NOTES Fiber-Optic-Based Sensing of Banded Luminescence in Corals PETER J. MILNE* and PETER K. SWART Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149 Index Headings: Luminescence; Fiber-optic probe; Laser-induced fluo- rescence; Coral. INTRODUCTION Ultraviolet irradiation of a sectional slice of the ara- gonitic skeleton cut along the growth axis of coral colonies reveals a series of alternating luminescent bands, the in- tensity of which appears to be related to environmental conditions at the former growth surface of the colony. Studies of the patterns of annual density banding in coral skeletons, as revealed by X-ray radiography, l.: have been used 3,4 in much the same way that growth rings in trees are used to provide indications of past environmental conditions. A second class of studies on coral skeletons attempts to quantify chemical and isotopic markers which are included within the calcium carbonate matrix to sim- ilar known climatic or environmental variables. The lu- minescence signal is generally believed to arise from the skeletal inclusion of varying amounts of naturally fluo- rescent dissolved organic matter (DOM) chromophores present in seawater. In particular, it has been argued 5-~ that the fluorescent banding observed in corals found growing adjacent to coastal regions impacted by periodic discharge of terrestrially derived fluorescent humic and fulvic acids may thus be used as a proxy for past river discharge and, hence, past rainfall. It may be noted, how- ever, that other components of calcium carbonate ma- trices, e.g., impurity Mn 2+,9,1° may contribute to some of the total observed luminescence signal. Since massive coral skeletons may be up to hundreds of years old, considerable interest arises with respect to what information they may record of past climatic con- ditions and, hence, rates of environmental change. As part of a larger study to characterize the controls of iso- Received 30 March 1994; accepted 7 July 1994. * Author to whom correspondence should be sent. topic composition of scleractinian corals found in the Caribbean, we sought a convenient way in which to: 1. Reproducibly quantify the luminescent banding in- tensity on coral sections. This capability is needed to compare signal strengths from different speci- mens. 2. Reproducibly position the measurement site to within a few millimeters on defined linear axes. This capability is needed in order to correlate the lumi- nescent banding with other microanalytical mea- surements such as X-ray density banding and 13C/ 12C and 180/160 isotopic marking. 3. Be able to monitor the luminescent emission in a spectrally selective manner. While the luminescent signal most probably arises from DOM fluores- cence, selective emission spectra may provide a way of discerning the presence of other fluorophores em- bedded within the coral matrix. EXPERIMENTAL Figure 1 shows a schematic diagram of the experimen- tal setup used in the current investigation. A low-pow- ered, sealed-plasma-cartridge, pulsed nitrogen laser (he, 337 nm; Laser Science Inc., Newton, MA) was used as an excitation source. This UV source was chosen because of the proximity of its emission line with the known ab- sorption profile of DOM fluorophores. The laser pulses (10 Hz) were coupled into a fiber optic by means of a fused-silica lens (f= 25 ram) mounted on a fiber coupler (Newport, Irvine, CA). A fused-silica, SMA-terminated, fiber-optic bundle (C-Technologies, Short Hills, N J) con- sisted of a central excitation fiber (600 #m, multimode UV silica) surrounded by six collection fibers at the sens- ing end. The collection fibers, arranged linearly rather than circularly at the receiving end, were matched to the entrance slit of a small in-line monochromator of the Fastie-Ebert design (f/3.9, 74-mm focal length, Mini- Chrom DMC 1, Optometrics USA Inc., Stony Brook, NY) with a holographic grating (200-800 nm). A UV-en- hanced silicon photocell with an integral operational am- plifier circuit (SDM-1, Optometrics) was used to detect luminescent emission light carried back through the fiber- optic bundle. A quartz window was mounted in the fiber- optic positioner to allow monitoring of the intensity of individual laser pulses. By monitoring the surface reflec- tion from this element with a second silicon photocell, we were able to acquire an intensity normalization signal. The sensing end of the fiber-optic bundle was held rig- 1282 Volume 48, Number 10, 1994 0003-7028/94/4810-128252.00/0 APPLIED SPECTROSCOPY © 1994 Society for Applied Spectroscopy