13 CO2 Isotopes
NOAA
Carbon-14 (or \({}^{14}C\)) is also known as radiocarbon, because it is the only carbon isotope that is radioactive. It is perhaps most famous for its use in radiocarbon dating of archeological artifacts ranging from mummies to cave drawings, and it plays a crucial role in studying fossil fuel carbon dioxide emissions as well.
Fossil fuels are, well, fossils, and are millions of years old. Because of this, all of the radiocarbon initially present has decayed away, leaving no \({}^{14}C\) in this ancient organic matter. All other atmospheric carbon dioxide comes from young sources–namely land-use changes (for example, cutting down a forest in order to create a farm) and exchange with the ocean and terrestrial biosphere. This makes \({}^{14}C\) an ideal tracer of carbon dioxide coming from the combustion of fossil fuels. Scientists can use \({}^{14}C\) measurements to determine how much \({}^{14}C\)O2 has been diluted with \({}^{14}C\)-free CO2 in air samples, and from this can calculate what proportion of the carbon dioxide in the sample comes from fossil fuels.
Unlike \({}^{14}C\), the amount of \({}^{13}C\) or \({}^{12}C\) in an artifact does not change over time since both \({}^{13}C\) and \({}^{12}C\) are stable isotopes. In other words, they do not decay. Because they are stable isotopes, a \({}^{13}C\) atom will always remain a \({}^{13}C\) atom, and the same is true for \({}^{12}C\).
Recall that there is much, much more \({}^{12}C\) than \({}^{13}C\) in the world –almost 99% of all carbon atoms are \({}^{12}C\). Even so, different carbon pools have different ratios of \({}^{13}C\) and \({}^{12}C\) – called isotopic fingerprints. The differences are small - one carbon pool may have 98.8% \({}^{12}C\) while another may have 99.2% \({}^{12}C\) - but modern machines, called isotope ratio mass spectrometers, can detect these differences quite easily. Pools with relatively more \({}^{13}C\) (less \({}^{12}C\)) are called “heavy” and those with less \({}^{13}C\) are called “light”.
Let’s look at the four main carbon pools with which climate scientists are concerned: the atmosphere, the terrestrial biosphere (land plants, animals, and soils), fossil fuels, and the ocean. The atmosphere has a certain ratio of \({}^{13}C\) to \({}^{12}C\). This ratio is affected by the isotopic fingerprint of the source of new carbon dioxide to the atmosphere. Some sources of carbon dioxide are “heavy” while others are “light”. The ratio in the atmosphere is also affected by the isotopic fingerprint of carbon dioxide sinks. Do these sinks take in a lot of \({}^{13}C\) or very little relative to the amount of \({}^{12}C\) they take up from the atmosphere?
13.1 Suess Effect
So, we know that the ratio of carbon isotopes in atmospheric carbon dioxide samples is from a mixture of sources, and we also know the unique isotopic fingerprint of each of those sources. Using these two pieces of information, scientists can figure out why trends in Δ14C and δ13C occur. Globally, as atmospheric carbon dioxide levels continue to increase, both Δ14C and δ13C are decreasing over time. This is called the Suess Effect – named after Dr. Suess who first discovered this phenomenon.
The steady downward trend in Δ14C of background air shows that the additional carbon dioxide added to the atmosphere must have a lower Δ14C value than what is already in the atmosphere. Well, we know that fossil fuels have a Δ14C signal of -1000‰, but that all other sources have a signal that is very close to that of ambient air (approximately + 45‰ in 2010, actually). Therefore, when CO2 from fossil fuels enter the atmosphere, the Δ14C value in the atmosphere goes down. We can precisely calculate how much the Δ14C value in the atmosphere goes down when fossil fuel CO2 is added. It turns out to be about a 3‰ decrease in Δ14C for every 1 ppm of fossil fuel CO2 added to the atmosphere.
Ocean
When CO2 is released from the ocean to the atmosphere, it tends to have a Δ14C value slightly lower than the atmosphere, because some of the 14C in it has had time to decay. Over the oceans, and especially in the Southern Hemisphere (which is mostly ocean!) this is the most important effect on Δ14C.
Respiration
However, over the land, there is another effect that we think about – carbon released from plants and soils. Carbon dioxide taken up from the atmosphere by plants is eventually released back to the atmosphere by respiration, but only after a few years (typically 10-20 years). This means that a very small amount of the 14C has decayed away, and the Δ14C value of the CO2 from respiration (when organisms use energy and release carbon dioxide as a byproduct) is different than the atmosphere. Scientists have calculated exactly how much this changes Δ14C in the atmosphere, and it turns out to be not much – to be sure though, when scientists use Δ14C measurements to calculate how much fossil fuel CO2 has been added to the atmosphere, they make a correction for this respiration effect.
