112lect8a - GY 112 Lecture Notes D. Haywick (2006) 1 GY 112...

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
D. Haywick (2006) 1 GY 112 Lecture Notes Stable Isotope Stratigraphy Lecture Goals : A) Stable isotopes of use to geology (fractionation) B) Delta values and isotopic standards C) Delta Oxygen Textbook reference: Levin 7 th edition (2003); Levin 8 th edition (2006), no information in the text. This is Doug only stuff. A) Stable Isotopes (geological significance) First of all, we need to outline what stable isotopes are relative to the unstable ones. Unstable isotopes like 235 U and 238 U breakdown over time. Stable isotopes (of which there are many), do not. They last essentially forever. As such, they are totally useless for establishing the absolute ages of rocks through radiometric techniques, but they do have amazing uses for other applications. For one, stable isotopes are useful for establishing the evolutionary history of pore water. This means you can do paleohydrology etc. You can also determine paleotemperatures , establish the geochemical variation of sea water over time, and map out sea level oscillations. The latter application is what we will be talking about in today's class. As mentioned earlier, there are several isotopes that are useful to geologists. Here are some of them and the rocks/materials that can be studied using them: Hydrogen: 1 H, 2 H ( 2 D) (water) Carbon: 12 C, 13 C (organic materials, petroleum, meteorites, carbonate systems) Oxygen: 16 O, 17 O, 18 O (water, ice, carbonate minerals, meteorites, hydrothermal systems) Nitrogen: 14 N, 15 N (biological systems, mantle rocks; diamonds -trace gases) Sulphur : 32 S, 33 S, 34 S, 36 S (meteorites, biological systems, magmatic rocks) Strontium : 86 Sr, 87 Sr (water) Some mention should be made about the relative proportions of the various isotopes in nature. There is usually one dominant stable isotope. For example, 12 C accounts for the vast majority of carbon (98.89%) where as 13 C accounts for very little (1.11%). The radioactive isotope of carbon ( 14 C) is not factored into this proportion because it is unstable and after awhile, decays to 14 N. Hydrogen is dominated by 1 H (99.984); "heavy hydrogen" or deuturium ( 2 H or 2 D) only naturally makes up 0.016% of all hydrogen. For oxygen, the relative abundances are as follows: 16 O = 99.763%; 17 O = 0.0375%; 18 O = 0.1905%. I could give you a bunch more numbers, but why bother; you get the idea. One of the neat things about stable isotopes that makes them so fundamentally important and useful is that the relative abundance of the various isotopes can be modified through different physical and biogenic processes. This is called fractionation and it is expressed by way of the fractionation factor ( α ), a ratio of the isotopic composition before and
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 6

112lect8a - GY 112 Lecture Notes D. Haywick (2006) 1 GY 112...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online