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Isotope geochemistry (Rev #8, changes)

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Wikipedia defines it as:

Isotope geochemistry is an aspect of geology based upon study of the relative and absolute concentrations of the elements and their isotopes in the Earth. Variations in the abundance of these isotopes, typically measured with an isotope ratio mass spectrometer or an accelerator mass spectrometer, can reveal information about the age of a rock or the source of air or water. Isotope ratios can even shed light on chemical processes in the atmosphere. Broadly, the field of isotope geochemistry is divided into two branches: stable and radiogenic isotope geochemistry.


Stable isotope geochemistry

For most stable isotopes, the magnitude of fractionation from kinetic and equilibrium fractionation is very small; for this reason, enrichments are typically reported in “per mil” (‰, parts per thousand). These enrichments δ\delta represent the ratio of heavy isotope to light isotope in the sample over the ratio of a standard.


Carbon [[Carbon}} has two stable isotopes, 12C and 13C, and one radioactive isotope, 14C. Carbon isotope ratios are measured against Vienna Pee Dee Belemnite (VPDB) They have been used to track ocean circulation, among other things. Carbon stable isotopes are fractionated primarily by photosynthesis (Faure, 2004). The 13C/12C ratio is also an indicator of paleoclimate: a change in the ratio in the remains of plants indicates a change in the amount of photosynthetic activity, and thus in how favorable the environment was for the plants.

The ratio is defined as

δC=(13C sample13C VPDB1)10 3 \delta C = (\frac{^{13}C_sample}{^{13}C_VPDB} -1)10^3

Where 13C VPDB^{13}C_VPDB is 0.0112372.

During photosynthesis, organisms using the C3 pathway show different enrichments compared to those using the C4 pathway, allowing scientists not only to distinguish organic matter from abiotic carbon, but also what type of photosynthetic pathway the organic matter was using.


Nitrogen has two stable isotopes, 14N, and 15N. The ratio between these is measured relative to nitrogen in ambient air. Nitrogen ratios are frequently linked to agricultural activities. Nitrogen isotope data has also been used to measure the amount of exchange of air between the stratosphere and troposphere using data from the greenhouse gas N2O.


Oxygen has three stable isotopes, 16O, 17O, and 18O. Oxygen ratios are measured relative to Vienna Standard Mean Ocean Water (VSMOW) or Vienna Pee Dee Belemnite (VPDB).[2] Variations in oxygen isotope ratios are used to track both water movement, paleoclimate,[1] and atmospheric gases such as ozone and carbon dioxide. Typically, the VPDB oxygen reference is used for paleoclimate, while VSMOW is used for most other applications.[1] Oxygen isotopes appear in anomalous ratios in atmospheric ozone, resulting from mass-independent fractionation. Isotope ratios in fossilized foraminifera have been used to deduce the temperature of ancient seas.

Welp, Keeling has used oxygen isotopes to get a better higher estimate on biomass and here is a diagram:

carbon cycle biomass

And here is a quote from the paper which explains their findings:

The model fit yielded a short turnover time of tN 50.4–0.8 yr for d18O-CO2 in the Northern Hemisphere, depending on the stations used in the analysis and with 1s errors not greater than 0.3 yr. Sensitivity analysis of the model parameter fit covariance suggested that tS is greater than 2 yr, but the upper bound was difficult to determine because of the 1-yr interhemispheric mixing time (tmix). We expected tN to be less than tS given the larger land biosphere and the greater exchange of CO2 with leaf and soil water in the north. Estimated turnover times for the two hemispheres based on gross exchanges with the ocean and the terrestrial biosphere indicated that the Southern Hemisphere turnover time is ,120% longer than that of the Northern Hemisphere. The model-derived, hemisphere specific turnover times correspond to a global tropospheric mean of 0.7–1.4 yr. Allowing for the 22% of the atmospheric mass contained in the stratosphere, which was not included in themodel, the estimated global atmospheric turnover time increases to 0.9–1.7 yr.


Sulfur has four stable isotopes, with the following abundances: 32S (0.9502), 33S (0.0075), 34S (0.0421) and 36S (0.0002). These abundances are compared to those found in Cañon Diablo troilite. Variations in sulfur isotope ratios are used to study the origin of sulfur in an orebody and the temperature of formation of sulfur–bearing minerals.


It can be used in environmental forensics, stable isotope signatures and there are many networks dedicated to this:


Envelope is a powerful tool for the interactive calculation and visualization of isotope distributions that is capable of simultaneously calculating distributions for an arbitrary number of species of a single peptide or oligonucleotide, each with a different labeling pattern.

Envelope can visualize experimental mass spectra, allowing the user to perform manual least-squares fits of calculated distributions to real experimental data. Envelope is useful for small-scale data analysis and planning experiments. Moreover it can be used as a teaching tool, and its user-friendly and interactive qualities make it well suited for use by research groups, in seminars, or in the classroom.


