ES 9506 Notes 1_removed PDF

Course Notes 1. Introduction Light stable isotopes allow us to trace interactions through systems Stable isotopes of any element will behave almost the same in chemical reactions, but the difference in mass can change behaviour. A...

Course Notes 1. Introduction Light stable isotopes allow us to trace interactions through systems Stable isotopes of any element will behave almost the same in chemical reactions, but the difference in mass can change behaviour. At equillibrium, their difference in behaviour is predictable Very small differences in isotopes can be a powerful tool Atoms Can use Bohr model for stable isotopes Differences in neutrons dictate isotopes Atom diameter: 0.1 nm, nucleus diameter: 10−5 nm Stable isotopes don’t change, radioisotopes have halflives Da = unified atomic mass unit, 1/12 the mass of 12 C atom Dalton is credited with coming up with atomic theory Notation Atomic number = number of protons = Z Number of neutrons = N Mass number = A A=N+Z A 4 Z E ( so 2 He is normal helium ​ ​ Elements of Interest Carbon, Hydrogen, Helium, Oxygen, Nitrogen, and Sulphur Chart of Nuclides Like a cross section of the periodic table, going out in a 3rd dimension Course Notes 1  Stable isotopes are most similar to ‘base’ element, farther from the stable isotopes, the more radioactive and faster the decay Atomic Weight Most of carbon is 12 C, with a little bit of 13 C so generally C weighs between 12.0096 - 12.0116 on average Different materials have different ratios, but the average of every single individual carbon atom on earth is this Atomic weight = proportional average of all masses contributed by naturally occuring isotopes of an element Carbon = 98.892% 12 C, 1.108% 13 C atomic mass of 13 C = 13.003355 Da, of 12 C is 12 Da So atomic weight is 12.0111 Binding Energy Mass of proton = 1.007825 Da, mass of neutron = 1.008665 Da, so mass of 13 C should be 13.107605 Actual mass of 13 C is 13.00355, smaller than the sum of its parts EB = ΔMc2  ​ E is binding energy Mass is converted into energy to hold protons and neutrons together and keep atoms stable Stability Valley - the Chart of the Nuclides Course Notes 2  y axis is proton number (Z), x axis is neutron number (N) The centre ‘valley’ is full of stable isotopes. The further from the valley an isotope/nuclide is, the more unstable it is (radioactive) When mass numbers get high enough, there are no stable isotopes left - all are radioactive Some (N-Z) graph have axes reversed Most light nuclei (Z 20, N/Z > 1 Nuclear Stability Symmetry rules are still being discovered Symmentry Rule #2 Atoms with even numbers of protons have higher abundances than those with odd numbers of protons Z-N combinations of even-even nuclides are most stable and therefore most abundant Even-odd and odd-even nuclides are about equally as abundant (just under 1/3 of the abundance of even-even) Odd-odd nuclides are very rare (1/10th of abundance of even-odd or odd-even) Found unusually stable nuclides (”Magic Numbers”) Some even numbers of Z-N nuclides 48 Ca is unusually stable 40 20 Ca: 96.94% ​ 48 20 Ca : 0.187% ​ Should be so unstable we can’t measure it before it decays, but its binding energy is much higher than normal (even higher than 40 20 Ca) ​ Tin has 10 stable isotopes, which is a lot Course Notes 4  Z = 50 (magic number) Even-even have higher abundance, and stable isotopes are more abundant than radioactive Nuclides further from the valley of stability have shorter and shorter half lives Solar System Abundance of the Elements Course Notes 5  Some elements don’t follow the normal pattern Fe is higher than expected Iron has the highest binding energy of all the elements, making it more abundant Li, Be, B, and F have very low despite being even Research on our own - why is this? Isotopes versus Isotopologues H2 16 O (9977/1000), H2 17 O, and H2 18 O are isotopologues - the same ​ ​ ​ compound with different isotopes in them Each isotopologue of water will behave slightly different H is assumed to be 1 H (protium), when 2 H (deuterium) is just 2 H Multiply Substituted Isotopologues (Clumped Isotope Geochemistry) Course Notes 6  More than one element has isotopic substitution Distribution of heavy isotopes in compound is directly related to the temperature that a compound formed in Application: Wheatley, Ont kept having spontaneous methane explosions Methane formed in the earth forms at high temps, formed by microbes forms at low temps so analysis can help find which is causing the explosions Common Light Stable Isotopes 1 H, 2 H, 12 C, 13 C, 16 O, 17 O, 18 O, 14 N, 15 N, 28 Si, 29 Si, 30 Si, 32 S, 33 S, 34 S, 36 S, 40 Ca, 42 Ca, 44 Ca, 48 Ca ‘Is Chemistry a Branch of Physics?’ An old debate that claims chemistry is the physics of electrons But - mass comes from nucleus and mass differences in isotopic reactions aren’t affected by electrons Fractionation Course Notes 7  Pterodactyl Fractionation Analogy Heavy pterodactyls fall out of the air first, so concentration of light pterodactyls, and continues until only the lightest pterodactyls are left in the air Isotopes of the same element react similarly, but the small differences will lead to fractionation (either staying in the reactant, or moving to the product) Plants preferentially take 12 C and leave behind 13 C (biological fractionation) Heavy isotopes of water come down in the rain, light isotopes stay in the cloud (physical fractionation) Equilibrium fractionation is a thermodynaamic property of atoms Depends on mass and temperature Use fractionation to trace movement through reactions/processes/cycles Nomenclature δ = (Runknown ​ − Rstandard )/Rstandard  ​ ​ R = 2 H/1 H (light isotope is generally the denometor, and the more common isotope) Standard for O, H = VSMOW (Vienna Standard Mean Ocean Water), for C = VPDB (Vienna Pee Dee Belemnite (a fossil)), for N = just air Units are º /ºº , parts per thousand/mil (french mil, not million) or mUr ​ (mili/micro Urey) in Coplen protocol (not common) α is Fractionation Factor α = Ra /Rb  ​ ​ R = 2 H/1 H (light isotope is generally the denominator, and the more common isotope) a and b are phases Course Notes 8  Ex, phase a = precipitation, phase b = water vapour Relationship between α and δ αa−b = (δa + 1000)/(δb + 1000) ​ ​ ​ ε is Enrichment Factor εa−b = αa−b -1 ​ ​ Ex, phase a is CO2 gas, phase b is H2 O water ​ ​ ε > 0 = CO2 is enriched in heavy isotope of O, compared to H2 O ​ ​ ε < 0 = CO2 is depleted in heavy isotopes of O compared to H2 O ​ ​ Generally given as per mil fractionation - εa−b = (αa−b -1) x 1000 ​ ​ Fractionation values are generally really small, so this makes the numbers easier to work with Oxygen isotope Example εCO 2 −H 2 O = αCO 2 −H 2 O − 1 ​ ​ ​ ​ ​ ​ εCO 2 −H 2 O = 1.04037 − 1 ​ ​ ​ εCO 2 −H 2 O = 0.4037 ​ ​ ​ ε > 0, so enriched in heavy isotopes Δ δa -δb = Δa−b ≈ 1000lnα1−b  ​ ​ ​ ​ Temperature-Dependent Fractionation 1000lnα1−b on y axis, 106 /T2 (T in Kelvin) on x axis has a linear relationship, ​ increasing as temperature decreases Course Notes 9  Can be used as a thermometer Isotopic Exchange Reactions Focus in on just the O isotopes to make equation for α Missing temperature, since isotope fractionation is mass and temperature dependent acalcice−H 2 O = 1.02881@25ºC for oxygen isotopes ​ Calcite = CaCO3  ​ δ18 Ocalcite = +27.0ppt ​ δ18 OH 2 O =? ​ ​ 1.02881 ∗ 1000 = +28.8ppt = Δa−b − δa − δb  ​ ​ ​ Δ18 Ocalcite−H 2 O = δ18 Ocalcite − δ18 OH 2 O  ​ ​ ​ ​ ​ +28.8 = 27.0 − δ18 OH 2 O  ​ ​ δ18 OH 2 O = +27.0 − 28.8 ​ ​ δ18 OH 2 O = −1.8ppt ​ ​ Course Notes 10  CF-IRMS: Continuous Flow Isotope Ratio Mass Spectrometry Faster, automated, smaller sample Autosampler → combustion tube → reduction tube → Mg perchlorate water trap (no water allowed in mass spec) → gas chromatography column → thermocouple detection → mass spec Gas Chromatography (GC)-Combustion CF-IRMS Compound specific - looking at individual compounds in organic matter Ex, looking at the individual AAs (compounds) in collagen (organic matter) Capillary GC separates compounds → combustion → water trap → reduction → CO2 trap → mass spec High Temp Conversion Elemental Analyser - TCEA-CF-IRMS Able to separate O and H from organic and inorganic materials/compounds using high temps Samples go through combustion over → water trap → GC column → mass spec LSIS-AFAR Lab at