Clumped isotopes are heavy isotopes that are bonded to other heavy isotopes. The relative abundance of clumped isotopes (and multiply-substituted isotopologues) in molecules such as methane, nitrous oxide, and carbonate is an area of active investigation. [1] The carbonate clumped-isotope thermometer, or "13C–18O order/disorder carbonate thermometer", is a new approach for paleoclimate reconstruction, [1] based on the temperature dependence of the clumping of 13C and 18O into bonds within the carbonate mineral lattice. [2] This approach has the advantage that the 18O ratio in water is not necessary (different from the δ18O approach), but for precise paleotemperature estimation, it also needs very large and uncontaminated samples, long analytical runs, and extensive replication. [3] Commonly used sample sources for paleoclimatological work include corals, otoliths, gastropods, tufa, bivalves, and foraminifera. [4] [5] Results are usually expressed as Δ47 (said as "cap 47"), which is the deviation of the ratio of isotopologues of CO2 with a molecular weight of 47 to those with a weight of 44 from the ratio expected if they were randomly distributed. [6]
Molecules made up of elements with multiple isotopes can vary in their isotopic composition; these variant molecules are called isotopologues. For example, consider the isotopologues of carbon dioxide. oxygen has three stable isotopes (16O, 17O and 18O) and carbon has two (13C, 12C).A 12C16O2 molecule (composed only with most abundant isotopes of constituent elements) is called a monoisotopic species. When only one atom is replaced with heavy isotope of any constituent element (ie, 13C16O2), it is called a singly-substituted species. Likewise, when two atoms are simultaneously replaced with heavier isotopes (eg., 13C16O18O), the it is called a doubly substituted or also a multiply substituted isotopologue. The multiply-substituted isotopologue 13C18O16O contains a bond between two of these heavier isotopes (13C and 18O), which is a "clumped" isotope bond.
The abundance of masses for a given molecule (e.g. CO2) can be predicted using the relative abundance of isotopes of its constituent atoms (13C/12C, 18O/16O and 17O/16O). The relative abundance of each isotopologue (e.g. mass-47 CO2) is proportional to the relative abundance of each isotopic species.
This predicted abundance assumes a non-biased stochastic distribution of isotopes, natural materials tend to deviate from these stochastic values, the study of which forms the basis of clumped isotope geochemistry.
When a heavier isotope substitutes for a lighter one (e.g., 18O for 16O), the chemical bond's vibration will be slower, lowering its zero-point energy. [7] [8] In other words, thermodynamic stability is related to the isotopic composition of the molecule.
12C16O32− (≈98.2%), 13C16O32− (≈1.1%), 12C18O16O22− (≈0.6%) and 12C17O16O22− (≈0.11%) are the most abundant isotopologues (≈99%) of carbonate ion, controlling the bulk δ13C, δ17O and δ18O values in natural carbonate minerals. Each of these isopotologes has different thermodynamic stability. For a carbonate crystal at thermodynamic equilibrium, the relative abundances of the carbonate ion isotopologues is controlled by reactions such as:
13C16O32− + 12C18O16O22− ⇌ 12C16O32− + 13C18O16O22− | (Reaction 1) |
The equilibrium constants for these reactions are temperature-dependent, with a tendency for heavy isotopes to "clump" with each other (increasing the proportions of multiply substituted isotopologues) as temperature decreases. [9] Reaction 1 will be driven to the right with decreasing temperature, to the left with increasing temperature. Therefore, the equilibrium constant for this reaction can be used as an paleotemperature indicator, as long as the temperature dependence of this reaction and the relative abundances of the carbonate ion isotopologues are known.
In conventional δ18O analysis, both the δ18O values in carbonates and water are needed to estimate paleoclimate. However, for many times and places, the δ18O in water can only be inferred, and also the 16O/18O ratio between carbonate and water may vary with the change in temperature. [10] [11] Therefore, the accuracy of the thermometer may be compromised.
Whereas for the carbonate clumped-isotope thermometer, the equilibrium is independent of the isotope compositions of waters from which carbonates grew. Therefore, the only information needed is the abundance of bonds between rare, heavy isotopes within the carbonate mineral.
