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Isotope geochemistry (Rev #4, changes)

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Contents

Idea

Wikipeidia 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.

Details

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 and δC\delta C zero.

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

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

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.

Sulfur

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.

Applications

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

  • Biosphere-Atmosphere Stable Isotope Network (BASIN)
  • The european CARIBIC

References