Isotopes VSI
Isotopes
- p11: DeVries (sun-spots, earth’s magnetic field, from tree rings), Seuss (dilution caused by buring fossil fuels), and bomb-testing effects p14
- p19: diet from fractioznation, C3 and C4 photosynthesizing plants
- p29: Credibility as accuracy and precision!
- p71-3: insolation spectral analysis vs. \(\delta^{18}O\) (a proxy for temperature) has a 71% \(R^2\)
- p74: CO2 levels and temperature, but lagging not leading
- p79: dinosaurs are warm blooded from isotopes!.
- p82: \(10^{-13}\) to \(10^{-15}\) level precision
- p91: age of earth.
- p94: isochron and concordia and very accurate dating, Chiculab 66M ya, but compounded with Decan flood traps.
- p103-4 natural nuclear reactors in Oklo, (Gabon, Africa)! p107: Atlantic too new for subduction faults hence no Atlantic ring of fire
- Heavier isotopes less likely to evaporate, more likely to condense
GPT Notes
Temperature and CO2 in ice ages
It is generally agreed that during the glacial-interglacial cycles of the past 400,000 years, the initial changes in temperature were driven by insolation changes due to the Milankovitch cycles, and these temperature changes then led to variations in CO2 levels. However, once CO2 levels changed, they amplified the initial temperature changes through the greenhouse effect, acting as a feedback mechanism. While the initial trigger for temperature changes was not CO2, the subsequent alterations in CO2 levels played a significant role in enhancing or dampening these temperature shifts. Thus, while CO2 changes were initially a reaction to temperature changes, they also significantly contributed to driving further temperature changes in a feedback loop.
Temperature impact of pro-cyclic changes in CO2 during glacial-interglacial cycles: the consensus is that while Milankovitch cycles initiated the temperature changes due to variations in Earth’s orbit and tilt, the subsequent changes in CO2 levels acted as a significant feedback mechanism, amplifying the temperature changes.
Amplification: Once CO2 levels started to change in response to initial temperature changes, they significantly amplified these temperature changes. This is because CO2 is a potent greenhouse gas, and changes in its concentration can lead to notable changes in the Earth’s energy balance.
Quantification: Quantifying the exact impact of CO2 changes on temperature during these cycles is complex due to the interplay of various factors. However, paleoclimate data, including ice core samples, suggest that the greenhouse gas feedback was substantial. For example, during the transition from glacial to interglacial periods, the increase in CO2 concentrations contributed significantly to the global temperature rise.
Scientific Studies: Studies have used climate models and paleoclimate data to estimate the sensitivity of Earth’s climate to CO2 changes. These studies generally find that the CO2 changes during glacial cycles played a crucial role in the Earth’s temperature changes, reinforcing the warming or cooling initiated by orbital variations.
In summary, while the initial temperature changes were not driven by CO2, the pro-cyclic changes in CO2 levels had a significant impact on the Earth’s temperature, acting to amplify the temperature changes initiated by variations in solar insolation.
There are several anti-cyclic mechanisms that can act to lower CO2 levels as temperature rises, particularly over long time scales. Before human impacts became significant, natural processes could reduce atmospheric CO2 in response to warming:
Increased Plant Growth: Warmer temperatures and higher CO2 levels can enhance photosynthesis, leading to increased plant growth and potentially more carbon sequestration in biomass and soils. However, this process is complex and can be influenced by other factors like water availability, nutrient availability, and ecosystem changes.
Weathering of Rocks: The chemical weathering of silicate rocks is a process that naturally removes CO2 from the atmosphere over long timescales. As temperatures rise, this weathering can accelerate, as it often involves reactions with water and is temperature-dependent. The resulting compounds can eventually be transported to the oceans, where they contribute to the formation of carbonate minerals, sequestering carbon.
Oceanic Processes: The ocean is a major carbon sink, absorbing CO2 from the atmosphere. As temperatures rise, the solubility of CO2 in ocean water decreases, but this is counterbalanced by biological processes. Increased temperatures can enhance the growth of phytoplankton, which consume CO2. When these organisms die, some of the carbon they sequestered sinks to the ocean floor, a process known as the biological pump.