Clumped isotopes analyses have traditionally been used in lieu of conventional δ18O analyses when the δ18O of seawater or source water is poorly constrained. While conventional δ18O analysis solves for temperature as a function of both carbonate and water δ18O, clumped isotope analyses can provide temperature estimates that are independent of the source water δ18O. Δ47-derived temperature can then be used in conjunction with carbonate δ18O to reconstruct δ18O of the source water, thus providing information on the water with which the carbonate was equilibrated. [18]
Clumped isotope analyses thus allow for estimates of two key environmental variables: temperature and water δ18O. These variables are especially useful for reconstructing past climates, as they can provide information on a wide range of environmental properties. For example, temperature variability can imply changes in solar irradiance, greenhouse gas concentration, or albedo, while changes in water δ18O can be used to estimate changes in ice volume, sea level, or rainfall intensity and location. [14]
Studies have used temperatures derived from clumped isotopes for varied and numerous paleoclimate applications — to constrain δ18O of past seawater, [18] pinpoint the timing of icehouse-hothouse transitions, [19] track changes in ice volume through an ice age, [20] and to reconstruct temperature changes in ancient lake basins. [21] [22]
Clumped isotope analyses have recently been used to constrain the paleoaltitude or uplift history of a region. [23] [24] [25] Air temperature decreases systematically with altitude throughout the troposphere (see lapse rate). Due to the close coupling between lake water temperature and air temperature, there is a similar decrease in lake water temperature as altitude increases. [26] [24] Thus, variation in water temperature implied by Δ47 could indicate changes in lake altitude, driven by tectonic uplift or subsidence. Two recent studies derive the timing of the uplift of the Andes Mountains and the Altiplano Plateau, citing sharp decreases in Δ47-derived temperatures as evidence of rapid tectonic uplift. [23] [27]
Measurements of Δ47 can be used to constrain natural and synthetic sources of atmospheric CO2, (e.g. respiration and combustion), as each of these processes are associated with different average Δ47 temperatures of formation. [28] [29]
Measurements of Δ47 can be used to better understand the physiology of extinct organisms, and to place constraints on the early development of endothermy, the process by which organisms regulate their body temperature. Prior to the development of clumped isotope analysis, there was no straightforward way to estimate either the body temperature or body water δ18O of extinct animals. Eagle et al., 2010 measure Δ47 in bioapatite from a modern Indian elephant, white rhinoceros, Nile crocodile and American alligator. [30] These animals were chosen as they span a wide range in internal body temperatures, allowing for the creation of a mathematical framework relating Δ47 of bioapatite and internal body temperature. This relationship has been applied to analyses of fossil teeth, in order to predict the body temperatures of a woolly mammoth and a sauropod dinosaur. [30] [31] The latest Δ47 temperature calibration for (bio)apatite of Löffler et al. 2019 [16] covers a wide temperature range of 1-80°C and was applied to a fossil megalodon shark tooth for calculating seawater temperatures and δ18O values. [16]
A key premise of most clumped isotope analyses is that samples have retained their primary isotopic signatures. However, isotopic resetting or alteration, resulting from elevated temperature, can provide a different type of information about past climates. For example, when carbonate is isotopically reset by high temperatures, measurements of Δ47 can provide information about the duration and extent of metamorphic alteration. In one such study, Δ47 from late Neoproterozoic Doushantou cap carbonate is used to assess the temperature evolution of the lower crust in southern China. [32]
Primitive meteorites have been studied using measurements of Δ47. These analyses also assume that the primary isotopic signature of the sample has been lost. In this case, measurements of Δ47 instead provide information on the high-temperature event that isotopically reset the sample. Existing Δ47 analyses on primitive meteorites have been used to infer the duration and temperature of aqueous alteration events, as well as to estimate the isotopic composition of the alteration fluid. [33] [34]
An emerging body of work highlights the application potential for clumped isotopes to reconstruct temperature and fluid properties in hydrothermal ore deposits. In mineral exploration, delineation of the heat footprint around an ore body provides critical insight into the processes that drive transport and deposition of metals. During proof of concept studies, clumped isotopes were used to provide accurate temperature reconstructions in epithermal, sediment hosted, and Mississippi Valley Type (MVT) deposits. [35] [36] These case studies are supported by measurement of carbonates in active geothermal settings. [35] [37] [38]
The temperature dependent relationship is subtle (−0.0005%/°C).[ citation needed ]
13C18O16O22− is a rare isotopologue (≈60 ppm [3]).
Therefore, to obtain adequate precision, this approach requires long analyses (≈2–3 hours) and very large and uncontaminated samples.
Clumped isotope analyses assume that measured Δ47 is composed of 13C18O16O22−, the most common isotopologue of mass 47. Corrections to account for less common isotopologues of mass 47 (e.g. 12C18O17O 16O2−) are not completely standardized between labs.
Isotope analysis is the identification of isotopic signature, abundance of certain stable isotopes of chemical elements within organic and inorganic compounds. Isotopic analysis can be used to understand the flow of energy through a food web, to reconstruct past environmental and climatic conditions, to investigate human and animal diets, for food authentification, and a variety of other physical, geological, palaeontological and chemical processes. Stable isotope ratios are measured using mass spectrometry, which separates the different isotopes of an element on the basis of their mass-to-charge ratio.
