Collaborative Research: Validation of the lacustrine branched GDGT paleothermometer

过去大陆温度的重建为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。这种重建为观测和预测的未来气候变化提供了地质背景，并揭示了各种强迫对全球气候的影响，例如太阳辐照度、火山事件和温室气体浓度的变化。沉积物中保存的有机分子提供了一种推断过去气候的工具，可以在空间和时间上广泛应用。最近开发的一项有前途的技术基于支链甘油二烷基甘油四醚（brGDGT）（细菌产生的膜脂）的组合，有可能扩大对过去大陆温度在各种气候状况、时间尺度和沉积环境中的了解。 brGDGT 代表了重要的气候档案，因为它们在湖泊中分布广泛且丰富，表现出与水温和 pH 值相关的结构差异，并且可以在湖泊沉积物中保存很长一段时间（数百万年）。然而，尚未严格评估 brGDGT 是否为过去的温度重建提供通用方法，并且尚未评估除温度和 pH 之外的环境参数的潜在影响。通过进行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，并研究氧气暴露、降解和微生物群落结构对 brGDGT 分布的影响。这项研究代表了验证 brGDGT 古温度计在湖泊中使用的必要基础工作。调查人员将与尤里卡合作！马萨诸塞州霍利奥克的 Girls, Inc. 计划将提供环境地球化学夏季实践研讨会，作为为期一个月的夏季科学营的一部分，旨在鼓励初中和高中女孩在 STEM 领域追求职业生涯。支链甘油二烷基甘油四醚 (brGDGT) 是普遍存在的化合物，存在于土壤、河流和湖泊等各种陆地环境中。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敏感，预示着可以使用 brGDGT 分布作为过去温度变化的代理。然而，许多挑战和未解决的问题限制了 brGDGT 温度计的应用。这些包括未知的生物来源、缺乏 brGDGT 结构对环境强迫作出反应的第一原理证据，以及成岩改变和/或对氧暴露敏感性的证据。该项目旨在通过现场和实验室方法解决其中一些基本挑战。在实验室环境中，研究人员将维持和操作介观实验，以确定 brGDGT 甲基化和环化是否对温度和 pH 值有反应。在现场，他们将监测目标湖中的 brGDGT 通量并进行受控降解实验。脂质分析将与 Illumina MiSeq 测序相结合，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，以研究 brGDGT 分布在不同沉积和气候状况下的表现。该项目的结果将构成对 brGDGT 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.