Collaborative Research: Validation of the lacustrine branched GDGT paleothermometer

对过去大陆温度的重建为了解地球气候系统及其对内部和外部强迫机制的敏感性提供了重要的见解。这些重建为观测到的和预估的未来气候变化提供了地质背景，并揭示了太阳辐照度、火山事件和温室气体浓度变化等各种强迫对全球气候的影响。保存在沉积物中的有机分子为推断过去的气候提供了一种工具，这种工具可能在空间和时间上都广泛适用。基于支化甘油二烷基甘油四醚（brGDGTs）（细菌产生的膜脂）的组合，最近发展起来的一项很有前途的技术，有可能扩大对各种气候制度、时间尺度和沉积环境中过去大陆温度的理解。brGDGTs代表了一个重要的气候档案，因为它们在湖泊中广泛而丰富，它们表现出与水温和pH值相关的结构差异，并且它们可以在湖泊沉积物中保存很长时间（数百万年）。然而，brGDGTs是否为过去的温度重建提供了一种通用的方法尚未得到严格的评估，温度和pH以外的环境参数的潜在影响尚未得到评估。通过基础现场和实验室实验，研究人员将阐明brGDGT对环境胁迫的结构响应，并研究氧暴露、降解和微生物群落结构对brGDGT分布的影响。这项研究为验证brdgt古温度计在湖泊中的应用提供了必要的基础工作。调查人员将与尤里卡！马萨诸塞州霍利奥克的女孩计划提供环境地球化学的暑期实践讲习班，作为为期一个月的暑期科学营的一部分，旨在鼓励初中和高中女孩在STEM领域追求职业。支链甘油二烷基甘油四醚（brGDGTs）是一种普遍存在的化合物，存在于各种陆地环境中，包括土壤、河流和湖泊。先前的研究，包括研究人员所做的工作，已经确定这些化合物的甲基化程度对温度很敏感，这预示着brGDGT分布可以作为过去温度变化的代表。然而，许多挑战和未解决的问题限制了brdgt温度计的应用。其中包括未知的生物来源，缺乏brGDGT结构对环境强迫反应的第一性原理证据，以及成岩改变和/或对氧暴露敏感的证据。该项目旨在通过实地和实验室的方法解决其中的一些基本挑战。在实验室环境中，研究人员将维持和操作中尺度实验，以确定brGDGT甲基化和环化是否对温度和ph有响应。在现场，他们将监测目标湖泊中的brGDGT通量并进行受控降解实验。脂质分析将与Illumina MiSeq测序配对，以了解微生物群落结构如何影响brGDGT分布。最后，他们将对一组湖泊沉积物进行探索性统计分析，研究brGDGT分布在不同沉积和气候条件下的表现。该项目的结果将构成对brGDGTs理解的根本性进步，并提高将其用作古温度计的能力。

The reconstruction of past continental temperatures provides critical insights into the Earth's climate system and its sensitivity to internal and external forcing mechanisms. Such reconstructions provide geological context for observed and projected future climate changes, and reveal the impact of diverse forcings, such as changes in solar irradiance, volcanic events, and greenhouse gas concentrations, on global climate. Organic molecules preserved in sediments provide a tool for inferring past climate that may be widely applicable in both space and time. A promising and recently developed technique based on assemblages of branched glycerol dialkyl glycerol tetraethers (brGDGTs), membrane lipids produced by bacteria, has the potential to expand understanding of past continental temperatures across a variety of climatic regimes, time scales, and depositional environments. brGDGTs represent an important climate archive because they are widespread and abundant in lakes, they exhibit structural differences related to water temperature and pH, and they can be preserved for long periods of time (millions of years) in lake sediments. However, it has not been rigorously assessed if brGDGTs provide a universal method for past temperature reconstruction and the potential influence of environmental parameters other than temperature and pH has not yet been evaluated. Through conducting fundamental field and laboratory experiments, the investigators will elucidate the structural response of the brGDGTs to environmental stresses and investigate the impacts of oxygen exposure, degradation, and microbial community structure on brGDGT distributions. This research represents necessary, fundamental work to validate the use of the brGDGT paleothermometer in lakes. Investigators will partner with the Eureka! Program of Girls, Inc. of Holyoke, MA to offer hands-on summer workshops in environmental geochemistry, as part of a month-long summer science camp designed to encourage middle and high school girls to pursue careers in STEM fields.Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are ubiquitous compounds, found in a variety of terrestrial environments includes soils, rivers, and lakes. Previous research, including work done by the investigators, has established that the degree of methylation of these compounds is sensitive to temperature, auguring the use of brGDGT distributions as a proxy for past temperature changes. However, a number of challenges and unresolved issues limit the application of a brGDGT thermometer. These include unknown biological source(s), a lack of first-principles evidence that brGDGT structures respond to environmental forcings, and evidence for diagenetic alteration and/or sensitivity to oxygen exposure. This project aims to address some of these fundamental challenges via both a field and laboratory-based approach. In a lab setting, researchers will maintain and manipulate mesocosm experiments to determine whether brGDGT methylation and cyclization responds to temperature and pH. In the field, they will monitor brGDGT fluxes in a target lake and conduct controlled degradation experiments. Lipid analyses will be paired with Illumina MiSeq sequencing in order to understand how microbial community structure influences brGDGT distributions. Finally, they will conduct exploratory statistical analyses on a set of lake sediments to study how brGDGT distributions behave in different sedimentary and climatic regimes. The results of this project will constitute a fundamental advance in the understanding of brGDGTs and improve the ability to use them as a paleothermometer.