Periodic Reporting for period 2 - RESpiReS (Reconstructing the Effect of Sulfide Respiration on Global Redox State: Insights from Experiments, Observations, and Models)
Reporting period: 2022-12-01 to 2024-05-31
Earth’s atmosphere currently contains approximately 80% molecular nitrogen (N2), 20% molecular oxygen (O2), and 400 parts per million---or 0.4%---carbon dioxide (CO2), along with other trace constituents. However, this has not always been the case. We know that Earth’s earliest atmosphere was devoid of molecular oxygen, instead being completely anoxic. Atmospheric oxygen levels subsequently increased in two stages---first, during the so-called “Great Oxygenation Event” around 2.3 billion years ago, and second, during the so-called “Neoproterozoic Oxygenation Event” between around 800 and 600 million years ago.
While geologists have garnered a good understanding of the timing and, to a lesser degree, the magnitude of these oxygenation events, the exact combination of geological, chemical, and biological mechanisms that allowed them to occur remain less constrained. While (nearly) all molecular oxygen on Earth is originally produced by photosynthesizing organisms, subsequent reactions such as respiration consume this O2 and prevent it from accumulating. Thus, the very existence of an oxygenated atmosphere requires that photosynthesis outpaces respiration, and that this was especially true during Earth’s two oxygenation events. Still, the exact reason(s) for this remain poorly known, largely due to a lack of quantitative constraints on photosynthesis and respiration through geologic time.
One such respiration mechanism is the conversion of the iron-sulfide mineral pyrite (FeS2) into sulfate (SO42-) during weathering of rocks on land, which simultaneously consumes molecular oxygen and releases carbon dioxide. Pyrite oxidation thus links the global carbon, sulfur, and oxygen cycles. The objective of this project is to develop chemical and isotopic tools to constrain and quantify pyrite oxidation, both on the modern Earth and throughout Earth’s history. We are specifically addressing this by combining well-constrained pyrite weathering experiments in the lab to understand isotope signatures with field studies of modern weathering environments (i.e. river basins) to determine controlling mechanisms and with geologic studies to reconstruct this process through time.
Beyond understanding the history of Earth’s atmospheric oxygen levels, this project is important for society in several ways: First, pyrite oxidation is a major natural process by which atmospheric carbon dioxide is produced, and better understanding the mechanisms that control this will yield insight into ongoing anthropogenic climate change. Second, by developing isotopic fingerprints for sulfur-cycle processes, we are developing a new tool that will aid in paleoclimate reconstructions---thus placing our modern climate perturbations within a historical context. Finally, the results of this work will develop a framework to analyze and interpret sulfur cycling in non-terrestrial materials, potentially including reconstructing traces of atmospheric O2 in other celestial bodies.
While geologists have garnered a good understanding of the timing and, to a lesser degree, the magnitude of these oxygenation events, the exact combination of geological, chemical, and biological mechanisms that allowed them to occur remain less constrained. While (nearly) all molecular oxygen on Earth is originally produced by photosynthesizing organisms, subsequent reactions such as respiration consume this O2 and prevent it from accumulating. Thus, the very existence of an oxygenated atmosphere requires that photosynthesis outpaces respiration, and that this was especially true during Earth’s two oxygenation events. Still, the exact reason(s) for this remain poorly known, largely due to a lack of quantitative constraints on photosynthesis and respiration through geologic time.
One such respiration mechanism is the conversion of the iron-sulfide mineral pyrite (FeS2) into sulfate (SO42-) during weathering of rocks on land, which simultaneously consumes molecular oxygen and releases carbon dioxide. Pyrite oxidation thus links the global carbon, sulfur, and oxygen cycles. The objective of this project is to develop chemical and isotopic tools to constrain and quantify pyrite oxidation, both on the modern Earth and throughout Earth’s history. We are specifically addressing this by combining well-constrained pyrite weathering experiments in the lab to understand isotope signatures with field studies of modern weathering environments (i.e. river basins) to determine controlling mechanisms and with geologic studies to reconstruct this process through time.
Beyond understanding the history of Earth’s atmospheric oxygen levels, this project is important for society in several ways: First, pyrite oxidation is a major natural process by which atmospheric carbon dioxide is produced, and better understanding the mechanisms that control this will yield insight into ongoing anthropogenic climate change. Second, by developing isotopic fingerprints for sulfur-cycle processes, we are developing a new tool that will aid in paleoclimate reconstructions---thus placing our modern climate perturbations within a historical context. Finally, the results of this work will develop a framework to analyze and interpret sulfur cycling in non-terrestrial materials, potentially including reconstructing traces of atmospheric O2 in other celestial bodies.
1. Design, build, and set-up our laboratory. Because my professorship at ETH Zurich was created as a result of being awarded an ERC grant, our first step was to build our laboratory space and install all instrumentation. To this end, we have successfully purchased and installed an isotope ratio mass spectrometer (IRMS) for high-precision isotope measurement, as well as several wet-chemistry tools for sample preparation and analysis (e.g. flow-through weathering reactors including all in situ sensors, an anoxic glove chamber, and a custom-built elemental analyzer coupled to a gas preparation device for sample introduction into the IRMS). In particular, the automated gas preparation device for sample introduction has been a major achievement and will likely be replicated by several other laboratories.
2. Hire all personnel for the project. This includes two Ph.D. students, Cornelia Mertens and Anna Somlyay, and one Postdoctoral researcher, Dr. Nir Galili. All personnel were hired within the first 18 months of the project and have begun performing various laboratory tasks.
