Piers Larcombe

Factors controlling suspended sediment on inner-shelf coral reefs, Townsville, Australia

This study was undertaken to describe the characteristics of suspended sediment concentrations (SSCs) of marine waters near inner-shelf fringing coral reefs and relate these to the prevailing oceanographic and meteorological conditions. Using logging optical backscatter nephelometers, SSCs were measured at fringing reefs at Magnetic Island and on the adjacent inner-shelf, Cleveland Bay, N.E. Australia. Continuous measure-ments were made over a period of 4 months, representing possibly the most comprehensive set of SSC data collected near coral reefs. Wind, current and wave data were also collected. Temporal and spatial variation in near-bed SSCs is high. Periods of strong southeasterly regional winds generate swells, which, within 1 km of the reefs, produce near-bed SSCs of well over 200 mg/l. At the fringing coral reefs at Arthur and Geoffrey Bays, SSCs were less than 5 mg/l for most of the time and rarely exceeded 40 mg/l, but there were a number of periods of over 24 h when near-bed SSCs continuously exceeded 20 mg/l. The height of locally produced, short-period wind-waves is the dominant control on the magnitude of near-bed SSCs at the reef sites, and thus the wind regime heavily influences conditions for coral communities. The magnitude of the tide is of lesser importance. However, it is likely that flushing of these bays by tidal currents is important in preventing a long-term build-up of SSC in the water around the coral reefs.

New evidence for episodic post-glacial sea-level rise, central Great Barrier Reef, Australia

Abstract We present an extensive database of 364 radiocarbon dates from coastal and marine sediments of the central Great Barrier Reef (GBR) shelf, of which 110 are previously unpublished. The elevation data have been reduced to a common datum (Australian Height Datum, AHD) and the various sources of error have been assessed. Using modern lithological and biological relationships with sea level, the elements of the radiocarbon database have been converted into sea-level indicators. The upper bound of the assembled dataset corresponds to a best-estimate sea-level curve, and the full dataset provides a narrow envelope for sea-level rise on the GBR shelf for the last 11–12 kyr (not including hydro-isostatic crustal flexing). The envelope is consistent with episodic rise in post-glacial sea levels. The rising post-glacial sea level probably included Stillstands (or minor falls), at ca. −45 m AHD (at ca. 10.5 kyr B.P.), −5 m (7.8 kyr B.P.), −2 m (ca. 6 kyr B.P.) and +1.7 m (5.5 kyr B.P.). There is evidence for a significant fall in sea level between stillstands at −11 m (8.5 kyr B.P.) and −17 m (8.2 kyr B.P.). Stillstand durations apparently ranged between The vertical spread in the derived sea-level data is very wide. The use of shell material for dating seems unreliable and prone to large and unpredictable errors. Data from bulk mangrove muds appear reliable for determination of ancient sea level, but may at times result in sea level being placed up to 4 m below the true level. In-situ biogenic carbonates such as preserved oyster beds and coral micro-atolls are the most reliable indicators of sea-level position, while deposits of mangrove mud give a useful first-order approximation of ancient sea levels. Caution should be used in drawing ‘sea-level curves’ from few data points. We conclude that the post-11–12 kyr B.P. relative rise in sea level was episodic on the central GBR continental margin. More data are required to define clearly sea-level change up to ca. −20 m at 9 kyr B.P.

Increased sediment supply to the Great Barrier Reef will not increase sediment accumulation at most coral reefs

Abstract The rate of terrigenous sediment supply to the central Great Barrier Reef (GBR) coastline has probably increased in the last 200 years due to human impact on the catchments of central Queensland. This has led some researchers and environmental managers to conclude that corals within the GBR are under threat from increased turbidity and sedimentation. Using geological data and information on sedimentary processes, we show that turbidity levels and sediment accumulation rates at most coral reefs will not be increased, because these factors are not currently limited by sediment supply.

