Historical timescale records of coral growth and skeletal carbonate deposition under conditions of high turbidity and terrigenous sediment influence

Lead Research Organisation: Manchester Metropolitan University
Department Name: School of Science and the Environment


There is a general perception that corals (and coral reefs) are highly susceptible to riverine inputs of terrigenous sediment, and that high rates of such inputs will negatively impact reef health and vitality (usually evidenced by low coral cover and/or high partial mortality rates). In coral reef settings where such inputs have been limited in the past and where corals are not adapted to deal with frequent sediment loading and reduced light penetration, this perception is likely to have considerable validity and may lead, over time, to shifts in coral community structure. However, there is an increasing body of sedimentological, geomorphological and palaeoecological data demonstrating not only long-term (>1,000 year) persistence of coral communities under conditions of high terrigenous sediment input and high turbidity, but also clear evidence of active and rapid reef-accretion. Under these conditions corals seem to be sufficiently adapted to these environmental conditions that coral cover is often high and well-developed reef structures can form. This has been demonstrated at sites in Thailand, Indonesia, Mozambique and at a range of sites along the nearshore (innermost shelf) areas of the Great Barrier Reef (GBR), Australia. An apparent paradox thus exists between the perceived negative effects of high turbidity and terrigenous sediment inputs on coral communities (which are widely referred to in the scientific literature) and the increasing sedimentary and palaeoecological evidence for historical timescale persistence of corals and of reef-building in these settings. This raises an intriguing question about coral carbonate production in these environments and about the nature of skeletal carbonate deposition. Are these coral communities able to produce reef structures, despite high terrigenoclastic sediment input and high turbidity regimes, because of particularly high coral growth and calcification rates? Little data exists from nearshore reefs of this type and there has been no attempt to quantify and compare these processes over temporal and spatial scales. This project thus aims to quantify coral extension (growth) and calcification rates, and to quantify the microskeletal characteristics (i.e., the size and density of key skeletal elements in the coral skeleton) from two of the dominant coral species associated with reef-building within nearshore, turbid-zone settings along the central GBR coastline. The focus for the research will be the two best-studied turbid-zone reefs in the region; Paluma Shoals and Lugger Shoal. Extensive datasets are available on the sedimentary environments, hydrodynamic conditions and contemporary community structures in each locality. In addition, radiocarbon (14C) date-constrained growth models exist for each site that allow data to be placed in a reliable chronological framework. Specifically, we will gather data on a massive coral species (Porites lobata) that makes a major contribution to contemporary reef-flat coral communities in both settings, and a branching coral species (Acropora pulchra) which previous research has demonstrated to have been a major framework contributor throughout the growth history of these reefs. The research will utilise novel Computerised (Axial) Tomography (CT) scanning and established Scanning Electron Microscopy (SEM) approaches to quantify coral growth rates and styles of coral skeletal deposition in these samples. Between-site comparisons will be made against data collected from the same species of Porites and Acropora that were collected from clear water sites at Low Isles during the 1928-1929 Great Barrier Reef Expedition. This extensive and well-catalogued coral repository is stored at the NHM and CT methodologies will allow us to examine the skeletal structures of these corals using non-destructive techniques.
Description The primary aim of this project was to quantify coral skeletal extension and calcification rates, and microskeletal characteristics (including density, element thickness, crystal size) within two major reef framework contributing coral species from two nearshore, turbid-zone reefs along the central Great Barrier Reef coastline. This data would then be utilized to develop novel historical timescale records of coral growth rates and carbonate deposition. One of the most novel, and technically challenging, aspects of the project was to utilise CT and established Scanning Electron Microscopy approaches to address these issues in the context of branched coral species.

Achievements: Specific achievements of the project have been as follows:

1. The development and refinement of Computerised (Axial) Tomography (CT) methodologies to determine density and growth, and thus calcification rates in branched (Acropora) corals (original Obj. 2),

2. The application of this technology to modern, fossil and museum specimens of branched Acropora (original Objs. 3 and 4), and

3. The development of high resolution descriptors of microskeletal architecture in a common, but previously poorly studied, reef flat massive coral (Goniastrea) (original Obj. 1), and the development of high resolution environmental (isotope and trace element) records from this species.


1. A large part of the time on this project was devoted to dealing with the technical and analytical challenges involved in scanning and analysing CT-generated data from branched coral specimens. Specifically, quantification of density from CT images is complicated by the need to correct for the effects of beam hardening, the process by which a greater portion of X-rays are absorbed at the outer portion of an object, relative to the center. The effect is to produce an image in which the outside of an object appears to have higher density than the inside.

2. To overcome this, a calcium carbonate standard was included with each scan, a beam hardening correction was calculated from this standard, and this was then applied to each coral fragment. This has allowed analysis of changes in coral skeletal micro-density within Acropora branches at the sub-mm level, something not previously possible using conventional techniques. Correction of the beam hardening effect is also necessary for obtaining accurate threshold values to differentiate between coral skeleton and skeletal voids within scans. We also undertook extensive experimentation, using a range of techniques, for calculating these threshold values. These have included manual thresholding, adaptive thresholding, and half-height averaging methods. The range of values of porosity produced utilizing these different methods have been compared to values obtained from conventional x-ray based techniques.

3. The final method developed for porosity calculation from CT scanning has been successfully applied to a range of specimens collected from nearshore GBR reef sites, as well as museum and reef core specimens. Changes in the thickness of architectural elements within Acropora branches have also been quantified and mapped from the CT scan images produced during the project. Both inter and intra-colony variability in the rates of change of porosity and architectural thickness within Acropora branches has been demonstrated, and appear to be connected to the growth rate of individual colonies, although further experimental work is needed to confirm this relationship.

4. We also undertook research to test the potential for recovering high quality palaeoenvironmental records from a common, but not widely considered for geochemical analysis, shallow-water coral Goniastrea aspera. Specifically, we were able to use standard x-ray and CT methodologies to define the skeletal microarchitecture of this coral, and to use this as a framework within which to consider the resolution of C and O isotopic, and trace element concentration data.
Exploitation Route The approaches we have developed have application to the use of Acropora skeletons in palaeoecological and palaeoclimatological research, with the major advances coming through method development and testing.
Sectors Environment

Description To generate 2 jnl publications as well as presentations at two conferences.
First Year Of Impact 2009
Sector Environment
Impact Types Societal