Urbanized reef environments pose challenges for coral-based paleoclimate reconstructions. Here, we test the hypothesis that Psammocora digitata preserves reproducible δ¹⁸O and δ¹³C signals comparable to those of conventional coral archives (e.g., massive Porites) under urban conditions. We present monthly-resolved δ¹⁸O and δ¹³C records spanning 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 identical δ¹⁸O and δ¹³C means, variances, and seasonal amplitudes, consistent with their 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, reflecting species-specific baselines. Seasonal δ¹⁸O variability is primarily driven by salinity (~60%), with temperature exerting a smaller influence (~40%). SG1 and SG2 show δ¹⁸O amplitudes of ~1.8‰, whereas Porites coral 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 (CDOM), ambient δ¹³C of dissolved inorganic carbon (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, including freshwater input or seasonal hydrological variability, rather than colony-specific metabolic effects. Taken together, these results show that Psammocora can provide robust, monthly-resolution isotopic records suitable for reconstructing coastal hydroclimate and biogeochemical variability.
 

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