Documenting the Early Diagenetic History of Articulate Brachiopods Using Transmission Electron Microscopy

9628364卡尔森古生代古气候和古海洋学同位素研究依赖于18O值来自原始的、未改变的腕足类方解石的假设。最近，用于检测低镁方解石成岩作用的标准测试(纹理和化学测试)的可靠性受到了质疑。有证据表明，随着地质时间的增加，古生代~(18)O的亏损趋势可能表明沉积后的蚀变，而不是海洋温度的主要趋势。根据光学和扫描电子显微镜图像的标准测试、微量元素分析和阴极发光显微镜的评估，“原始的”腕足方解石经常显示出同位素变化的迹象。显然，指示早期成岩作用的精细溶解/沉淀作用将改变原始的同位素组成，而不影响钙质组构。透射电子显微镜是唯一一种能够通过在原子水平上成像晶体超微结构来探测精细成岩作用的技术。TEM分析提供了分辨率为0.2纳米的样品的光学图像和X射线衍射图。X射线衍射用于确定晶格结构，而透射电子显微镜图像可以揭示晶体和孪晶界等结构特征。本研究的目的是利用透射电子显微镜对腕足类化石的细微尺度成岩作用进行表征。将现存腕足类中原始低镁方解石的透射电子显微镜特征与上更新世成岩蚀变低镁方解石亲属中发现的透射电子显微镜特征进行对比，将建立识别精细成岩作用的标准。对腕足类化石的研究应该揭示出成岩低镁方解石的特征，如微孔隙度增加，孪晶界和位错位置的横切关系，以及相对于原始低镁方解石更高的位错密度。这些典型的早期诊断标准将与标准成岩测试以及现存和化石腕足类的同位素值进行比较，以测试标准测试的可靠性，以诊断改变原始同位素组成所需的精细成岩作用。本文所述研究的意义在于，建议的透射电子显微镜检查提供了一种直接而独特的方法来识别细小规模的成岩作用，这种成岩作用改变了原始的同位素组成，而不改变钙质组构。成功识别指示同位素变化的成岩特征的能力，应该对我们准确解释腕足类化石的同位素数据的能力产生深远的影响。

9628364 Carlson Paleozoic isotopic studies of paleoclimate and paleoceanography rely on the assumption that 18O values are obtained from original, unaltered brachiopod calcite. Recently, the reliability of the standard tests (textural and chemical) used to detect diagenesis in low magnesium calcite has been questioned. It has been suggested that the well-documented Paleozoic trend of depleted 18O with increasing geologic time may indicate post-depositional alteration rather than a primary trend in ocean temperatures. As assessed by the standard tests of optical and scanning electron microscopic images, trace element analyses, and cathodoluminescent microscopy, "pristine" brachiopod calcite often exhibits signs of isotopic alteration. Apparently, fine-scale dissolution/precipitation indicative of early diagenesis will alter the original isotopic composition without affecting the calcitic fabric. Transmission electron microscopy (TEM) is the only technique able to detect fine-scale diagenesis by imaging the crystal ultrastructure at an atomic level. TEM analysis provides both optical images of specimens at a resolution of 0.2 nm and X-ray diffraction patterns. X-ray diffraction is used to determine crystal lattice structures, whereas, TEM images can reveal structural features such as grain and twin boundaries. The purpose of this study is to use TEM to characterize fine-scale diagenesis in fossil brachiopods. A comparison of TEM features of original low magnesium calcite in extant brachiopods with TEM features identified in Plio-Pleistocene relatives of diagenetically altered low magnesium calcite will establish criteria to identify fine-scale diagenesis. Examination of fossil brachiopods should reveal features that characterize diagenetic low magnesium calcite such as increased microporosity, cross-cutting relationships at twin boundaries and dislocation sites, and a higher dislocation density with respect to original low magnesium calcite. These criteria that typify early diag enesis will be compared with standard diagenetic tests and with isotopic values from extant and fossil brachiopods to test the reliability of the standard test to diagnose fine-scale diagenesis necessary to alter the original isotopic composition. The significance of the research described herein is that the proposed TEM examination provides a direct and unique means of identifying fine-scale diagenesis that alters the original isotopic composition without altering the calcitic fabric. The ability to successfully identify diagenetic features that indicate isotopic alteration should have a profound effect on our ability to accurately interpret isotope data from fossil brachiopods.