3. Publish initial scientific manuscripts. Concurrent with the laboratory build, we have published the first manuscripts on high-precision oxygen isotope analyses as a result of this project. Specifically, this includes one manuscript (Hemingway et al., 2022 GCA) in which we modeled isotope fractionation factors using computational chemistry software and one manuscript (Sutherland et al., 2022 GCA) in which we experimentally constrained similar fractionation factors.
4. Prepare and submit subsequent scientific manuscripts. We have been working on two manuscripts which will likely be submitted by the end of February, 2024. This includes: (1) a manuscript reconstructing the amount and isotope composition of dissolved organic carbon in the ocean over geologic time, and (2) a manuscript constraining the amount and isotope composition of pyrite formed in marine sediments today and through time. Both manuscripts are led by junior researchers on this project (1, Dr. Nir Galili; 2, Cornelia Mertens).
5. Prepare and disseminate isotope standards to the scientific community. This effort has been led by Ph.D. student Anna Somlyay and will lead to significantly improved inter-laboratory data comparability by providing the same standard materials for all laboratories.
2. Hire all personnel for the project. This includes two Ph.D. students, Cornelia Mertens and Anna Somlyay, and one Postdoctoral researcher, Dr. Nir Galili. All personnel were hired within the first 18 months of the project and have begun performing various laboratory tasks.
3. Publish initial scientific manuscripts. Concurrent with the laboratory build, we have published the first manuscripts on high-precision oxygen isotope analyses as a result of this project. Specifically, this includes one manuscript (Hemingway et al., 2022 GCA) in which we modeled isotope fractionation factors using computational chemistry software and one manuscript (Sutherland et al., 2022 GCA) in which we experimentally constrained similar fractionation factors.
4. Prepare and submit subsequent scientific manuscripts. We have been working on two manuscripts which will likely be submitted by the end of February, 2024. This includes: (1) a manuscript reconstructing the amount and isotope composition of dissolved organic carbon in the ocean over geologic time, and (2) a manuscript constraining the amount and isotope composition of pyrite formed in marine sediments today and through time. Both manuscripts are led by junior researchers on this project (1, Dr. Nir Galili; 2, Cornelia Mertens).
5. Prepare and disseminate isotope standards to the scientific community. This effort has been led by Ph.D. student Anna Somlyay and will lead to significantly improved inter-laboratory data comparability by providing the same standard materials for all laboratories.
1. Instrument and analytical design. Our IRMS sample preparation device offers clear advantages relative to traditional instrumentation. Specifically, by automating sample preparation steps and miniaturizing several components, we are able to achieve higher precision with less sample size compared to other laboratories. Additionally, by omitting gas-phase fluorination agents, our setup offers further safety advantages.
2. Development of community standards. Given the high-precision nature of the IRMS analyses used in this project, small differences in protocols and preparation steps between laboratories can lead to large offsets. To overcome this, we developed a suite of mineral standards with varying isotopic compositions (i.e. by synthesizing these in equilibrium with water of varying isotopic compositions) to be circulated to the community. By ensuring all laboratories use and report data to the same set of community standards, this work will significantly improve inter-laboratory data comparison.
3. Flow-through reactor design. To date, we have designed, built, and tested flow-through “chemostat” laboratory weathering reactors for the purpose of constraining isotope effects in response to various environmental conditions (e.g. oxygen levels, pH, etc.). These reactors offer significant improvement over existing “batch” experiments, for example: high-precision and continuous O2 monitoring, complete control over fluid residence times, and in situ monitoring of short-lived reactive oxygen species. This setup will lead to improved understanding of several weathering processes.
4. Model improvement. In addition to laboratory work, we are improving global sulfur-cycle models. Specifically, this includes a diagenetic model to describe pyrite formation and corresponding isotope compositions in marine sediments. This work aims to determine global pyrite (i) formation rates and (ii) isotope compositions in the modern ocean and (iii) use this knowledge to interpret pyrite formation environments throughout Earth’s history. These results will update our understanding of geologic sulfur cycling.
2. Development of community standards. Given the high-precision nature of the IRMS analyses used in this project, small differences in protocols and preparation steps between laboratories can lead to large offsets. To overcome this, we developed a suite of mineral standards with varying isotopic compositions (i.e. by synthesizing these in equilibrium with water of varying isotopic compositions) to be circulated to the community. By ensuring all laboratories use and report data to the same set of community standards, this work will significantly improve inter-laboratory data comparison.
3. Flow-through reactor design. To date, we have designed, built, and tested flow-through “chemostat” laboratory weathering reactors for the purpose of constraining isotope effects in response to various environmental conditions (e.g. oxygen levels, pH, etc.). These reactors offer significant improvement over existing “batch” experiments, for example: high-precision and continuous O2 monitoring, complete control over fluid residence times, and in situ monitoring of short-lived reactive oxygen species. This setup will lead to improved understanding of several weathering processes.
4. Model improvement. In addition to laboratory work, we are improving global sulfur-cycle models. Specifically, this includes a diagenetic model to describe pyrite formation and corresponding isotope compositions in marine sediments. This work aims to determine global pyrite (i) formation rates and (ii) isotope compositions in the modern ocean and (iii) use this knowledge to interpret pyrite formation environments throughout Earth’s history. These results will update our understanding of geologic sulfur cycling.