Comparison of d18O measured on coral core collected at Moorea (French Polynesia) (Boiseau et al., 1998) and measured and estimated temperatures. Fig. 8a, between 1980 and 1990, seasonal measured data are compared to instrumental seawater temperature (Boiseau et al., 1998). Fig. 8b, over the last century, annual averaged measured data, originated from the same data series than seasonal data, are compared to estimated temperature in the (1°, 1°) grid containing Moorea (Kaplan et al., 1998). The two curves are displayed to obtain the best matching. Isotopic scale of the two isotopic profiles is common to the two profiles, while measured and estimated temperature scales cover 7°C and 2°C respectively. There is a mismatch between annual and monthly calibrations given on a unique isotopic scale, illustrating the non-linearity between monthly and annual d18O profiles over the time.
Light, an environmental parameter playing a crucial role in coral aragonite growth and d18O 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 d18O–annual temperature calibrations of all considered coral genera may be compared because their assessment assumes homogenous light levels. Temperature prevails on d18O because it influences d18O in two ways: firstly it acts as is thermodynamically predicted implying a d18O decrease; and secondly it induces an enhancement of photosynthesis causing d18O increase. When the highest annual temperature occurs simultaneously with the highest annual irradiation, the annual d18O amplitude is shortened. The annual d18O–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 d18O–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 d18O 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 d18O–monthly and annual temperature calibrations, we revealed that monthly calibration results from the superimposition of seasonal and annual variability over time. Seasonal d18O strongly impacted by seasonal light fluctuations, may be obtained by removing interannual d18O only weakly affected by light. Such features necessitate the reconstitution of tools frequently utilised, such as the coupled d18O – Sr/Ca or pseudo-coral concepts.
Référence : Juillet-Leclerc A., 2018. The role of light as vital effect on coral skeleton oxygen isotopic ratio. Biogeoscience Discussion. Doi: 10.5194/bg-2018-433
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