River runoff reconstructions from novel spectral luminescence scanning of massive coral skeletons

Abstract

Inshore massive corals often display bright luminescent lines that have been linked to river flood plumes into coastal catchments and hence have the potential to provide a long-term record of hinterland precipitation. Coral luminescence is thought to result from the incorporation of soil-derived humic acids transported to the reef during major flood events. Corals far from terrestrial sources generally only exhibit dull relatively broad luminescence bands, which are attributed to seasonal changes in coral density. We therefore tested the hypothesis that spectral ratios rather than conventional luminescence intensity provide a quantitative proxy record of river runoff without the confounding effects of seasonal density changes. For this purpose, we have developed a new, rapid spectral luminescence scanning (SLS) technique that splits emission intensities into red, green and blue domains (RGB) for entire cores with an unprecedented linear resolution of 71.4 lm. Since humic acids have longer emission wavelength than the coral aragonite, normalisation of spectral emissions should yield a sensitive optical humic acid/aragonite ratio for humic acid runoff, i.e., G/B ratio. Indeed, G/B ratios rather than intensities are well correlated with Ba/Ca, a geochemical coral proxy for sediment runoff, and with rainfall data, as exemplified for coral records from Madagascar. Coral cores also display recent declining trends in luminescence intensity, which are also reported in corals elsewhere. Such trends appear to be associated with a modern decline in skeletal densities. By contrast, G/B spectral ratios not only mark the impact of individual cyclones but also imply that humic acid runoff 123Coral Reefs (2010) 29:579-591 DOI 10.1007/s00338-010-0629-y increased in Madagascar over the past few decades while coral skeletal densities decreased. Consequently, the SLS technique deconvolves the long-term interplay between humic acid incorporation and coral density that have confounded earlier attempts to use luminescence intensities as a proxy for river runoff.

Spatial linkages between coral proxies of terrestrial runoff across a large embayment in Madagascar

“…Annual density bands were visualised by X-radiograph-positive prints, and the growth axis of the coral slab was defined as the line perpendicular to these bands. The average growth rate of all four coral cores was approximately 13 ± 2 mm y −1 (Grove et al, 2010 and Table 1). With a diamond coated drill, subsamples were taken every 1 mm parallel to the growth axis, equivalent to approximately monthly resolution.…”

“…Subsequent reports indicated that luminescence may also result from changing skeletal densities Taylor, 2001, 2005). More recently, spectral luminescence scanning (SLS) has shown that both processes contribute to luminescence and that humic acids can be normalised for the effects of changing skeletal density to provide an indicator of humic acid runoff (Grove et al, 2010). As the luminescent emission signal of HA is slightly longer than aragonite, taking the green / blue (G / B) ratio gives an estimate of the amount of HA relative to the skeletal density (Grove et al, 2010).…”

“…More recently, spectral luminescence scanning (SLS) has shown that both processes contribute to luminescence and that humic acids can be normalised for the effects of changing skeletal density to provide an indicator of humic acid runoff (Grove et al, 2010). As the luminescent emission signal of HA is slightly longer than aragonite, taking the green / blue (G / B) ratio gives an estimate of the amount of HA relative to the skeletal density (Grove et al, 2010). SLS resolves density effects associated with luminescence intensities, such as declining trends in intensity with coral age (Lough, 2011a, b;Jones et al, 2009).…”

“…SLS was performed, as described by Grove et al (2010), on bleached coral slabs using a line-scan camera fitted with a Dichroic beam splitter prism, separating light emission intensities into three spectral ranges; blue (B), green (G) and red (R). The four coral cores used for this study were also analysed in the study of Grove et al (2010) to test the performance of the SLS technique.…”

“…The four coral cores used for this study were also analysed in the study of Grove et al (2010) to test the performance of the SLS technique. Digital core images were analysed with the Line Scan Software Version 1.6 (Avaatech), providing RGB intensity values for individual pixels of 71 µm in length.…”