Evaluating coral proxy fidelity at monthly resolution in urban reefs: A 20-year δ18O and δ13C comparison of Psammocora and Porites

Urbanized and turbid reef environments pose challenges for coral-based paleoclimate reconstructions, highlighting the need to broaden the range of target taxa beyond conventional massive Porites and to develop additional archives suitable for climate proxy applications. In this study, we present monthly-resolved δ¹⁸O and δ¹³C records covering the past 20 years, from two Psammocora digitata corals (SG1, SG2) and one Porites core (SG3) collected from a turbid nearshore reef in Singapore. All three records show clear seasonal cycles, with the Psammocora exhibiting higher intra-coral reproducibility. SG1 and SG2 share nearly identical δ¹⁸O and δ¹³C means, variances, and seasonal amplitudes, consistent with similar hydrographic conditions at ~3 m depth. The shallower Porites core (SG3, ~2 m) is offset toward lower δ¹⁸O and δ¹³C values and displays reduced δ¹⁸O seasonal amplitude, perhaps reflecting species-specific baselines. Seasonal δ¹⁸O variability is primarily driven by salinity changes (~60%), with temperature exerting a smaller influence (~40%). SG1 and SG2 show δ¹⁸O amplitudes of ~1.8‰ and closely matched interannual patterns, while SG3 records a smaller amplitude (~1.2‰) consistent with its lower variance. δ¹³C exhibits larger overall variability (standard deviations 0.65–0.71‰; seasonal amplitudes ~3.0–3.9‰), responding to changes in underwater light, turbidity, colored dissolved organic matter, ambient δ¹³C_DIC, and minor colony-level physiological effects. In each coral, δ¹⁸O and δ¹³C show significant positive correlations (r = 0.59–0.68), reflecting shared environmental drivers—such as freshwater input or seasonal hydrological variability—rather than colony-specific metabolic effects. Taken together, these results show that Psammocora can provide robust, high-resolution isotopic records suitable for reconstructing hydroclimate and biogeochemical variability in sediment-rich coastal environments.