While these processes can act to reduce CO2 levels as temperatures rise, they typically operate over very long timescales and may not be sufficient to counteract rapid increases in CO2 and temperatures. Additionally, the effectiveness of these mechanisms can be influenced by various feedback loops and interactions within the Earth system. For example, while increased plant growth can sequester CO2, it is also influenced by other factors like nutrient availability, which can limit the process.
Isochrons
The element isotope with the longest recorded half-life is tellurium-128 (\(^{128}\)Te). Its half-life is about \(2.2 \times 10^{24}\) years, which is over a trillion times longer than the age of the universe. This extremely long half-life means that \(^{128}\)Te is very nearly stable for all practical purposes.
A Pb-Pb isochron diagram is a tool used in geochronology to determine the age of the Earth and meteorites, providing insights into the age of the solar system. This method relies on the decay of uranium isotopes into lead isotopes. Specifically, it often uses the decay of ^238U to ^206Pb and ^235U to ^207Pb.
Here’s how a Pb-Pb isochron diagram is used:
Isotope Ratios: The diagram plots ratios of lead isotopes (207Pb/204Pb vs. 206Pb/204Pb) from various samples. ^204Pb is a non-radiogenic lead isotope, meaning it’s not produced by radioactive decay and thus serves as a reference.
Isochron Line: When these isotope ratios are plotted, samples that formed at the same time will fall on a straight line, known as an isochron. The slope of this line is related to the age of the samples because it reflects the accumulation of ^207Pb and ^206Pb over time due to the decay of ^235U and ^238U.
Age Determination: The slope of the isochron can be used to calculate the age of the samples. This is based on the known decay rates of uranium isotopes. The intercept of the isochron provides information about the initial lead isotopic composition at the time of the system’s formation.
Application to the Solar System: By applying this method to meteorites, particularly those considered as primordial material from the early solar system, scientists can estimate the age of the solar system. This is based on the assumption that these meteorites formed at the same time as the solar system and have remained closed systems with respect to uranium and lead.
The Pb-Pb isochron method is particularly valuable because it is less susceptible to lead loss than other dating methods, making it one of the most reliable techniques for dating ancient rocks. The ages obtained from meteorites using this method suggest that the solar system is about 4.56 billion years old.
Geochronology
U-Pb geochronology is a widely used radiometric dating method that utilizes the decay of uranium isotopes (^238U and ^235U) into lead isotopes (^206Pb and ^207Pb, respectively). This method is highly regarded due to the dual decay chains, which provide a cross-check to ensure the accuracy of dating results.
How U-Pb Geochronology Works:
Decay Equations: The decay processes are described by:
- ^238U decays to ^206Pb with a half-life of about 4.47 billion years.
- ^235U decays to ^207Pb with a half-life of about 704 million years.
Isotope Ratios: The ages of geological samples are determined by measuring the ratios of parent uranium isotopes to their daughter lead isotopes in a mineral, commonly zircon (ZrSiO4), which is resistant to weathering and geological processes.
Concordia Diagram: A powerful tool in U-Pb geochronology is the concordia diagram, where 207Pb/235U is plotted against 206Pb/238U. If a sample has remained a closed system, the data points will fall on the concordia curve, indicating the age of the sample. Discordant points can signal lead loss or gain, providing insights into the geological history of the sample.
Applications of U-Pb Geochronology:
Dating Geological Events: U-Pb is used to date a range of geological events, including volcanic eruptions, mountain building processes, and sediment deposition, providing insights into the history and evolution of Earth’s crust.
Meteorites and Planetary Science: By dating meteorites, scientists can establish the age of different solar system bodies and events, offering clues about the formation and evolution of the solar system.
Calibrating the Geological Time Scale: U-Pb dating of zircon in volcanic layers provides precise age markers that are used to calibrate the geological time scale, linking geological events to specific time intervals.
Economic Geology: Understanding the timing of mineralization processes in ore deposits through U-Pb dating can inform exploration strategies and the economic viability of mining operations.
Overall, U-Pb geochronology is a cornerstone of geological sciences, offering a reliable means to understand the temporal aspects of geological, planetary, and even solar system processes.
Natural fission
Natural nuclear fission reactor in Oklo, Gabon. Discovered in 1972, this phenomenon is a fascinating example of a natural nuclear fission reactor. Here are the key details:
Discovery and Location: Scientists analyzing uranium ore from a mine in Oklo, Gabon, noticed that the uranium-235 isotope content was anomalously low. Upon further investigation, they discovered that several natural fission reactors had operated in this region about 2 billion years ago.
