Multi-colony coral skeletal Ba/Ca from Singapore’s turbid urban reefs: Relationship with contemporaneous in-situ seawater parameters

The ratio of barium to calcium in coral skeletons (Ba/Cacoral) is broadly used as a proxy for tracking terrestrial/river runoff. There are, however, inconsistencies in Ba/Cacoral records that have prompted caution in its reliability as an environmental proxy. Direct comparisons between in-situ seawater measurements and coral Ba/Ca are therefore needed to ensure accurate proxy calibration and interpretation. The current study represents the first to test Ba/Cacoral against years-long monthly-resolution contemporaneous measurements of several in-situ seawater parameters i.e. dissolved seawater barium (BaSW), temperature, salinity, suspended sediments, sedimentation rate and photosynthetically active radiation. We analysed the Ba/Cacoral of six Porites lutea corals sampled from two turbid reefs in Singapore, and explored relationships with in-situ seawater parameters over the period 2008–2015. Our study found poor agreement in Ba/Cacoral from replicate corals sampled from the same reef, and only one of the six colonies analysed showed significant but weak Ba/Cacoral–BaSW and Ba/Cacoral–Ba/CaSW relationships. There was also no clear relationship between Ba/Cacoral and skeletal luminescence G/B (a coral proxy strongly linked with salinity and river runoff). This implies that (1) the incorporation of terrestrially derived humic-like substances into the coral is independent of Ba, and (2) it is likely factors other than freshwater discharge/flood events are driving the poor reproducibility of Ba/Cacoral and disconnect with BaSW at our study sites. We found a positive relationship between Ba/Cacoral and organic suspended solids for 4 of the 6 colonies analysed that suggests biological mechanisms such as feeding could be driving skeletal Ba incorporation. A negative relationship between Ba/Cacoral and total sedimentation rate was also found, suggesting that within reef sediment fluxes may be influencing spatio-temporal variability of the Ba-supply. Ba incorporation into coral skeleton, especially in dynamic, turbid settings such as those found in Singapore, is likely more complex than previously thought. Our results highlight the possibility of high heterogeneity in coral responses to environmental conditions, and the need for careful selection of colonies and a site-specific, replicated approach when attempting to apply Ba/Cacoral as a trace element proxy.