Magnesium in the lattice of calcite-shelled brachiopods
Magnesium in the lattice of calcite-shelled brachiopods
复制标题
DOI：
10.1016/j.chemgeo.2008.08.007
复制
发表时间：
2008-11-30
期刊：
影响因子：
3.9
通讯作者：
Finch, Adrian A.
中科院分区：
文献类型：
--
作者：
Cusack, Maggie;Perez-Huerta, Alberto;Finch, Adrian A.
古气候信息通常是通过δ O-18组成提取的，δ O-18组成是海水温度的代表。与δ O-18相比，Mg/Ca比值作为腕足动物海水温度指标的潜力被忽视了。用Mg/Ca比值作为温度指标的假设是，随着温度的升高，方解石晶格中更多的Mg取代了Ca。腕足动物的壳，像所有的生物矿物一样，是有机和无机成分的混合物。这就提出了镁是由有机成分承载的可能性。或者，镁可以作为单独的矿物相存在，而不是方解石晶格的真正组成部分，或者以非理想或可变配位的方式并入方解石中。本文利用同步加速器x射线吸收近边光谱（XANES）在镁钾边缘测定了两种低镁方解石壳腕足动物retusa和Notosaria nigricans以及一种高镁方解石壳的noocrania anomala体内镁的局部环境。XANES在一套含镁标准的Mg- k边缘指纹腕足类贝壳粉末中镁的局部环境以及原位分析。在所有情况下，很明显镁不是由有机成分承载的，而是在壳的无机成分内。这些数据支持了用腕足动物镁钙比作为温度代理的可能性。(C) 2008年Elsevier B.V.出版
Palaeoclimate information is often extracted from Rhynchonelliform brachiopod shell calcite, in particular the inner secondary layer, via the delta O-18 composition, which is a proxy for seawater temperature. Compared to delta O-18, the potential for Mg/Ca ratio, as a brachiopod seawater temperature proxy, has been neglected. The use of Mg/Ca ratio as a temperature proxy assumes that, with increasing temperature, more Mg Substitutes for Ca in the calcite lattice. Brachiopod shells, like all biominerals, are composites of organic and inorganic components. This raises the possibility that magnesium is hosted by the organic components. Alternatively, magnesium may be present as a separate mineral phase, rather than a true component of the calcite lattice, or incorporated into calcite in non-ideal or variable coordination. Here we use synchrotron X-ray absorption Near Edge Spectroscopy (XANES) at the Mg K-edge to determine the local environment of magnesium in two species of brachiopod with low Mg-calcite shells, Terebratulina retusa and Notosaria nigricans and one species with a high Mg-calcite shell, Novocrania anomala. XANES at the Mg K-edge of a Suite of Mg-bearing standards fingerprints the local environment of magnesium in the brachiopod shell powders as well as in situ analyses. In all cases, it is evident that magnesium is not hosted by organic components but is within the inorganic component of the shell. These data support the possibility of using brachiopod Mg/Ca ratios as a temperature proxy. (C) 2008 Published by Elsevier B.V.