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

要很好地理解过去的气候，就需要准确地重建海洋温度和生物生产力等因素。一种这样的古温度方法是“TEX86代理”，它是基于保存下来的古细菌的有机分子，古细菌是海洋微生物的重要类别。TEX86记录对于研究过去1亿年的气候特别有价值，特别是在古代暖期，没有其他温度指标可用。尽管在现代海洋中，从TEX86预测的温度与实际观测的海洋表面温度(SSTs)之间通常有很好的一致性，但TEX86可能会产生显著的区域或季节性误差。在过去与海洋含氧量低相关的温室气候中，也观察到了不同的观点；特别是，TEX86有时预测在这些时间间隔期间气温会更低，尽管海洋实际上正在变暖。为了加深对TEX86温度记录的理解，该项目将研究TEX86的环境和生态控制，从而加强其在古海洋学中的价值。这些结果可能不仅与气候研究有关，而且还与理解古细菌在追踪营养物质分布和海洋生物地球化学历史方面的重要性有关。这项研究的中心假设是，TEX86的温度误差是由于古细菌生长速度的地区差异造成的，这是氨氧化的函数，因此也是海洋出口产量的函数。这意味着TEX86是一个综合了温度和海洋生物地球化学的信号。主要的分析目标是通过测量单个TEX86化合物的同位素比率(称为IGDGT)，将稳定的碳和氢同位素信息纳入TEX86温度替代记录。与温度重建相结合的同位素测量将共同实现生产率信号与温度信号的分离。具体地说，这项工作将审查三个不同的记录，这些记录与不同时间尺度上海洋生物地球化学结构的重大变化有关。记录1：地中海的上新世-更新世腐殖层，其平均海洋温度没有明显的波动，但营养状况发生了重大的时间变化。记录2：更新世和全新世赤道东太平洋，热带环境缺乏季节性，但显示出温度和生产力的长期变化。记录3：古新世-始新世最高温度(PETM)及其相关的碳同位素漂移。这三项记录共同实现了主要变量温度和生产率的分离。为了实现这些目标，这项研究需要几种新的方法：用于碳同位素分析的Spooling Wire Microburning-IRMS(SWIM-IRMS)，同时测量TEX86和UK37的一体化LC-MS方法，以及用于氢的同位素分析的高温GC-IRMS；因此，先进的同位素技术也在该项目中发挥作用。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.