Base de données de recherche des TNO

Régions: North Slave Region

étiquettes: stromatolites, three-dimensional analyses, Earth history

Objectif(s): To better understand the relationships between environment, biology, and the form of stromatolites; and to provide insights about past environmental conditions or the evolution of life.

Description du projet: This licence has been issued for the scientific research application No. 5571. For much of Earth’s history, microbes have been the only forms of life on our planet. As a result of this dominance, the sedimentary rock record is full of physical and chemical evidence of microbial activity. Stromatolites, which are laminated buildups constructed, or aided, by microbes, are one kind of microbial fossil. Researchers believe that some combination of physical (e.g., wave energy) and biological factors (e.g., microbial response to changing light levels) control the form---or the shape and structure---of microbial buildups. Unfortunately, the relationships between environment, biology, and the form of stromatolites are poorly understood. In order to use stromatolite shape and structure to provide insights about past environmental conditions or the evolution of life, The team must better understand these relationships. The East Arm of Great Slave Lake is an ideal natural laboratory for the study of stromatolites. The rocks of the Pethei Group include a wide variety of well-exposed stromatolite buildups of various sizes, including isolated individuals, meter-scale buildups, and kilometer-scale reef systems. Previous workers have shown that the forms and orientations of these stromatolites vary based on location. By combining detailed field observations with three-dimensional (3D) reconstructions, the research team can begin to understand exactly how different biological and environmental factors controlled stromatolite shape. For decades, geochemists have worked to understand how Earth's climate varied over the last two billion years. Ancient seawater chemistry may have impacted climate change by burying or storing CO2 in the form of carbonate minerals. The rocks of the Pethei Group represent a valuable opportunity to better understand these interactions. The rocks exposed along Great Slave Lake are unique because they preserve sediments deposited at the same time, both in deep and shallow waters of the ancient basin. This fact enables the team to, island by island, ``walk" up the ancient seafloor, through almost 1 kilometer of water depth. The research team believes that the texture, features, and chemical composition of the rocks will reveal trends from deep to shallow settings that reflect changes in ancient seawater chemistry, exposure to a low-oxygen environment, and physical mixing. By studying the Pethei Group, the research team aims to understand how the sediments in the rocks formed prior to the evolution of skeletal animals. Since most of Earth's history featured oceans without any animal life, the findings will help in more accurately interpreting other ancient sedimentary units that are substantially different from Earth’s modern seafloor. These observations will also help to understand atmosphere-ocean climate interactions over Earth’s history, and may give greater insight into the aftermath of mass extinctions, which are times when animal life is severely reduced and microbial communities temporarily dominate the seafloor. During fieldwork, the research team plans to measure layers of rock with a Jacob's Staff, produce detailed geologic maps, and take photographs and notes. The research team also intends to collect two kinds of physical samples (using rock hammers): 1. Thumb-sized chips for thin sections and for geochemical analyses; and, 2. Oriented centimeter-scale blocks for 3D reconstructions. Every sample both will be geolocated (via GPS) and associated with a specific layer of rock, which will allow the team to place analyses or reconstructions within a broader context. As is standard practice, the research team will ensure that they only take samples from inconspicuous locations in a way that limits damage to the rocks. In addition to making observations and collecting samples, the research team plans to image outcrops using a drone that is equipped with a high resolution digital camera. Dr. Mehra, who is licensed to fly drones in the United States and has several hundred hours of experience, will program and pilot all flights. The team plans to process the collected imagery using Structure from Motion (SfM; a process that produces 3D reconstructions from multiple overlapping photographs) to build digital models. Using these models, the team will measure the size, shape, and spacing of microbial buildups, which will help illustrate how these structures formed and changed their environments 1.9 billion years ago. The elements and isotopes that make up carbonate rocks reveal a lot about the past climates, paleo-environments, and biologies of Earth. The research team will measure the chemistry of individual thumb-sized chips using a variety of laboratory methods, which will require the team to drill and powder each sample. The shape of the different parts of stromatolites (such as individual layers) provides information about the biological and physical processes (such as trapping and binding of sediment) that work together to produce a microbial buildup. One way to extract the 3D shape of such features is with serial grinding and imaging. This technique involves polishing the surface of a sample (removing approximately 0.03 millimeters of material at a time), taking a photo of the freshly ground surface with a high resolution digital camera, and repeating these two steps until a stack of images is all that remains. Using machine learning algorithms, the research team will turn the images into a 3D digital model. The research team intends to engage NWT stakeholders in every step of the research process, including, but not limited to: consulting the management board on study design and sampling locations, presentations to the community on the objectives and (eventually) findings, and contributions to interpretative programs for the National Reserve. The fieldwork for this study will be conducted - July 10 - August 15, 2024