AFAR for biology based stuff Stable Isotope Analytical Revolutions Laser-based systems, no mass spec needed Most common for analysis of water, CO2, CH4, etc (molecules directlyO) Cavity Ring Down Spectroscopy - CRDS Chamber lined with highly reflective mirrors, and laser than can introduce a given wavelength into the chamber The laser bounces around so path length is much longer than direct length Course Notes 11  Mirros reflect ~99% of light, but leak a tiny amount When laser is turned off, the light decays away and the time this take is determined by the sample Laser is tuned to be unique to a molecule’s frequency When sample is added, both mirror and sample are absorbing laser The decay of light (ring-down time) will be shorter with sample than without, since more things were absorbed Difference between ring-down time with and without sample will be unique to the isotopologue measured Can measure things like atmospheric water in real time Off-Axis Integrated Cavity Output Spectroscopy - OA-ICOS Secondary Ion Mass Spectrometry - SIMS Used for in situ analysis for geochem, cosmochem, geology, material sciences Makes a ‘splatter’ of ions to be sent through magnetic tubes and them counted Can be done on a very small scale Measure oxygen and lead isotopes in zircon crystals 40 um in size to find age (lead) and isotopic composition (oxygen) to find conditions of formation (Laser Ablation) Multi-collector Inductively Coupled Plasma Mass Spec - (LA)-MC-ICPMS Used for ‘heavy’ metal stable isotopes (Fe, Cu, Zn, Mg, Ca, Co, Ni, Mo, Sn) Fractionation is much smaller and requires more delicate analysis Used in cosmochem, geochem, planetary science, ecology, medicine 2. Stable Isotopes in the Atmosphere & Hydrosphere Course Notes 12  Can trace movement of water through the water cycle Ice cores, tooth enamel, etc Why is there Fractionation? Potential energy curve for diatomic hydrogen (H2) Course Notes 13  Hydrogen atoms have a positive attraction, falling along the potential energy curve Atoms want to minimize the amount of energy needed to associate with another atom H-H takes more energy to associate than D-D (D is deuterium) Interatomic distance can only come so close before repulsion occurs Lowest actually possible potential energy is the zero-point energy, the minimum potential energy that must exist Even at 0ºK, atoms have vibrational frequency (E= 1/2 hv, planks constant) Distance from zero-point energy to dissociation energy is closer for H-H than D-D, so less energy to break bond between lighter isotope (H-H) than heavier isotopes (D-D) Energy values are different for each system, but principle stays the same Equilibrium Isotope Fractionation Generalisation Light isotopes are more reactive (LIAR) Takes less energy to change state Large mass differences show greater fractionation H → D system will have greater fractionion than O16 → O18 system due to the relative difference in mass H → D is 2x the mass, whereas O16 → O18 is only a 12% increase in mass Elements with solids, liquids, and gases stable over a wide temp range show more isotopic variation Heavy isotopes perfer solid over liquid, and liquid over gas Lighter isotopes will preferentially evaoprate in the water system, heavier isotopes will freeze faster Course Notes 14  Heavy isotopes prefer the highest oxidation state Biological systems are best explained using non-equilibrium (kinetic) fractionation Light isotopes are favoured in reaction product relative to starting material Vapour pressure of water is inversely proportional to mass Lower mass isotopologue, higher vapour pressure Higher vapour pressure, preferentially enters vapour stage During evaporation, water vapour is enriched in 16O and 1H relative to the liquid Mostly an equilibrium phenomenon, but some kinetic effects to be considered Equilibrium Isotope Fractionation Factors for Water (liquid-vapour) Fractionation factor decreases as temperature increases Oxygen Isotope Fractionation of Precipitation Course Notes 15  Ocean is 0‰ (VSMOW) Vapour from ocean δ is 9‰ lower than ocean water, to be expected since lighter isotopes evaporate first So vapour in first cloud is -9‰ at 20ºC, but when rain