Presentation on theme: "Lecture 5 Stable Isotopes"— Presentation transcript:
1Lecture 5 Stable Isotopes Isotopes of ElementsTypes of IsotopesChart of the NuclidesMeasurementsDelta NotationIsotope FractionationEquilibriumKineticRaleighSee E & H Chpt. 5
2Key questions: What are isotopes? What are the types of isotopes? How do we measure isotopes?How do we express measurements of isotopes?What is isotope fractionation and how do we express it?What is equilibrium isotope fractionation?What is kinetic isotope fractionation?What is Raleigh distillation?What are some applications of stable isotopes?
3Isotopes of ElementsAtomic Number = # Protons = defines which element and its chemistryAtomic Weight = protons + neutrons = referred to as isotopesDifferent elements have different numbers of neutrons and thus atomic weights.Example: Carbon can exist as 12C, 13C, 14CHow many protons and neutrons in each of the C isotopes?12C = 6P, 6N13C = 6P, 7N14C = 6P, 8N1 chemical, many isotopes!
4Where do Isotopes come from? In the beginning (Big Bang), light elements of H and He were formed (and a little bit of Li)Nuclear reactions (ie: fusion) in stars created the remaining elements (and are still creating), some of which have since decayed to more stable elementsThere are 92 naturally occurring elements – some are stable, some are not
5Types of Isotopes Isotopes can be categorized into 2 categories: Stable isotopes – Isotopes that do not decay over the timescale of earth history (4.5 billion years)Radioactive isotopes – Isotopes that spontaneously convert into other nuclei at a discernable rate
6The chart of the nuclides (protons versus neutrons) for elements 1 (Hydrogen) through 12 (Magnesium).Valley of StabilityMost elements have morethan one stable isotope.b decay1:1 lineXXa decayNumber of neutrons tendsto be greater than thenumber of protons
7Full Chart of the Nuclides Valley of Stability1:1 line
8Examples for H, C, N and O: Atomic Protons Neutrons % Abundance Weight (Atomic Number) (approximate)Hydrogen H 1P 0ND P 1NCarbon 12C 6P 6N13C 6P 7N14C 6P 8NNitrogen 14N 7P 7N 99.615N 7P 8NOxygen 16O 8P 8N17O 8P 9N18O 8P 10N 0.201/2 = 5730 yr% Abundance is for the average Earth’s crust, ocean and atmosphere
9Isotope Ratio Mass Spectrometer (IRMS) How we measure stable isotopes – the IRMS1. Input as gases2. Gases Ionized3. Gases/ions accelerated in vacuum4. Gases bent by magnetic field according to mass5. Gases detected188.8.131.52.4.Isotopes are measured as ratios of two isotopes.Standards are run frequently to correct for instrument stability
10Nomenclature – δ Notation Report stable isotope abundance as ratio to most abundance isotope (13C/12C)- Why? The ratio can be measured very precisely.BUT – any differences in the isotope ratio can be very very small so we use δ (“del”) notationδ = “delta” or “del” (if you’re real savvy), units are per mil (‰)WhereH = moles of heavy isotopeL = moles of light isotopeR = H/Lδ tells us how much the sample deviates from the standard
12Standards VaryEach standard has a well defined H/L ratio
13Example 1:The IRMS standard for C is PDB (13C/12C = )Your sample has an 13C/12C =What is δ13C in ‰ for the standard?For the sample?
14Isotopic Fractionation All isotopes of a given element have the same chemical propertiesSmall differences in the distribution of the isotopes in materials because heavier isotopes form stronger bonds and move slightly slowerA heavier mass = stronger bondYou would need a stronger string to hold two bowling balls together than you would need to hold two golf balls togetherIsotope Fractionation = process that results is differences in delta values in products and reactantsExample: condensation of water vaporH2O(g) <=> H2O(l)In a closed water sample:δ 18O of H2O(g) = -1‰ (Atlantic)δ 18O of H2O(l) = -10‰ (Atlantic)Because of isotope fractionation!