Magnesite is a mineral with the chemical formula MgCO
3. Iron, manganese, cobalt, and nickel may occur as admixtures, but only in small amounts.
In the study of past climates ("paleoclimatology"), climate proxies are preserved physical characteristics of the past that stand in for direct meteorological measurements and enable scientists to reconstruct the climatic conditions over a longer fraction of the Earth's history. Reliable global records of climate only began in the 1880s, and proxies provide the only means for scientists to determine climatic patterns before record-keeping began.
In chemistry, isotopologues are molecules that differ only in their isotopic composition. They have the same chemical formula and bonding arrangement of atoms, but at least one atom has a different number of neutrons than the parent.
Kinetic fractionation is an isotopic fractionation process that separates stable isotopes from each other by their mass during unidirectional processes. Biological processes are generally unidirectional and are very good examples of "kinetic" isotope reactions. All organisms preferentially use lighter isotopes, because "energy costs" are lower, resulting in a significant fractionation between the substrate (heavier) and the biologically mediated product (lighter). For example, photosynthesis preferentially takes up the light isotope of carbon 12C during assimilation of atmospheric CO2. This kinetic isotope fractionation explains why plant material (and thus fossil fuels, which are derived from plants) is typically depleted in 13C by 25 per mil (2.5%) relative to most inorganic carbon on Earth.
A paleothermometer is a methodology that provides an estimate of the ambient temperature at the time of formation of a natural material. Most paleothermometers are based on empirically-calibrated proxy relationships, such as the tree ring or TEX86 methods. Isotope methods, such as the δ18O method or the clumped-isotope method, are able to provide, at least in theory, direct measurements of temperature.
Oxygen isotope ratio cycles are cyclical variations in the ratio of the abundance of oxygen with an atomic mass of 18 to the abundance of oxygen with an atomic mass of 16 present in some substances, such as polar ice or calcite in ocean core samples, measured with the isotope fractionation. The ratio is linked to ancient ocean temperature which in turn reflects ancient climate. Cycles in the ratio mirror climate changes in the geological history of Earth.
In geochemistry, paleoclimatology and paleoceanography δ18O or delta-O-18 is a measure of the deviation in ratio of stable isotopes oxygen-18 (18O) and oxygen-16 (16O). It is commonly used as a measure of the temperature of precipitation, as a measure of groundwater/mineral interactions, and as an indicator of processes that show isotopic fractionation, like methanogenesis. In paleosciences, 18O:16O data from corals, foraminifera and ice cores are used as a proxy for temperature.
Equilibrium isotope fractionation is the partial separation of isotopes between two or more substances in chemical equilibrium. Equilibrium fractionation is strongest at low temperatures, and forms the basis of the most widely used isotopic paleothermometers : D/H and 18O/16O records from ice cores, and 18O/16O records from calcium carbonate. It is thus important for the construction of geologic temperature records. Isotopic fractionations attributed to equilibrium processes have been observed in many elements, from hydrogen (D/H) to uranium (238U/235U). In general, the light elements are most susceptible to fractionation, and their isotopes tend to be separated to a greater degree than heavier elements.
The Global Meteoric Water Line (GMWL) describes the global annual average relationship between hydrogen and oxygen isotope (oxygen-18 [18O] and deuterium [2H]) ratios in natural meteoric waters. The GMWL was first developed in 1961 by Harmon Craig, and has subsequently been widely used to track water masses in environmental geochemistry and hydrogeology.
Magmatic water, also known as juvenile water, is an aqueous phase in equilibrium with minerals that have been dissolved by magma deep within the Earth's crust and is released to the atmosphere during a volcanic eruption. It plays a key role in assessing the crystallization of igneous rocks, particularly silicates, as well as the rheology and evolution of magma chambers. Magma is composed of minerals, crystals and volatiles in varying relative natural abundance. Magmatic differentiation varies significantly based on various factors, most notably the presence of water. An abundance of volatiles within magma chambers decreases viscosity and leads to the formation of minerals bearing halogens, including chloride and hydroxide groups. In addition, the relative abundance of volatiles varies within basaltic, andesitic, and rhyolitic magma chambers, leading to some volcanoes being exceedingly more explosive than others. Magmatic water is practically insoluble in silicate melts but has demonstrated the highest solubility within rhyolitic melts. An abundance of magmatic water has been shown to lead to high-grade deformation, altering the amount of δ18O and δ2H within host rocks.
Hydrogen isotope biogeochemistry (HIBGC) is the scientific study of biological, geological, and chemical processes in the environment using the distribution and relative abundance of hydrogen isotopes. Hydrogen has two stable isotopes, protium 1H and deuterium 2H, which vary in relative abundance on the order of hundreds of permil. The ratio between these two species can be called the hydrogen isotopic signature of a substance. Understanding isotopic fingerprints and the sources of fractionation that lead to variation between them can be applied to address a diverse array of questions ranging from ecology and hydrology to geochemistry and paleoclimate reconstructions. Since specialized techniques are required to measure natural hydrogen isotopic composition (HIC), HIBGC provides uniquely specialized tools to more traditional fields like ecology and geochemistry.
