Beyond ocean temperature: Extracting new dimensions of paleoclimatic information from archaeal lipids and their isotopic compositions

要想对过去的气候有一个很好的了解，就需要对海洋温度和生物生产力等因素进行精确的重建。其中一种古温度方法是“TEX86代用法”，它基于古细菌（一种重要的海洋微生物）保存下来的有机分子。TEX86记录对于研究过去1亿年的气候特别有价值，特别是在其他温度代用物无法获得的古代温暖时期。虽然在现代海洋中，TEX86预测的温度与实际观测到的海表温度（SSTs）之间普遍存在良好的一致性，但TEX86可能产生显著的区域或季节误差。在过去与海洋低氧水平有关的温室气候中，也观察到不同意见；特别是，TEX86有时预测在这样的间隔期间温度会更低，即使海洋实际上正在变暖。为提高对TEX86温度记录的认识，本项目将研究TEX86的环境和生态控制，从而加强其古海洋学价值。这一结果可能不仅与气候研究有关，而且对了解古细菌在追踪营养分布和海洋生物地球化学历史方面的重要性也有重要意义。本研究的中心假设是，TEX86的温度误差是由于古细菌生长速度的区域差异造成的，这是氨氧化的函数，因此是海洋出口生产的函数。这表明TEX86是一个温度和海洋生物地球化学的综合信号。主要的分析目标是通过测量单个TEX86化合物（称为iGDGTs）的同位素比率，将稳定的碳和氢同位素信息纳入TEX86温度代理记录中。同位素测量与温度重建相结合，将使生产力信号与温度信号解耦。具体来说，这项工作将检查三个不同的记录，这些记录与海洋生物地球化学结构在不同时间尺度上的主要变化有关。记录1：地中海的上新世-更新世湖冲力形成，在此期间，海洋平均温度没有显著波动，但营养物制度的主要时间变化已被推断。记录2：更新世和全新世东赤道太平洋，一个缺乏季节性的热带环境，但显示出温度和生产力的长期变化。记录3：古新世-始新世热最大值（PETM）及其相关的碳同位素偏移。这三个记录共同允许分离主要变量温度和生产率。为了实现这一目标，研究需要几种新的方法：用于碳同位素分析的线轴微燃烧- irms (swims - irms)，同时测量TEX86和UK37的一体化HPLC-MS方法，以及用于氢同位素分析的高温GC-IRMS；因此，先进的同位素技术也在这个项目中发挥作用。该项目支持博士后研究人员的职业发展。

A good understanding of past climate requires accurate reconstruction of factors such as ocean temperature and biological productivity. One such paleotemperature method is the "TEX86 proxy", which is based on the preserved organic molecules of Archaea, an important class of ocean microbes. TEX86 records have been particularly valuable for studying climate of the last 100 million years, especially during ancient warm episodes where other temperature proxies are unavailable. Although in the modern ocean there generally is good agreement between temperatures predicted from TEX86 and the actual observed sea surface temperatures (SSTs), TEX86 can yield significant regional or seasonal errors. Disagreement also has been observed during past greenhouse climates associated with low oxygen levels in the oceans; in particular, TEX86 sometimes predicts colder temperatures during such intervals, even though the ocean actually is warming. To improve understanding of TEX86 temperature records, this project will examine environmental and ecological controls on TEX86, thereby strengthening its value for paleoceanography. The results may be relevant not only to climate studies, but also to understanding the importance of Archaea in tracing nutrient distributions and the biogeochemical history of the ocean. The central hypothesis of this research is that temperature errors in TEX86 are due to regional differences in growth rate for Archaea, which is a function of ammonia oxidation and therefore of marine export production. This implies that TEX86 is a combined temperature and marine biogeochemistry signal. The major analytical goal is to incorporate stable carbon and hydrogen isotopic information into TEX86 temperature proxy records, by measuring isotopic ratios for the individual TEX86 compounds (called iGDGTs). Isotopic measurements in tandem with temperature reconstruction will collectively allow productivity signals to be decoupled from temperature signals. Specifically, the work will examine three distinct records that are associated with major shifts in biogeochemical structuring of the ocean over different time scales. Record 1: the Pliocene-Pleistocene sapropel formation in the Mediterranean Sea, for which no significant fluctuations in mean ocean temperature but major temporal changes in the nutrient regime have been inferred. Record 2: the Pleistocene and Holocene Eastern Equatorial Pacific, a tropical environment lacking seasonality but showing secular changes in both temperature and productivity. Record 3: the Paleocene-Eocene Thermal Maximum (PETM) and its associated carbon isotope excursion. These three records collectively allow for separation of the major variables temperature and productivity. To achieve the goals, the research requires several novel methods: Spooling Wire Microcombustion-IRMS (SWiM-IRMS) for isotopic analysis of carbon, all-in-one HPLC-MS approaches to measuring TEX86 and UK37 simultaneously, and high-temperature GC-IRMS for isotopic analysis of hydrogen; advanced isotopic technologies therefore also play a role in this project. The project supports career development for a postdoctoral researcher.