Stable oxygen isotopes in Porites corals monitor weekly temperature variations in the northern Gulf of Aqaba, Red Sea

Abstract

In order to assess the ability of Porites corals to accurately record environmental variations, high-resolution (weekly/biweekly) coral δ18O records were obtained from four coral colonies from the northern Gulf of Aqaba, which grew at depths of 7, 19, 29, and 42 m along one transect. Adjacent to each colony, hourly temperatures, biweekly salinities, and monthly δ18O of seawater were continuously recorded over a period of 14 months (April 1999 to June 2000). Contrary to water temperature, which shows a regular and strong seasonal variation and change with depth, seawater δ18O exhibits a weak seasonality and little change with depth. Positive correlations between seawater δ18O and salinity were observed. The two parameters were related to each other by the equation δ18O Seawater (‰, VSMOW) = 0.281 × Salinity − 9.14. The high-resolution coral δ18O records from this study show a regular pattern of seasonality and are able to capture fine details of the weekly average temperature records. They resolve more than 95% of the weekly average temperature range. On the other hand, attenuation and amplification of coral seasonal amplitudes were recorded in deep, slow-growing corals, which were not related to environmental effects (temperature and/or seawater δ18O) or sampling resolution. We propose that these result from a combined effect of subannual variations in extension rate and variable rates of spine thickening of skeletal structures within the tissue layer. However, no smoothing or distortion of the isotopic signals was observed due to calcification within the tissue layer in shallow-water, fast-growing corals. The calculations from coral δ18O calibrations against the in situ measurements show that temperature (T) is related to coral δ18O (δ c ) and seawater δ18O (δ w ) by the equation T (°C) = −5.38 (δ c − δ w ) −1.08. Our results demonstrate that coral δ18O from the northern Gulf of Aqaba is a reliable recorder of temperature variations, and that there is a minor contribution of seawater δ18O to this proxy, which could be ignored.