Collaborative Research: Development of a novel way for understanding ancient Earth atmospheres and marine sulfate using the stable isotope of Oxygen (17O) in marine barite.

海洋沉积物包含了气候演变和地球表面在过去1.25亿年间如何变化的记录。这个故事的一部分被锁在海洋重晶石（BaSO4）中，这是一种从海水中沉淀出来的矿物质，含有硫和氧。通过对海水硫酸盐（SO4）稳定同位素的研究，确定了对地球生命至关重要的生物地球化学过程和其他化学循环的变化。然而，由于这种硫酸盐的性质及其结合气候、微生物代谢和风化等多种过程信息的能力，传统的氧同位素（18O/16O）和硫同位素（34S/32S）测量结果的解释通常不是唯一的。这项研究探索了一种新的、独立的方法，通过添加一种以前难以测量的、因此通常被忽视的氧同位素：17O，来解开储存在海洋重晶石中的信息。直到最近，随着越来越灵敏的质谱仪和新的实验室方法的发展，这些测量才成为可能。锁定在海洋重晶石中的17O信号有可能识别大气中氧气/二氧化碳的比例，以及生物圈活动的强度，以及这些参数如何随时间变化，这是理解当前大气成分和与全球变暖相关的过程的重要知识。本初步研究分析了10个海洋岩心顶部的重晶石，以定量评估重晶石是否是海洋硫酸盐的忠实记录者。为了进一步验证17O是否可以为揭示各种环境过程的影响提供可靠的代理，将对岩心顶部研究中分析的其他岩心和下岩心的样品进行分析。本研究进一步发展和探索了利用海相重晶石构建过去1.25亿年（即从白垩纪到新生代）海水硫酸盐17O指纹的可靠记录的潜力。如果成功，这项工作，结合更常用的测量氧（18O/16O）和硫（33S/32S和34S/36S）同位素比率的结果，有可能提供随时间变化的大气O2/CO2组成和地球生物圈活动的见解。虽然以前对海洋重晶石的17O测量很难进行，而且有很高的不确定性，但这种限制现在已经被质谱法的进步和实验室程序的发展所克服，这些程序被调整为提高17O的分析精度。该研究的目标是从太平洋海底收集的沉积物岩心中精心挑选的重晶石样本中测量17O，并量化现代岩心顶部重晶石与同期水柱硫酸盐之间的偏移量（如果有的话）。它还将测量17O的岩心变化，以检查同位素特征随时间的变化，并将这些结果与共存孔隙水的结果进行比较，以检查可能的成岩作用。结果将在海洋记录中验证这一新代理。重晶石样品将从10个岩心顶部的沉积物中提取，使用酸浸和其他分离技术除去除重晶石以外的所有含氧相。将使用扫描和分析电子显微镜等技术检查所得重晶石的纯度。同位素工作样品的制备将包括氟化和高分辨率同位素比质谱仪分析。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Marine sediments contain a record of climate evolution and how Earth's surface has changed over the last 125 million years. Part of this story is locked up in marine barite (BaSO4), a mineral that precipitates from seawater and contains sulfur and oxygen. Through studies of the stable isotopes in seawater sulfate (SO4), changes in biogeochemical processes and variations in other chemical cycles that are important for life on Earth have been determined. However, due to the properties of this sulfate and its ability to incorporate information from processes as diverse as climate, microbial metabolism and weathering, interpretation of traditional oxygen isotope (18O/16O) and sulfur isotope (34S/32S) measurements commonly result in interpretations that are not unique. This research explores a new, independent means for unraveling the information stored in marine barite via the addition of a previously difficult-to-measure, and hence generally overlooked, isotope of oxygen: 17O. Only recently, with the development of increasingly sensitive mass spectrometers and new laboratory methodologies, have these measurements become possible. The 17O signal locked inside marine barite has the potential to identify the oxygen/carbon dioxide ratio of the atmosphere as well as the intensity of biospheric activity and how these parameters have changed over time, important knowledge for understanding present atmospheric compositions and processes related to global warming. This pilot study analyzes barite in the core tops of ten marine cores to quantitatively evaluate whether barite is a faithful recorder of marine sulfate. To further validate whether 17O can provide a reliable proxy for unraveling the influences of various environmental processes, analyses of samples from additional cores and down core of the those analyzed in the core top study will be carried out. This research further develops and explores the potential of using marine barite to construct a reliable record of the 17O fingerprint of seawater sulfate over the last 125 million years (i.e., from the Cretaceous through the Cenozoic). If successful, the work, in combination with the results of more commonly measured oxygen (18O/16O) and sulfur (33S/32S and 34S/36S) isotope ratios, has the potential to provide insights into the O2/CO2 composition of the atmosphere over time and activity of Earth's biosphere. Although previously 17O measurements of marine barite were difficult to make and had high uncertainties, this limitation has now been overcome by advances in mass spectrometry and the development of laboratory procedures that are tuned to increase the analytical precision of 17O. Goals of the research are to measure 17O in carefully selected barite samples from sediment cores collected from the floor of the Pacific Ocean and quantify the offset, if any, between modern core-top barite and contemporaneous water column sulfate. It will also measure down core variations of 17O to examine variations in the isotope signature with time and compare these with the results from co-existing pore waters to examine possible diagenetic effects. The result will be validation of this new proxy in the marine record. Samples of barite will be extracted from sediment from the tops of ten cores, using acid leaching and other separation techniques to remove all oxygen-bearing phases but barite. The resulting barite will be checked for purity using techniques including scanning and analytical electron microscopy. Preparation of samples for isotope work will include fluorination and analysis on a high resolution isotope ratio mass spectrometer.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.