Magnesium stable isotope fractionation in marine biogenic calcite and aragonite
Magnesium stable isotope fractionation in marine biogenic calcite and aragonite
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DOI：
10.1016/j.gca.2011.07.017
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发表时间：
2011-10
影响因子：
5
通讯作者：
F. Wombacher;F. Wombacher;A. Eisenhauer;F. Böhm;N. Gussone;M. Regenberg;W. Dullo;A. Rüggeberg
中科院分区：
文献类型：
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作者：
F. Wombacher;F. Wombacher;A. Eisenhauer;F. Böhm;N. Gussone;M. Regenberg;W. Dullo;A. Rüggeberg
海洋生物成因的文石和方解石的镁稳定同位素组成的调查，包括珊瑚，硬海绵，底栖poraneous和浮游有孔虫，颗石软泥，红藻，和海胆和腕足动物的测试样品。采用MC-ICP-MS进行分析，外部重复性为±0.22‰（δ 26 Mg为2SD; n=37），来自珊瑚参考样品（JCp-1）。方解石珊瑚和硬海绵中的镁同位素分馏与已发表的方解石洞穴沉积物的数据一致，平均Δ 26 Mg方解石-海水=−2.6±0.3‰，似乎与温度的关系较弱。除了一个例外（瓦氏菌属），文石珊瑚和硬海绵相对于海水也显示出均匀的Mg同位素分馏，Δ 26 Mg生物成因文石-海水=−0.9±0.2。镁同位素在高镁方解石从红藻，海胆，也许一些porcelaneous有孔虫，以及在所有低镁方解石（有孔虫，cocoliths和腕足动物）显示出显着的生物影响。对于浮游有孔虫，镁同位素数据与平衡条件下有机物质对镁的固定是一致的，但似乎与镁从液泡中的去除不一致。然而，我们的首选模型表明，浮游有孔虫合成的生物分子，增加了镁掺入的能量屏障。在该模型中，避免了从空泡溶液中去除大量Mg的需要。对于来自海胆的高镁方解石，无定形碳酸钙的沉淀可能是其较弱的Mg同位素分馏的原因。不考虑叠加的生物效应，它似乎是阳离子轻同位素富集CaCO 3主要是由于化学动力学同位素效应，有关的阳离子在扭结网站的掺入。在该模型中，CaCO 3中阳离子同位素分馏的系统学与阳离子掺入所需的活化能有关，这可能反映了阳离子和晶体表面的脱水以及掺入位点的成键.该动力学结合模型预测（i）对生长速率没有内在依赖性，除非缓慢生长时的显著反反应使同位素分馏朝向平衡同位素分配的特征降低（这可以在方解石中的Ca同位素中观察到），（ii）同位素分馏随温度升高而小幅降低，如果较高温度促进逆反应，则可能放大;以及（iii）对由添加剂如阴离子或生物分子或由无定形CaCO 3的初始形成引起的活化屏障变化的敏感性。
This survey of magnesium stable isotope compositions in marine biogenic aragonite and calcite includes samples from corals, sclerosponges, benthic porcelaneous and planktonic perforate foraminifera, coccolith oozes, red algae, and an echinoid and brachiopod test. The analyses were carried out using MC-ICP-MS with an external repeatability of ±0.22‰ (2SD for δ26Mg; n=37), obtained from a coral reference sample (JCp-1). Magnesium isotope fractionation in calcitic corals and sclerosponges agrees with published data for calcitic speleothems with an average Δ26Mgcalcite–seawater=−2.6±0.3‰ that appears to be weakly related to temperature. With one exception (Vaceletia spp.), aragonitic corals and sclerosponges also display uniform Mg isotope fractionations relative to seawater with Δ26Mgbiogenic aragonite–seawater=−0.9±0.2. Magnesium isotopes in high-Mg calcites from red algae, echinoids and perhaps some porcelaneous foraminifera as well as in all low-Mg calcites (perforate foraminifera, coccoliths and brachiopods) display significant biological influences. For planktonic foraminifera, the Mg isotope data is consistent with the fixation of Mg by organic material under equilibrium conditions, but appears to be inconsistent with Mg removal from vacuoles. Our preferred model, however, suggests that planktonic foraminifera synthesize biomolecules that increase the energetic barrier for Mg incorporation. In this model, the need to remove large quantities of Mg from vacuole solutions is avoided. For the high-Mg calcites from echinoids, the precipitation of amorphous calcium carbonate may be responsible for their weaker Mg isotope fractionation. Disregarding superimposed biological effects, it appears that cation light isotope enrichments in CaCO3principally result from a chemical kinetic isotope effect, related to the incorporation of cations at kink sites. In this model, the systematics of cation isotope fractionations in CaCO3relate to the activation energy required for cation incorporation, which probably reflects the dehydration of the cation and the crystal surface and bond formation at the incorporation site. This kinetic incorporation model predicts (i) no intrinsic dependence on growth rate, unless significant back reaction upon slow growth reduces the isotope fractionation towards that characteristic for equilibrium isotope partitioning (this may be observed for Ca isotopes in calcites), (ii) a small decrease of isotope fractionation with increasing temperature that may be amplified if higher temperatures promote back reaction and (iii) a sensitivity to changes in the activation barrier caused by additives such as anions or biomolecules or by the initial formation of amorphous CaCO3.