The role of light as vital effect on coral skeleton oxygen isotopic ratio

Abstract. Light, an environmental parameter playing a crucial role in coral aragonite growth and δ18O formulation, is always neglected in the geochemical literature. However, by revisiting already published studies, we demonstrated that light might be considered as a vital effect affecting coral aragonite oxygen isotopic ratios.

Re-examining data series included in a publication by Weber and Woodhead (1972), we stressed that annual δ¹⁸O–annual temperature calibrations of all considered coral genera may be compared because their assessment assumes homogenous light levels. Temperature prevails on δ¹⁸O because it influences δ¹⁸O in two ways: firstly it acts as is thermodynamically predicted implying a δ¹⁸O decrease; and secondly it induces an enhancement of photosynthesis causing δ¹⁸O increase. When the highest annual temperature occurs simultaneously with the highest annual irradiation, the annual δ¹⁸O amplitude is shortened. The annual δ¹⁸O–annual temperature calibration is also explained by the relative distribution of microstructures, centres of calcification or COC and fibers, according to morphology, and in turn taxonomy. We also investigated monthly δ¹⁸O–monthly temperature calibrations of Porites grown at the same sites as by Stephans and Quinn (2002), Linsley et al. (1999, 2000) and Maier et al. (2004). Multiple evidence showed that temperature is the prevailing environment forcing on δ¹⁸O and that the mixture of temperature and light also determines the relative distribution of microstructures, explaining the relationships between Porites calibration constants. By examining monthly and annual δ¹⁸O–monthly and annual temperature calibrations, we revealed that monthly calibration results from the superimposition of seasonal and annual variability over time. Seasonal δ¹⁸O strongly impacted by seasonal light fluctuations, may be obtained by removing interannual δ¹⁸O only weakly affected by light. Such features necessitate the reconstitution of tools frequently utilised, such as the coupled δ¹⁸O–Sr / Ca or pseudo-coral concepts.