How It Worked: Natural conditions at Oklo were just right for nuclear fission to occur spontaneously. About 2 billion years ago, the concentration of U-235 in natural uranium was about 3% (compared to about 0.7% today), which is sufficient for a chain reaction under the right conditions. The presence of water acted as a neutron moderator, and the specific arrangement of uranium and surrounding geology created a self-sustaining nuclear fission reaction.
Operation and Duration: These natural reactors operated intermittently for hundreds of thousands of years. They would go critical, generate heat and radiation, and then shut down as water boiled away, interrupting the reaction. The process would start again once the reactor cooled down and water returned.
Significance: The Oklo reactors provide valuable insights into nuclear reactions and the conditions under which they can occur naturally. They also offer a unique natural laboratory for studying the movement and containment of nuclear fission products over geological time scales, which is relevant for modern issues like nuclear waste disposal.
Isotope Ratios: The fission reactions at Oklo altered the isotope ratios in the surrounding rock, which has allowed scientists to study the products of nuclear fission in a natural setting. This has implications for understanding the distribution of elements on Earth and the natural processes affecting them.
The Oklo natural nuclear reactor remains a subject of interest for scientists in fields ranging from geology and physics to environmental science, showcasing a rare and intriguing natural phenomenon.
The Chondritic Uniform Reservoir (CHUR) is a theoretical concept used in geochemistry and cosmochemistry to represent the primordial isotopic composition of the solar system before the differentiation of its various components, like planets and asteroids. The term “chondritic” refers to chondrites, which are a class of stony meteorites that have not been significantly altered by melting or differentiation and are thought to closely represent the composition of the early solar system.
In isotope geochemistry, CHUR is often used as a reference point or standard for comparing the isotopic compositions of different materials. For example, in neodymium (Nd) isotope studies, the 143Nd/144Nd ratio of a sample is often expressed relative to that of CHUR to assess the degree and timing of silicate differentiation processes on Earth or other planetary bodies.
The concept of CHUR is fundamental for understanding the processes that have led to the isotopic diversity observed in the solar system today and provides insights into the formation and evolution of its various components.
Δ47 isotopologues and thermoregulation of dinosaurs
The use of Δ47 isotopologues in paleoclimatology and paleobiology involves measuring the clumped isotope compositions, specifically carbon and oxygen isotopes, in carbonate minerals. The Δ47 value refers to the abundance of 13C-18O bonds in carbonate minerals relative to what would be expected in a random distribution of isotopes, which is temperature-dependent.
Here’s how Δ47 isotopologues are used to gather evidence regarding the thermoregulation of dinosaurs:
Principle: The clumping of heavy isotopes in carbonate minerals is temperature-dependent. At lower temperatures, there’s a higher propensity for heavy isotopes (^13C and ^18O) to bond together, resulting in a higher Δ47 value. Conversely, at higher temperatures, these isotopes are less clumped, leading to lower Δ47 values.
Application to Dinosaurs: To infer dinosaur body temperatures, scientists analyze the Δ47 values in bioapatite (a type of calcium phosphate mineral found in bones and teeth). By measuring the clumped isotope composition of bioapatite from dinosaur fossils, researchers can estimate the temperature at which the apatite formed, which is closely related to the dinosaur’s body temperature.
Endothermy in Dinosaurs: If dinosaurs were endothermic (warm-blooded), their body temperatures would have been regulated internally and likely higher than their ambient environment. By comparing the Δ47-derived temperatures of dinosaurs to those of their contemporaneous environment (inferred from other geochemical markers or nearby organisms considered to be ectothermic), scientists can assess whether dinosaurs maintained higher metabolic rates indicative of endothermy.
Studies using this method have provided evidence supporting the hypothesis that some dinosaurs were endothermic, as their estimated body temperatures were higher than the ambient temperatures, suggesting metabolic heat production and internal temperature regulation akin to modern birds and mammals. This approach offers a direct geochemical method to probe the thermophysiology of extinct organisms, contributing valuable insights into their biology and ecology.
Deets
- Rob Ellam
- Volume 476
- Published 2016