Isotopic Evolution of Proterozoic Sediments and Seawater

建议在整个新元古代（0.54 -1.0 Ga）和一些稍老的地层（1.0-1.1 Ga）中，继续研究特征良好、保存完好的地层样品中C、Sr和Nd的同位素变化。本文还提出了一种通过测量碳酸盐岩中以微量硫酸盐形式存在的S同位素来确定新元古代海水S同位素记录的时间变化的方法。新元古代全球变化记录了海相碳酸盐记录的古海水中强烈的碳、锶同位素变化。层序的精确同位素表征也是世界范围内地层精确相互对比的有力工具。基于相同样品的S、C、Sr和Nd同位素变化的相互相关性对于理解驱动新元古代同位素变化的全球系统强迫至关重要。一份为期三年的工作声明描述了S质谱分析的发展工作，以及我们的标准技术在C、Sr和Nd同位素的各种序列中的应用。该项目的三年工资支持，如下项目描述所述，要求为一名研究生提供，为一名研究助理提供50%的支持，每年为PI提供一个月的支持。所有样品都可以从合作者或内部获得。样本的选择将与正在或曾经积极参与地层剖面研究的研究小组合作进行。在本提案中，我们要求资助实验室工作，以便：1)建立新元古代海水中C、Sr和Nd同位素变化的详细记录，以及基于精心筛选的海洋石灰石和磷岩样本的ce异常；2)详细了解两次（三次）新元古代主要冰期（- 0.06 Ga、-0.75 Ga和-0.9 Ga）与海相碳酸盐岩中记录的C-、Sr、S-和nd同位素变化之间的关系；3)分析未变质微钻孔碳酸盐样品中提取的微量硫酸盐中的S同位素。碳酸盐s同位素记录的研究将是有用的，因为前寒武纪沉积序列中层状硫酸盐的碎片记录为理解前显生宙地球历史中海水s同位素丰度的长期变化提供了不完整的基础。由于热电离质谱法的新进展，现在有可能分析碳酸盐中微量硫酸盐的S同位素，这将被修改以适应地质物质的分析。与NIST的Bob Kelly合作开发这些微技术需要资金。我们的C、Sr和Nd同位素测量的标准方法在出版物中有很好的记录，证明了本提案的技术目标。新元古代海水中同位素变化率的精确知识将用于进一步了解地球系统在感兴趣的时间框架内的演化。具体目标如下：1)追踪外生地球化学旋回中的主要物质通量，了解新元古代沉积储层与海洋和大气的演化关系；2)确定侵蚀速率、有机碳埋藏速率和热液蚀变速率及其与全球构造的关系；3)研究强同位素变化与已知冰期、磷矿和带状铁沉积事件的关系；4)探讨大气氧的平衡是否与地壳S和C储层耦合，或者是否如我们先前的研究所支持的那样，C和S循环不耦合导致大气O2丰度的广泛变化；5)加强了这些新元古代序列之间同位素地层和生物地层对比的联系。在新元古代，海水Nd和Sr的收支似乎从主要的水热输入（如太古宙）转变为主要的大陆（河流）输入。基于过去和未来研究数据的建模将用于约束热液循环速率、大陆侵蚀速率、有机碳埋藏速率以及新元古代大陆侵蚀产物平均年龄的变化。还将考虑对地球大气中O2和CO2演化的影响。本实验室前期工作表明，可以获得新元古代沉积序列的高分辨率Sr和C同位素曲线。这项研究将继续引领我们对新元古代演化认识的重大突破。

Jacobsen 9418445 It is proposed to continue research on the isotopic variations in C, Sr and Nd in well-characterized and well-preserved stratigraphic samples from successions spanning the entire Neoproterozoic (.54-1.0 Ga) as well as some slightly older strata (1.0-1.1 Ga). It is also proposed to develop a method for determination of temporal variations in the S-isotopic record of Neoproterozoic seawater by measurement of S bound as trace sulfate in carbonate rocks. Global change during the Neoproterozoic is record of strong C- and Sr-isotopic variations in paleoseawater recorded in marine carbonates. Precise isotopic characterization of sequences is also a powerful tool for precise inter-correlation of strata world-wide. Inter-correlation of S, C, Sr, and Nd isotopic variations based on the same samples is essential for understanding forcings in the global system that drive the isotopic variations in the Neoproterozoic. A three year work statement describes developmental work in mass spectrometry for S and application of our standard techniques to a variety of sequences for C, Sr, and Nd isotopes. Three year salary support for this project, described in the following project description is requested for one graduate student, 50% support for one research associate and one month per year for the PI. All samples are available from collaborators or in house. Sample selections will be made in collaboration with research groups who are, or have been, involved in active research on the stratigraphic sections. In this proposal we request funding for laboratory work in order to : 1) establish a detailed record of C, Sr, and Nd isotopic variations as well as Ce-anomalies in Neoproterozoic seawater based on carefully screened samples of marine limestones and phosphorites; 2) understand in detail relationships between the two/(three?) major Neoproterozoic glacial events (-.06, -0.75, and -0.9 Ga (?)) and C-, Sr, S- and Nd-isotopic variations recorded in marine carbonates; 3) analyze S isotope s in trace sulfate extracted from unaltered micro-drilled carbonate samples. Study of the carbonate S-isotope record will be useful because the fragmentary record of bedded sulfates in Precambrian sedimentary successions provides an incomplete basis for understanding secular variations in the S-isotopic abundance of seawater through pre-Phanerozic Earth history. The analysis of S isotopes of trace sulfate incorporated in carbonate is now possible due to new advances in thermal ionization mass spectrometry which will be modified to accommodate the analysis of geological materials. Funding is requested for development of these micro-techniques in collaboration with Bob Kelly of the NIST. Our standards methods for C, Sr, and Nd isotopes measurements are well documented in publications which justify the technical objectives of this proposal. Precise knowledge of rates of isotopic change in Neoproterozoic seawater will be used to further understanding of Earth System evolution during the time frame of interest. The following specific objectives will be pursued: 1) trace the major mass fluxes in the exogenic geochemical cycle and understand the relationship between the evolution of the sedimentary reservoir, the oceans and the atmosphere during the Neoproterozoic; 2) determine rates of erosion, organic C burial and hydrothermal alteration and their relation to global tectonics; 3) investigate the relationship of strong isotopic variations to known events of glaciations, phosphorite and banded iron-formation deposition; 4) explore whether the balance of atmospheric oxygen was coupled to the crustal reservoirs of S and C, or whether, as supported by our earlier studies, the C and S cycles were uncoupled leading to wide variations in the abundance of atmospheric O2; 5) strengthen the tie between isotope-stratigraphic and bio-stratigraphic correlations between these Neoproterozoic successions. During the Neoproteroic, seawater Nd and Sr budgets appear to have changed from dominantly hydrother mal inputs (as in the Archean) to dominantly continental (river) inputs. Modeling based on data obtained from past and proposed research will be used to constrain the changes in rates of hydrothermal circulation, continental erosion rates, burial rates of organic C and variations in the mean age of continental erosion products through the Neoproterozoic. Implications for the evolution of O2 and CO2 in the Earth's atmosphere will also be considered. Previous work in this laboratory demonstrated that high resolution Sr and C isotopic curves may be obtained for Neoproterozoic sedimentary successions. The proposed research will continue to lead to major breakthroughs in our understanding of Neoproterozoic evolution.