Quantifying the response of optical backscatter devices and transmissometers to variations in suspended particulate matter

Abstract Optical instruments have been used effectively in studies of sediment dynamics for several decades. Without accurate instrument calibrations, calculated concentrations of suspended particulate matter (SPM) may be unreliable, with implications for interpretations of sedimentary processes and sediment fluxes. This review aims to quantify the effect of variations in SPM characteristics on the response of optical instruments (optical backscatter sensors OBS and transmissometers) and to note the implications for users of these instruments. A number of factors have a significant impact on instrument response, for example; a change in grain size from medium sands to fine silts may lead to a×100 increase in instrument response; flocculation of fine particles may decrease instrument response by×2; and the presence of plankton in suspension may lead to poor instrument calibrations of SPM concentration. Calibrations carried out in environments either with multi-modal bottom sediments, where flocculation of fine-grained sediments is likely, or where the hydrodynamics or grain type are highly variable must also include a determination of the changing nature of the suspended load in space and time. A more complete understanding of instrument response to SPM and of calibration requirements may enable optical devices to be used to a greater potential as long-term measures of SPM concentration, and also enable improvements in calculations of net sediment fluxes.

Marginal and non-reef-building coral environments

This special issue of Coral Reefs stems from a thematic session held at the European Meeting of the International Society for Reef Studies held in Cambridge in September 2002. A wide range of papers were presented, covering aspects of oceanography, sediment transport and accumulation, sedimentary settings, species assemblages, coral physiology, and geological evolution. These presentations emphasized the diversity and potential significance of the coral communities that occur in a wide range of what might be described as ‘‘marginal’’ settings, and demonstrated the geological and ecological significance of marginal and non-reefbuilding coral environments. Most papers published in this issue were submitted from researchers who presented work at the Cambridge meeting; however, some additional contributions were also accepted. When we were organizing the thematic session on ‘‘marginal’’ and non-reef-building coral environments at Cambridge, and then in editing this Special Issue, we wondered whether the title of this Special Issue, and specifically the word ‘‘marginal,’’ could be a little misleading (we expand a little on this thought in sections below). In this introductory paper, the term ‘‘marginal’’ marine is used in a broad sense, to describe settings where coral communities or framework reefs occur either close to well-understood (or strongly perceived) environmental thresholds for coral survival (sensu Kleypas et al. 1999) or in areas characterized by ‘‘suboptimal’’ or fluctuating environmental conditions. These include settings characterized by high or low temperatures, salinities, or nutrient levels, or by low light penetration or low aragonite saturation states. We are aware that much of this issue generally considers only one component of an ecological biota, so the use of the term ‘‘coral communities’’ here and in many of the papers in this issue might equally be read as ‘‘coral biotopes,’’ ‘‘coral assemblages,’’ or ‘‘ecological assemblages of corals.’’ Further, we use ‘‘framework reefs’’ here to explicitly denote those coral assemblages that have produced accretion through the production of reef framework, and we do not intend this as a contribution to any debate on what does or does not constitute a reef. The settings under consideration in this issue include settings where external factors act to change parameters such as temperature, salinity, nutrient load, and suspended sediment concentrations over a variety of magnitudes and timescales (e.g. ‘‘permanently,’’ seasonally, over lunar and other cycles, and episodically). Such settings may be considered by some as ‘‘marginal’’ in terms of reef-building potential, for example, the production of reef framework, but they are clearly important for maintaining a rich diversity of coral community and reef types, and as localized sites of coral and carbonate sediment production and, in some cases, accumulation.

Lowstand rivers need not incise the shelf: An example from the Great Barrier Reef, Australia, with implications for sequence stratigraphic models

A key tenet of many sequence stratigraphic models is that sea-level lowering causes widespread fluvial incision of the subaerially exposed continental shelf, and that river-borne terrigenoclastic sediments bypass the lowstand shoreline via canyons to the continental slope and basin floor. Consequently the occurrence of incised channels is considered a fundamental criterion for the recognition of sequence boundaries in ancient shelf successions. Contrary to this, we argue that rivers may not necessarily incise during glacio-eustatic lowstands if they flow out onto a coastal plain flanked by a broad, low-angle shelf. On the Great Barrier Reef shelf, fluvial incision did not occur during the last glacial maximum (LGM), but instead, subaerial accommodation was created and infilled as contemporary rivers graded to the “LGM-bayline.” Incision was restricted to the lowstand shelf break, where canyons of limited extent formed by nickpoint retreat.

