On the nature and causes of luminescent lines and bands in coral skeletons: II. Contribution of skeletal crystals

Slices cut from skeletons of massive Porites display two types of luminescence when illuminated by ultra-violet (UV) light: (1) faint luminescent banding associated with the annual skeletal density banding pattern and (2) narrow lines of strong luminescence associated with monsoonal runoff of fresh water from nearby land. Barnes and Taylor [Barnes, D.J. Taylor, R.B. 2001a. On the nature and causes of luminescent lines and bands in coral skeletons. Coral Reefs 19, 221–230] showed how larger skeletal holes could give rise to increased luminescence—thus accounting for the link between skeletal density banding and faint luminescent banding. Work described here tests the notion that strongly luminescent lines are also regions of lower skeletal density. Experiments involving real and artificial coral skeletons indicated that likely changes in hole size in real skeletons cannot account for the amount of luminescence associated with luminescent lines. Larger particles (< 50 μm) of powdered skeleton from cut from luminescent lines were more luminescent than similar particles cut from adjacent less luminescent skeleton. However, very small particles (< 3 μm) from the two regions of skeleton showed no difference in luminescence. Since skeletal crystals would have been largely destroyed by powdering skeleton to very small particle sizes, most of the luminescence of strongly luminescent lines is probably associated with changed crystal size and packing, with changed crystal chemistry, or with a combination of these possibilities.