15 and ε Nomenclature If = 1, no fractionation Fractionation Factor = If = 1, no fractionationIf >1, more heavy in productIf <1, more heavy in reactant is unitlessDifference fractionation Factor = εIf ε = 0, no fractionationIf ε > 0 , more heavy in productIf ε < 0 , more heavy in reactantε is in permil (‰)
16Example #2: condensation of water vapor H2O(g) <=> H2O(l)In a closed water sample:δ 18O of H2O(g) = -1‰δ 18O of H2O(l) = -10‰What is ε and of this reaction?
17Two kinds of Isotope Fractionation Processes Equilibrium Isotope effectsOccurs in equilibrium reactions (reactions can go both ways) if the system is in equilibriumChemical equilibriumPhase changes (closed system)Distributes isotopes in a system so that the total energy of the system is minimizedHeavier isotope equilibrates into the compound or phase in which it is most stably boundWithin a molecule (CO2 vs HCO3-)Between molecules (CO2(g) vs CO2(aq) )Usually applies to inorganic species. Usually not in organic compoundsDue to slightly different free energies for atoms of different atomic weightUsually temperature dependent!Differences in vibrational energy is the source of the fractionation.Heavier isotopes wind up in the compound where it is bound more strongly
18Example #3: Condensation of Water Vapor in a closed container H2O(g) <=> H2O(l)H216O(l) + H218O(g) ↔ H218O(l) + H216O(g)In a closed container:δ18O of H2O(g) = -1‰δ18O of H2O(l) = -10‰Is this reaction an example of an equilibrium isotope effect? How can you tell?Does the 18O “prefer” to be in the gas or liquid phase? Why?
19Example #4: Bicarbonate system The carbonate buffer system involving gaseous CO2(g), aqueous CO2(aq), aqueous bicarbonate HCO3- and carbonate CO32-.One step of that reaction:CO2(aq) + H2O ↔ HCO3- + H+δ 13C of CO2(aq) = 1‰ = at 0ºC and at 30ºC(The IRMS standard for C is PDB (13C/12C = ))Is this reaction an example of an equilibrium isotope effect? How can you tell?What is the final δ13C of HCO3- at 0ºC at 30ºC?Is 13C more stable as CO2(aq) or HCO3-?Is there more or less fractionation at higher temperatures?
202. Kinetic Fractionation Occurs inunidirectional (irreversible) reactionsreversible reactions that are not yet at equilibriumdiffusion or differential bond breakingHeavier isotopes move more slowly (KE = ½ mv2)Therefore react more slowlyReaction products are depleted in the heavy isotope relative to the reactantsAll isotopes effects involving organic matter are kineticWhy do heavier isotopes move more slowly?Same kinetic energy, despite isotopeE = ½ mv2If E is the same and mass increases, the v must decrease
21Examples of Kinetic Fractionation Three types of kinetic fractionation:1. Unidirectional reactionsExample:Carbon fixation via photosynthesis:12CO2 + H2O -> 12CH2O + O2 faster13CO2 + H2O -> 13CH2O + O2 slowerOrganic matter gets depleted in 13C during photosynthesis (decreases in 13C)2. Reversible reactions that are not yet at equilibriumEvaporation of water vapor if not in equilibrium (net evaporation ie: N .Atlantic)H216O(l) -> H216O(g) fasterH218O(l) -> H218O(g) slowerWater vapor gets depleted in 18O during net evaporation (decreases in 18O)3. DiffusionDiffusion of H2Oacross a cell membraneH216O(l) outside cell -> H216O(l) inside cell fasterH218O(l) outside cell -> H218O(l) inside cell slower
22Equilibrium Fractionation vs Kinetic Fractionation The difference depends on the reason for the fractionationEquilibrium fractionation occurs so that the total energy of the system is minimized via forming the most stable bonds possibleEquilibrium is related to bond stability of the isotopeKinetic fractionation occurs because smaller molecules move faster than heavier molecules and therefore react more slowlyKinetic is related to the speed of the isotope
24d13C of atmospheric CO2 versus time See Quay, 1992, Science
25Raleigh Fractionation A combination of kinetic and equilibrium isotope effectsKinetic when water molecules evaporate from sea surface (net evaporation b/c system is not in equilibrium)Equilibrium effect when water molecules condense from vapor to liquid formA isotope fractionation reaction where products are isolated immediately from the reactants will show a characteristic trend in isotopic composition.