Terrigenous sedimentation and coral reef growth: a conceptual framework

Abstract The role of terrigenous (and non-framework carbonate) sedimentation has not been prominent in models of coral reef growth and evolution. We derive and discuss a semi-quantitative model which relates coral reef growth to sedimentation. The model is independent of coral biology and is based upon the relative net rates of framework and non-framework sediment accumulation and/or removal. The model might enable some forecasting of long-term responses to changes in sedimentation regime and other environmental factors. The occurrence of turbid-zone reefs is successfully explained in regions of very high turbidity but with little or no net sediment accumulation. Potential future use of the model may include aiding prediction of the effects of other geological and oceanographic factors on the growth or demise of coralline communities.

Intertidal Mangrove Foraminifera From The Central Great Barrier Reef Shelf, Australia: Implications for Sea-Level Reconstruction

Contemporary foraminiferal samples and environmental information were collected from three fringing mangrove environments (Sandfly Creek Transect 1 and 2, and Cocoa Creek) in Cleveland Bay, and an estuarine mangrove environment (Saunders Creek) in Halifax Bay, on the central Great Barrier Reef (GBR) coastline, Australia, to elucidate the relationship of the foraminiferal assemblages with the environment. The data support the vertical zonation concept, which suggests that the distribution of foraminifera in the intertidal zone is usually a direct function of elevation, with the duration and frequency of subaerial exposure as the most important factor. An agglutinated foraminiferal assemblage dominated by Miliammina fusca, Trochammina inflata, Ammotium directum and Haplophragmoides sp. exists at the landward edge of the field sites, in a zone between just above Mean Low Water of Neap Tides to Highest Astronomical Tide level (a vertical range of 1.8 m). In addition, a foraminiferal assemblage dominated by Ammonia aoteana is found at all sites, existing between just below Mean Low Water of Neap Tides and Mean High Water of Neap Tides (a vertical range of 0.8 m). These assemblages may be used to reconstruct sea level from fossil cores from the area.

Contemporary foraminiferal distributions of a mangrove environment, Great Barrier Reef coastline, Australia: implications for sea-level reconstructions

Abstract Contemporary foraminiferal samples and associated environmental information were collected from Cocoa Creek, a mesotidal fringing mangrove environment on the Great Barrier Reef (GBR) coastline, Australia, to elucidate the relationship of the foraminiferal assemblages with elevation and environment. There is a strong and highly significant relationship between elevation and the foraminiferal assemblages, supporting the intertidal vertical zonation concept, which suggests that the distribution of foraminifera in the intertidal zone is usually a direct function of elevation, with the most important controlling factors being the duration and frequency of subaerial exposure. Multivariate analyses separate the intertidal foraminiferal assemblages into three elevational zones, with Zone I the highest and Zone III the lowest: Zones I and II are dominated by agglutinated species Trochammina inflata and Miliammina fusca , respectively; and Zone III is dominated by calcareous species, notably Ammonia tepida and Elphidium discoidale multiloculum . These assemblage zones are similar to those found in both tropical and temperate intertidal environments. A predictive transfer function has been developed to allow reconstruction of former sea levels for tropical environments, based upon the relationship between foraminiferal assemblage and elevation. Results suggest that a precision of ±0.07 m should be attainable, superior to most similar studies from temperate, mid latitude environments. Our work has produced the first foraminifera-based transfer function for environmental interpretations for tropical Australia, and allows the potential development of a new generation of high-resolution sea-level reconstructions for the post-glacial sequences of the GBR shelf.

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).