Hydrothermal effects on isotope and trace element records in modern reef corals: A study of Porites lobata from Tutum Bay, Ambitle Island, Papua New Guinea

The coral reef in Tutum Bay, Ambitle Island, Papua New Guinea, is exposed t o the vigorous discharge of hydrothermal fluids (up to 98 degrees C). This study investigates eight Porites lobata samples that were collected through out the area of active venting at varying distances from vent sites. A samp le from a "non-hydrothermal" coral (C-29) collected 10 km north of Tutum Ba y, was analyzed for comparative purposes. Density banding is moderately well developed in these corals and subannual bands are common. For corals from Tutum Bay, delta(13)C ranges from -4.5 to -1.0 parts per thousand and delta(18)O from -6.0 to -3.8 parts per thousan d, which are relatively depleted values for shallow water Porites. The comp arison sample, C-29, has delta(13)C values ranging from -1.8 to -0.5 parts per thousand and delta(18)O values of -5.4 to -4.6 parts per thousand. Conc entrations 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, Cy Fe, Mn, and Zn were detected in some samples. Sr was detected in all samples a nd 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 Sr-87/Sr-86 values than the "non-hydro thermal" sample (C-29). The observed isotope patterns indicate that the hydrothermal input into Tut um Bay influences the physico-chemical conditions in the surrounding coral reef: Tutum Bay Porites lobata show distinctly different delta(13)C and Sr- 87/(86S)r records when compared to the "non-hydrothermal" sample and other corals front. Papua New Guinea and elsewhere. Direct and indirect synergist ic effects, including temperature, isotopic and chemical composition of ven t fluids and the influence of pCO(2) on the expression of photosynthetic an d kinetic stable isotope fractionation factors, are the most likely explana tion for the "hydrothermal" signal seen in these corals. The delta(18)O val ues are significantly correlated with the estimated amounts of hydrothermal exposure.