26Raleigh Fractionation - Concept Vapor depleted in 18O compared to ocean waterAir masses transported to higher latitudes where it is cooler.Rain enriched in 18O, removed from system (cloud)Cloud gets lighterRain enriched in 18O, removed from system (cloud), but less enriched
27Raleigh Fractionation – Characteristic trend Example: Evaporation – Condensation Processesd18O in cloud vapor H2O(g) and condensate (H2O(l) rain)plotted versus the fraction of remaining vapor for a Raleigh process. Idealized:20ºC – All vapor -9‰Just colder than 20ºC – Condensate starts to form, more enriched in 18O, but is removed from the system (rained out)The vapor continues to condense as the temperature decreases – becoming more and more depleted in 18OFractionation increases with decreasing temperatureSame pattern for D/H isotopes - different scale because more fractionation during the condensation (ε = +78‰ rather than +9‰)This trend is used to reconstruct local paleotemperature from in Antartica and Greenland from ice cores
28d18O variation with time in Camp Century ice core. d18O was lower in Greenland snowduring last ice age15,000 years ago d18O = -40‰10,000 to present d18O = -29‰Reflects1. d18O of precipitation2. History of airmass – cumulative depletion of d18O
29Applications of Stable Isotopes There are many applications of stable isotopes – especially in the study of past conditions on earthThree case studies in oceanography:Paleothermometer from foraminifera shellsOrigin of organic matterEstimate primary production in marine systems
30Case study: 18O of forams in sediment to reconstruct paleotemperature HCO3- + Ca2+ ↔ CaCO3(s) + H+Fractionation of 18O is temperature dependent and well quantified in labsThe 18O of CaCO3 precipitated in forams reflects the temperaturePreserved in marine sedimentsComplicated because although thisrelationship is well defined, depends on a known 18O of water . That may changedue to ice volume.
31Case Study: Estimation of temperature in ancient ocean environments CaC16O3(s) + H218O CaC18O16O2 + H216OThe exchange of 18O between CaCO3 and H2OThe distribution is Temperature dependentlastinterglacialHolocenelast glaciald18O of planktonic and benthic foraminiferafrom piston core V (160ºE 1ºN)Planktonic and Benthic differ due to differencesin water temperature where they grow.Assumptions:1. Organism ppted CaCO3 in isotopic equilibriumwith dissolved CO32-2. The δ18O of the original water is known3. The δ18O of the shell has remained unchangedPlanktonic forams measure sea surface TBenthic forams measure benthic T
32Case study: 18O of forams in sediment to reconstruct paleotemperature Does the 18O of water in the ocean change over time?Large scale Raleigh distillationNet transfer of water from ocean to continental ice sheets make ice very depleted in 18O and the oceans enriched in 18O, increasing the d18O of water about 1‰In some cores, pore water can be measured directly, which gets around this issue.
33Case study: 13C of bulk organic matter to determine source Many people are interested in the preservation in organic matter in marine sedimentsBy looking at the d13C of an organic material, it can say something to how it was produced (marine or terrestrial) because the starting material is so different in d13CComplicated by C4 and CAM plants.C4 = grassesCrassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathwaythat evolved in some plants as an adaptation to arid conditions
34Case study: Profiles of DI13C and 18O to estimate primary productivity The profiles of DI13C and 18O can be used to estimate primary productivityMore photosynthesis in surface results in a heavier DI13C, resulting in a more positive d13C in surface DICDuring respiration, 16O is preferentially taken up, resulting in a more positive d18O “left over” in the water (obvious at O2 minimum)Why does the d13C decrease slightly at the O2 minimum?North Atlantic data