occurs the heavy isotopes leave (at 0‰) making the cloud smaller and lighter (-15‰) In second cloud, fractionation difference between vapour and rain is larger (10‰) than first cloud (9‰) Third cloud is difference of 15‰ with a phase change to solid instead of liquid (and lower temperature) Vapour is distilled through condensates changing in isotopic variation Most clouds form over the ocean and then move over land, leading to this whole process Rayleigh Distillation Model Course Notes 16  Rv /Ro = f(α−1)  ​ ​ (δ18 Ov + 1000)/(δ18 Oo + 1000) = f(α−1)  ​ ​ Rv - vapour remaining in air mass (δ) ​ Ro = vapour in original air mas (δ) ​ f = fraction of vapour remaining in air mass α = isotopic fractionation between liquid and vapour (at a given temp) More condensation removed from cloud, more depleated vapour becomes in 18O, then more depleated precipitation Fractionation increases Assumes air mass (cloud) is a closed system during formation Nothing being added or subtracted except for the single factor (precipitation) Not always true in reality but ignored for modelling Course Notes 17  Transpiration from plants, re-evaporation, condensation en route, etc Assumes equilibrium Atmospheric Circulation & Latitude Dependence of Precipitation δ18O and δ2H About 50% of moisture lost at equator Only 10% moisture lost at high latitues At equator, δ values are pretty high (-2 and -6), whereas high latitues are very low (-15 and -110) Global Distribution of Precipitation of δ^{18}O Course Notes 18  Latitude effects Very low values at poles, with highest values around the equator Drinking water and eating plants from an area will affect someone’s isotopic composition, allowing for migration tracing Altitude effects West coast of South America has lower than expected due to mountain ranges affecting moisture mass (cloud) movement Amount effects Things like monsoons moving large quantities of moisture at once Global Meteroric Water Line (GMWL) Distribution of H and O compositions fall along a line of δ2 H = 8 δ18 O +10 Course Notes 19  So δ2 H of any sample can be found if you know the δ18 O value Why is slope 8? Why doesnt GMWL pass through composition of the seawater (red dot, 0‰ VSMOW) Condensation and Evaporation Water vapour over seawater (0‰) has lower δ2 H and δ18 O than at equilibrium, as evaporation is non-equilibrium under most conditions Global meteroric water line - δ2 H = 8*δ18 O + 10‰ Crosses δ18 O 0 line at +10 δ2 H Course Notes 20  Actual marine atmospheric vapour at equator is lower than would be calculated with Rayleigh Distilation Model Kinetic isotope effects (mass of isotopologue, etc) Blue box has relative humidity of approx 82-85% Condensation of vapour is considered an eqilibrium process, so distance from vapour to rain (condensation) would be fractionation (controlled by temperature) After condensation, remaining vapour is lower Ocean water doesn’t fall along GMWL Deuterium (d) Excess More deuterium than expected, so +10 δ2 H at 0 δ18 O Deuterium Excess GMWL doesn’t intersect δ2 H and δ18 O seawater due to equilibrium and kinetic fractionation of evaporation Kinetic effects based on molecular mass ( 1000C Temp is so high that fractionation is effectively 0, so mantle water should be about the same as rock (6‰) What about δ2 H? Some minerals (like phlogophite from kimberlite) contain H and should have isotopic comp. similar to mantle δ2 H Course Notes 45  Kimberlite is formed by explosions deep in the mantle and carry diamonds from high temp to surface, and brings phlogophite too Phlogophite δ2 H = ~50‰ Using hot springs and volcanic eruptions, found δ2 H stayed the same while δ18 O increased Most starting points intercepted the meteroric water line at the isotopic composition of that area Juvenile water has expected δ values, and doesn’t start at MWL like others Freshwater and mantle water may be mixing at geysers, since lines up in H with juvenile water In Niland (hot spring), water is originating from fresh water but getting more and more enriched in 18 O as temperature increases Rocks with high δ18 O is exchanging with water (low δ18 O) at high temps, the hotter it is, the more exchange occurs Hot springs come from local meteoric recharge in the ground that’s been heated, not from the mantle itself Has a low W:R ratio since water is changing in δ18 O, so lots of rock compared to water Why doesn’t δ2 H change? Most minerals in igneous rocks are made of silicates which are high in O but very few have H There isn’t any hydrogen to exchange with Water-Rock ratio If W/R is high (lots of water, little rock), rock δ18 O will decrease and move towards water’s δ18 O If W:R is low, water δ18 O will increase and move towards rock’s δ18 O 18 18 If W:R is close to 50:50, water δ O increases and rock δ O decreases Course Notes 46  Temperature Without heat, no exchange occurs Higher temperatures = more exchange occurs Fractionation between water and rock is dependent on temperature, fractionation decreases as temp increases Hot Spring Water Geothermal Power A heat source in the earth causes groundwater to circulate as hot water rises Rising hot water exchanges with rocks to inc. δ18 O and then erupt as a hot spring (if W:R is low/ lots of rock compared to water) Porewater (water that becomes trapped in rocks) Sediment cores can be taken of lake or ocean bottoms and are filled with water, fossils, minerals, etc Things trapped in sediment can be used as proxies to learn about the environment they formed in Sediment layers of cores can be dated and act as a calendar/ruler Water trapped in pores usually comes from whatever water source deposited it Ocean porewater Can be taken at huge scales Course Notes 47  Common pattern appears, δ18 O decreases deeper in the core Shows that what is now open ocean likely used to be a brackish water environment and has shifted due to sea water rising and falling Seen both near current coasts and in the middle of the atlantic, continents have shifted a lot The older brackish environment had a lower δ18 O value that was trapped as porewater and preserved Glacier melt can also contribute Continental glaciers are freshwater and form at poles with very low δ18 O, so if a huge glacier melts into the ocean it can mix and lower the δ18 O and increase the sea levels Need to take context of formation into account, not just isotopes themselves Secondary Mineral Formation in Pore Space Along with water inside pore spaces, other minerals can form there too So porewater is no longer representative of it’s starting δ18 O Minerals formed are enriched in 18 O (level of enrichment depends on temperature) Preferential uptake of 18 O means water is depleted of the heavier isotope, and the amount of depletion depends on Course Notes 48  fractionation (which itself is temperature dependent), and W:R ratio Deeper into the earth, higher temperature so more mineral formation and fractionation, further depleting water When sediment and water interface, there’s a high W:R ratio but when more sediment piles on the W:R ratio is lower as depth increases Behaves as a closed system, like Rayleigh Distillation Formation Water and Brine Rock units are called formations, so water traped inside is called formation water Very old pore water Very high salinity, 5-30% (ocean is 3.5%) Found several km deep Rocks are marine in origin So expect formation water to be old sea water with δ18 O of ~0 ppt Course Notes 49  δ18 O of brines All slopes extrapolate back to about the current local values on the GMWL Doesn’t fall at seawater values - why? 5 theories: Evaporation δ18 O and δ2 H don’t extrapolate back to seawater Usually evaporation lines have more shallow slopes and sit to the right of GMWL Also usually have slopes between 4-6, but these are all more varied Can’t explain data by itself but might be a factor Isotopic Exchange Exchange between MW and O-bearing phases (carbonates) and MW and H-bearing phases (clays and hydrocarbons aka clastics) Course Notes 50  Dependent on isotopic composition of phases, W:R or water to hydrocarbon ratio, and temperature Carbonates are 18 O-rich, but clastics have low δ2 H So, isotopic exchange should cause δ2 H to decrease in formation water, would cause negative slopes δ2 H is increasing, so exchange isn’t the main factor at play Formation of New Minerals Mineral formation preferntially takes up 18 O, so water should become depleted and δ18 O would decrease δ18 O is increasing, so this isn’t the cause Ultrafiltration New sediment added to a system weighs down and compacts shale systems This weight and squishing sends water from inbetween minerals up in the system Like stepping on mud and watching all the water come up around your foot Shale particles act like an electric-charged membrane Course Notes 51  Salts are prevented from moving up, and heavy isotopes stay behind while light isotopes pass through So increases δ18 O and δ2 H BUT: in experiments, can only enrich δ18 O by 1-2 ppt and δ2 H by 10-15 ppt, so not enough to explain the increases seen Application: Nuclear waste storage proposed in rock repositories that hasn’t had any evidence of water coming in for 450 million years, so likelihood of leaking through water is so tiny Tested by taking cores and looking at isotopic changes in the layers If porewater is becoming more negative, it suggests freshwater is incoming some how and therefore not a good site Mixing Between meteroric (fresh) water and juvenile water (water from mantle)? Course Notes 52  Mixing gulf coast with JW? no Mixing california and illinois with JW? no Mixing alberta and JW? possible Precambrian igneous rocks extend >40 km to mantle, so water needs to travel a long way to mix with meteroic water So works on the graph but no, not realistic in context Between meteroric (fresh) water and sea water? Course Notes 53  Mixing doesn’t work for any option Between meteroic water and evaporated seawater (brine)? When evaporating freshwater, creates a classic LEL line When evaporating sea water, creates a curve that decreases after a while Highly evaporated brines Course Notes 54  Evaporating seawater concentrates the solutes within it (sodium, calcium, Mg, chloride, etc), which causes water molecules to organise themselves around these charged solutes Hydration shells of water form around the solutes when the concentration becomes high enough Creates a third phase (liquid water, water vapour, and water in hydration shells) instead of just two (liquid and vapour) Hydration shell takes up heaviest isotopes so remaining free water decreases in δ18 O after concentration is high enough (critial concentration) Possible mixing explaination! Course Notes 55  (black) a Na-Cl brine at 4x sea water concentration at high humidity (~80 RH) reaches critical concentration at 10x seawater concentration (blue) larger curve in arid concentrations Mixing between meteroic water and highly evaporated ocean water best explains the chart Ontario Abandonded Works Program - The Problem Lots of abandonded oil and gas (O&G) wells across Ontario that lack any records of original depth, date, etc O&G comes bubbling up but we don’t know where it came from Pump hydrolic cement down to plug the hole, costs ~$1 million to do, but cheaper if you don’t need to plug all the way down. If we know a well is shallow, plug down to just under the well. Combination of formation water and the δ18 O of the O&G used to determine depth Formations have unique set of O, H, and C isotopic compositions Course Notes 56  Aquifers in SW Ontario are separated by impermeable layers that prevent them from mixing together SW Ontario Formation Water Course Notes 57  Shallow aquifers follow GLMWL (great lakes) Finding these isotopic compositions in formation water around a well means a shallow plug is needed Some very very low δ values in shallow aquifers, from pleistocene meltwaters of glaciers For deeper aquifers, the isotopic composition of the O&G itself can give the depth Modern Ocean Water Assumed to be 0 ppt for δ18 O and δ2 H with salinity of 3.5% Not actually homogenous Course Notes 58  Can still monitor sea animal migration Variation caused by: Evaporation (δ2 H = s x δ18 O) Highest evaporation in red areas (high δ18 O and δ2 H), which correspond with tropic of cancer and tropic of capricorn Heat and wind in those areas cause increased evaporation Relationship between salinity and ocean water δ18 O and δ2 H Higher salinities increase δ18 O and δ2 H values, as predicted for evaporation Different slope for δ18 O and δ2 H though Course Notes 59