Isotopes Generals Review
What is an 'extinct radionucleotide'? How can we determine the age of a sample with it? What are the other assumtions?
For radionuclitides that have decayed away completely we can still ascertain the amount of radionucleatide that was initially incorporated and thus date the sample relative to another line. In addition to the usual assumptions about isochrons, you need to assume that all isotopes of the element in question were incorporated at into the rock the same way and there was no fractionation of the pool before as well.
What is mass dependent fractionation?
A chemical process that separates isotopes in a way that depends on mass.
Describe the 14C isotope system. How is it applied in biogeochemistry? Describe the trends it creates in the environment.
A cosmogenic dating scheme that has a relativley short half life (~5700 years). Older carbon follows normal decay systematics. But it is complicated by a few processes, and the 14C year does not equal a calendar year because we have to assume constant production of 14C in the atmosphere. This is not true (change in production rate is probably due to changes in the geomagnetic fields). The carbon in the atmosphere can also interact with different carbon resivors. C in the atmoosphere is small compared to othe resivors. We correct to a materials whose ages are known.
What is a geochron? How can you assess time from it? Write the equation for the geochron used to date the earth. What assumptions are needed?
A geochron combines the daughter atoms from two decay processes to determine the age of a group of samples of the same origin. Just like an isochron you must assume that the initial composition of rock is isotopically consistent (235U:238U is the same) but chemically diverse (U:Pb varies). With this info, the modern Uranium ratio, and an initial Pb isotope composition obtained from a near uranium free substance, you can solve for time.
Draw an isochron. How can this be used to tell time since a sample formed? What are the assumptions needed?
An isochron consists of a daughter and parent isotope measured in a sample. The equation for the line is set by the decay equation between one parent and daughter. Knowing both the daughter, parent, and having a distribution of instal conditions to get the intercept you can find "t".
Describe the (87/86Sr) vs (87Rb/86Sr) isotope system. How is it applied in biogeochemistry? Describe the trends it creates in the environment.
Application 1 - Dating An isochron example that allows you to date things as long as you have a isotopically homogenized system, multiple physical samples, and (for all isochrons) is only sensitive up to 5-7 half lives Application 2 - Mantle Partitioning Sr is more compatible with mantle, so its stable daughter 86Sr stays in mantle; Rb pref enters the melt, which cools to form crust, where it decays to 87Sr over time. Crust has higher 87Sr/86Sr than mantle.
Describe the (234U/238U) vs (230Th/238U) isotope system. How is it applied in biogeochemistry? Describe the trends it creates in the environment.
Application 1 - Diagenesis You can investigate the amount of 234 that was leached from a sample by using the equiline. Both 234U and 230U are daughter products of 238U. So, everything wants to get to the equipoint in the middle, which is at secular equilibrium. In the section that has more 234/238U, there is an addition of 238U to the lattice and this is in excess (this is called the excess zone). Where there is more 230Th/238U 234 U is being lost by breaks in the lattice during leaching (this is called the leach zone).
Describe the (234U/238U) vs (230Th/238U) isochron system. How is it applied in biogeochemistry? Describe the trends it creates in the environment.
Application 1 - Doubly Radioactive Isochron Case Over time the samples will evolve toward the equiline and the 238 U will be constant relative to 230Th at this point due to its long half life
Describe the (26Mg/24Mg) vs (27Al/24Mg) isotope system. How is it applied in biogeochemistry? Describe the trends it creates in the environment.
Application 1 - Extinct Radionuclide Isotopes The parent isotope has already decayed away. But we can still use it because the stable daughters of the parent can give us a hint. Any "extra" 26Mg in the rock indicates that the sample was formed when there was a lot of 26Al around (thus it is older). The slope is the 26Al/27Al. This only gives relative dates unless you have another isochron system.
Describe the (207Pb/204Pb) vs (206Pb/204Pb) isotope system. How is it applied in biogeochemistry? Describe the trends it creates in the environment.
Application 1 - Geochron Allows you to date the earth as long as your samples fill the assumptions needed for an isochron. It is important to note that 207Pb and 206Pb come from different U decay series. So, if your sample has more 238U at the beginning, 207 would be produced more initially since 235 has a considerably shorter half-life.This leads to the curved lines we saw in class. Shows Earth is ~4.5 Byr and was discovered by Claire Patterson
Describe the (230Th/232Th) vs depth isotope system. How is it applied in biogeochemistry? Describe the trends it creates in the environment.
