Growth characteristics of massive corals from the Great Barrier Reef

Brief description

Annual density banding in massive coral skeletons provides a means to date and retrospectively analyse coral growth and geochemical composition. Annual density bands, consisting of high and low density skeleton are revealed when a slice from a coral is X-rayed. Analysis of these annual density bands enables the calculation of the following annual growth characteristics: average skeletal density, linear extension rate and calcification rate. The living tissue layer occupies a discrete layer at the outer edge of coral colonies. The thickness of the living tissue layer is an indication of the "health" of the coral colony and has been shown to decrease in thickness with environmental stress. This dataset contains skeletal density, extension rate, calcification rate and tissue thickness measurements for long and short massive Porites coral cores as well as whole Porites colonies from the Great Barrier Reef. The purpose of this research is to investigate variation in growth characteristics for massive corals along environmental gradients. [Temporal, spatial scales] Sample Processing: 1. Coral Cores Coral cores are mounted on metal trays and three slices ~7mm thick are removed using a solvent cleaned diamond tipped blade on an electric saw. Each slice is X-rayed at a commercial radiographers and a positive print made of the X-ray. An assessment is made to choose the best slice for further analysis. This is based on the clarity of the annual density bands visible in the X-ray positive, as well as the overall quality of the core. An optimum track for density analyses, chosen to cross the clearest presentation of annual density bands along the length of the core, is marked on the X-ray positive. Density is measured along each marked track with a gamma densitometer with a 4mm diameter gamma-ray beam. The gamma count, measured at 0.0254cm intervals, combined with the known thickness of the coral core slice provided absolute skeletal density (in g.cm3) at that point on the core slice. A data series of absolute density vs. distance is produced for each coral core slice. On the Great Barrier Reef, skeleton of highest density forms during the summer season of highest water temperature and the low-density band forms in the winter season of coolest water temperature. Successive density peaks are then dated backwards from the date of collection. This then allows for the calculation of annual extension rate (mm/y) and annual calcification rate (gCaCO3/cm2/y). 2. Whole Porites Colonies. Colonies are cut vertically into halves with a chainsaw fitted with a masonry blade. One half of the colony is placed on the moveable bed of a circular saw and at least two 6-7cm slices are removed from vertical planes close to the centre of the whole colony. Further processing is similar to that of core slices, except that two tracks are chosen for density analysis. Both tracks start at the origin of the colony, with one track close to the vertical growth axis and the other close to the horizontal axis. All coral slices, remaining parts of each core or colony and X-rays are archived at AIMS. This database has been superseded by newer methods of analysis, described in De'ath AG, Lough JM and Fabricius KE (2009) Declining coral calcification on the Great Barrier Reef. Science. 323: 116-119, and found on record "Coral Calcification In Massive Porites Of The Great Barrier Reef, Over A 400 Year Period" linked below.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Statement: A subjective dating Confidence Index (see Lough JM and Barnes DJ (1992) Comparisons of skeletal density variations in Porites from the central Great Barrier Reef. Journal of Experimental Marine Biology and Ecology 155: 1-25) is applied to each analysed coral core slice. A rating of 1 indicates confidence that all annual density bands have been correctly identified and dated. A rating of 2 indicates reasonable confidence that the majority of bands have been correctly identified and dated but there may be problems in one or two years. A rating 3 indicates low confidence in dating due to difficulties with identifying several to many annual growth bands.

Notes

Credit
Barnes, David J, Dr (Principal Investigator)

Credit
Lough, Janice M, Dr (Custodian)