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Isotope Geochemistry > Exam 1 > Flashcards

Exam 1 Flashcards

1
Q

What is delta for seawater?

A

zero

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2
Q

When we decrease temperature, we get more? How does this effect delta?

A

precipitation. Makes clouds lighter due to evap of light isotopes and rainout of heavy isotopes (temp drives precip + precip drives delta)

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3
Q

How does CO2 compare to 2H/1H values in ice cores?

A

They are the same

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4
Q

Why are 2H/1H more positive in warm climate?

A

humid, not as much loss of water vapor

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5
Q

What type of fossils in limestone do we use and why?

A

Formenifera; calcite; tells us temp from 18O/16O values

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6
Q

When did 53Cr become abundant and why?

A

750 Mya; oxidation events; redox forming Cr3+ and Cr6+

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6
Q

What form of Cr is immobile and how can we tell distance from contaminant source?

A

Cr3+ is immobile; Cr6+ will be enriched relative to the source

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7
Q

What can He tell us about mountains?

A

If we find He it means that uplift and erosion has happened fast

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8
Q

Where does Sr come from and what can it tell us about fish?

A

Sr comes from rocks and soil into the water. Sr is in the fish ear bone and can show migration patterns

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9
Q

How can we age date trees?

A

14C

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10
Q

What is 87Sr from?

A

daughter of radioactive decay

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11
Q

How do we age date really old rocks?

A

U-Pb of zircons

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12
Q

What is the relationship between capital delta and temperature?

A

As temperature increases capital delta decreases

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13
Q

How does 1/2mv^2 relate to D2 vs. H2

A

D2 has a big mass and a slow velocity whereas H2 has a small mass and a fast velocity (must be equivalent)

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13
Q

What isotope does evaporation favor?

A

the light isotope

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14
Q

What isotope is the product enriched in during microbial reduction?

A

the light isotope

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15
Q

When precipitating calcite from water the calcite is enriched in what isotope and why?

A

The heavier isotope b/c the heavy isotopes prefer the more strongly bonded structure

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16
Q

How do you know fractionation has occured?

A

Different isotope values between reactants and products

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17
Q

Why do we use lowercase delta?

A

Sometimes isotope fract. can be in the 6th decimal place where <0.1% differences are important therefore delta allows us to have reasonable numbers and avoid having to write the first few zeros that appear in measurements

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18
Q

What is per mill equivalent to?

A

ppt

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19
Q

How do we measure the difference btwn lowercase deltas?

A

big delta = delta(product) - delta(reactant)

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20
Q

How can we measure the size of isotopic fract.?

A

alpha = R(product)/R(reactant)

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21
Q

How can we relate big delta to alpha?

A

delta ~ 1000ln(alpha)

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22
Q

What does alpha = 1 mean?

