The coral reef in Tutum Bay, Ambitle Island, Papua New Guinea, is exposed to the vigorous discharge of hydrothermal fluids (up to 98°C). This study investigates eight Porites lobata samples that were collected throughout the area of active venting at varying distances from vent sites. A sample from a “non-hydrothermal” coral (C-29), collected 10 km north of Tutum Bay, was analyzed for comparative purposes.

Density banding is moderately well developed in these corals and subannual bands are common. For corals from Tutum Bay, δ13C ranges from −4.5 to −1.0‰ and δ18O from −6.0 to −3.8‰, which are relatively depleted values for shallow water Porites. The comparison sample, C-29, has δ13C values ranging from −1.8 to −0.5‰, and δ18O values of −5.4 to −4.6‰. Concentrations of As, Co, Cr, Ga, Ge, Mo, Nb, Ni, Pb, Rb, Se, W, Y and Zr were always below their respective proton probe detection limits. Ba, Br, Cu, Fe, Mn, and Zn were detected in some samples. Sr was detected in all samples and concentrations ranged from 6970 to 8240 ppm. Strontium isotope ratios of selected samples (C-5, C-8 and C-29) are very close to seawater, but Tutum Bay corals (C-5 and C-8) have lower 87Sr/86Sr values than the “non-hydrothermal” sample (C-29).

The observed isotope patterns indicate that the hydrothermal input into Tutum Bay influences the physico-chemical conditions in the surrounding coral reef. Tutum Bay Porites lobata show distinctly different δ13C and 87Sr/86Sr records when compared to the “non-hydrothermal” sample and other corals from Papua New Guinea and elsewhere. Direct and indirect synergistic effects, including temperature, isotopic and chemical composition of vent fluids and the influence of pCO2 on the expression of photosynthetic and kinetic stable isotope fractionation factors, are the most likely explanation for the “hydrothermal” signal seen in these corals. The δ18O values are significantly correlated with the estimated amounts of hydrothermal exposure.

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