Temperature control on high-resolution SIMS oxygen isotopic compositions in Porites coral skeletons

Oxygen isotope composition (δ¹⁸O) in Porites coral skeletons at the micrometer scale has been proposed to be uninfluenced by temperature in previous studies. Considering that temperature is the main controlling factor of coral δ¹⁸O at the macro scale, the effect of temperature on the δ¹⁸O variations at the micrometer scale should be evaluated carefully. To better understand the climatic and biological significance of high-resolution δ¹⁸O in coral skeletons, in situ δ¹⁸O values were analyzed in three modern Porites corals from the South China Sea and the Great Barrier Reef by a Cameca IMS 1280-HR secondary ion mass spectrometry (SIMS). A twice vacuum impregnation procedure for sample mount preparation and a subsection calibration method based on multiple controlling points for coral matrix effect correction were used for SIMS analysis. The derived SIMS coral δ¹⁸O time series exhibit consistent long-term variations with the monthly δ¹⁸O (measured by conventional gas isotope ratios mass spectrometry), monthly Sr/Ca, and daily sea surface temperature, first highlighting the dominant temperature control on micrometer-scale coral δ¹⁸O. The 1.5‰ fluctuation amplitude of SIMS coral δ¹⁸O at the scales of 400–800 μm can be explained by the combined SST effects of ~10-day variation and day–night fluctuation, whereas the effects of seawater δ¹⁸O and growth rate on SIMS coral δ¹⁸O seem not dominant. Some abnormally negative δ¹⁸O values possibly resulted from the primary ion bombardment on indistinguishable centers of calcification.