Isotopes

Discussion

nuclear models

This page is mostly just a pile of notes. There is very little prose here.

proton πρῶτον "at first"

Even numbers of protons and even numbers of neutrons are most stable. There are no stable elements heavier than bismuth (Z = 83). Two elements below Z = 83 do not exist naturally. Surprise, surprise, they have odd atomic numbers.

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isotopic ratios (isotopic signatures)

oxygen-18/oxygen-16

Nearly all the oxygen on the earth is of one particular isotope, 16O which makes up 99.76% of all the oxygen around us. The other two stable isotopes (18O and 17O) are found only in trace amounts (0.20% and 0.04% respectively). These abundances are stable in the lithosphere, but vary in the atmosphere.

"Oxygen is a unique tracer of the chemical and physical history of geological and extra-terrestrial material since it has three stable isotopes, (masses 16, 17 and 18), which are easily fractionated by numerous processes and it exists abundantly in gases (e.g. carbon dioxide, oxygen), liquids (primarily water) or solids (virtually all rock forming minerals). Measurement of the ratios of the three isotopes can therefore be used to interpret the physical processes experienced by the sample"

"Light water (water with 16O) evaporates more easily. The 18O/16O fraction will be smaller in the snow that falls on a glacier than it is in the ocean from which the water evaporated. As glaciers grows worldwide there is less and less 16O in the ocean, so the 18O/16O ratio of the ocean gets larger. And so does the d18O ratio. As the world's glaciers grow in volume 18O values become larger. The oxygen isotope record was recording the size of the ice sheets."

"Precipitation has a ratio of oxygen isotopes present. 18O is heavier and 16O is lighter. If the rain is cold it will have a higher ratio of 18O:16O and if it is warm the amount of 16O increases in the ratio. This is due to many mechanisms Look for papers by Daasgarad 1964 to elaborate on this. [Dansgaard, W. "Stable isotopes in precipitation." Tellus. 16 (1964): 436-468.]"

The ratio of 18O/16O changes by 700 ppm/℃ of mean ocean temperature.

it is estimated that each 1 part per thousand change in δ18O represents roughly a 1.5-2 °C change in tropical sea surface temperatures (Veizer et al. 2000).

hydrogen-1/hydrogen-2 (hydrogen/deuterium)

Because isotopic fractions of the heavier oxygen-18 (18O) and deuterium (2H) in snowfall are temperature-dependent and a strong spatial correlation exists between the annual mean temperature and the mean isotopic fraction of 18O or 2H in precipitation, it is possible to derive temperature records from the records of those isotopes in ice cores.

Deuterium values are expressed as δD, which is defined as:

δD = {[(2H/1H)sample - (2H/1H)V-SMOW] (2H/1H)V-SMOW} X 1000

where (2H/1H)sample is the ratio of deuterium to ordinary hydrogen in sample corresponding to a particular datum, and (2H/1H)V-SMOW is the ratio of deuterium to ordinary hydrogen in Vienna Standard Mean Ocean Water (V-SMOW).

The deuterium content distribution is well documented over East Antarctica and over a large range of temperatures (-20° to -55° C); there is a linear relationship between the average annual surface temperature and the snow deuterium content. The slope of this δD/surface temperature relationship was found by Jouzel et al. (1993, 1996) and Petit et al. (1999) to be 9‰ per °C. Further details on the methodology are presented in Jouzel et al. (1987), Lorius et al. (1985), and Petit et al. (1999).

The record presented by Jouzel et al. (1987), based on data in a 2083-meter ice core from the Russian Vostok station in central east Antarctica, was the first such record to span a full glacial-interglacial cycle. Drilling continued at Vostok until January 1998, reaching a depth of 3623 m, and a corresponding time of ~420 kyr BP. More recently, a 740-kyr deuterium record has been extracted from an ice core taken at Dome C (EPICA Community Members, 2004). Deuterium fractions were determined in meltwater from 55-cm long sections of the ice core the surface down to the bottom of the core.

http://cdiac.ornl.gov/trends/temp/domec/domec.html

Heavy water is more readily condensed or deposited from vapor, causing its distribution to differ somewhat from ordinary, light water.

carbon-13/carbon-12

"Carbon atoms occur in three different masses, or isotopes. Unlike high-temperature processes in deep Earth, low- temperature, biological processes, such as photosynthesis, are sensitive to the differences in mass, and actively sort different carbon isotopes. Thus, the ratios of carbon isotopes in organic materials — plants, animals, and shells — vary, and also differ from those in the carbon dioxide of the atmosphere and the oceans."

sulfur-34/sulfur-32

Similarly, sulfur comes in two stable isotopes: sulfur 32 and sulfur 34. Sulfur eating bacteria prefer the slightly lighter sulfur 32. (It's easier to chemically pull the lighter isotope out of a molecule than the heavier.) The concentration of this isotope in their waste products is as much as 7 percent greater than it is in their food sources. When sulfur rich minerals are found in nature it can be determined if the sulfur came had a geologic or biologic origin by looking at the ratio of 34S:32S.

nitrogen-15/nitrogen-14

herbivores vs. carnivores

Tracers