Nuclear Nuclear power has a much higher Δ14C value than the atmosphere (which is what caused the 14C “bomb spike” in the 1960s).
13C
The relative proportion of 13C in our atmosphere is steadily decreasing over time. Before the industrial revolution, δ13C of our atmosphere was approximately -6.5‰; now the value is around -8‰. Recall that plants have less 13C relative to the atmosphere (and therefore have a more negative δ13C value of around -25‰). Most fossil fuels, like oil and coal, which are ancient plant and animal material, have the same δ13C isotopic fingerprint as other plants. The annual trend–the overall decrease in atmospheric δ13C–is explained by the addition of carbon dioxide to the atmosphere that must come from the terrestrial biosphere and/or fossil fuels. In fact, we know from Δ14C measurements, inventories, and other sources, that this decrease is from fossil fuel emissions, and is an example of the Suess Effect.
Seasonal Variations
Total atmospheric carbon dioxide levels (not isotopic ratios, but just total carbon dioxide) show strong seasonal variations. In the summer (in the northern hemisphere–where most of the Earth’s land sits), carbon dioxide decreases as it is fixed by plants via photosynthesis. In the fall and winter, carbon dioxide increases as many plants stop photosynthesizing and some of the carbon dioxide they fixed is released through respiration from plants, animals, and soils. Seasonal δ13C variations show the opposite pattern. δ13C increases in the summer and decreases in the winter.
When plants take up carbon dioxide, they prefer 12C over 13C. This leaves relatively more 13C in the atmosphere, which increases the δ13C of the atmosphere. However, in the winter, when the plants release more carbon dioxide than they consume, this carbon dioxide entering the atmosphere is relatively poor in 13C. This decreases the δ13C of the atmosphere during the fall and winter of each year since the carbon dioxide released from the plants is relatively rich in 12C–decreasing the ratio of 13C to 12C in the atmosphere. (Northern Hemissphere).
13.2 Isotopic discrimination of land photosynthesis
Keeling Significance
Climate change and rising CO2 are altering the behavior of land plants in ways that influence how much biomass they produce relative to how much water they need for growth. This study shows that it is possible to detect changes occurring in plants using long-term measurements of the isotopic composition of atmospheric CO2. These measurements imply that plants have globally increased their water use efficiency at the leaf level in proportion to the rise in atmospheric CO2 over the past few decades. While the full implications remain to be explored, the results help to quantify the extent to which the biosphere has become less constrained by water stress globally.
Keeling Abstract
A decrease in the 13C/12C ratio of atmospheric CO2 has been documented by direct observations since 1978 and from ice core measurements since the industrial revolution. This decrease, known as the 13C-Suess effect, is driven primarily by the input of fossil fuel-derived CO2 but is also sensitive to land and ocean carbon cycling and uptake. Using updated records, we show that no plausible combination of sources and sinks of CO2 from fossil fuel, land, and oceans can explain the observed 13C-Suess effect unless an increase has occurred in the 13C/12C isotopic discrimination of land photosynthesis. A trend toward greater discrimination under higher CO2 levels is broadly consistent with tree ring studies over the past century, with field and chamber experiments, and with geological records of C3 plants at times of altered atmospheric CO2, but increasing discrimination has not previously been included in studies of long-term atmospheric 13C/12C measurements. We further show that the inferred discrimination increase of 0.014 ± 0.007‰ ppm−1 is largely explained by photorespiratory and mesophyll effects. This result implies that, at the global scale, land plants have regulated their stomatal conductance so as to allow the CO2 partial pressure within stomatal cavities and their intrinsic water use efficiency to increase in nearly constant proportion to the rise in atmospheric CO2 concentration.
13.3 Isotopic signature of Anthropocene
Dean Abstract
We consider whether the Anthropocene is recorded in the isotope geochemistry of the atmosphere, sediments, plants and ice cores, and the time frame during which any changes are recorded, presenting examples from the literature. Carbon and nitrogen isotope ratios have become more depleted since the 19th century, with the rate of change accelerating after ~ ad 1950, linked to increased emissions from fossil fuel consumption and increased production of fertiliser. Lead isotope ratios demonstrate human pollution histories several millennia into the past, while sulphur isotopes can be used to trace the sources of acid rain. Radioisotopes have been detectable across the planet since the 1950s because of atmospheric nuclear bomb tests and can be used as a stratigraphic marker. We find there is isotopic evidence of widespread human impact on the global environment, but different isotopes have registered changes at different times and at different rates.
Dean (2014) Is there an isotopic signature of the Anthropocene?(pdf)