Geochemistry Exam 1

Front 1

Fire impacts

Back 1

Releases lots of CO2

Clears forest leaving ash layer

Post:

Soil compaction

Impermeable top-layer from ash

Loss of slope stability Inc erosion

Front 2

Big bang evidence

Back 2

Elemental composition, red shifts, background radiation

Front 3

Atom abundance

Back 3

Front 4

Big bang nucleosynthesis

Back 4

-Universe had to cool to 3000k before H and He were stable

-Collisions of He produce small amounts of Li, Be and B

-No elements heavier than B created till stars appeared

Front 5

Stellar nucleosynthesis

Back 5

-gravity leads to matter accumulation and increased local heat

-matter + heat = nuclear fusion reactions= C and No formation

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Explosive nucleosynthesis

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- in large mass stars, tipping point reach where they rapidly collapse

-core rebounds sending massive shockwave outward and increasing heat

- breakdown of some atoms (Fe)

- releases huge quantity of neutrons =formation of heavier elements (>Ni)

Front 7

Iron excess

Back 7

- fusion reactions up to iron release energy

-fusions reactions to create iron or above consume energy

- leads to buildup of iron cores in stars

- large star build up> supernova> iron excess

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Cosmic Ray spallation

Back 8

-Free particles (protons) and small atoms (He) from pace smashing into larger atoms-Breaks them into smaller pieces (li, Be, & B) that decay and may be used to date things)

Front 9

Radionuclides

Back 9

- large discrepancies between neutrons and protons= usually unstable- always decay with equal or smaller atomic weight

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Alpha decay

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1 alpha particle = 1 4/2He+ nucleus reduces A (top #) by 4 and Z ( bottom #) by 2

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Beta negatron decay

Back 11

- occurs when N > P, Z increases by 1, A remains same

Front 12

Position beta decay

Back 12

occurs when N < P, Z decreases by 1, A remains same

Front 13

Electron capture beta decay

Back 13

occurs when N < P, Z decreases by 1, A remains same

Front 14

Damage potential of radiation types

Back 14

Alpha- largest, most dangerous, easily stopped (ex cannot cross skin barrier)

Beta- much smaller and less damage than alpha, but travel further/penetrate deeper

Gamma- least damage, but penetrates through almost everything

Front 15

How many moles of iron in 550g of pyrite, FeS2?

Back 15

Fe = 55.845 amu, S = 32.06 amuFeS2 = 55.845+2*32.06 = 119.965 amuMass PercentFe = 55.845/119.965 = 0.4655 * 100 = 46.55%S = (2*32.06)/119.965 = 0.5345 * 100 = 53.45%550 * 0.4655 = 256.025 g Fe à 256.025/55.845 = 4.58 moles Fe in 550 grams of pyrite

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Radiometric dating assumptions

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1.The decay constant (λ) is well-constrained and has not changed over geologic time2.Measurements of D and N are accurate and representative of the sample being dated3.The system has remained closed to the parent and daughter nuclei since it formedD*/N ratio due only to radioactive decay4.D0 can be estimated realistically\

Front 17

Rb/Sr dating

Back 17

Half-life: 4.88*10^10 yearsRb-87 substitutes for K, commonly/easily in igneous rocks

Front 18

Isochron plot

Back 18

Isochron Plots: (Sr-87/Sr-86) = (Rb-87/Sr-86)*((e^λt)-1)+ (Sr-87/Sr-86) y = m * x + b

Front 19

U/Pb dating

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Most precise radiometric dates ( because zircons and 2 radioactive U isotopes)-> less than 1% uncertainty= +/- 10s of thousands to millions of years !->In Zircon (ZrSiO4), U substitutes for Z-> Zircon actively rejects Pb, all Pb in zircon is the daughter product*U-238 → Pb-206 half-life: 4.468*10^9 years =99.27% UU-235→ Pb-207 half-life: 7.04*10^8 years =0.72% U

Front 20

Concordia/Discordia

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Concordia: ratio daughter/parent pairs should form specific line Discordia:Line indicating age of formation and major heating resetting event

Front 21

Radiocarbon dating

Back 21

Carbon-12 Stable Carbon-13 Stable Carbon-14 Unstable, half-life: 5730 years -> N-14Used to date carbon-bearing sources younger than 60,000 years (max 10 ½ lives)

Front 22

C-14 production

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Cosmic ray intensity (prod. Increase with solar activity)Draw of charged particles to the poles = more C-14 at poles (due to earth’s magnetic field)

Front 23

Suess effect

Back 23

fossil fuels flooding atmosphere with stable CO2-> relative abundance of C-14 drops

Front 24

Devries effect

Back 24

increase in C-14 abundance due to changes in Earth’s magnetic field

Front 25

Nuclear testing

Back 25

releases burst of neutrons inducing C-14 production

Front 26

Fractionation

Back 26

“Differences in the relative abundance of different isotopes of an element between two reservoirs”Isotope fractionation-Preferential movement of heavy versus light isotopes largely dependent on chemical bonding and mass (often mass-dependent)-Larger proportional mass difference = larger fractionationAt given temp:-Lighter isotopes have more kinetic energy than heavy isotopes -Lighter isotoper easier to escape system -Heavier gets captured in crystalline solid

Front 27

Stable isotope fractionation

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Uses H, C, N, O, & SLarge mass diff between isotopesLess common stable isotope still abundant enough to measure very accurately These elements make up majority of biological & mineral objectsUseful for reconstruction environmental change, magma sources, digenetic processes, biological pathways

Front 28

Fractionation factor

Back 28

α(A-B) = R(A) / R(B) A & B are diff reservoirs, R is ratio of heavy to lightVaries with temp, chemical composition, crystal structure/chemical bonds, & pressure

Front 29

Delta notation of fractionation

Back 29

Directly compare samples by differences between samples and standards in delta notation

δXsample = {(Rsample – Rstandard) / Rstandard } * 1000Units: 0/00 (aka parts per mil)

Front 30

Water isotopologues

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Front 31

Types of water fractionation

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Equilibrium Fractionation - reaction of two reservoirs with different compositions mixing to reach chemical equilibrium ( ex: oceans behave like equilibrated systems)Kinetic Fractionation - unidirectional exchange across systems Biologically mediated pathways & evaporation

Front 32

Delta notation of ocean water for 2H

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α(ow-wv) = (2H/1H)ow / (2H/1H)wv = 1.074Ocean water enriched 74°/°°in 2H compared to water vapor above ocean

Front 33

Delta notation for ocean water of 18O

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α(ow-wv) = (18O/16O)ow / (18O/16O)wv = 1.0092Ocean water enriched 9°/°°in 18O compared to water vapor above ocean

Front 34

Temperature fractionation relationship

Back 34

Fractionation more pronounced in slow reactions ( at poles/ where it’s cold)-> cold regions concentrate light isotopes

Front 35

Rayleigh Distillation

Back 35

R = R0f^(α-1)R0 = initial 18O/16O or 2H/1H ratio of water vaporR = 18O/16O or 2H/1H ratio of water vapor after precipitationα = (18O/16O)rain/ (18O/16O)vapor or (2H/1H)rain/ (2H/1H)vaporf = atomic fraction of water vapor remaining in system compared to original amountf = 1 before any precipitation, f = 0 when no water vapor remains

Front 36

Global meteoric water line

Back 36

Because precipitation is largely sourced from the same reservoir (oceans) there are linear changes in δO and δH across the Earth

(1/1)/(2/16)= 8

Front 37

Isotope forensics equation

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δX = δX0 + Ɛln(f)^Calculates delta of isotope change of either part of systemδX0 is the initial element’s deltaƐ is the enrichment factor (- values are for calculating delta of remaining low-energy phase, + values are for calculating delta of remaining high-energy phase)f is the fraction of original source remaining

Front 38

Review horse case study

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N/a

Front 39

Marine isotopes

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Paleo temperatures: Changing isotope ratios at a given location should represent changes in temperatureSources of Oxygen often found in calcite-bearing organisms

Front 40

Marine isotopes and u/th dating

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Intermediate decay product on the way to Pb (lead)Th not soluble so all Th in limestone is decay product of U only

Front 41

Marine isotopes 18O record source

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Corals, Conodonts (extinct eel-like vertebrates), forams(1-celled organisms w calcite shell)

Front 42

Marine isotopes biological effects

Back 42

Diff species of same group live in diff environments (benthic vs pelagic zones)Within species, individuals in modern day migrate oftenChanges in environment over the course of life Ex: in-shore nursery to offshore life

Front 43

Forams and marine isotopes

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Planktic forams can be found over the entire ocean, not just nearshore making them idealLarge sample size to testLiving/ closely-related species so fractionation can be studied

Front 44

CCD

Back 44

CCD is depth of balance between CaCO3 supply and dissolution rates(modern depth ~4,500 m)Below CCD there is more CO2 which acidifies the water

Front 45

Heavy 18O=

Back 45

= Ice age - light 18-O is in the ice so seawater becomes heavier

Front 46

Neodymium

Back 46

144-Nd is unstable (2.229*10^15 yr), 143-Nd is stableHalf-life ratios driven by fractionation Values reported as Ɛ(Nd)ƐNdsample = {(143Ndsample/144Ndsample – 143NdCHUR/144NdCHUR )/ 143NdCHUR/144NdCHUR } * 10000CHUR = Chondrite Meteorite (Standard ratio)

Front 47

Sea Level Proxy

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ƐNd dependent on sources ( continental vs magmatic)ƐNd used as proxy based on:Nd residence time is shorter than inter-ocean basin mixingƐNd negatively correlated with age because of evolution of Earth’s mantle and radioactivity of SmYounger crust has higher ƐNd valuesOlder crust has lower ƐNd valuesSea level drop → more exposure & weathering of continents negative shifts in ƐNdSea level rise → less exposure & weathering of continents positive shifts in ƐNdIssuesNd measuring craton versus magmatic input with assumption that sea level is the major controlBut craton source dependent on sediment supply rate which is influenced by local basin topography, climate, and relation of sample to delta frontsHigh degree of spatial variation along a coastline

Front 48

Redox state

Back 48

Front 49

Redox indicators

Back 49

Front 50

Carbon isotopes/extinction

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Carbon is the 4th most common element (50% all living tissue)Largest reservoirs of carbon are sedimentary rocks

Front 51

Carbon flux

Back 51

Air → biomass (photosynthesis)Biomass → air (cellular respiration)Biomass → rocks (organic matter burial)Air → Oceans (spontaneous gas exchange)

Front 52

Carbon residence times

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Atmosphere residence time ~5 yearsBiological residence time ~10 yearsEarth residence time >10 million years

Front 53

Carbon cycle feedback loop

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Front 54

Broken feedback loop

Back 54

Burning of fossil fuels releasing CO2 at a rate 100-300x faster than pre-industrial levelsMuch faster than negative feedback loop can keep up Leads to runaway greenhouse effect

Front 55

Mass extinctions and climate change

Back 55

All mass extinctions associated with major climate changeEnd-Ordivician (massive cooling)Permian (6-10°C increase) >90% species extinct

Front 56

Isotope excursions

Back 56

Carbon: Earth’s carbon system relatively robust to changes in δ13C (± 2 o/oo considered significant over 100,000 yr)Negative isotopic excursions have coincided with two mass extinctions (end-Permian & Late Triassic)

Front 57

Negative isotope excursions

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Negative isotopic excursionsMake reservoir lighterIncrease in carbonate prod and burialMethane hydrate releaseoutgassing from LIPs

Front 58

Positive isotope excursions

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Cooling events/ reservoir heavierDecrease carbonate production and weathering (sea level drop)Burial of organic matterOrganisms are light but as they accumulate in sediment, they leave behind an increasingly heavy ocean reservoir for later organisms

Postive isotopic excursions associated with two mass extinctions (end-Oridivician, Late Devonian)

Front 59

Siberian Traps

Back 59

Cover an area ⅔ the size of continental United States Erupted in Pulses ~250 myaWould have pumped enormous amounts of CO2 into the atmosphere

Front 60

Baking carbon

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Large field of intrusions in carbon-rich rocks leads to release of CO2 in far greater quantities than from eruptionsAlso occurred during Triassic extinction and possibly part of K-Pg extinction story

Front 61

Magnitude excursion vs extinction

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