Carbon-13. C3 and C4 plants.
The isotopes of a chemical element are the various configurations of its atoms. There are three carbon isotopes in nature: 12C, 13C and 14C. These are three varieties of the same chemical element, carbon, whose nuclei contain the same number of protons (six), but a different number of neutrons (six, seven and eight, respectively). Thus, besides having the same chemical properties, the isotopes have different atomic mass: twelve, thirteen and fourteen, respectively).
Almost 99% of atmospheric CO2, contains the less heavy carbon, 12C. A small part, 1.1% of CO2, is somewhat heavier, since it contains 13C. Finally, there is also in the atmosphere in a very small proportion, a type of CO2 that contains 14C, which is radioactive and unstable, and whose applications have typically being in paleochronology.
Below, we will see where lies the interest for 13C in paleoclimatological research.
Terrestrial vegetation and marine phytoplankton, in the process of photosynthetic absorption of CO2, discriminate against heavy molecules prefering 12C to 13. In this way, the carbon trapped in continental flora contains a smaller proportion of 13C than the carbon in atmospheric CO2.
Similarly, in the ocean, organic plankton carbon also has a smaller proportion of 13C compared to the inorganic carbon dissolved in the ocean (DIC, dissolved inorganic carbon).
This discrimination during photosynthesis is, nevertheless, variable and depends on the existing CO2 levels, just as much in the air as in the sea. The higher the CO2 concentration in the atmosphere or in the sea the bigger the discrimination.
The symbol d13C marks the deviation of isotopic concentration of 13C in any sample, living or fossil, with respect to a standard measurement. This is the carbon contained in the carbonate from the shell of a specific marine fossil called PDB (Pee Dee Belemnite), or VPRB, belonging to a Cretacic geological formation in North Carolina, whose value has been established by the International Atomic Energy Agency, based in Vienna.
The formula for d13C (in ‰) is as follows:
(13C/12C)sampled
– (13C/12C)standard
——————————–––––––––––––––
x 1.000
(13C/12C)standard
being (13C/12C)standard the isotopic ratio of PDB. The absolute 13C/12C ratio of the standard VPDB standard is 0.0112372. Material with ratios 13C/12C > 0.0112372 have positive d13C values, and those with ratios 13C/12C < 0.00112372 have negative d13C in vegetation and in paleosoil.
C3 and C4 plants.
Because of discrimination during photosynthesis, the d13C of terrestrial organic matter (in vegetation and in soils) has a mean value of -26‰. The d13C of atmospheric CO2 is close to -6‰. In inorganic sediment carbonates in the sea, the d13C is 1‰ and for organic sediment carbon it is -23‰.
However, these are mean values. For example, -26‰ is a mean value of the overall continental vegetation since, depending on how their photosynthesic process is materialized, the plants belong to two large groups, C3 and C4, with very different values for d13C.
The denominations are because in the plants of group C3, the first photosynthesized organic compound has 3 atoms of carbon while in group C4, there are 4. (There is also a third, very minor, group called CAM, a combination of C3 and C4 where some cactus and succulents belong to.)
Most plants (85%) (e.g. trees and crops) follow the C3 photosynthesis pathway and have lower values of d13C, between -22‰ and -30‰.
The remaining 15% of the plants are of type C4. The majority are tropical herbs and have high values of d13C, between –10 ‰ and –14 ‰.
Therefore, the d13C carbon value of the paleosoils depends largely on the type of plant that grew on them. It is less when the C3 plants were dominant and higher when those of type C4 proliferated. The study of the variations of d13C in the continental paleosoils can give an indication of the type of plant, C3 or C4, that dominated in specific periods.
Indirectly, the value d13C of the paleosoils can also indicate the evolution of the concentration of atmospheric CO2. It so happens, that with elevated concentrations of CO2, the type C3 plants are favoured with respect to plants of type C4, since the C3 pathway plants require less energy to carry out photosynthesis. On the contrary, when the CO2 concentration is low, the C4’s increase since they possess a mechanism of concentrating CO2 that favours them. So, the lower the d13C in the paleosoil, the higher the probability that the CO2 concentrations were high, and vice versa.
