Collaborative research: Validation of the Lacustrine Branched GDGT Paleothermometer

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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.