Sub-annual fluorescence measurements of coral skeleton: relationship between skeletal luminescence and terrestrial humic-like substances

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.

Dissolved organic matter from tropical peatlands reduces shelf sea light availability in the Singapore Strait, Southeast Asia

“…Future research needs to corroborate our estimate by reconstructing past variation in terrigenous CDOM in this re gion, e.g. using luminescence data from coral skele ton cores as a terrigenous CDOM palaeo-proxy (Kaushal et al 2020(Kaushal et al , 2021. Nevertheless, our study already indicates that the disturbance of tropical peatlands has likely resulted in CDOM-mediated coastal browning in Southeast Asia, and that peatland disturbance therefore entails an additional environmental impact beyond the large increases in CO 2 emissions from peat and peatland DOC oxidation (Hooijer et al 2010, Murdiyarso et al 2010, Wit et al 2018.…”

Influence of Freshwater Discharges on Biogeochemistry and Benthic Communities of a Coral Reef Ecosystem (La Réunion Island, Indian Ocean)

“…Consequently, we may probably consider the humic-like material as a good tracer of sand groundwater (SL), and more generally of stagnant and/or superficial freshwaters, such as HR and 3BR. Interestingly, Kaushal et al (2020) have shown that this (terrestrial) humic-like material was readily incorporated in coral skeletons, suggesting that coral cores may have the potential to reconstruct past variations in freshwater fluxes.…”

Dissolved organic matter from tropical peatlands impacts shelf sea light availability on coral reefs in the Singapore Strait, Southeast Asia

“…Our estimate of the anthropogenic contribution to the observed CDOM-mediated light attenuation relies on earlier reports that the peatland tDOC flux has increased by slightly over 50% as a result of land conversion (Moore et al 2013, Yupi et al 2016). Future research should therefore aim to corroborate our estimate by reconstructing past variation in terrigenous CDOM in this region, which may become possible through measurements of humic acid concentrations in coral skeleton cores (Kaushal et al 2020). Nevertheless, our study already indicates that the disturbance of tropical peatlands has likely resulted in CDOM-mediated coastal browning in Southeast Asia, and that peatland disturbance therefore entails an additional environmental impact beyond the large increases in CO2 emissions from peat and peatland DOC oxidation (Hooijer et al 2010, Murdiyarso et al 2010, Wit et al 2018.…”