Apparently, before the Miocene (15 million years ago), the C4 plants were almost non existent. As a result, it is thought that the reduction of CO2 in the Miocene, caused perhaps by higher weathering linked to the upwelling of the Thibet, could have originated the development of C4 plants, and that the advance of tropical flora, which are typically of type C4, favoured the evolution of mammals.
Neverheless, there are discrepancies in some studies as it appears that the changes in the water supply to the vegetation (more or less aridity) were perhaps more important, than the variations of CO2, for the evolution of proportions in C3 and C4 plants in some regions of the earth. That seems to be the case in Africa. The supply of water to the interior of the continent could in turn evolve in parallel with the temperature variations in the tropics of the Atlantic ocean. Therefore, the differences in d13C in the paleosoild in large time scales, could also be due to climatic changes that, via altering the hydrological conditions, have also altered the type of predominant plants in a specific region.
d13C in the sea
The oceanic organic matter, that is, the soft parts of marine plankton – produced by phytoplankton during photosynthesis and subsequently grazed by zooplankton – has a mean value for d13C of –23 ‰. Petroleum, derived from fossil plankton, also has very low d13C values, and methane, also formed by burried organic matter, reaches values for d13C of –50 ‰ .
The dissolved inorganic carbon (DIC) in the sea is also trapped by marine organisms and forms shells or calcarious skeleton. But in contrast with what happens during photosynthesis, in the process of calcite precipitation into shells of marine organisms there is no isotopic discrimination with respect to the ratio 13C/12C of marine waters. So, the value of d13C in the carbon of these shells, which is +1 ‰, approximates that of the standard, 0 ‰.
Now, in the surface, the organic matter created in the photosynthetic activity of plankton depletes the 12C of the water in higher proportion than the 13C. So, the DIC in the water remains relatively enriched in 13C and, at the surface, leads to values of d13C that are occasionally slightly positive (up to + 4 ‰, if the productivity is elevated). In contrast, deep water receives large amount of organic matter that falls from above, poor in 13C. So, after remineralizing the d13C of the DIC is reduced and reaches a null, or slightly positive, value in the deep of the oceans.
Thus, there is a difference in the values of d13C between the surface waters (positive values) and the deep (negative values). When the oceanic productivity is intense the difference is bigger. When the productivity collapses (for example in the case of massive extinction of plankton) the difference disappears.
Comparison of the d13C value of planktonic fossils and benthic fossils (inhabitants of surface and deep waters, respectively) can provide indication of the intensity of oceanic productivity in the past. The oceanographer Shackleton and other researchers examined this question measuring the gradient in a sounding of the Equatorial Pacific, and found that the gradient increases in glacial eras, which would demonstrate an increase of biological productivity in those seas during glaciation. In turn, the photosynthetic productivity would explain in part the absorption by the oceans of the large quantities of atmospheric CO2, which would reduce the CO2 concentration in the air.
The key to the change of functioning of the deep marine currents is the evolution of d13C in the benthic foraminifera.
Currently, the value of d13C of shell carbon from benthic foraminifera in the Atlantic is larger than in the Pacific. It is in the Nordic Seas and the Labrador Sea, zones of sinking and formation of the NADW (North Atlantic Deep Water) mass, where d13C has the higher value.
This mass of water, in its route towards the south across the deep, collects organic carbon that falls from the surface level and is low in d13C. In this manner the deep NADW current becomes poorer in d13C, so that the lowest values are reached in the Pacific, at the end of its long transoceanic trajectory.
So then, analysis of d13C in fossil foraminifera during the last Glaciation at several latitudes and oceanic levels inform us about variations in the intensity of those deep currents. Thus, in soundings performed in the Antarctic Circumpolar Ocean, an abrupt increase of d13C is observed when passing from the Last Glaciation to the Holocene. This is caused by arrival of the NADW, poor in 13C, to the Antarctic.
