To study long-term variations in surface ocean d13C, we investigated coral skeletal d13C records of 20 colonies of the shallow-water coral Porites spp. from the northern Gulf of Aqaba and the northern Red Sea. The coral colonies represent different water depths, a wide range of different periods (Last Interglacial, Holocene, the last centuries, the last decades), and various growth rates. Records from modern and fossil corals show irregular seasonal cycles, attributed mainly to the seasonal cycle of light. No attenuation in the amplitude of the seasonal skeletal d13C cycle with depth is evident, and no significant correlation between mean annual coral d13C and water depth was observed in the modern corals. The mean coral extension rates show no clear relationship with mean skeletal d13C values. The average skeletal d13C value of modern corals was -2.74 ± 0.49 per mil, offset from both calcite and aragonite equilibrium values by about 5.22 per mil and 6.26 per mil, respectively. Modern corals reveal a clear trend toward lighter skeletal d13C values since the year 1974. At longer timescales, the skeletal d13C values from Last Interglacial and Holocene corals and from coral records extending back to the mid- to late 18th century reveal much heavier d13C values compared to values from 1960 to the present. The trend toward lighter skeletal d13C values over the last decades can be attributed to changes in the d13C of the dissolved inorganic carbon of the ambient seawater due to the addition of anthropogenically derived CO2 (13C Suess effect) to the atmosphere. Other factors such as the metabolic effects of corals may account for the modulation of skeletal d13C on shorter timescales. A centuries-long coral record from the northern Red Sea reveals a magnitude of decrease in skeletal d13C comparable to global trends (1960–1990), whereas a centuries-long coral record from the northern Gulf of Aqaba indicates a larger decrease over the same period, probably due to local effects. In conclusion, the combined carbon isotope records obtained from Porites spp. corals from the northern Red Sea seem to be suited to provide information on long-term world-wide changes in atmospheric CO2.