Isotopic reference materials are compounds with well-defined isotopic compositions and are the ultimate sources of accuracy in mass spectrometric measurements of isotope ratios. Isotopic references are used because mass spectrometers are highly fractionating. As a result, the isotopic ratio that the instrument measures can be very different from that in the sample's measurement. Moreover, the degree of instrument fractionation changes during measurement, often on a timescale shorter than the measurement's duration, and can depend on the characteristics of the sample itself. By measuring a material of known isotopic composition, fractionation within the mass spectrometer can be removed during post-measurement data processing. Without isotope references, measurements by mass spectrometry would be much less accurate and could not be used in comparisons across different analytical facilities. Due to their critical role in measuring isotope ratios, and in part, due to historical legacy, isotopic reference materials define the scales on which isotope ratios are reported in the peer-reviewed scientific literature.
Position-specific isotope analysis, also called site-specific isotope analysis, is a branch of isotope analysis aimed at determining the isotopic composition of a particular atom position in a molecule. Isotopes are elemental variants with different numbers of neutrons in their nuclei, thereby having different atomic masses. Isotopes are found in varying natural abundances depending on the element; their abundances in specific compounds can vary from random distributions due to environmental conditions that act on the mass variations differently. These differences in abundances are called "fractionations," which are characterized via stable isotope analysis.
Shuhei Ono is a professor of earth, atmospheric, and planetary sciences at the Massachusetts Institute of Technology. In his research, he measures isotopes of sulfur and other elements to investigate water-rock-microbe interactions, seafloor hydrothermal systems, the deep biosphere, and global sulfur cycles.
Carbonate-associated sulfates (CAS) are sulfate species found in association with carbonate minerals, either as inclusions, adsorbed phases, or in distorted sites within the carbonate mineral lattice. It is derived primarily from dissolved sulfate in the solution from which the carbonate precipitates. In the ocean, the source of this sulfate is a combination of riverine and atmospheric inputs, as well as the products of marine hydrothermal reactions and biomass remineralisation. CAS is a common component of most carbonate rocks, having concentrations in the parts per thousand within biogenic carbonates and parts per million within abiogenic carbonates. Through its abundance and sulfur isotope composition, it provides a valuable record of the global sulfur cycle across time and space.
Methane clumped isotopes are methane molecules that contain two or more rare isotopes. Methane (CH4) contains two elements, carbon and hydrogen, each of which has two stable isotopes. For carbon, 98.9% are in the form of carbon-12 (12C) and 1.1% are carbon-13 (13C); while for hydrogen, 99.99% are in the form of protium (1H) and 0.01% are deuterium (2H or D). Carbon-13 (13C) and deuterium (2H or D) are rare isotopes in methane molecules. The abundance of the clumped isotopes provides information independent from the traditional carbon or hydrogen isotope composition of methane molecules.
Photosynthesis converts carbon dioxide to carbohydrates via several metabolic pathways that provide energy to an organism and preferentially react with certain stable isotopes of carbon. The selective enrichment of one stable isotope over another creates distinct isotopic fractionations that can be measured and correlated among oxygenic phototrophs. The degree of carbon isotope fractionation is influenced by several factors, including the metabolism, anatomy, growth rate, and environmental conditions of the organism. Understanding these variations in carbon fractionation across species is useful for biogeochemical studies, including the reconstruction of paleoecology, plant evolution, and the characterization of food chains.
In stable isotope geochemistry, the Urey–Bigeleisen–Mayer equation, also known as the Bigeleisen–Mayer equation or the Urey model, is a model describing the approximate equilibrium isotope fractionation in an isotope exchange reaction. While the equation itself can be written in numerous forms, it is generally presented as a ratio of partition functions of the isotopic molecules involved in a given reaction. The Urey–Bigeleisen–Mayer equation is widely applied in the fields of quantum chemistry and geochemistry and is often modified or paired with other quantum chemical modelling methods to improve accuracy and precision and reduce the computational cost of calculations.
Silicon isotope biogeochemistry is the study of environmental processes using the relative abundance of Si isotopes. As the relative abundance of Si stable isotopes varies among different natural materials, the differences in abundance can be used to trace the source of Si, and to study biological, geological, and chemical processes. The study of stable isotope biogeochemistry of Si aims to quantify the different Si fluxes in the global biogeochemical silicon cycle, to understand the role of biogenic silica within the global Si cycle, and to investigate the applications and limitations of the sedimentary Si record as an environmental and palaeoceanographic proxy.