Diagenesis in live corals from the Gulf of Aqaba. I. The effect on paleo-oceanography tracers

The effect of early diagenesis on trace element abundance in coral skeleton was studied in live coral heads (Porites) from the Nature Reserve Reef (NRR), flat, Gulf of Aqaba, northern Red Sea. Petrography of the corals shows diagenetic features of dissolution, recrystallization, and secondary aragonite precipitation (pore filling), which are most extensive in the oldest part of the coral. Coral porewaters were extracted with a special setup and were analyzed for chemical composition. The total alkalinity and Sr deficit in pore water as compared to reef water is consistent with both precipitation of secondary aragonite and recrystallization of primary skeleton. The present rate constant of pore filling by secondary aragonite was estimated by a water replacement experiment to be 0.0015 y(-1), which equals to pore filling rate of 1.5 +/- 0.3 kg aragonite per year. The corals show clear seasonal fluctuations in Sr/Ca ratios that: are interpreted as reflecting changes in sea surface temperature (SST). Yet, the secondary aragonite is characterized by a significantly higher Sr/Ca ratio than the average ratio in primary aragonite. Thus, measuring a mixed sample of pristine and secondary aragonite may produce erroneous (about 2 degrees C lower) SST estimates by the Sr/Ca thermometers. It appears that the Sr/Ca ratio, a major proxy for paleo-environmental marine studies, is sensitive to subtle pore-filling and replacement of the original coral matrix by secondary aragonite in the marine environment. Copyright (C) 2000 Elsevier Science Ltd.