Isotopes Flashcards
(1) Explain why pyrite weathering on land changes the sulphur isotope composition in seawater. (A) Does d34S in seawater sulphate increase or decrease due to pyrite weathering?
A) SO42- + reducing bacteria. They prefer the lighter one and convert this to H2S and HS. When H32S forms pyrite it is also enriched in the lighter on.
During oxidation pyrites have a low d34S. The sea water has a positive high d34S. After weathering the d34S decreases.
2) (A)Write the expression for δ18O. Which standards can be used for the δ18O measurement? (B) Describe fractionation during evaporation of water (hydrogen and oxygen) near the equator, during transport and later precipitation over land. Use terms such as: delta value, fractionation and light/heavy (C) Describe mass-dependent fractionation.
A) δ 18O = (( 18O/ 16O)sample / ( 18O/ 16O)standard -1) ×1000
The standards are VSMOW: Vienna Standard Mean Ocean Water or VPDB: Vienna Pee Dee Belemnite
B) During evaporation, there will be a fractionation so that the lighter water, (1) H2(16)O, get enriched in the evaporated air. Therefore increasing d18O in the water and decreasing d18O in the atmosphere. Later on, due to Rayleigh fractionation, the precipitation will be enriched in 18O and D. As the evaporated water moves towards the poles, the vapour will become increasingly enriched in 16O and H and depleted in 18O and D, causing the delta value to decrease over time.
Write the equation for radioactive decay. Compare and describe the dating range of 14C and 210Pb. Describe how 210Pb can be used to date sediment. When does the “radioactive clock” start? How is 210Pb transported to the sediments?
210Pb half-life time of 22.26 years. Part of the 238U decay series. Decays to 210Bi. Radioactive clock starts when 222Rn escapes into atmosphere -> decays to 210Pb -> removed from atmosphere (10 days) -> glaciers and sediments.
14C half-life time of 5730 years. Formed by 14N and cosmic rays in form of neutrons. Decays back to 14N by beta radiation. 14C formation quicker in the past (due to solar activity) -> calibration needed. Radioactive clock starts when organism dies.
Dating possible for 6-7 times the halflife
5) The isotopic composition of stable isotopes is usually expressed using the d notation. Write the expression for d13C (in other words, how is d13C defined)? (A) Which are the two most important reactions where fractionation of C and S isotopes occurs in nature? (B) Which is the isotope standard for the stable carbon isotopes? (C) How is the d13C value of atmospheric CO2 affected by photosynthesis during summer and decomposition of organic matter during winter?
δ 13C = (( 13C/ 12C)sample / ( 13C/ 12C)standard -1) ×1000
A) For C it’s photosynthesis. 6CO2 (g) + 6H2O –> C6H12O6 +6O2
For S it’s sulfate reduction. 2CH2O + SO4 2- –> H2S + 2HCO3 –
B) VPDB - Vienna Pee Dee Belemnite (VPDB)
C) More 12C will be taken up during photosynthesis during summer which decreases the amount of 12C in the atmosphere. Due to it’s weaker bonding. Increased d13C.
During decomposition more 12C will be released due to it’s weaker bonding. Increased 12C in the atmosphere -> Decreased d13C
6) The isotopic composition of stable isotopes is usually expressed using the d notation. (A) Write the expression for d34S (in other words, how is d34S defined)? (B) Which is the isotope standard for the stable sulphur isotopes, and what is the d34S value for this standard? (C) Is the d34S value in solid FeS normally higher or lower than in dissolved SO42– ions? Explain your answer.
A) δ 34S = (( 34S/ 32S)sample / ( 34S/ 32S)standard -1) ×1000
B) CDT- Cañon Diablo troilite. d34S of standard is 0.
C) Compounds with higher oxidation state are enriched in 34S, meaning that SO42- has more 34S than S. Therefore the d34S value is higher in SO42-.
Why do these isotopic variations occur and how should the variations be interpreted? (C) Measurements of δ18O in calcium carbonate shells in the ocean sediments are anti-correlated with the ice-core values. How can this be explained?
) Isotope fractionations occur due to that the mass difference between the isotopes affect the covalent bond strength. The vibration frequency of heavier atoms is lower causing the bond to be stronger. Light atoms have a weaker bond and are therefore the higher fraction to change reservoir. The heavy isotopes in gases are less reactive and have a lower diffusion rate.
C) The ice-cores will be enriched in 16O due to precipitation while the calcium carbonate shells will be enriched in 18O due to 18O being heavier and staying in the ocean. Therefore, the d18O values found in climate archives of ice-cores vs carbonate shells are anti-correlated.
What does isotopes give information about?
Stable isotopes: isotope fractionation gives information about geochemical processes, trace origin of elements
Radioactive isotopes: age determination, trace origin of elements
Describe the bonding of isotopes
Mass difference (different number of neutrons) affect bond strength. Vibration frequency of heavy atoms is lower causing a stronger bond. Heavier isotopes are less reactive and have lower diffusion rates.
What are the isotopes of H, C, N, O, S and why do they vary?
2H, 1H – affected by climate and environmental processes. Temperature dependent.
13C, 12C, (14C radioactive) – mainly affected by photosynthesis.
15N, 14N
18O, 16O, (18O) – affected by climate and environmental processes. Distinguish water reservoirs. Temperature dependent.
34S, 32S, (33S, 36S) – mainly affected by sulfate reduction and exchange reactions between sulphur ions.
How are climate records shown in ice cores and carbonate shells?
Ice has 5% less 18O than ocean water due to Rayleigh fractionation. Lighter isotopes in the frozen water means that temperatures were colder – more lighter isotopes are retained in the ice (more negative d18O). Increasing d18O in ice core indicate increase of temperature at the time of formation.
Carbonate shells with high d18O indicate colder climate.
(B) Explain why photosynthesis on land changes the carbon isotope composition in seawater. Does d13C in seawater bicarbonate ions increase or decrease due to photosynthesis on land?
