Mineralogy, early marine diagenesis, and the chemistry of shallow-water carbonate sediments
Mineralogy, early marine diagenesis, and the chemistry of shallow-water carbonate sediments
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矿物学、早期海洋成岩作用以及浅水碳酸盐沉积物的化学性质
DOI：
10.1016/j.gca.2017.09.046
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发表时间：
2018-01-01
影响因子：
5
通讯作者：
Swart, P. K.
中科院分区：
文献类型：
--
作者：
Higgins, J. A.;Blattler, C. L.;Swart, P. K.
浅水碳酸盐沉积物构成了地质记录中大部分的沉积碳酸盐，并且是地球化学和气候历史的广泛使用的档案。解释古代碳酸盐沉积物地球化学的主要限制之一是沉积后成岩作用改变的可能性。在这项研究中，我们使用对沉积碳酸盐及相关孔隙流体中钙（Ca - 44/Ca - 40或δCa - 44）和镁（Mg - 26/Mg - 24或δMg - 26）同位素比值的配对测量，作为一种工具来了解巴哈马和澳大利亚西南部新近纪浅水碳酸盐沉积物的矿物学和成岩历史。我们发现这些地点的整体碳酸盐沉积物的Ca和Mg同位素组成呈现出与矿物学和早期海洋成岩作用相关的系统性地层变化。整体沉积物Ca同位素中观察到的变化最好由早期海洋成岩作用的程度和方式的变化来解释，从一种成岩碳酸盐矿物的组成由流体化学决定（流体缓冲）的情况到一种成岩碳酸盐矿物的组成由前体沉积物化学决定（沉积物缓冲）的情况。我们的结果表明，这一过程，连同碳酸盐矿物学（文石、方解石和白云石）的变化，在决定地质记录中浅水碳酸盐沉积物地球化学示踪剂（δC - 13、δO - 18、主要、次要和微量元素）的区域和全球地层表现方面起着基础性但未被充分认识的作用。我们的结果还提供了证据，表明一个富含Ca - 44的大型浅水碳酸盐汇可以解释河流和深海碳酸盐沉积物的δCa - 44/40值之间的不匹配，并对海水的δCa - 44/40值取决于原生碳酸盐沉淀的矿物学（例如“文石海”和“方解石海”）这一假设提出质疑。最后，我们对沉积白云石的研究结果表明，Ca和Mg同位素的配对测量可能为白云石化过程中的质量转移提供一种独特的地球化学指纹，以便更好地理解这些神秘但广泛存在的碳酸盐矿物中保存的古环境信息。（C）2017爱思唯尔有限公司。保留所有权利。
Shallow-water carbonate sediments constitute the bulk of sedimentary carbonates in the geologic record and are widely used archives of Earth's chemical and climatic history. One of the main limitations in interpreting the geochemistry of ancient carbonate sediments is the potential for post-depositional diagenetic alteration. In this study, we use paired measurements of calcium (Ca-44/Ca-40 or delta Ca-44) and magnesium (Mg-26/Mg-24 or delta Mg-26) isotope ratios in sedimentary carbonates and associated pore-fluids as a tool to understand the mineralogical and diagenetic history of Neogene shallow-water carbonate sediments from the Bahamas and southwest Australia. We find that the Ca and Mg isotopic composition of bulk carbonate sediments at these sites exhibits systematic stratigraphic variability that is related to both mineralogy and early marine diagenesis. The observed variability in bulk sediment Ca isotopes is best explained by changes in the extent and style of early marine diagenesis from one where the composition of the diagenetic carbonate mineral is determined by the chemistry of the fluid (fluid-buffered) to one where the composition of the diagenetic carbonate mineral is determined by the chemistry of the precursor sediment (sediment-buffered). Our results indicate that this process, together with variations in carbonate mineralogy (aragonite, calcite, and dolomite), plays a fundamental and underappreciated role in determining the regional and global stratigraphic expressions of geochemical tracers (delta C-13, delta O-18, major, minor, and trace elements) in shallow-water carbonate sediments in the geologic record. Our results also provide evidence that a large shallow-water carbonate sink that is enriched in Ca-44 can explain the mismatch between the delta Ca-44/40 value of rivers and deep-sea carbonate sediments and call into question the hypothesis that the delta Ca-44/40 value of seawater depends on the mineralogy of primary carbonate precipitations (e.g. 'aragonite seas' and 'calcite seas'). Finally, our results for sedimentary dolomites suggest that paired measurements of Ca and Mg isotopes may provide a unique geochemical fingerprint of mass transfer during dolomitization to better understand the paleo-environmental information preserved in these enigmatic but widespread carbonate minerals. (C) 2017 Elsevier Ltd. All rights reserved.