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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >pH control on oxygen isotopic composition of symbiotic corals
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pH control on oxygen isotopic composition of symbiotic corals

机译：pH控制共生珊瑚氧同位素组成

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Boron, carbon and oxygen isotopic compositions were determined at the micrometre scale by high-resolution ion microprobe in a sample of modern coral (massive hermatypic coral, Porites lutea). The ion probe data show for B and O much larger isotopic variations at the micrometre scale than those measured at the millimetre scale by conventional techniques: δ~(18)O_(PDB) values range from -10.6 ± 0.9‰ to -0.2 ± 0.5‰ and δ~(11)B values range from +18.6 ± 1.5‰ to +30.6 ± 1.6‰. By contrast, δ~(13)C values show the same range of variations, from -4.6 ± 0.65‰ to -2.2 ± 0.67‰ at the micrometre and millimetre scales. The range of δ~(11)B values indicates that significant pH variations, from ≈ 7.1 to ≈ 9.0, are present at the sites of calcification. The largest δ~(18)O variations correspond to the highest δ~(11)B values, i.e. to the highest pHs. This measurement of pH allows modelling the oxygen isotopic fractionation occurring during aragonite precipitation. Taking into account the rate of O isotopic equilibrium between dissolved carbonate species (H_2CO_3, HCO_3~- and CO_3~(2-)) and water via the two reactions of hydration and hydroxylation, the full range of δ~(18)O values measured at the micrometre scale can be modelled for residence times of dissolved carbonates in the calcifying fluid ranging between ≈ 1 h and at maximum ≈ 12 h. The pH controls the δ~(18)O of the growing carbonate through the relative fractions of dissolved carbonate species and through the kinetics of their isotopic equilibration with water via hydration and hydroxylation. The so-called 'vital effect' systematically observed for δ~(18)O in corals can thus be understood as representing an average of rapid pH variations due to coral biology during coral growth. Selectively measuring δ~(18)O values in the zones of coral skeletons that have low δ~(11)B values (i.e. formed at low pH) should significantly improve the quality of palaeoclimatic reconstructions based on δ~(18)O values.
机译：硼，碳和氧的同位素组成是通过高分辨率离子微探针在现代珊瑚（大规模放血珊瑚，Porites lutea）样品中以微米为单位测定的。离子探针数据显示，B和O的微米级同位素变化远大于常规技术在毫米级上的同位素变化：δ〜（18）O_（PDB）值范围为-10.6±0.9‰至-0.2±0.5 ‰和δ〜（11）B值的范围为+18.6±1.5‰至+30.6±1.6‰。相比之下，在微米和毫米级别，δ〜（13）C值显示相同的变化范围，从-4.6±0.65‰到-2.2±0.67‰。 δ〜（11）B值的范围表明钙化部位存在明显的pH值变化，从≈7.1到≈9.0。最大的δ〜（18）O变化对应于最高的δ〜（11）B值，即最高的pH。 pH值的这种测量可以模拟文石沉淀过程中发生的氧同位素分馏。考虑到水合和羟基化这两个反应在溶解的碳酸盐种类（H_2CO_3，HCO_3〜-和CO_3〜（2-））与水之间的O同位素平衡速率，测量了整个δ〜（18）O值以微米为单位，可以模拟溶解的碳酸盐在钙化流体中的停留时间，范围在≈1 h至最大≈12 h之间。 pH通过溶解的碳酸盐种类的相对分数，以及通过水合和羟基化作用与水进行同位素平衡的动力学来控制生长中的碳酸盐的δ〜（18）O。因此，可以系统地观察到珊瑚中δ〜（18）O的所谓“生命效应”，可以理解为代表珊瑚生长过程中由于珊瑚生物学引起的快速pH变化的平均值。在δ〜（11）B值较低（即在低pH下形成）的珊瑚骨骼区域中选择性测量δ〜（18）O值应显着提高基于δ〜（18）O值的古气候重建质量。

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