CC-licencedCreative Commons licence

Abstract. Conventionally, measurements of carbon isotopes in atmospheric CO2 (δ13CO2) have been used to partition fluxes between terrestrial and ocean carbon pools. However, novel analytical approaches combined with an increase in the spatial extent and frequency of δ13CO2 measurements allow us to conduct a global analysis of δ13CO2 variability to infer the isotopic composition of source CO2 to the atmosphere (δs). This global analysis yields coherent seasonal patterns of isotopic enrichment. Our results indicate that seasonal values of δs are more highly correlated with vapor pressure deficit (r = 0.404) than relative humidity (r = 0.149). We then evaluate two widely used stomatal conductance models and determine that the Leuning Model, which is primarily driven by vapor pressure deficit is more effective globally at predicting δs (RMSE = 1.6‰) than the Ball-Woodrow-Berry model, which is driven by relative humidity (RMSE = 2.7‰). Thus stomatal conductance on a global scale may be more sensitive to changes in vapor pressure deficit than relative humidity. This approach highlights a new application of using δ13CO2 measurements to validate global models.

Abstract. The terrestrial carbon (C) cycle has received increasing interest over the past few decades, however, there is still a lack of understanding of the fate of newly assimilated C allocated within plants and to the soil, stored within ecosystems and lost to the atmosphere. Stable carbon isotope studies can give novel insights into these issues. In this review we provide an overview of an emerging picture of plant-soil-atmosphere C fluxes, as based on C isotope studies, and identify processes determining related C isotope signatures. The first part of the review focuses on isotopic fractionation processes within plants during and after photosynthesis.

The second major part elaborates on plant-internal and plant-rhizosphere C allocation patterns at different time scales (diel, seasonal, interannual),including the speed of C transfer and time lags in the coupling of assimilation and respiration, as well as the magnitude and controls of plant-soil C allocation and respiratory fluxes. Plant responses to changing environmental conditions, the functional relationship between the physiological and phenological status of plants and C transfer, and interactions between C, water and nutrient dynamics are discussed. The role of the C counterflow from the rhizosphere to the aboveground parts of the plants, e.g. via CO2 dissolved in the xylem water or as xylem-transported sugars, is highlighted. The third part is centered around belowground C turnover, focusing especially on above- and belowground litter inputs, soil organic matter formation and turnover, production and loss of dissolved organic C, soil respiration and CO2 fixation by soil microbes. Furthermore, plant controls on microbial communities and activity via exudates and litter production as wellas microbial community effects on C mineralization are reviewed. A further part of the paper is dedicated to physical interactions between soil CO2 and the soil matrix, such as CO2 diffusion and dissolution processes within the soil profile.

Finally, we highlight state-of-the-art stable isotope methodologies and their latest developments. From the presented evidence we conclude that there exists a tight coupling of physical, chemical and biological processes involved in C cycling and C isotope fluxes in the plant-soil-atmosphere system. Generally, research using information from C isotopes allows an integrated view of the different processes involved. However, complex interactions among the range of processes complicate or currently impede the interpretation of isotopic signals in CO2 or organic compounds at the plant and ecosystem level. This review tries to identify present knowledge gaps in correctly interpreting carbon stable isotope signals in the plant-soil-atmosphere system and how future research approaches could contribute to closing these gaps.

Copyrighted publications

Abstract:The stable isotope ratios of atmospheric CO2 (18O/16O and 13C/12C) have been monitored since 1977 to improve our understanding of the global carbon cycle, because biosphere–atmosphere exchange fluxes affect the different atomic masses in a measurable way1. Interpreting the 18O/16O variability has proved difficult, however, because oxygen isotopes in CO2 are influenced by both the carbon cycle and the water cycle2. Previous attention focused on the decreasing 18O/16O ratio in the 1990s, observed by the global Cooperative Air Sampling Network of the US National Oceanic and Atmospheric Administration Earth System Research Laboratory. This decrease was attributed variously to a number of processes including an increase in Northern Hemisphere soil respiration3; a global increase in C4 crops at the expense of C3 forests4; and environmental conditions, such as atmospheric turbulence5 and solar radiation6, that affect CO2 exchange between leaves and the atmosphere. Here we present 30 years’ worth of data on 18O/16O in CO2 from the Scripps Institution of Oceanography global flask network and show that the interannual variability is strongly related to the El Niño/Southern Oscillation. We suggest that the redistribution of moisture and rainfall in the tropics during an El Niño increases the 18O/16O ratio of precipitation and plant water, and that this signal is then passed on to atmospheric CO2 by biosphere–atmosphere gas exchange. We show how the decay time of the El Niño anomaly in this data set can be useful in constraining global gross primary production. Our analysis shows a rapid recovery from El Niño events, implying a shorter cycling time of CO2 with respect to the terrestrial biosphere and oceans than previously estimated. Our analysis suggests that current estimates of global gross primary production, of 120 petagrams of carbon per year7, may be too low, and that a best guess of 150–175 petagrams of carbon per year better reflects the observed rapid cycling of CO2. Although still tentative, such a revision would present a new benchmark by which to evaluate global biospheric carbon cycling models.

category: earth science