpretations of reef evolution from the simpler marginal reefs with data collected from the Pompey Complex, a model of evolution on a stepped continental shelf margin is developed, involving initiation as ribbon reefs, formation of both ebb and flood tide deltas (which have formed the foundation for further reef growth), and incorporation of at least one line of previously submerged reefs on the open ocean side by progradation of the deltaic structures to form large lagoonal reefs. Although the reefs cover a smaller area than the extensive reefs of the continental shelf, which could have grown only at higher Quaternary sea levels, the smaller area of shelf-marginal reefs may contain a longer record of coral growth than that of the better-known shelf reefs.

Holocene sea levels in eastern Australia — A discussion

Abstract The data for a Holocene higher sea level(s) in eastern Australia is summarized, and the evidence brought forward against this interpretation is reviewed. Differences are resolved if seaward flexure (Phipps) is accepted as explaining the lack of such emerged features in the central part of the east coast, and the emerged features of Queensland (Hopley) and Victoria (Gill) are accepted.

Sea Level Change on the Great Barrier Reef: An Introduction

Evidence for Holocene shorelines from the Queensland coast, off which the Great Barrier Reef lies, has epitomized the problems of eustatic fluctuations over the last 6000 years. While some areas of southern and central Queensland show evidence of no sea level higher than the present over this period, other areas, particularly within 150 km of Townsville on the mid-North coast, have provided radiometrically dated evidence for an emergence of up to 4.9 m. The area in which the 1973 Expedition worked has been described previously by several authors, and evidence for higher shorelines in the form of cemented platforms, raised reefs and related features suggesting higher sea levels, though without isotopic dating, has been noted. Research was aimed at confirming and accurately measuring and dating such evidence and relating it to the pattern described elsewhere. Any divergences must then be explained in terms of spatially and temporally varying oceanographic or geomorphic conditions and Earth movements of tectonic and/or isostatic origin.

Anthropogenic influences on Australia's Great Barrier Reef

SUMMARY Although legislation has been introduced to allow stricter control of activities within the Great Barrier Reef region, paradoxically there are increasing pressures due to greater use of the Reef, and population expansion and economic development on the adjacent mainland. In addition, greater understanding of the functioning of the reef system has allowed for recognition of more subtle anthropogenic effects on the Reef. On the outer Reef some localised degradation may result from intense use, accidents or from pre‐legislation activities. Runoff from the mainland has the potential to introduce increased turbidity levels, reduced salinities and some chemical pollutants particularly on nearshore reefs and in the Cairns region. Through global scale atmospheric and oceanic circulation the Great Barrier Reef is open to more distant sources of perturbation.