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 (NR R), flat, Gulf of Aqaba, northern Red Sea. Petrography of the corals shows diagenetic features of dissolution, recrystallization, and secondary aragon ite precipitation (pore filling), which are most extensive in the oldest pa rt of the coral. Coral porewaters were extracted with a special setup and w ere analyzed for chemical composition. The total alkalinity and Sr deficit in pore water as compared to reef water is consistent with both precipitati on of secondary aragonite and recrystallization of primary skeleton. The pr esent 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 fi lling rate of 1.5 +/- 0.3 kg aragonite per year. The corals show clear seas onal fluctuations in Sr/Ca ratios that: are interpreted as reflecting chang es in sea surface temperature (SST). Yet, the secondary aragonite is charac terized by a significantly higher Sr/Ca ratio than the average ratio in pri mary aragonite. Thus, measuring a mixed sample of pristine and secondary ar agonite may produce erroneous (about 2 degrees C lower) SST estimates by th e Sr/Ca thermometers. It appears that the Sr/Ca ratio, a major proxy for paleo-environmental mari ne studies, is sensitive to subtle pore-filling and replacement of the orig inal coral matrix by secondary aragonite in the marine environment. Copyrig ht (C) 2000 Elsevier Science Ltd.