Light and temperature effects on δ11B and B / Ca ratios of the zooxanthellate coral Acropora sp.: results from culturing experiments

[摘要] The boron isotopic composition (δ¹¹B) of marine carbonates (e.g.corals) is increasingly utilised as a proxy for paleo-pH, with the strongcorrelation between δ¹¹B of marine calcifiers and seawater pH nowwell documented. However, the potential roles of other environmentalparameters that may also influence both the boron isotopic composition andboron concentration into coral aragonite are poorly known. To overcome this,the tropical scleractinian coral Acropora sp. was cultured under 3different temperatures (22, 25 and 28 °C) and two light conditions(200 and 400 μmol photon m⁻² s⁻¹). Theδ¹¹B indicates an increase in internal pH that is dependent onthe light conditions. Changes in light intensities from 200 to400 μmol photon m⁻² s⁻¹ seem to indicate an apparentdecrease in pH at the site of calcification, contrary to what is expected inmost models of light-enhanced calcification. Thus, variations in lightconditions chosen to mimic average annual variations of the naturalenvironments where Acropora sp. colonies can be found could bias pHreconstructions by about 0.05 units. For both light conditions, a significantimpact of temperature on δ¹¹B can be observed between 22 and25 °C, corresponding to an increase of about 0.02 pH-units, while nofurther δ¹¹B increase can be observed from 25 to 28 °C.This non-linear temperature effect complicates the determination of acorrection factor. B / Ca ratios decrease with increasing light, consistentwith the decrease in pH at the site of calcification under enhanced lightintensities. When all the other parameters are constant, boron concentrationsin Acropora sp. increase with increasing temperatures and increasingcarbonate ion concentrations. These observations contradict previous studieswhere B / Ca in corals was found to vary inversely with temperature,suggesting that the controlling factors driving boron concentrations have notyet been adequately identified and might be influenced by other environmentalvariables and/or species-specific responses.

[学科分类] 地球化学与岩石