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Biogeosciences An interactive open-access journal of the European Geosciences Union
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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 19 Oct 2018

Research article | 19 Oct 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

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

Anne Juillet-Leclerc Anne Juillet-Leclerc
  • LSCE Domaine du CNRS, 91198 Gif-sur-Yvette, France

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.

Anne Juillet-Leclerc
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Anne Juillet-Leclerc
Anne Juillet-Leclerc
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Short summary
My manuscript is the result of a reflexion process about the meaning of coral skeleton tracers. I realized that in contrast to biologists, geochemists always neglect the light role on proxies. Light influence is difficult to separate from temperature. Thus, I used coral culture in controlled conditions. Once the process identified, I read differently the literature dedicated to the coral skeleton δ18 and found several evidences demonstrating that light could explain the vital effect.
My manuscript is the result of a reflexion process about the meaning of coral skeleton tracers....