Isotopic Evolution of Proterozoic Sediments and Seawater

最近的研究指出，最新的元古代（0.5-0.9 Ga）是一个巨大的进化变化时期。纳米比亚、斯匹次卑尔根、南非和中国保存完好的沉积层序已被详细研究，因为它们含有重要的经济沉积物和第一批后生动物的遗迹。此外，在这些层序中发现了强烈的碳和锶同位素变化，这可能对世界范围内的同古地层对比有用。为了更好地描述元古宙后期的同位素变化速率，我们建议继续研究具有较好特征和保存较好的Nama组、Polarisbreen组和Kango组以及陡山沱组和灯影组地层样品中C、Sr和Nd的同位素变化。在对最新元古代海水同位素变化率有了更好的了解之后，我们的目标是：1)追踪外生地球化学旋回中的主要物质通量，了解晚元古代沉积储层、海洋和大气演化之间的关系；2)确定侵蚀速率、有机碳埋藏速率和热液蚀变速率及其与全球构造的关系；3)研究了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)加强了这些晚元古代序列之间化学地层和生物地层对比的联系。在元古代，海水Nd和Sr的收支似乎从主要的热液输入（如太古宙）转变为主要的大陆（河流）输入。该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晚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。在前人的工作中，我们已经证明了在晚元古代沉积序列中可以获得高分辨率的Sr和C同位素曲线，并相信本研究将为我们对晚元古代演化的认识带来重大突破。

Recent investigation point to the latest Proterozoic (0.5-0.9 Ga) as being a time of vast evolutionary change. Well-preserved sedimentary successions in Namibia, Spitsbergen, South Africa and China have been studied in detail as they contain important economic deposits and remnants of the first metazoan faunas. In addition, strong carbon- and strontium-isotopic variations are found in these sequences and may be useful for the correlation of coeval strata world-wide. We propose to continue research on the isotopic variations in C, Sr and Nd in well-characterized and well- preserved stratigraphic samples from the Nama, Polarisbreen and Kango groups as well as the Doushantuo and Dengying formations to better characterize the rate of isotopic change during the latter part of the Proterozoic Eon. Given a better knowledge of the rate of isotopic changes in latest Proterozoic seawater our goals are to: 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 Late Proterozoic, 2) determine rates of erosion, organic carbon 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, and 4) strengthen the tie between chemo- stratigraphic and bio-stratigraphic correlations between these Late Proterozoic successions. During the Proterozoic, seawater Nd and Sr budgets appear to have changed from dominantly hydrothermal inputs (as in the Archean) to dominantly continental (river) inputs. Modeling of the data obtained from this research will be used to constrain the changes in rates of hydrothermal circulation, continental erosion rates, burial rates of organic carbon and variations in the mean age of continental erosion products through the late Proterozoic. Implications for the evolution of O2 and CO2 in the Earth's atmosphere will also be considered. We have demonstrated with our previous work that high resolution Sr and C isotopic curves may be obtained for late Proterozoic sedimentary successions and are confident that this research will lead to major breakthroughs in our understanding of Late Proterozoic evolution.