A

no isotope fractionation

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23
Why is the classical model for bond energy incorrect?
Bonds only absorb quantum energies and isotopes have diff. bond energies
24
What is zero point energy?
Lowest energy
25
What is the energy equation?
E = (n + 1/2)hv
26
What is the relationship btwn vibrational frequency/energy and heavy isotopes?
heavy isotopes have lower vibrational frequency/energy (low ZPE + hv)
27
Bonds with lighter isotopes are what?
weaker
28
Why does it take less energy to break a bond with a lighter isotope?
ZPE of light isotopes is higher
29
Which environment is stronger? CO2 or H2O?
CO2
30
What is Keq. = 2 alpha for?
Simple collision rxn btwn a light and heavy isotope
31
How would 18O behave btwn a CO and O2? Why?
18O would be enriched in CO bc of the stronger bond and a collision leads to energy transfer. Lowest energy would be all 18O in CO
32
If a rxn runs for a long time what will happen?
System will work to achieve the lowest energy state
33
Does the equilibrium constant equal the equilibrium fractionation factor?
No
34
How does energy relate to ZPE of two molecules colliding?
Energy decreases bc of differences in energy wells and diff. ZPE of molecules
35
How is delta = delta (p) - delta (r) different from delta ~ 1000ln(alpha)
The first equation is used for real measurements whereas the second is related to equilibrium theory
36
What is epsilon used for?
Kinetic fractionation
37
What do we use a boltzmann factor for?
Accounting for energy changes as a function of temperature. We need to know the energies of different molecules to examine isotope fractionation change
38
How can we get an isotopic fractionation factor?
using an equilibrium constant
39
What is a partition function?
The sum of all energies (which molecules are at higher energy) + the boltzmann factor (isotopic fractionation)
40
How do energy levels change as temp. increases?
More population of higher energy levels
41
How are Kequil. and the partition function related?
Partition functions replace concentrations in Keq. and then these functions can be broken into trans., rot., and vib. motions
42
What is Kvib.?
1 + h/2kt (boltz.) [v(light) - v(heavy)] - [v(light) - v(heavy)] (v = vibrational frequencies)
43
Why are we ignoring trans. and rot. motion?
Liquid and solid dont have trans. and theres little rot. motion
44
Methan has 4 vib. modes. How can we calc Keq.?
Get vib. frequency of diff. modes and then use those to calc the partition functions
45
What temp. relationship does isotopic fractionation follow at low temp.?
1/T
46
What temp. relationship does isotopic fractionation follow at high temp.?
1/T^2
47
What causes isotope fractionation
Difference in chemical bonds of light and heavy isotopes
48
What is the relationship between the mass of light and heavy isotopes?
change in mass/m^2
49
How big is isotopic fractionation for heavy elements?
Small
50
What is isotope clumping?
Heavy isotopes are clumped together
51
Why do heavy isotopes clump?
Leads to an extra low vibrational energy
52
What is the relationship btwn clumping and temperature?
Clumping more at low temps and clumping less at high temps
53
How do isotopes behave in a temp gradient?
cool zone enriched in heavy isotopes and hot zones are enriched in light isotopes
54
Why do heavy isotopes generally populate upper energy levels?
With lower vibrations their are smaller energy gaps making it easier to access higher quantum levels
55
Does hv depend on temperature?
No, it depends on quanitized energy
56
What are examples of equilibrium fractionation?
precip. of calcite and quartz, droplet and cloud formation
57
What are examples of kinetic fractionation?
Evaporation, sulfate reduction
58
Why do lighter isotopes diffuse faster?
kinetic energy is the same for both isotopes therefore the velocity of the light must be higher
59
What isotopes prefer to evaporate?
Light
60
How can we find the kinetic rxn constant
Ae^(-Eb/Kt) (aka boltzmann factor including a kinetic energy barrier)
61
How big is the kinetic energy barrier of light isotopes?
small b/c of higher ZPE
62
How is alpha different in kinetic vs. equili
Kinetic uses ratio of instanenous product/reactant
63
How can we get capital delta of clumped isotopes
The measured value minus the random chance value
64
What does kinetic alpha depend on?
back rxn, rxn mechanism, pH
65
What is a rate limiting step?
A step in a multi-step rxn that has a high activation energy making it hard for a rxn to progress
66
What if the rate limiting step is last?
back rxn occurs and isotopes become close to equilibrium
67
What could cause heavy isotopes to be favored in the products for a kinetic rxn
If the steps have fractionation that favors heavy isotopes such as strong bonds or the rate limiting step favoring lighter isotopes
68
what is the relationship btwn products and reactants in a closed system?
Reactants become inenriched in heavy isotopes leading products to slowly become enriched in heavy isotopes
69
What are the assumptions for a rayleigh distillation model?
closed system, no back rxn, and system is well mixed
70
for carbon isotopes why is alpha flipped?
so that epsilon is not always negative
71
What does it mean if all Cr(v) converts to Cr(III)?
No isotopic fractionation has occurred
72
What is the rayleigh equation?
R = Ro f^alpha - 1
73
What is the most important redox on earth?
photosynthesis
74
What is smoothing out the temp at the equator and poles?
latent heat of H2O
75
What causes rain to get lighter?
evap + vapor is light then as rain is removing heavy the vapor is getting lighter
76
Is precip. a kinetic or equili. rxn?
equili. w/vapor
77
What is the trend of a rayleigh model?
The residual vapor gets more neg leading to rain following a downward tren with an epsilon of 10.2
78
what is the equation for the GMWL
delta(D)=8delta18O + 10
79
If delta(D) is negative is that a glacial or warm period?
Glacial
80
Is the world a rayleigh model?
No, we can have mixing of different air masses in different locations
81
How can you have kinetic and equilibrium fractionation all in the same water column?
Changes in humidity in the transition zone
82
What are the x and y axis for GMWL
x - delta (D) y - delta 18O
83
What is d - excess used for?
tells us deviations from GMWL
84
What is d - excess for VMOW and GMWL
VSMOW = 0 GMWL = 10
85
What climate is above the GMWL?
Dry source
86
What climate is below the GMWL?
Wet source
87
Where to water rock interactions plot on the GMWL
to the right w/ higher delta 18O
88
High d - excess indicates?
low humidity of source
89
What is the amount effect?
Amount of rain dictates the delta 18O values
90
What is the orographic effect?
As air masses rise precip becomes lighter
91
What is the main driver of precip?
temp
92
What do precip isotopes values relate to?
temp., precip variation over months, proximity to local sources, low humidity + evap, precip intensity
93
Why would you see a seasonal signal in a carst?
fast moving water through a carst
94
Where does modern precip. relative to glacial meltwater?
The modern plots in the middle and glacial plots more neg.
95
How would you test if a mountain range has not yet uplifted?
measure delta 18O of calcite in soil. Soils will have a higher delta 18O compared to modern soils
96
What is the per mill difference between vapor and rain droplets?
about 10 per mill
97
What can cavity ring down measure?
C, O, H
98
What can gas source mass spec measure?
H, C, N, O, S, Cl
99
What can TIMS measure?
RRE
100
What can ICPMS measure?
everything expect Ar, H, O, C, N
101
What can SIMS measure?
solids of U-Pb
102
What can AMS measure?
14C, 36Cl, 10Be, 27Al

Isotope Geochemistry (2 decks)

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