Isotopic Evolution of Proterozoic Sediments and Seawater

元古代沉积物和海水的同位素演化

• 批准号: 9418445
• 负责人: Stein Jacobsen
• 金额: $ 6.95万
• 依托单位: Harvard University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 1995
• 资助国家: 美国
• 起止时间: 1995-08-15 至 1997-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9418445&HistoricalAwards=false
• 关键词: Isotopic; Evolution; Proterozoic; Sediments; Seawater

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.

雅各布森9418445 建议继续研究整个新元古代（0.54 -1.0 Ga）以及一些稍老的地层（1.0-1.1 Ga）中特征良好且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S结合态的微量硫酸盐来确定新元古代海水S同位素记录随时间变化的方法。 新元古代的全球变化是海相碳酸盐岩中记录的古海水中强烈的C-和Sr-同位素变化的记录。 层序的精确同位素特征也是全球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同位素变化的相互关联对于理解驱动新元古代同位素变化的全球系统中的强迫是必不可少的。 三年的工作报告描述了质谱法的发展工作的S和我们的标准技术的应用程序的各种序列的C，Sr和Nd同位素。 该项目的三年工资支持，在下面的项目描述中描述，要求一名研究生，一名研究助理和PI每年一个月的50%的支持。 所有样品均可从合作者或内部获得。 将与正在或曾经参与地层剖面积极研究的研究小组合作进行样本选择。 在这项建议中，我们要求资助的实验室工作，以：1）建立一个详细的记录的C，Sr和Nd同位素的变化，以及Ce异常新元古代海水的基础上，仔细筛选样品的海洋石灰岩和磷块岩; 2）详细了解两者之间的关系/（三个？）新元古代主要冰川事件（-0.06、-0.75和-0.9Ga（？））3）分析了未蚀变微钻孔碳酸盐岩样品中微量硫酸盐的硫同位素组成。 研究碳酸盐S-同位素记录将是有用的，因为层状硫酸盐在前寒武纪沉积序列的零碎记录提供了一个不完整的基础，了解长期变化的S-同位素丰度的海水通过前寒武纪地球历史。 由于热电离质谱法的新进展，现在可以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材料的分析。 要求与NIST的Bob Kelly合作，为这些微技术的发展提供资金。 我们的C、Sr和Nd同位素测量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范围内的演化。 具体目标如下：（1）追踪新元古代外生地球化学循环中的主要物质通量，了解新元古代沉积储层、海洋和大气演化之间的关系;（2）确定侵蚀速率、有机碳埋藏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的冰川、磷块岩和条带状铁建造沉积事件的关系; 4）探索大气氧的平衡是否与S和C的地壳水库相耦合，或者是否如我们早期研究所支持的那样，C和S旋回不耦合，导致大气O2丰度的广泛变化; 5）加强了这些新元古代层序之间同位素地层学和生物地层学相关性之间的联系。 在新元古代，海水Nd和Sr的预算似乎已经改变了占主导地位的水热输入（如在太古代）占主导地位的大陆（河流）输入。 将利用从过去和拟议研究中获得的数据进行建模，以限制热液循环速率、大陆侵蚀速率、有机碳埋藏速率的变化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演变的影响。 本实验室以往的工作表明，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。 拟议中的研究将继续导致我们对新元古代演化的理解取得重大突破。