CRC REEF RESEARCH CENTRE TECHNICAL REPORT No. 22

On The Nature Of Luminescence In Coral Skeletons.

D.J. Barnes, Australian Institute of Marine Science
R.B. Taylor, Australian Institute of Marine Science

FOREWORD

Coral researchers have long known that there is a wealth of information on past climatic conditions recorded within the skeletons of massive reef corals. Skeletal density, chemical characteristics, organic inclusions, isotopic composition and optical properties have all been identified as potential recorders of past events and conditions. The process of discovering the "language" in which the history of these coral colonies is recorded has been much more complicated that many scientists anticipated. From a management point of view, however the rewards are substantial. Long term records of climate variations and possible anthropogenic perturbations provide an essential background against which present day measurements and short-term trends can be compared. Such comparisons enable us to determine if an event is unprecedented in its severity or frequency when compared to a historical record which predates European influence on the system. In the absence of such records, managers must make informed guesses and run the risk of under or over estimated the significant of a perturbation event.

Fluorescent bands in near-shore massive corals are know to be well correlated with river run-off events, and until recently, the cause of these bands were thought to be directly attributable to the incorporation of humic acids associated with flood waters. In this carefully conducted series of experiments, Drs Barnes and Taylor demonstrate that this explanation is not correct, and that luminescence in skeletons is a function of variation in the skeletal architecture. This discovery both explains anomalous results of other researchers, and opens new opportunities for the application of luminescent banding work in non-coastal areas.

The clever scientific detective work described in this report is somewhat technical in nature, but it is vital to the development of effective tools for understanding how reefs respond to changes in the environment. It is commendable that the Cooperative Research Centre for the Ecologically Sustainable Development of the Great Barrier Reef has sponsored this research.

Jamie Oliver
Director, Information Support
Great Barrier Reef Marine Park Authority

EXECUTIVE SUMMARY

Work reported here was carried out as part of a CRC Reef project to use luminescent (fluorescent) bands in coral skeletons to provide information about the frequency, extent and magnitude of land influences on the Great Barrier Reef (GBR). We wanted to determine criteria for designing equipments to excite and record variations in luminescence in skeletal slices removed from Porites colonies collected at locations along the length and across the width of the GBR. Early results of this work did not accord with some of the generally accepted notions about coral skeletal luminescence. Accordingly, we investigated the nature and causes of this luminescence.

It was found that indentations in the surface of laboratory grade calcium carbonate powder could preproduce all features of coral luminescence. The yellow luminescence seen in slices of coral skeletons, and the blue luminescence measured in such slices, are properties of mineral calcium carbonate. In corals, enhanced luminescence is associated with regions with larger numbers of holes and indentations. The luminescent lines associated with monsoonal river flows in corals from the Great Barrier Reef are narrow regions of lower density skeleton ie, regions with greater amounts of holes and indentations. These narrow, low-density regions presumably result because significantly lower salinities reduce coral calcification without concomitant reduction in skeletal extension. Offshore corals, not subject to regular, periodically lowered saliniteis, show luminescent banding in which higher luminescence is associated with the lower density portion of the annual skeletal density banding pattern.

Long wavelength ultraviolet (UV) light from fluorescent tubes used to display coral fluorescent banding contains significant amounts of violet and blue light. Luminescence is excited in coral skeletons by UV, violet, blue and even green light. Light returning from indentations and holes in coral skeletons will have been subject to a greater number of reflections than light returning from the surface. Each bounce from a surface increases the probability of absorption of the light (UV, violet and blue) and its subsequent re-emission at longer wavelengths. Light returned from surface features of skeletal slices has been subject to far fewer reflections and contains relatively more short wavelengths and relatively less long wavelengths. Thus light returned from surfaces appears blue while light returned from holes appears yellow. Luminescent bands in coral skeletal slices are regions where less skeleton is exposed at the surface and there are more holes (which appear more yellow), relative to regions to either side (which appear more blue).

Luminescence in coral skeletal slices is, essentially, a measure of the density of nearsurface layer of a skeletal slice. Radiographic measurements (X-ray, gamma densitometry) of density in very thin slices tend to be noisy because, in very thin slices, information associated with skeletal architecture dominates over density information. Luminescence and reflectance are recorded when attempts are made to measure luminescence of skeletal slices. Allowance can be made for reflectance by repeating measurements at wavelengths at which the contribution of luminescence is very small. Thus, variations in luminescence can be a useful proxy for variations in near-surface density in coral skeletal slices because allowance can be made for architectural effects.

THIS PUBLICATION IS CITED AS:
Barnes, D.J. & Taylor, R.B. (1998)
On the nature of luminescence in coral skeletons.
Technical Report No. 22
Townsville; CRC Reef Research Centre Ltd, 38 pp.

ISBN 1 876054 81 6

A full copy of this report may be obtained from the author(s), and through the following libraries: