Abstract
Some massive coral core slices reveal luminescent bands under ultraviolet light, which have been attributed to terrestrial humic acids in the skeleton. Coral luminescence has therefore been used to reconstruct past climate and hydrological variability. However, it has remained unresolved how closely coral luminescence at sub-annual resolution is related to terrestrial humic acid concentrations. This study presents a solution-based fluorescence method to quantify terrestrial humic substances in less than 4 mg of coral powder. The results show that in corals from Malaysia and Singapore, the luminescence green-to-blue ratio is correlated with skeletal concentrations of terrestrial humic substances (R2 > 0.40, p < 0.001) at two sites that are exposed to terrestrial dissolved organic matter from peatlands on Sumatra. In contrast, coral cores from two other sites located far from major terrestrial organic matter sources show lower green-to-blue values and no convincing correlation with fluorescence intensity of terrestrial humic substances in the skeleton. Abiogenic aragonite precipitation experiments with both terrestrial and marine organic matter sources confirmed that terrestrial humic substances are readily incorporated into aragonite, but not fluorescent organic matter from marine sources. The results of this study suggest that in coral cores with high luminescence green-to-blue ratios (> 0.6) and large downcore variability (range of ≥ 0.05), the green-to-blue ratio is strongly linked to variation in terrestrial humic substances. Coral cores therefore have the potential to reconstruct past variation in terrigenous dissolved organic carbon fluxes.
Highlights
Some aragonite skeletons of massive corals such as Porites spp. are important paleo-archives, as they record many environmental properties via changes in the skeletal chemistry during growth (Druffel 1997; Lough 2010)
We found that the correlation for the Kusu and Port Dickson cores was highest with the terrestrial humic-like substances’ (HULIS) fluorescence peak, while for the Redang and Tioman cores, the correlation was highest with the fluorescence at longer ex/em wavelengths shown by Aldrich and Fluka humic acids (Table 2)
The four coral cores analysed in this study all showed maximum humic-like substances (HULIS) fluorescence intensity in the spectral region of ex/em 300–350/
Summary
Some aragonite skeletons of massive corals such as Porites spp. are important paleo-archives, as they record many environmental properties via changes in the skeletal chemistry during growth (Druffel 1997; Lough 2010). Subsequent studies showed that variation in luminescence intensity in coral cores correlates with historical records of river discharge and rainfall for corals growing near the Burdekin River on the Great Barrier Reef (Isdale 1984; Isdale et al 1998, 2016; Lough et al 2002; Lough 2007), and more recently with salinity around the Thai-Malay peninsula (Tanzil et al 2016) This demonstrates that coral luminescence can be used as a proxy for climate and hydrological variables. HULIS fluorescence intensity, often acquired through parallel factor analysis (PARAFAC) of fluorescence excitation emission matrices (Murphy et al 2013), is a well-established measure of tDOC in coastal oceans (Walker et al 2009; Yamashita et al 2011; Osburn et al 2016) This suggests that coral luminescence might act as a proxy for past changes in tDOC concentration in coastal waters
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