Investigation of luminescent banding in solid coral: the contribution of phosphorescence

This study investigates the nature and components of annual luminescent ban ding in massive Porites coral skeletons, with a view to refining the techni que for using this banding to reconstruct past environmental conditions. Th ree-dimensional excitation-emission-matrix spectroscopy and optical fibre b eam delivery have been used to investigate the luminescence properties of t he bright and dull bands of solid coral. Six characteristic excitation/emis sion peaks have been identified: 280/450-600, 340/450, 370/470, 390/485, 42 0/505 and 450/530 nm. The first peak corresponds to protein-type fluorescen ce. The others are characteristic of humic acid luminescence. The differenc e in luminescence intensity between bright and dull bands has been quantifi ed and characterised spectroscopically. The luminescence of the bright band s is up to 25% more intense than their neighbouring dull bands with the gre atest increase in relative intensity in the long wavelength emission region , between 500 and 600 nm. The contribution of long-lived phosphorescence to the total luminescence intensity has been determined by time-resolved meas urements on the 100 ms timescale, Both bright and dull bands show long-live d phosphorescence with decay times up to 1.5 s, This phosphorescence accoun ts for about 10% of the total luminescence intensity of bright bands. The d ifference in phosphorescence intensity between bright and dull bands is sub stantially greater than the difference in total luminescence intensity: the phosphorescence of bright bands is up to twice as intense as that of dull bands. This suggests that phosphorescence plays an important role in defini ng luminescent banding ill coral. Furthermore, the large observable differe nce in phosphorescence between bright and dull bands indicates that measure ment of phosphorescence profiles across growth bands in corals may prove to be a moro sensitive indicator of past environmental conditions than measur ements of total luminescence.