Application 1 - Sedimentation Rate The 230Th that reaches the seafloor is from two sources (1) decay of parent isotope (238U) and (2) from particle scavening. (230Th/232Th) = (230Th/232Th))intial*e^(-lamba230*z/s) where z is depth in seds and s is sedimentation rate. Helps determine sedimentation rate.
Describe the δ13C isotope system applied to the Suess effect. How is it applied in biogeochemistry? Describe the trends it creates in the environment.
As we burn fossil fuels, we are putting radiodead carbon into the atmosphere. The material is deleted in δ13C (as we are burning C3 plants and most plants now are C4) and has virtually no 14C. (δ13C (C3) = -27 ‰ on average, δ13C (C4) = -13 ‰ on average)
Describe the δ13C isotope system applied to extent of NADW. How is it applied in biogeochemistry? Describe the trends it creates in the environment.
By looking at δ13C in benthic forams over time, you can see that the δ13 changes dramatically during the LGM. During glacial time periods (specifically the LGM) there was lower δ13C values and higher δ18O in the forams. At the end of the LGM, the then δ13C increased. This reflects an increase in the proprtion of (young) NADW that entered into the system. This suggests that NADW is muted during glacial periods (NADW generates heat to the atmosphere, so it could drive glacial cycles). All of that being said, the variation in delta13 is still much lower than 14C
Describe the 182Hf/182W isotope system. How is it applied in biogeochemistry? Describe the trends it creates in the environment.
Core Differentiation - 182W more compatible with core because it is a siderophile. 182Hf enters melt to form mantle.
Describe the δ18O isotope system paleo application to temperature and ice. How is it applied in biogeochemistry? Describe the trends it creates in the environment.
Glacial Times: More ice and the ice sheets have a lower δ18O and seawater likely has a higher δ18O. Interglacial: Less ice. The ice sheets have a higher δ18O and the seawater is heavier. Ice volume seems to correspond to Rayleigh fractionation patterns. The benthic forams record deep water delta18O and planktonic record surface water. These can then be linked to temperature (lower Temperature in deep leads to a larger fractionation). The temp. in the deep water is always the +/- same temperature, so you can get at the difference between water and carbonate
Describe how an isochron might be impacted by a mixing signal. Which property does this violate? How can it be corrected for?
Having two sources for an isochron means that the line is not "isotopically homogeneous". However, this can be corrected for if you know the starting components A and B
Describe the 147Sm/143Nd isotope system. How is it applied in biogeochemistry? Describe the trends it creates in the environment.
Mantle Partitioning - Sm is more mantle compatible, so remains in mantle. Over time it decays, leading to higher 143nd/144nd in mantle than in the crust
Describe the 187Os/187Re isotope system. How is it applied in biogeochemistry? Describe the trends it creates in the environment.
Os is more compatible so it remains in the mantle, Re entres the melt. High Re/Os in the crust and low in the mantle. Variations of Re and Os are HUGE. Mantle Re/Os = 1 and Crust is = 100 or more. Dating applications are limited due to low Os concentrations in minerals
What is the decay equation for for one simply parent daughter pair. What can this be used for?
The decay equation here can be used to construct an isochron which determines the age of a sample. Dtot is the total amount of daughter in the sample. Do is the initial amount of daughter in the sample that is preformed and did not come from decay. N is the current amount of parent isotope. The decay constant is for this radioactive parent.
What is the equation for the average life expectancy of a radio isotope? What is this referred to as?
This value (𝜏) is often called mean life or e-folding rate. This is when 1/e of the sample remains (36.8%)
Describe the δ13C isotope system applied to photosynthesis. How is it applied in biogeochemistry? Describe the trends it creates in the environment.
Two processes affect fractionation in photosynthesis. 1) Diffusion is KIE and imparts signature of ~4 per mille. Then the formation of OM will fractionate differently for C3 and C4. C4 has a pre concentration step that makes the system nearly closed, this results in less fractionation than in C3 plants that has a more open system
Describe the δ13C isotope system applied to water mass age and circulation. How is it applied in biogeochemistry? Describe the trends it creates in the environment.
δ13C is enriched in the surface waters as biology takes up δ12C. δ12C is then removed to the deep water more efficiently than δ13C during remineralization. Thus, the amount of δ13 C can tell you the age of a water mass. Waters with high amounts of δ13C are younger since remineralization hasn't had time to pump in a lot of δ12C. As waters age, 12C overwhelms the system and δ13C is less abundant.