Measured coral luminescence as a freshwater proxy: comparison with visual indices and a potential age artefact

Illuminating slices from massive coral skeletons under ultraviolet light can reveal bright luminescent lines in nearshore corals affected by freshwater river flows. The occurrence and intensity of these lines in long-lived corals can then be used to reconstruct past river flow and rainfall, extending the instrumental records of past tropical climate variability considerably. Earlier studies from the Great Barrier Reef, Australia, have used visual assessments of luminescent line intensity to develop semi-quantitative (though potentially subjective) indices of spatial and temporal variations in freshwater flows. Annual visual assessments and relative coral luminescence intensity (measured by fluorescence spectroscopy) and growth variables are first compared for 89 Porites coral colonies from 30 reefs throughout the length and breadth of the GBR. This demonstrates that simple visual assessments can provide useful information, in the absence of quantitative measurements, of this proxy freshwater tracer. The annual range of measured luminescence between the preceding winter minimum and summer maximum, rather than annual average or annual maximum luminescence is shown to be the most robust measure of freshwater flow. Second, from analyses of the coral colony data and over 40-century-long or longer coral core records, attention is drawn to a potential age artefact in annual average and annual maximum measured coral luminescence. These variables show a significant decline through time, similar to the observed decline in average skeletal density. Although the reasons for this decline are unknown, it could compromise interpretation of long-term variations in freshwater flows and subsequent climatic inferences. This artefact does not appear to affect the annual luminescence range which, it is concluded, is a robust proxy for inter-annual variations in river flow and rainfall.