Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba, Japan.
Anal Bioanal Chem. 2011 Nov;401(9):2755-69. doi: 10.1007/s00216-011-5264-0. Epub 2011 Jul 30.
High-precision Mg isotope measurements by multiple collector inductively coupled plasma mass spectrometry were applied for determinations of magnesium isotopic fractionation of biogenic calcium carbonates from seawater with a rapid Mg purification technique. The mean δ(26)Mg values of scleractinian corals, giant clam, benthic foraminifera, and calcite deep-sea corals were -0.87‰, -2.57‰, -2.34‰, and -2.43‰, suggesting preferential precipitation of light Mg isotopes to produce carbonate skeleton in biomineralization. Mg isotope fractionation in deep-sea coral, which has high Mg calcite skeleton, showed a clear temperature (T) dependence from 2.5 °C to 19.5 °C: 1,000 × ln(α) = -2.63 (±0.076) + 0.0138 (±0.0051) × T(R(2) = 0.82, p < 0.01). The δ(26)Mg values of large benthic foraminifera, which are also composed of a high-Mg calcite skeleton, can be plotted on the same regression line as that for deep-sea coral. Since the precipitation rates of deep-sea coral and benthic foraminifera are several orders of magnitude different, the results suggest that kinetic isotope fractionation may not be a major controlling factor for high-Mg calcite. The Mg isotope fractionation factors and the slope of temperature dependence from deep-sea corals and benthic foraminifera are similar to that for an inorganically precipitated calcite speleothem. Taking into account element partitioning and the calcification rate of biogenic CaCO(3), the similarity among inorganic minerals, deep-sea corals, and benthic foraminiferas may indicate a strong mineralogical control on Mg isotope fractionation for high-Mg calcite. On the other hand, δ(26)Mg in hermatypic corals composed of aragonite has been comparable with previous data on biogenic aragonite of coral, sclerosponges, and scaphopad, regardless of species differences of samples.
采用多接收电感耦合等离子体质谱法对高精度镁同位素进行了测量，结合快速镁净化技术，对海水生源碳酸钙中镁同位素分馏进行了测定。石珊瑚、巨蛤、底栖有孔虫和深海珊瑚碳酸钙的平均 δ(26)Mg 值分别为-0.87‰、-2.57‰、-2.34‰和-2.43‰，表明生物矿化过程中优先沉淀轻的镁同位素生成碳酸盐骨架。具有高镁方解石骨架的深海珊瑚的镁同位素分馏在 2.5°C 至 19.5°C 之间表现出明显的温度（T）依赖性：1,000×ln(α)=-2.63(±0.076)+0.0138(±0.0051)×T(R(2)=0.82，p<0.01)。同样由高镁方解石骨架组成的大型底栖有孔虫的 δ(26)Mg 值也可以绘制在与深海珊瑚相同的回归线上。由于深海珊瑚和底栖有孔虫的沉淀速率相差几个数量级，因此结果表明动力学同位素分馏可能不是高镁方解石的主要控制因素。深海珊瑚和底栖有孔虫的镁同位素分馏因子和温度依赖性斜率与无机沉淀方解石石笋的相似。考虑到元素分配和生物源碳酸钙的钙化速率，无机矿物、深海珊瑚和底栖有孔虫之间的相似性可能表明高镁方解石的镁同位素分馏受到强烈的矿物控制。另一方面，由文石组成的造礁石珊瑚的 δ(26)Mg 值与以前关于珊瑚、硬骨海绵和栉齿蛤生物文石的研究结果相当，而与样品的物种差异无关。
