Isotopic Evolution of Proterozoic Sediments and Seawater

雅各布森9418445建议继续研究整个新元古代(0.54Ga-1.0Ga)和一些稍老的地层(1.0Ga-1.1Ga)特征良好和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定新元古代海水中微量硫酸盐与S结合的方法来确定新元古代海水S同位素记录的时间变化的方法。新元古代的全球变化记录了海相碳酸盐中记录的古海水中强烈的碳和锶同位素变化。层序的精确同位素特征也是世界范围内精确地层相互对比的有力工具。基于同一样品的S、碳、锶、钕同位素变化的相互对比对于理解驱动新元古代同位素变化的全球体系中的作用力是必不可少的。一份为期三年的工作说明书描述了S在质谱学方面的开发工作，以及我们的标准技术在各种碳、锶和钕同位素序列中的应用。对这个项目的三年工资支持，如以下项目描述所述，为一名研究生提供50%的支持，为一名研究助理提供50%的支持，为PI每年提供一个月的支持。所有样品都可以从合作者那里获得，也可以在公司内部获得。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。在这项提案中，我们要求为实验室工作提供资金，以便：1)根据仔细筛选的海相石灰岩和磷岩样品，建立新元古代海水中C、Sr、ND同位素变化以及Ce异常的详细记录；2)详细了解两者之间的关系。主要新元古代冰川事件(-0.06、-0.75和-0.9Ga(？))以及海相碳酸盐中记录的碳、锶、S和钕同位素变化；3)对未蚀变微孔碳酸盐样品中提取的微量硫酸盐中的S同位素S进行分析。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屑记录为通过前显生代地球历史了解海水S同位素丰度的长期变化提供了不完整的基础。热电离质谱技术的新进展使碳酸盐中痕量硫酸盐的S同位素分析成为可能，它将被改进以适应地质材料的分析。与NIST的鲍勃·凯利合作开发这些微技术需要资金。我们用于碳、锶和钕同位素测量的标准方法在出版物中有很好的记录，这些出版物证明了本提案的技术目标。关于新元古代海水同位素变化率的精确知识将被用来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集层、海洋和大气演化之间的关系；2)确定侵蚀、有机碳埋藏和热液蚀变的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、磷矿和带状铁沉积事件的关系；4)探讨大气氧平衡是否与S和C的地壳储集层耦合，或者是否如我们先前的研究所支持的那样，C旋回和S旋回的解耦导致大气O2丰度的巨大变化；5)加强了这些新元古代序列之间的同位素地层和生物地层对比之间的联系。在新元古代，海水ND和Sr的收支似乎已从主要的热液输入(如太古宙)转变为主要的大陆(河流)输入。基于过去和拟议研究数据的模拟将被用来限制热液循环速率、大陆侵蚀速率、有机碳埋藏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演化的影响。本实验室的前期工作表明，新元古代沉积序列可以获得高分辨率的锶、碳同位素曲线。拟议中的研究将继续在我们对新元古代演化的理解上取得重大突破。

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.